Loss of power when engine is under moderate to heavy load at normal operating temperature

Tiny
EK2000D
  • MEMBER
  • 1995 PONTIAC GRAND PRIX
  • 3.1L
  • V6
  • FWD
  • AUTOMATIC
  • 135,000 MILES
I am working on my aunt's 1995 Grand Prix (she has no extra money, so I am donating my time and money in an attempt to repair the drivability issue with her car) with the "in-between" OBDI&II diagnostic port. I can't pull any codes that then engine may be setting. Not that this is necessarily a problem, as the check engine light is not on (according to my aunt, it has not historically come on), and it does not come on during vehicle operation.

The car starts up fine. When the engine temperature is cold (coolant temperature gage is below "C"), it idles higher than when the engine is at normal operating temperature (anywhere between 190 and 210 degrees F). When the engine temperature is cold, the engine can be revved in neutral (by stabbing the accelerator), and you have the expected snap increase in engine rpms. As the engine is warming up, stabbing the accelerator results in the engine hesitating / stumbling before the engine rpms increase. As long as the revs are above idle, the accelerator can be stabbed, and the engine rpms immediately increase. If, however, the engine rpm is allowed to return to idle, the next stab of the accelerator results in the aforementioned hesitation / stumble before the rpms start to increase. This hesitation / stumble gets gradually worse as the engine approaches normal operating temperature. All of the above events take place when the car is in neutral (park).

If the car is driven when the engine is at normal operating temperature, the car will accelerate smoothly if you press the accelerator about 1/4 of the way down. Pressing the accelerator down further than this will cause the engine to start to periodically stumble. The further down your press the accelerator, the worse the stumbling becomes. If the accelerator is floored, the engine has trouble maintaining the vehicle's rate of progress from before you stepped on the accelerator, and it never gets better / remains stuck in a violent bout of hesitation (no matter how long you keep your foot on the floor. I kept it down for about 1/8 of a mile with absolutely no change). If the car starts to pull a hill on the interstate, the stumbling prevents the vehicle from accelerating up the hill. And, if the hill is sufficiently steep, the engine will not be able to maintain the vehicle's speed. You never get any black smoke out of the exhaust during engine operation, which makes me think (OK, guess) that the engine may be running lean when it stumbles.

I have worked on several different versions of GM's 60-degree V6 over the years, and (over two months ago) figured I could easily determine the cause of the hesitation. Things I have done to the motor so far (in roughly the order I did them):

- Upper and lower intake manifold gasket replacement with the new upgraded FelPro metal gasket set (I was going to do this anyway, as my aunt's car was showing the classic leaking intake manifold sign of oil in the coolant);
- Removal and cleaning of the throttle body and all associated sensors (anti-stall sensor (?) Up top, and the throttle position sensor on the bottom);
- Replacement of the cam shaft position sensor (noticed one of the wires on the top of the sensor was broken while preforming the intake manifold gasket replacement);
- New spark plugs (my aunt had the local dealer do that, not me. And she paid almost $15 per AC Delco plug when I could have gone to Autozone and purchased them for $6. Grrrrrr, she doesn't have that kind of money to spend!);
- Swapped out spark plug wires (have many extra plug wires lying around the garage, and swapped many of the wires on my aunt's car out while diagnosing a bad ignition coil pack. Also replaced said coil pack with one of my many junk-yard coil packs lying around the garage);
- Visually inspected all wires leading to and from the ignition control module (on which the coil packs sit) for signs of missing loom, wire pinching or chafing. Nothing appeared out of the ordinary;
- Beat on the top of the engine management computer while increasing engine revs to see if the hesitation changed (it didn't);
- Beat on the throttle position sensor while increasing engine revs to see if the hesitation changed (it didn't);
- Tested for good vacuum at the intake manifold and the vacuum reserve canister (vacuum was within spec at idle, and would briefly fall to zero when you stabbed the accelerator before recovering);
- Tested for properly functioning fuel pressure regulator (fuel pressure jumped up about 12-15 psi when the vacuum line was removed from the regulator. I did this about two months ago with a seemingly good functioning fuel pressure tester, and not with the tester mentioned in the next bullet point);
- Tested for proper fuel rail pressure (this was a bit of a challenge, because the Autozone lend-a-tool fuel pressure tester didn't seem to be operating quire right. The last borrower had ripped the needle and needle gasket out of the fitting that screws into the fuel rail. I was able to create a new gasket with some vacuum hose, and reseated the needle, but fuel pressure seemed to rise slowly in the gauge. Ultimately, the fuel pressure rose to the lower end of the acceptable operating range, about low 30's psi);
- Replaced the fuel filter (I think this was unnecessary, as the old filter wasn't very heavy);
- Replaced the fuel pump / pump screen / fuel tank emission hoses (second guessed myself because of the issues with the fuel pressure gauge. Just finished this project this afternoon, and it had absolutely no effect on engine performance. Aaargh!)

