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I have a 92′ GMC Sonoma 2.8L tbi. I had issues with the truck loping and sluggish when warm. I had talked to a man at the parts store and he told me my IAC Valve was going bad. Not knowing the real issue I listened to him and bought the part. After installing the new valve and spraying the port with throttle body cleaner, the truck stuttered at crank but did not start. I opened my chilton s10 book and had learned that the new IAC Valve was an 1/8th of an inch short than of the old one. Thinking the old one might work if I put it back in I tried but it wouldn’t crank then either, idk if it has messed up my ecm or what. please help asap
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Ok, so now you’ve learned the hard way two things: 1) don’t buy/replace a part unless it is confirmed to be bad, and 2) parts salesmen sell parts. While it is true that many are knowledgeable about automotive issues, they are not employed as mechanics and should not be relied upon for over-the-counter diagnostics.
Re-install your old IAC, making sure that the passage that the pintle seats against is clean and free from carbon buildup. Pull the ECM fuse(s) or disconnect the battery for at least 15 minutes. Reinstall fuse(s) or reconnect battery and then see if it starts. If not, there is a different issue unrelated to the IAC that is causing your no start. Even with a bad IAC the engine should start. It might stall right out, but it should at least start. Remember that the IAC acts like a controlled vacuum leak to adjust idle speed by bleeding air into the intake. So, without the IAC even installed it should try to start. Check to make sure that other electrical connectors in the vicinity of the IAC did not inadvertently get disconnected. Make sure the fuel pump is priming, and that you can see the TBI nozzles spraying fuel into the intake during cranking. It sounds to me like this (and your original issue) is a fuel delivery problem.
If the IAC is somehow stuck in a closed position, that could prevent your car from starting by depriving the engine of oxygen while the throttle is closed.
There are two ways to test this idea:
1. open the throttle while cranking
2. pull the brake booster hose without opening the throttleEither one should bypass the IAC and allow air into the engine to start it. Then you could go about diagnosing an air delivery problem during start.
If your car doesn’t start with these tests then it’s possible that something else changed during your IAC swap and re-swap. So you should diagnose and fix that problem.
Either way, take out the part you bought and replace it with your old one before testing.
Thanks guys, I have my battery on a charger so when I do my diagnostics tomorrow I will have full cranking amps. I have taken notes on what you guys have said, I will be in touch in the next 24h. Again thank you, there’s nothing like having a genuine help with something that has had me completely stressed since it is my daily driver.
I have great news, civvy, I did what you said and cranked it with the throttle open. It cranked right up.
Now I have the original problem which is just loping and sluggish at warm when: disengaging the clutch and pulling off from a stand still, idling at operating temperature. And if its warm the starter turns really slow and lopes until it builds speed and cranks, and then sometimes and doesn’t even build up enough speed. could this be a starter going bad?
Thank you again guys.
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