99sierra 6.0l Gas

jaygeem · June 20, 2009

My 99 Sierra 6.0L gas bucks and pops when cold for the first 1/2 to1 mile when the throttle is pressed moderately. If I just very slightly press the throttle it will run OK. After that first mile everything seems normal. The dealer did a complete induction cleaning, replaced the fuel regulator and fuel pump, no help! Now he wants to replace the fuel rails but he's not sure that will work. This remove and replace is getting expensive. I'm holding off on the rails right now. There are no faulty codes showing.

Any help??

angelic0- · June 20, 2009

i've noticed mine doing that for the first 10-15 seconds after start....

then it smoothens out and no faulty codes either..

and it's only got 18000 miles on it

bill190 · June 21, 2009

Well there is a much better method to "guess and replace parts"!

That is to test things to see if they are working ok. And better still is to follow the factory troubleshooting instructions for the specific problem.

To do this, get a GM factory service manual set (3 or 4 books) for your specific year/model vehicle (order from a GM dealer). You will also need a multimeter and know how to use it. Or learn by searching google.com for the words: how to use a multimeter

With that said, the engine gets its air at idle from the PCV valve (and associated air lines) and the IAC valve. Check the PCV valve that it is clean, that the lines it is connected to are not clogged on the inside, and where these connect, it is not clogged inside (air can flow).

And there is an opposite air hose going to the crankcase for part of this PCV valve ventilation. Check the other hose and opening to be sure air can flow freely.

Then check the IAC valve.

The IAC valve is a computer controlled "stepper motor" which slides a "pintle" in/out of an opening to allow more/less air into the throttle body. This adjusts the idle speed.

It pretty much requires expensive electronic equipment to test. But there is also an inexpensive way to see if it is basically operating or not.

But first clean where the pintle slides in/out. Remove the IAC and clean the sliding pintle part only with throttle body cleaner. DO NOT get any cleaning fluid on the electrical portion of the IAC as this can wreck it.

DO NOT turn on the ignition or start the vehicle with the IAC removed or the pintle will go flying off into the bushes!

Then clean inside the opening where the IAC pintle goes. And clean the air opening at the back of this opening. Make sure that when this valve is open, air can get through. Use throttle body cleaner only. (Throttle bodies have a coating which can be damaged by other cleaners.)

Reinstall the IAC valve.

Next test it to see if it is basically working...

You can hear an IAC valve operate with a little help. But only after shutting off the engine.

When the engine is running, then shut off, the IAC valve moves fully forward, then retracts half way. (After engine shuts off)

You can hear this with a paper cup and string. Just take a paper cup, punch a small hole in the bottom, thread a string through the hole, tie a knot on the end of the string, then attach the other end of the string to the IAC valve or the IAC valve electrical connector. Place the cup to your ear and pull the line taught lightly. Then have someone else start the vehicle, then turn it off. And you hear the IAC valve operate through the vibrations on the string and this is amplified by the paper cup.

(Also you can use a doctor's stethoscope.)

Pictures...

String just wrapped around IAC electrical connector and not touching anything else...

Hold cup to ear....

bill190 · June 21, 2009

BTW... The previous is if you are having problems with your foot off the accelerator at idle. And barely press on the accelerator, then it runs ok. (This would let air in bypassing the IAC and PCV valves.)

I'm not clear if this is the problem or if you are barely pressing on the accelerator as opposed to pressing on it a lot, but idle is fine?

Anyway if it is an idle problem or the other problem, for further testing/troubleshooting, you would need the factory manual. Then find the symptoms, then test what it says to test/check.

There are all sorts of electrical sensors which control the IAC valve, there is a coolant temperature sensor which tells the computer the engine is cold, etc. These can all be checked with a multimeter. But you need the books to show you where these are located, which wires to test, how to test, and what your test results should be.