I don't like throwing parts at a problem. I have done my best to verify voltages and pressures before purchasing replacement parts. I have now checked everything I can think of, except for the up-stream O2 sensor (this being OBD1-ish, I don't think it has a downstream sensor). I just spend $100 on the fuel pump and strainer, and it didn't fix the problem. This makes me angry (with myself for not being more patient). I don't want to purchase an O2 sensor for another $75 / $100 if it won't fix the problem.

As far a diagnostic tools are concerned, I have a vacuum gauge and a voltmeter. I can borrow the fuel pressure gauge if need be. I do not have access to an ocilliscope.

Can you please suggest anything else I may need to check (and how to check it) before I purchase the O2 sensor? Thank you in advance for your assistance.

Eric
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Saturday, December 28th, 2013 AT 2:12 PM

25 Replies

Tiny
JACOBANDNICKOLAS
  • EXPERT
You have basically done everything. I do, however, have a couple questions. Since this is a OBD change year, does your vehicle have a DIC (digital ignition control) unit that the coils mount to? Also, have you checked engine compression? Cleaned injectors?
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Saturday, December 28th, 2013 AT 2:49 PM
Tiny
EK2000D
  • MEMBER
JacobandNickolas,

Thank you for your reply. I apologize in advance, but I sometimes struggle to identify parts by their proper industry-given names.

I know that the ignition coils on this engine are mounted to a bracket that bolts to the outside of the rear cylinder bank. When you mount the coils to the bracket, their mounting screws go through what I consider to be an ignition module (it sits between the bracket and the coils, and the 12-volt feed, camshaft position sensor and crankshaft position sensor all plug into this module). I do not know if this module is digital, but I can check if you tell me what to look for.

I checked compression on the front bank of cylinders (easy access), and all cylinders were within spec (about 160 psi if I recall correctly). I don't have any reason to think there's a problem with the rear bank, but will check compression there if you feel it would be helpful.

I had each individual injector removed from the fuel rail while I was replacing the lower intake manifold gaskets (I replaced the upper and lower O-rings on the injectors, just because I was there). The injectors didn't appear to be clogged or dirty (that's just from looking at them. I don't know how good of an analysis that is). I've had my aunt run a couple of bottles of Chevron Techron fuel system cleaner through the fuel system. No change in performance. I'm open to learning how to test injector performance.

Eric
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Saturday, December 28th, 2013 AT 3:08 PM
Tiny
EK2000D
  • MEMBER
Also, (I almost forgot to mention) just in case it was a clogged catalytic converter, I disconnected the header pipe from the converter pipe, and then started the engine. The engine sounded fantastic (sounded like a dragster!). But, no change in the hesitation / stumbling. I don't think it has to do with the converter.
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Saturday, December 28th, 2013 AT 3:28 PM
Tiny
JJIMENEZ88911
  • MEMBER
Agreed seems you have covers most bases I was leaning towered cat being plugged until I read further down. Now aside from disconnecting of header have you checked and monitored vacuum under acceleration although intake vacuum may show normal at idle watch for drastic drop under acceleration does not sound like cat plugged but to rule out EGR flow issue or even so check EGR valve.
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Saturday, December 28th, 2013 AT 8:42 PM
Tiny
EK2000D
  • MEMBER
Thank you for the reply. I appreciate everyone's patience as I work through this repair.

I am going to check for proper EGR valve function first, as it is the easier of the two suggestions to test AND I found a Wells PDF with a diagram of my 3-solenoid digital EGR valve with a four wire connection (no reference voltage circuit, I guess) along with solenoid testing instructions. I will also check the EGR passageway to the intake manifold for blockage. I am sure I cleaned this as part of the intake manifold gasket replacement, but will check it again. I will be testing the EGR valve and checking the passage this afternoon, and will report back with my findings.

Before I forget, I have also swapped out the MAP sensor (the sensor that sits just behind the upper intake manifold) with a known good one from another 3.1l, and it had no effect on the hesitation / stumbling issue. Just wanted to mention that.

Assuming the EGR valve and passageway checks out, I'll test vacuum under acceleration (will have to rig some extra long vacuum hoses to my vacuum gauge that I will be holding in the car as I drive. Should be fun).

