1990 Formula 5.0L TPI stalling issues
06-07-2018, 11:25 AM
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1990 Formula 5.0L TPI stalling issues
I have a 1990 Formula Firebird 5.0L TPI. It’s a low mile car but it’s got a nasty gremlin. It stalls when it’s warmed up. It runs fantastic when it’s cold. Once the car is up to operating temp it sometimes stumbles a bit but it eventually quits. As soon as the fan comes on it dies. If you hit the brakes when it’s warm it dies. It seems electrical, but I am at a loss. I have replaced the following.
ECM
TPS
IAC
cap rotor, wires, plugs
ecm sensor
thermostat
o2 sensor
ignition module
coil
nothing has made any difference. Had a few people say I should do the distributor but I am not sure that would be the fault. It did look a bit rusty when I pulled the cap off but could that cause the issue I am describing? Any advice would be appreciated.
ECM
TPS
IAC
cap rotor, wires, plugs
ecm sensor
thermostat
o2 sensor
ignition module
coil
nothing has made any difference. Had a few people say I should do the distributor but I am not sure that would be the fault. It did look a bit rusty when I pulled the cap off but could that cause the issue I am describing? Any advice would be appreciated.
06-07-2018, 11:55 AM
#2
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Car: 1991 Z28
Engine: 305 TPI
Transmission: 700R4
Axle/Gears: 3:42 Auburn
Re: 1990 Formula 5.0L TPI stalling issues
Are they the original Multec injectors? If so they could be your culprit. You should run a OHM test on them when the engine and injectors are hot.
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Trans am 1 (09-15-2019)
06-07-2018, 12:07 PM
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Re: 1990 Formula 5.0L TPI stalling issues
Good day, yes the car is all original equipment. I did a fuel pressure and leakdown test and it passed ok. How would I know if the injectors are bad and what correlation would that have with the fan/brake electrical load?
06-07-2018, 01:11 PM
#4
Re: 1990 Formula 5.0L TPI stalling issues
You'd measure the electrical impedance across the terminals with a multi-meter.
I think they should be 13 to 15 ohms IIRC. You'd look for one that is way out of family.
My guess is you'll find it on the driver side, if anything.
What happens is when the engine is cold, the ECM is in open loop and not correcting for overly rich or lean mixtures.
The scenario is this...
If you have a bad injector(s) causing a lean or rich mixture, the car may run decently at first (during open loop). But when the ECM gets into closed loop operation (and say you're running overly rich), the ECM now reacts to the O2 sensor, and will start reducing the injector pulse width to lean out the mixture. Problem with batch fire (where all injectors fire simultaneously) is the ECM has no choice but to lean out the entire engine to deal with the bad injector(s). It'll keep leaning it out until the O2 reports the correct air/fuel mixture (which is being driven by the cylinder(s) with the bad injector). But by that time, the rest of the engine may be so lean that it doesn't want to run at all (which is what you're seeing).
I think they should be 13 to 15 ohms IIRC. You'd look for one that is way out of family.
My guess is you'll find it on the driver side, if anything.
What happens is when the engine is cold, the ECM is in open loop and not correcting for overly rich or lean mixtures.
The scenario is this...
If you have a bad injector(s) causing a lean or rich mixture, the car may run decently at first (during open loop). But when the ECM gets into closed loop operation (and say you're running overly rich), the ECM now reacts to the O2 sensor, and will start reducing the injector pulse width to lean out the mixture. Problem with batch fire (where all injectors fire simultaneously) is the ECM has no choice but to lean out the entire engine to deal with the bad injector(s). It'll keep leaning it out until the O2 reports the correct air/fuel mixture (which is being driven by the cylinder(s) with the bad injector). But by that time, the rest of the engine may be so lean that it doesn't want to run at all (which is what you're seeing).
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Trans am 1 (09-15-2019)
06-07-2018, 01:12 PM
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Car: 1991 Z28
Engine: 305 TPI
Transmission: 700R4
Axle/Gears: 3:42 Auburn
Re: 1990 Formula 5.0L TPI stalling issues
You need a multi meter and select the OHM function. Remove the 2 wire clip from the top of the injector. Place the leads from your meter on the 2 prongs on the top of the injector. I believe a good reading should read 14-17 on the meter. All 8 should be within 1 OHM of each other. The coil inside the injector sometimes fails when hot and thus no injector fire. 1 bad injector can short the whole bank as they are all tied together in a batch firing system
The only correlation is the injectors run off the same electrical system. IMHO I don't think the fan/brake system has any thing to do with your stalling unless it is drawing the whole electrical system down therefore not providing enough voltage for the whole system to operate. If that is the case you have an electrical short or bad ground not a engine issue.
The only correlation is the injectors run off the same electrical system. IMHO I don't think the fan/brake system has any thing to do with your stalling unless it is drawing the whole electrical system down therefore not providing enough voltage for the whole system to operate. If that is the case you have an electrical short or bad ground not a engine issue.
06-07-2018, 03:50 PM
#7
Re: 1990 Formula 5.0L TPI stalling issues
check the fuel pressure if you haven't already. From your description of the symptoms, my money is on a fueling issue, not a spark issue.
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06-07-2018, 04:07 PM
#8
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Car: 1990 Formula Firebird
Engine: 5.0 L TPI
Transmission: 5 speed
Axle/Gears: 3:42
Re: 1990 Formula 5.0L TPI stalling issues
06-07-2018, 04:59 PM
#9
Re: 1990 Formula 5.0L TPI stalling issues
Ok. I'm still skeptical it's a problem with the distributor. Looks like the coil has already been replaced.
Have you checked the system voltage when the car is warmed up. Typically voltage will be a little higher when cold. But if something is happening where the system voltage is dropping significantly after warm up you could also have problems.
Have you checked the system voltage when the car is warmed up. Typically voltage will be a little higher when cold. But if something is happening where the system voltage is dropping significantly after warm up you could also have problems.
06-18-2018, 07:11 AM
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Car: 1991 Z28
Engine: 305 TPI
Transmission: 700R4
Axle/Gears: 3:42 Auburn
Re: 1990 Formula 5.0L TPI stalling issues
Did you ever get this sorted out? I developed the same issue shortly after you posted this. Also you said you did a leak down test. Do you have the actual numbers you went by?
07-23-2018, 05:09 PM
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Car: 1989 Firebird SSA
Engine: 2.8L V-6
Transmission: 700R4
Axle/Gears: Stock
Re: 1990 Formula 5.0L TPI stalling issues
I'm having a similar problem with my SSA Firebird. (2.8L V6) When she's at NOT she runs just fine but at a stop light or driving slow in a parking lot as the temp reaches about 216-220 (when the electric fan is supposed to kick in, I reckon) the car stalls. No warning at all. No stumble, hesitation just dead. If I put the car in park then the RPMs will drop significantly (and seemingly randomly) but the engine will catch itself and rise back to normal idle. Any ideas?
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