1988 Pontiac Sunbird



October, 20, 2009 AT 6:23 PM

Engine Mechanical problem
1988 Pontiac Sunbird 4 cyl Front Wheel Drive Manual 206000 miles

At start-up (in neutral) the engine will idle fast, then slow, then fast, then slow, then die. Sometimes it will start up fine, then the very next time the above problem will happen. This has been happening more and more often. Now it will seldom start properly at all (though it will ALWAYS start, and idle fast and slow for a few seconds and then die). There are no error codes.

I've had 2 mechanics work on it. One fixed some vacuum leaks and cleaned a bunch or oil out of the distributor, the other found the fuel pump was failing intermittently. Both problems have been fixed (fuel pump, fuel pump strainer, and fuel filter replaced, vacuum hoses replaced, distributor cleaned), but the problem keeps returning with no changes. And it's getting worse.

I've replaced the distributor cap and rotor, tested the ignition control module twice at Auto Zone (no problems found with it) and replaced the coolant temp sensor. The battery is brand new. The MAP sensor and O2 sensors are also new.

During start-up, while the engine is idling fast and then slow the headlights (if testing at night) will get brighter and then dimmer dramatically (I figure that is just because the change in idle speed). Then after a few moments the engine will just die. Pressing the accelerator down has absolutely no effect on the problem, but will cause the engine to race (if I press it at the moment the idle is fast). But the engine will still die every time this problem happens--regardless of what I do. After 5 to 30 times starting up the engine, it will sometimes start up and idle just fine.

Once it does start up fine, it will continue to run fine until stopped, then it's a gamble whether it will start up again or not--usually not, at this point. Though if it has been running a while and is nice and hot, it seems to be more likely that it will start up again and idle fine, if started immediately and not allowed to cool down.

I've checked the IAC for carbon build-up and cleaned a small amount of carbon off of it, but no change. Though I have not yet checked to see if the IAC plunger moves like it should.

Note: Just before this problem started happening the very first time, I had drove up a very long and steep hill for about 20 minutes and noticed the engine losing power and overheating by the time I reached the top. There was also a smell of burning oil. I have no idea if that's connected.

8 Answers



October, 20, 2009 AT 9:11 PM

Have the alternator checked. The " flickering" of the lights at night may indicate a faultering voltage regulator.
There seems to be an oil leak somewhere. It is most-likely the valve cover gasket. Look to see if any oil is leaking on any sensors like the knock sensor.
What is the coolant level? Does coolant ever need to be toped off?



October, 22, 2009 AT 2:32 AM

A bad alternator or voltage regulator could cause the engine to go fast then slow then die during idle? Just want to make sure I understand you.

Note: The lights aren't exactly " flickering, " just getting brighter or dimmer in exact proportion to the speed of the idle (and the idling only lasts a few seconds before the engine dies). Kind of like if you had the lights on a dimmer switch and turned them up and then down--in sync to the engine speed. Is that what you meant?

The oil is only leaking around the oil cap itself. There aren't any other oil leaks that I can find.

The coolant level is fine and does not need to be refilled very often at all.

I'll get the things you mentioned checked.



October, 22, 2009 AT 11:27 PM

If the voltage goes too low there will not be enough voltage to fire the spark plugs. The ignition coil is nothing more than a step up transformer; meaning it recieves a certain voltage and " steps it up" to a much higher voltage. It does this proportionatly to the applied voltage --- If the coil is a 30000 volt coil, this means that for every 12 volts that goes into the coil - 30000 volts comes out. If you put 6 volts into the same coil, you will get 15000 volts.I know that this sounds a little out there, but it seems that you have checked almost everything that could cause this problem, so checking the alternator is a good idea. BTW, the voltage regulator is built in the alternator.
Did you change the spark plugs? They may have been fouled by the intermittent fuel pump failure. Use A/C Delco plug ONLY in a GM ignition system.
Yes, that is what I meant by the lights flickering.
How did the oil get into the distributer if it is (or was) only leaking from the oil cap?



