Firebird won't run with alternator connected

Tiny
THADIUS856
  • MEMBER
  • 1990 PONTIAC FIREBIRD
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 106,000 MILES
Had a previous donation thread which hasn't received any reply within the last 7 days, so I'm opening a new one.

Brought my 1990 Firebird 3.1L across country. Ran fine until I got here, then failed smog (5x allowable CO at 2500rpm - properly warmed), and then died the same day. It will start for a few seconds, but will not run otherwise.

Disconnecting the alternator allows it to run, albeit not all that smoothly, for a few minutes until the battery starts dieing. Alternator tested good three times at two different shops out-of-vehicle (14v). In the vehicle it shows less than 8v charge when running (from the cluster).

First shop was stumped. After replacing the radiator (hole pouring fluid) and O2 sensor (dead), they ran out of ideas. ECM swapped for known good to no effect. New plugs, wires, cap, rotor, fuel filter, air filter.

Towed to second shop. They claimed it's a bad alternator (internal short), and cited smog failure on high resistance in 5 injectors. I've been reading that high CO @ 2500 is more indicative of a bad cat.

What tests can I perform to help diagnose this problem, without throwing money at part swapping?
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Monday, August 8th, 2011 AT 2:04 AM

8 Replies

Tiny
CARADIODOC
  • EXPERT
I can share some insight into the generator issue. There's no way to sugar-coat it; GM went from the the second best generator to by far the world's worst pile ever developed beginning with the '87 model year, and they have no plans to improve it. Due to the design, they develop huge voltage spikes that interfere with sensor signals and computers. That's why engines often run better with the generator plug disconnected.

Besides the fact these things generate big voltage spikes, the job of the battery is to dampen and absorb those spikes. It's real common to see repeat generator failure caused by the spikes taking out the internal diodes and voltage regulator. To reduce the number of repeat failures, the battery must be replaced at the same time. As they age, they lose their ability to absorb those spikes.

These spikes have nothing to do with the generator's ability to produce its rated output current and voltage. Just because it tests okay, it's still those spikes that are the problem. If those spikes radiate into a sensor wire, the Engine Computer is going to respond as though that is part of the signal. If the oxygen sensor signals include some of those spikes, the computer is going to modify fuel metering incorrectly. An incorrect mixture will increase emissions.
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Monday, August 8th, 2011 AT 4:42 AM
Tiny
THADIUS856
  • MEMBER
Replaced the alternator with one from the parts yard. No effect. Took it back, less $6 for 1 yr warranty, got another. Still no effect. Haven't taken it back yet, but will.

Removed intake manifold, held throttle full open and sprayed in WD-40, released throttle. Vehicle started and ran for 5-10 seconds very smoothly, then starved and died again.

Created an over-the-top access port to the fuel pump when I replaced it last year, so I pulled up the carpet and opened it up. Pump primes when key turned to accessory position, and runs for 1-2 seconds after engine starves.

(original thread has resumed with original expert - everything below is also asked in that thread)

Now not so trusting of the mechanic's fuel pressure reading. I think my next step is to check pressure at the Shraeder valve just before the pressure regulator.

I've never checked pressure here. Any specific tips for testing? Any specific type of pressure tester I need to pick up? What is the range of acceptable pressures at this location under what conditions?
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Friday, August 12th, 2011 AT 4:05 PM
Tiny
CARADIODOC
  • EXPERT
The generator is not the solution. They all produce huge voltage spikes but it's the battery that absorbs them. You said disconnecting the generator made the engine run better. That is the common clue that you need a new battery.

Don't know what normal fuel pressure is but I suspect it will vary between 40 and 50 psi depending on manifold vacuum.
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Friday, August 12th, 2011 AT 6:37 PM
Tiny
THADIUS856
  • MEMBER
Replaced the battery a day before I replaced the alternator. Now has a new Napa Legends 75 battery. It didn't seem to have much effect, though the old dual-terminal Orbital did allow me to get some prorate warranty back.

Been doing much electrical on the house this weekend, so not much time for the car. Been calling around for a pressure tester, and while every parts store says they rent them, nobody seems to have one. Will keep looking.

Swapped the fuel pump replay with another identical one under the hood. No effect. Swapped back. Checked my splices on the fuel lines at the pump, and they look fine.

We *think* we smell fuel at the vacuum port on the intake manifold, which would seem to indicate a bad fuel pressure regulator. But again, without the tester, we can't be sure without opening her up.