Continued thank-yous.
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Sunday, December 29th, 2013 AT 9:08 AM
Tiny
JACOBANDNICKOLAS
  • EXPERT
Hi Again:

I believe what you have described to me is the DIC. It is what the coils mount to and receive power from. If that is what you are dealing with, remove it and have it checked. Also, you have checked the ignition module too, correct?
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Sunday, December 29th, 2013 AT 11:57 AM
Tiny
EK2000D
  • MEMBER
All, this is my afternoon update.

I have finally found something that appears to be wrong. More in a second.

But first, I did remove and check the EGR valve. I charged the voltage input connector on the valve with 12 volts (from the car battery using a jump wire). When I grounded each of the solenoid connectors, I heard (and felt) a solid "click" indicating that all three solenoids were functioning. I also checked the resistance across the three solenoids, and they were all in spec (between 12 and 22 ohms, depending on the solenoid). I also re-probed the passage between the EGR valve and the intake manifold (opened the throttle body butterfly and visually watched the wire that I had threaded through from the EGR valve base move into and out of the throttle body). Everything was as clean as a whistle.

Next, I conscripted my significant other to hold a vacuum gauge while I drove the car down the road. I had run about 5 feet of emissions / vacuum hose from the accessory vacuum port on the intake manifold to the hand-held vacuum gauge that was being held by my passenger. When the car was started (engine was already at normal operating temperature), I had about 18 in. Hg of vacuum (or -60 kpa if you prefer).

When I was cruising down the road at 45 mph, I had about 15 in. Hg of vacuum. When I was cruising down the road at 55 mph, I had about 12-15 in. Hg of vacuum.

Now, when I would accelerate normally from a stop (1/4 throttle), the vacuum would drop to 7-10 in. Hg and would not recover until I back off of the throttle. Depressing the throttle further while accelerating would continuously lower the in. Hg until, at full throttle, there would be no (or possibly negative?) Vacuum continuously until I took my foot off of the throttle.

I thought, when you accelerate, the vacuum was supposed to temporarily drop and then recover. This is not what I witnessed when driving the car.

I also thought that, while doing the intake manifold gasket replacement, I checked all vacuum connections. So if there's a vacuum leak somewhere, where would I start when looking for it?

Thank you again for your assistance.

Oh, almost forgot. I have another DIC I can use to swap out the one on my aunt's car to see if that has any effect on the condition. It has a set of good ignition coils already mounted on it, so it shouldn't take me to long to check this. I will let everyone know.
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Sunday, December 29th, 2013 AT 12:55 PM
Tiny
JACOBANDNICKOLAS
  • EXPERT
I hope it does the job. Let me know.
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Sunday, December 29th, 2013 AT 5:13 PM
Tiny
EK2000D
  • MEMBER
OK, a Monday update. Swapped out the DIC (along with the mounted ignition coils) on the '95 Pontiac Grand Prix with a known good one (used it on another car in the past). There was no change in the hesitation / stumbling. It's still there, and it's still bad. The car is still unsafe to drive at highway speeds or up steep grades because of this amazing lack of power on acceleration.

I did some research on the internet earlier this evening, and determined that zero vacuum at wide-open-throttle isn't necessarily a bad thing. So I don't know if my vacuum measurements during yesterday's drive were necessarily bad (18 in. Hg @ idle, 10-15 in. Hg @ cruising speeds, 0 in. Hg at full throttle, and 22 in. Hg during full deceleration). I'm thinking the vacuum test didn't turn up anything unusual. Can anyone please confirm these vacuum test results as being acceptable?

I'm going to test the signals for the engine computer to the EGR Valve to ensure the EGR Valve solenoids are getting good (grounded) signals from the computer. If this all checks out, I'm going to test the MAP sensor to ensure good signals from the computer (downloaded directions from the internet for both procedures).

Is there something else that a mechanic feels I should be investigating instead of spending time checking the EGR Valve / MAP sensor electronics? Please let me know.

Thank you,

Eric
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Monday, December 30th, 2013 AT 5:59 PM
Tiny
EK2000D
  • MEMBER
Ok, another Monday update.

I have been trying to test the functionality of my digital EGR Valve while it's on the car. According to the article in "Wells Counter Point, Volume 3, Issue 4, October 1999", I should be able to continuously ground each of the three solenoids individually, and I should witness a "momentary drop in idle speed", after which point "The computer will immediately compensate for the drop in idle speed by opening up the idle air-control valve to increase rpm."

