Random Start Up Problems

itsmeagain5789 · June 9, 2009

Every once in a while, maybe once a week at no expected time, the truck will not want to start up. I will press the gas in slightly and try startin it like that. RPM goes up for about one second, then drops back down to zero. It takes probably 3 minutes to get it started. It has done it mostly on cold start ups, but once when it was warm. Runs great after it starts up though.. I have replaced injectors, fuel pump, and fuel filter, and the guys at the shop can't figure it out either since it never does it when i drop it off for them to check it. Check engine light comes on every once in a while too for about 5 minutes then goes away. anyone have a similar experience?

edit: today when it did it, I took off the air filter and inspected the injectors, and told my friend to try to start it up. When he did the injectors made a big wooshing sound, like it was suckin in something. not sure if thats normal or not, but i've never heard it before

Cowboy09 · June 9, 2009

Check the code. Your computer keeps a log of it. I don't have my book handy, but there's a process to check it where it'll flash at you and you count the flashes (assuming yours is OBDI, which if I recall correctly it is).

As far as symptoms, what do you mean it doesn't want to start? Does it turn over and just not fire up or what?

bill190 · June 9, 2009

If everything is working properly, you should be able to start it with your foot off the gas pedal.

When it will not start with your foot off the gas, do you press down on the accelerator slightly and it starts, then you take your foot off the accelerator and it dies?

Or do you press slightly on the accelerator, it starts and you continue to press slightly on the accelerator and it dies?

Basically for idle, the throttle is completely closed. The engine gets it air from the PCV valve and lines and the IAC valve.

So first check/clean the PCV valve and lines running to the valve covers. Be sure the lines are not clogged and air can get through.

Then remove the IAC (Idle Air Control) valve. This has a "pintle" which moves in and out of the hole. Be sure the hole is clean so the pintle can freely move back and forth. Then there are holes inside for air to flow. Use throttle body cleaner to clean these out so air can get through when the IAC valve is open.

As to checking the IAC valve for operation, there should be 4 electrical connections. Two pairs of wires. Use an ohm meter to measure resistance between two connections. You should get around 50 ohms. If nothing, try that pin with another of the remaining two pins. See if you get around 50 ohms. Then the other two pins should be 50 ohms between them. So 50 ohms between two connections. And 50 ohms for the other two. (About 50 ohms).

Also you can take a regular doctors stethoscope and place it on the IAC valve. With the IAC valve installed, have someone start the engine, then turn it off. When the engine is shut off, the IAC valve moves forward in the hole and hits the stop, then comes back half way. You can hear this with the stethoscope. DON'T start the vehicle with the IAC removed or that pintle will go shooting out of the IAC valve!

If the IAC valve is not working, check the throttle to be sure it is clean and can fully close. Then also check the TPS (Throttle Position Sensor) with a multimeter. When the TPS is fully closed, it tells the engine computer the vehicle is at idle. And thus to activate the IAC valve.

So if there is a problem with the TPS, the computer will not activate and control the IAC valve, nor will it control idle. (Thus you would need to press on the accelerator to let air in!) Or if the IAC valve was not working.

Beyond this, there are all sorts of electrical sensors which the computer uses to regulate the IAC valve, fuel/air mixture, and timing. These can all be tested with a multimeter to see if they are operating OK or not. You would need a GM factory fuel and emissions manual for your year/model vehicle for further testing (as I'm not going to type all that stuff in). But this will tell you how to test each component. You can order a factory service manual set (4 books) from a GM dealer.

The IAC valve for my truck looks like this...

itsmeagain5789 · June 9, 2009

If everything is working properly, you should be able to start it with your foot off the gas pedal.

When it will not start with your foot off the gas, do you press down on the accelerator slightly and it starts, then you take your foot off the accelerator and it dies?

Or do you press slightly on the accelerator, it starts and you continue to press slightly on the accelerator and it dies?

Basically for idle, the throttle is completely closed. The engine gets it air from the PCV valve and lines and the IAC valve.

So first check/clean the PCV valve and lines running to the valve covers. Be sure the lines are not clogged and air can get through.

Then remove the IAC (Idle Air Control) valve. This has a "pintle" which moves in and out of the hole. Be sure the hole is clean so the pintle can freely move back and forth. Then there are holes inside for air to flow. Use throttle body cleaner to clean these out so air can get through when the IAC valve is open.

As to checking the IAC valve for operation, there should be 4 electrical connections. Two pairs of wires. Use an ohm meter to measure resistance between two connections. You should get around 50 ohms. If nothing, try that pin with another of the remaining two pins. See if you get around 50 ohms. Then the other two pins should be 50 ohms between them. So 50 ohms between two connections. And 50 ohms for the other two. (About 50 ohms).

Also you can take a regular doctors stethoscope and place it on the IAC valve. With the IAC valve installed, have someone start the engine, then turn it off. When the engine is shut off, the IAC valve moves forward in the hole and hits the stop, then comes back half way. You can hear this with the stethoscope. DON'T start the vehicle with the IAC removed or that pintle will go shooting out of the IAC valve!

If the IAC valve is not working, check the throttle to be sure it is clean and can fully close. Then also check the TPS (Throttle Position Sensor) with a multimeter. When the TPS is fully closed, it tells the engine computer the vehicle is at idle. And thus to activate the IAC valve.

So if there is a problem with the TPS, the computer will not activate and control the IAC valve, nor will it control idle. (Thus you would need to press on the accelerator to let air in!) Or if the IAC valve was not working.

Beyond this, there are all sorts of electrical sensors which the computer uses to regulate the IAC valve, fuel/air mixture, and timing. These can all be tested with a multimeter to see if they are operating OK or not. You would need a GM factory fuel and emissions manual for your year/model vehicle for further testing (as I'm not going to type all that stuff in). But this will tell you how to test each component. You can order a factory service manual set (4 books) from a GM dealer.

The IAC valve for my truck looks like this...

i forgot to mention that i replaced the idle air control valve as well. one of the codes were scanned and that was what was malfunctioning. i will hook it up to the computer sometime this week to see what this new code says. thanks for all that. I will clean the pcv valve aswell.