October, 23, 2009 AT 6:05 PM

Well, that's a good question (about the oil leaks). The mechanic said he thought the distributor might not be sealed very well around the center rod that turns it (I forget the terminology), so the oil might have gotten in that way, but he didn't find any oil farther back in the distributor, only in rotor area, so he was perplexed. He said it might have sprayed up into it from below, but didn't see anywhere that it looked like that was happening. So it's a bit of a mystery. I did have the engine washed a few years back, so maybe it got in there that way. There were some oil leaks around the top valve cover gasket a couple years ago that I fixed with a new gasket, so that might have been it.

It doesn't seem to be collecting in there now.

Thanks for the info about the spark plugs! A friend put some Bosch dual electrode plugs in (all 4). He said it would give better gas mileage/power. I wasn't sure that was a good idea. Could that have damaged/fouled something? They've been in about 2 years.

Can fouled spark plugs cause an intermittent problem like this? I'll put the A/C Delco ones in right away!

So are you saying that because the ignition coil only steps up the voltage that it probably isn't the problem?

FYI, I haven't tested the ignition coil or the alternator yet. (Having trouble getting the car to a spot where I can get that tested).

Thanks for all the advice! I really appreciate it.

I've put so much into this car, I'd hate to have to junk it yet. Half the parts are new.



October, 24, 2009 AT 3:11 PM

My experience with Bosch spark plugs hasn't been a good one. IMO, they are junk. A/C Delcos are a little more expensive, but they are the best choice for GM ignition systems.
On the coil -- NO. I am saying that if there is not enough voltage on the low side of the coil, it will not produce enough voltage to fire the spark plugs.
Test the coil and alternator. If they are ok, Then test the tps (throttle position sensor).

If there is no check engine light " on", it is difficult to determine the problem. Did you scan the computer for codes? The engine light could be burned out.

Remember to test the item first before you replace it.



November, 1, 2009 AT 3:06 AM

I'll get those things tested, my mechanic should have some time free within a week.

Yes, the codes have been scanned. No errors are being recorded for this problem. Bummer. (Though errors have been recorded for other problems in the past)

Again, thanks for all your help!



December, 1, 2009 AT 7:01 PM

I tested a bunch of things on the car today, and was able to get this " intermittent" problem to happen EVERY time.

Here is what I learned: --***Fuel is NOT being injected at a constant rate*** THIS IS WHAT'S CAUSING TO PROBLEM (though I don't yet know the cause of THAT cause)
--but ONLY when the engine temperature is warm.
--and ONLY if the engine has not just been running fine a few seconds before (must wait 10 minutes after engine has been running and turned off, to reproduce the problem).

--The problem will NEVER happen when it's very cold outside and the car has been sitting, and the engine is ice-cold.

--The problem will ALWAYS happen if it's either warm outside, **OR** I have run the engine for 15 minutes to get it warm and THEN let it sit for 10 minutes (simulating warm-weather startup conditions).

--When the problem happens, if I manually drip gas into the intake from a drip-bottle, it will eliminate the problem and get past the startup phase and then run on its own (without the drip-bottle). This is because when it (the problem) happens the fuel injector is spraying gas intermittently (spraying, then not spraying, spraying, then not spraying). The fuel injector is definitely NOT the problem (unless it is failing at only one specific temperature, and that seems unlikely).

So. It must be the computer or some fuel delivery electronic system failing at a certain temperature.

Things I have tested or replaced and know to be good: --Fuel pump (replaced)
--Fuel filter (replaced)
--Coolant temp sensor (replaced)
--MAP sensor (replaced)
--O2 sensor (replaced)
--ignition coil (tested, spark is strong, the system is just not sending enough fuel when the problem happens)
--alternator (giving 14.35 volts once engine is running)
--distributor ignition module chip (tested)
--Throttle Position Sensor (I unplugged it while the problem was happening, and it made NO difference).