Well, when I ground the solenoid powered by Terminal D, the engine rpms slow, but do not recover to their normal state until I un-ground the solenoid. If I ground the solenoids powered by Terminals B or C, the car will stall (instantly with the grounding of Terminal C, and after several seconds of the grounding of Terminal B).

So, it appears that, for some reason, the engine is NOT compensating for the drop in idle speed. What could cause this condition? The engine is up to operating temperature. The only thing I don't know how to check is if the engine is idling in "closed loop".

Does the above information help anyone with my potential problem?

Thank you,

Eric
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Monday, December 30th, 2013 AT 6:46 PM
Tiny
EK2000D
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One last Monday update:

Checked the MAP sensor voltage as I manipulated the vacuum experienced by the sensor. All sensor voltages checked out to spec with the experienced vacuum, so the sensor is OK.

What should I check next in order to determine the cause of the engine hesitation / stumble upon acceleration?

Eric
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Monday, December 30th, 2013 AT 7:33 PM
Tiny
EK2000D
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Another update (if anyone is still monitoring this feed):

I tested the Idle Air Control Valve for continuity between the two different circuits (it tested OK).

I have been to the local library, and made a copy of the ECM wiring diagram. I have been confirming that voltage is at least making it to the various sensors and valves on the engine. What I don't know is the exact voltage I should be seeing at each of the sensors / valves. Does anyone know where I can find expected voltages while the engine is in operation?

It seems that the engine is operating under some kind of preset sensor values, and is just ignoring any information being provided by the sensors. I've unplugged the Idle Air Control Valve and the Throttle Position Sensor while the engine is running, and it has no effect on the engine operation. As mentioned earlier, grounding individual EGR solenoids will cause the engine to stall. Pulling the fuel pump relay while the engine is idling doesn't stall out the engine (although the wiring diagram does show a wire running straight from the ECM to the fuel pump without going through the relay first, so that may explain this). Does anyone know where I should be looking (using the wiring diagram) to try and confirm proper ECM operation?

Eric
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Sunday, January 5th, 2014 AT 12:39 PM
Tiny
JACOBANDNICKOLAS
  • EXPERT
Eric:
I'm still here. Won't leave until we figure this out. Listen, do you have specific sensors you are checking voltage to? I will try to find them in my manuals for you.

Let me know.

Joe
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Sunday, January 5th, 2014 AT 2:42 PM
Tiny
EK2000D
  • MEMBER
Joe,

Well, given that I believe the issue involves the air/fuel ratio, I've been concentrating my efforts on the MAP Sensor, TPS, Idle Air Control Valve and EGR Valve.

I did another test this afternoon (in the 34 degree rain. Nice) of the TPS while it was mounted to the car. I was able to verify that the 5-volt reference circuit is good, and that the ground circuit was also good. I then both slowly and quickly opened and closed the throttle body flap, and watched the voltage from the TPS signal circuit fluctuate between 0.630 volts and 4.500 volts. The fluctuations appeared smooth, although my digital voltmeter took a few seconds to update the signal voltage from the TPS on the display (meaning, if I quickly opened the flap, my voltmeter would temporarily show 0 volts before showing the new, higher voltage). I figured my voltmeter may be a bit wonky, so, to rule out a bad TPS, I 1) hooked up an ancient analog battery tester to the circuit, and it showed the needle moving smoothly up and down as I adjusted the flap, and 2) I installed my spare TPS on the throttle body, and it provided the exact same results (for both meters) when manipulating the flap.

I feel (correct me if you disagree) that my TPS is OK, and it has a good connection to the ECM. Further more, the wiring diagram shows that the TPS and the MAP Sensor use the same 5-volt reference circuit and ground circuit on the ECM. So, I would like to assume that, if the two circuits are good for the TPS, they should also be good for the MAP Sensor (I have already verified that the MAP Sensor is putting out correct voltage as vacuum to the sensor is changed). I am hoping I can rule out any issues with the Intake Air Temp Sensor, MAP Sensor and TPS as a result.

I know that the EGR Valve itself is working, but I have no idea how to tell if the ECM is sending it the correct signals to open or close the various solenoids. Not sure if you feel this is something worth investigating, but I will test for solenoid activation while the engine is running if you can give me an idea for how to do this.

I would also like to test the signals that the ECM is sending to / receiving from both the camshaft position sensor and the crankshaft position sensor. Both of these sensors are directly connected to the fuel injector circuits, so if one of both of them is not operating correctly, I would think that could cause a problem. So, knowing what the correct voltages for these sensors should be would be helpful.