Will be in touch!
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Monday, August 15th, 2011 AT 4:45 PM
Tiny
CARADIODOC
  • EXPERT
That smell is a clue. Pull the vacuum hose off the regulator and look for signs of wetness. If it's leaking, the pressure will still be normal but that raw fuel will get into the engine and cause an excessively rich condition.
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Monday, August 15th, 2011 AT 10:47 PM
Tiny
THADIUS856
  • MEMBER
Checked vacuum hose at fuel pressure regulator again. Don't smell fuel this time after cranking several times. I think I may have just been taking a whiff of the WD-40 that was on the fingers I was using to hold the hose.

Purchased an El Cheapo fuel pressure tester at Harbor Freight ($20 - 20% Sunday paper coupon = $16+tax) that came with adequate attachments, since no parts store seems to have their rental tester in stock. After a little fuel squirting the block because I forgot to use Teflon tape (woops, fixed that PDQ), we got a reading.

39-41 psi key-on
38-45 psi cranking and (attempting to) idle
29 seconds to drop from 40 psi to 20 psi after removing key.
Readings were consistent every time

Looks like we can count out the fuel pump, relay, and filter as a cause. Everything aft of the Schrader valve on the rail must be OK.

Again, I think the fact that I can remove the air intake, spray in WD-40 and get 5-10 seconds of perfectly smooth idle to be a pretty big clue.

Will pick up a multimeter in the morning, just in case.

Ideas?
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Monday, August 22nd, 2011 AT 11:04 PM
Tiny
THADIUS856
  • MEMBER
Picked up the multimeter. A previous mechanic said the resistance was high on the injectors. How do I test this without removal like he did?

Fuel flow rate looked good. The cup didn't overflow in seconds, but it pumped ample amounts of fuel for the engine size.
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Wednesday, August 24th, 2011 AT 4:36 PM
Tiny
CARADIODOC
  • EXPERT
I've been to classes where they talked about measuring the resistance of injectors but then they always mentioned there was some other underlying cause that fixed whatever problem they were having.

One real big problem GM has is mismatched injectors. Chrysler buys theirs from Bosch in flow-matched sets of four, six, or eight for each engine, and problems are unheard of. GM just grabs a handful out of a huge bin, and the flow rates are all different. They work well enough when the engine is new, but as characteristics change and wear takes place, those differences become more pronounced. If one flows a little less fuel volume, that cylinder runs a little lean. The oxygen sensor picks up the unburned oxygen in the exhaust. In response, the Engine Computer commands more fuel from all of the injectors. Five cylinders will be running rich which can lead to excessive unburned hydrocarbons out the tail pipe.

The fix for that is a set of flow-matched injectors. One company that specializes in that is owned by Jim Linder in Indianapolis. They rebuild and match injectors. GM injectors are their top seller. Many customers say their engines never ran so smoothly until they had new injectors installed.

As for testing the fuel pump, to be accurate, flow-testing has to be done after the pressure regulator in the return line. The ability to supply just enough fuel for the engine to run on isn't sufficient. If you tested the flow rate at the test port, you'll be bleeding off pressure before the regulator opens and lets fuel flow back to the tank. The pump has to be able to supply the needed volume at the needed pressure. (Think of a little toy air compressor that can deliver a whole pile of air flow if the hose is wide open, but if you try to build any respectable pressure, it can't deliver enough volume to run a small air tool). If you draw the fuel from the return line, you'll measure how much fuel can be supplied at the required pressure.

To test the injectors electrically, unplug them and use the ohm meter to measure resistance between the two terminals on each one. Typical is somewhere in the area of 6 to 12 ohms, as I recall. Instead of a correct value, look for consistency between them. The meter leads will have a couple of ohms of resistance too so that has to be factored into the readings. I never measured injector resistance, so I don't know what that's going to tell you. If they're buried and inaccessible, look for a connector in the wiring harness that you can get to. You'll have to figure out which terminal is the common one and put one meter lead there, then find the other six, one for each injector, and measure each of those.

It sounds like you might have moved to California. Various parts of the country have different fuel formulations depending on the whims of the uninformed politicians, and California is the worst. Besides a ton of new groundwater contamination problems they caused, the fuel is well-known to cut fuel mileage by over ten percent. Between pump pressure and volume, and mismatched injectors, you may have had a minor problem all along that went unnoticed until you started using the different fuel.
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Wednesday, August 24th, 2011 AT 9:27 PM

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