Things I tried at exactly the moment when the problem is happening: --While problem happening, unplugged MAP sensor. NO EFFECT. (Through engine ran a bit rougher when this was unplugged, as expected, the problem was exactly the same, just with some added roughness in addition to the original problem)
--While problem happening, unplugged MAT sensor. NO EFFECT.
--While problem happening, unplugged Throttle Position sensor. NO EFFECT.
--While problem happening, unplugged Coolant Temp sensor. NO EFFECT.
--While problem happening, used bottle of gas with thin drip-straw to manually drip gas into intake. PROBLEM ELIMINATED--though not fixed, of course.

Note: When I say " NO EFFECT" I mean there was no effect on the problem whatsoever (same as if I hadn't unplugged anything). Though the " Serve Engine" light was activated by unplugging those things, and error codes were registered in the computer as you'd expect.

The problem either happens or it doesn't. It's like a " switch" is turned on or is not turned on. Causing the problem to happen or not happen.

If the engine is warm (but has been turned-off for at least ten minutes without running), the startup cycle will not work right and fuel will not be sprayed out of the injector at a constant rate--causing the engine to idle fast, then slow, then fast, then slow, then die (all in about 8 seconds). (E.G. Fuel injector spraying, then not spraying, spraying, then not spraying, during this startup period)

This happens EVERY TIME these conditions are met--Like clockwork.

Does this tell you anything? It seems like we're close to a solution?

I suspect the Engine Control Computer is bad, or perhaps some sensor (that I don't know about) is feeding wrong info to the computer. I did have the computer replaced 5 years ago, but I can't imagine what else it could be! (With my limited knowledge of cars)

QUESTION: How does the computer get temperature info about the engine temperature? It's NOT getting it from the coolant temp sensor as far as I can tell.

Could it be something like: --Faulty sensor sending wrong temp to ECM?
--Bad Manifold Air Temperature (MAT) sensor?
--Idle Air Control Valve (I tested it some, and it appears to move, but I still need to measure the resistance on the leads)
--ECM computer bad?
--Fuel Injector Pressure Regulator?
--Fuel Pump Relay? (Keeping fuel pump from getting reliable electricity at startup?)
--Something disrupting electricity to fuel pump and/or fuel injector?

Seems like it's got to be something more " electronic.&Quot;



December, 13, 2010 AT 3:56 PM

Well I replaced the ECM and OIL PRESSURE SENSOR, and for almost a year I have had no further problems. So I believe it's safe to say that the problem was caused by one or both of those things being faulty.

I also noticed that my oil pressure gauge pointer (on the dash board) had flipped all the way around so that it was stuck under the pin that it's supposed to rest on. So obviously it had received a voltage spike (or something like that) that had made it go farther up than it was supposed to go--until it flipped all the way around 360 degrees.

After searching for many hours online through the schematics in the shop manual for my car, I noticed that the ECM reads the oil pressure during the startup phase, and when it senses that the oil pressure has reached a certain level it activates the fuel pump relay and from that point on the fuel pump receives power without the ECM being involved.

Since the problem was that my fuel pump was getting intermittent power, and thus causing the fuel injector to spray fuel then stop, spray fuel then stop, and thus causing the engine to run, then sputter, run then sputter, run then die. It makes sense (after looking at the schematics) that replacing the ECM and/or oil pressure sensor would fix the problem.

It also explains why, even before I had fixed the problem, the engine would finally start running fine after I had tried to start it 30 or so times--since that created enough oil pressure to activate the fuel pump relay and bypass the faulty devices that were causing the problem.

I don't know if there was another problem that led to the failure of these devices in the first place, but if I were to take a wild guess, it may have been the extremely old battery I had, or getting a jump start without using proper jump start techniques. Or it may be something I had yet to find (let's hope not). I'm NOT an expert, so these may have nothing to do with it. But I DID replace the battery before replacing the ECM and Oil Pressure Sensor (Though, only replacing the battery had no effect on the problem, and did not make it go away--just to be clear).

I am very grateful for the help of a friend of mine who worked for decades as a Head Mechanic for Boeing Aircraft Corp. It was his assistance that really helped me work through the problem logically and methodically, using proper system diagnostic procedure.

But I did want to follow up and post the solution here for anyone having a similar problem. I hope this helps you if you have the misfortune of trying to fix a problem a complicated as mine, and gives you a possible answer.

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