Not helping matters is that, every time I disconnect the Idle Air Control Valve for testing (as I did yesterday), the engine idle is ridiculously high when the car is restarted. And, the idle doesn't come down as the engine warms up. It actually gets higher. I'm not sure what I did the last time I had this problem, but I have manually pulled the plunger on the Idle Air Control Valve out as far as it could go so that it was restricting air flow into the intake. That usually keeps the neutral idle speed around 2500 rpms. I plan on disconnecting the battery negative for about 10 minutes the next time I get a chance to work on it, then go through the idle re-learn process after I start the car. Hopefully this helps with the racing idle. But, because this is another reason why I'm not sure the ECM is working correctly. You'd think the Idle Air Control Valve would be receiving instructions to move from the ECM, but I'm not convinced that's occurring.

Thank you, and I look forward to your reply.

Eric
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Sunday, January 5th, 2014 AT 5:08 PM
Tiny
JACOBANDNICKOLAS
  • EXPERT
Eric:
I presently am at work. I have a 2nd after my regular job so I won't get home till around 8:00PM tonight. We are getting some crazy sub zero temps here in PA today and tonight, so I may close the shop up early and get home. However, if I don't, I will see what I can find for you when I get home. Is that okay? Please let me know.
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Monday, January 6th, 2014 AT 5:19 AM
Tiny
EK2000D
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Joe,

I am right there with you on the temps. We are supposed to hit -15 F tonight, which is very cold for southern Ohio. I probably won't be able to do additional testing until Tuesday night at the earliest (and if it's still cold, I'll probably hold off). Don't stay late for me today. We have some down time here.

Eric
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Monday, January 6th, 2014 AT 8:46 AM
Tiny
JACOBANDNICKOLAS
  • EXPERT
Okay, I'm home. Lol It's -5 here at this point. Can you believe the shop was busy tonight regardless of the temps? The heater maintained 50 degrees until I opened one of the garage doors. Then you can figure what happened. Lol

Okay, I looked in my manual and here is what I found for what you asked.

MAP 3.6 to 5.3v

Temp Sensor / temp to resistance values / measure across sensor terminals @ 194 degrees F = 241 ohms

TPS The manual says it should have "low voltage" ( I have no idea what that means) but it should increase to 4.5 volts as the throttle is opened.

Let me know if this helps at all. If it doesn't, I'll keep looking.

Keep warm.
Joe
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Monday, January 6th, 2014 AT 5:32 PM
Tiny
EK2000D
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Joe,

No, don't open the garage door. ! I'll bet that was cold.

I'll test the MAP sensor voltage when conditions allow here (down below -10 already here).

I'll also test the Temp sensor once I can get the engine up to operating temperature (not likely in the next 48 hours).

Your TPS voltages were exactly what I measured, so I'm going to rule out a bad TPS.

Any expected voltages for the crankshaft / camsharft position sensors? (Without freezing to death!).

Eric
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Monday, January 6th, 2014 AT 6:36 PM
Tiny
JACOBANDNICKOLAS
  • EXPERT
Wow is it cold! Getting ready to get back to the shop. Ugh!

Regardless, the crank should show between 900 and 1200 ohms

As far as the camshaft sensor, I can't find anything other than this. If the cam sensor is not sending a signal, there is a 1 in 6 chance that the injectors are firing correctly. (Which could cause the issue you are describing). If this is the case, the engine will run in a closed loop condition allowing the vehicle to make it home.
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Tuesday, January 7th, 2014 AT 12:06 PM
Tiny
EK2000D
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Temperature relief is on it's way in the form of "High 30's" this weekend. Can't believe I'm looking forward to that.

OK, I'll check the resistance of the crankshaft position sensor.

Hmmm, if the signal from the camshaft position sensor is bad or missing, the engine will run in a "closed loop". I have suspected the engine is running in a closed loop all of the time, so we may have something here. I'll check for proper voltage (well, at least consistent voltage, since we don't know the correct amount), first at the sensor, and then at the ignition coil assembly.

According to the wiring diagram, the camshaft position sensor has three wires, one that is full-time hot (probably 12-volt) originating from the fuel injector fuse, and two others. One directly back to the ECM, and one tied to both the ECM and crankshaft position sensor. I'll check for good 12-volts on the pink wire first, and then check the change in voltage on the other two wires (I wish I owned an ociliscope).

Then, I'll report back. Work is dragging me out of town for a couple of days, so I'll report back Friday night / Saturday morning.

Thanks,

Eric
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Tuesday, January 7th, 2014 AT 3:55 PM

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