'92 GMC Sonoma 4.3 V-6. Cold starts only if gas is poured (a little) into the TBI. Cranking it sprays NO fuel into the "Carb". Once started it runs perfectly. SOMETIMES on some mornings or evenings it will start. ALWAYS it will warm start if recently run. NO Codes showing. Fuel Pump? TBI? Injectors? What do we check? What do we fix?
Country: United StatesMake: GMCModel: SonomaYear: 1992Engine: 4.3 V-6
Crank engine and check for fuel spray at "Carb" (used a strip of dry paper) no fuel.
Pour a thimble of gas down the TBI and it'll start first time every time. Check for fuel when running, and the paper strip gets wet.
Truck runs perfectly, just won't start when cold. . . no fuel sprayed.
Thanks for trusting us with your problem. I have trained in the automotive field and currently work as a shop foreman. I will be helping you today.This is a indication of a bad fuel pump. You will need to check and verify you are getting power to the fuel pump. I suspect you are. What is happening you are not getting enough for it to run as you cannot see anything. So when you add fuel it forces the engine to run allowing just enough fuel to get it going. Now if the fuel pump is running but you are just not getting fuel this can be injector issue. There is a very common cause here with a failed ignition control module which signals fuel injector pulse. When it will not start connect a test lamp across the injector back probing the wires it should flash lightly if not the ICM is at faulty here. But if the fuel pump is not running when it has proper power This will correct the problem you are having. Since you indeed have spark and injector pulse you have to replace the fuel pump for intermittent runnig issue.
Let me know If I can assist you further. Please take a moment to positively rate the answer. Thank you!
The fuel pump runs. When I turn the key, it clearly can be heard to be running, for a while.
Than you indeed have a issue with the injector likely the ignition control module. Can you test to see if you have a flashing test lamp when it will not start and you are cranking the engine?
I have 2 VOM's one digital and one analog, May I use the needle type and watch for it to waver? How do I hook it up, and where?
Well I am sure the problem is the ignition module. But without a scope it will be impossible to verify the square wave. If you can gain access to a handheld this is all you need to do. Ask any true Gm mechanic about these they will confirm my diagnosis. With the engine running on alternate fuel, check RPM in scan data. If the RPM is not there, check for a 0-5v square wave on circuit #430, Purple/White wire with the engine cranking or running. If the signal is missing, check the wire for an open or short to ground. If the wire is OK, replace the ignition module. If the RPM reference is OK, replace the Powertrain Control Module. Very low but possible on the PCM. 99.9% of the time its the ignition module. Thanks.
Had to "sleep on it" to digest your answers. 1) For now it looks to you, like a probe into the injectors looking for a flashing light (pulsing electrical signal, when cranking) is what we seek. Is it once per revolution? If no signal at the injector wire, then follow up on that line of troubleshooting. Does that mean retiming the distributor too, if the ECM is changed?2) You began with "bad fuel pump". Why might a pump be "bad" when starting, but "Good" when running. What makes you certain it was not your first diagnosis? Do you wish me to "check" the fuel pressure/flow?I appreciate your help in catching me up to your current thinking.
Robin, Thanks for your reply, Did you get the second email. Just ignore the injector test since we know the pump works and it will start on alternative source. I am quite certain you have the popular ignition module failure.
Thanks, XXXXX XXXXX ignore the 1) part, and move on to the Ignition Module.Are you suggesting that it fails to pulse the injectors at cranking but will work at normal RPM?Is this where I try a "noid light" on the injector whilst cranking? Your advice seems almost to have led us to a conclusion.
That is correct. The module is in charge of the injector. It will run when alternate fuel is forced into the bowl. But it does not get signal that the fuel is coming in. They have a know issue. The ignition module fails causing this exact concern. Unless you have a scope we cannot verify the square wave on the purple wire which is needed to verify the ignition module. You can check for as signal with a test lamp but this is very hard to verify without a scope. I wish there was a easy test but they were designed this way by GM. Thanks
ECM is "cheap", how do I swap it out, and will I have to re-time the distributor?
Click here This is the location, basically you remove and take your prom out (the chip) and install the prom in the replacement ECM. No programming is needed or timing. Your most likely culprit is the ignition module though Click here Removal of distrubutor is not needed to replace the module. Thanks
OK, Just to be sure I get it (I was confusing ECM with IM). I will replace the Ignition Module on the distributor. Pop the cap, unscrew the old module, screw in the new one, plug in the wires and problem solved. Right? Of course it could be something else, but this is your "suggestion #A". Right?
That is my #1 suggestion.
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