Odd idle problem: wife's Yukon XL

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Jul 29, 2004
I bought my wife a Yukon XL with the 5.3L V-8 about 8 months ago. It's been a great vehicle and it seems almost as good as new.

The 5.3L is a really smooth and powerful engine, too, but there's been a nagging problem it's had: Sometimes it idles really low and acts like it wants to die. It never dies, but it always seems like it will. Plus, it doesn't do this problem all the time, but so far I haven't been able to determine a common denominator with it.

I've replaced the spark plugs, PCV, air filter, etc., and still haven't been able to figure out the problem. There are no CELs coming on either. Someone told me this seems to be a common problem with the 5.3Ls so I was hoping someone here might be able to point me in the right direction.
Almost seems like something a PCM reflash might cure when the engineers at GM figure out the issue. I also doubt you're the only one.
I forgot to mention it's a 2001 model and it currently has about 86,000 miles on it. We bought it when it had about 72,000 miles and this problem has been present from that day forward.

Everything else about the vehicle is awesome. I just wish I could figure out this embarrassing issue. Friends have said, "Hey man, what's wrong with your car? Why's it doing that?"

And for a BITOGer like me, that's totally unacceptable.
Sounds like maybe there's a vacuum leak somewhere in your intake system?

Same symptoms one time on an EFI vehicle I owned. A crusty old german mechanic who I really trusted said it was a vacuum leak. He pulled it into the shop and looked for leaks. When he found a suspected location for a leak, he pulled out a gas cutting torch...set it on low (without lighting it of course) and hovered the nozzle over the suspected leak. Right away the leak sucked in the gas/oxygen and the engine's idle increased as a result. He tried it a few more times with the same result to confirm it.

I ended up ordering a new hose to replace the leaking one, and the problem was solved.

I'm not sure if this would help, but I found it quite interesting and sensible at the time.
One other point. Have you had the codes read for "pending codes"? These are codes that are stored by the computer but don't illuminate the CEL?

Seems like another problem could be an intermittent Idle Air Control Valve. I don't know much about GM EFI, but I suspect your truck might have one of these. This electrical valve (if your truck has it) allows air to flow around the throttle body giving the computer control to stabilize the engine RPM. If this actuator has an intermittent fault, it could cause this sort of problem as well. I'm not sure if the computer would throw a code for an intermittent problem or a "lazy" actuator like this.

In any case, something is wrong, and it shouldn't be too difficult to track down the problem.
AstroVic, same thing happened on a '00 Silverado 5.3. It would idle bad and the rpm's seemed to dip and go back to normal. I changed the fuel filter, PCV valve, cleaned the TB and decarbonized the engine with Seafoam deep creep, ran cleaners in the tank, nothing worked.

The 5 year extended warranty was about to expire, so I took it in and the dealer replaced the fuel pressure regulator.

It fixed it and there has been no problems yet.

i would bet $$$ that if you replaced the fuel pressure regulator, it would fix it.

Good luck.
Find someone with a real scantool to check for a high long term fuel trim on either bank.

If one exists, likely a vacuum leak is the cause.
I agree with Islandvic.

I have the same problem with my 96 Saturn but have been delaying it. Yet, I think its catching up with me as I barely passed smog last time.

I probably won't replace it until I have to though, which is when I'm due for a smog check next year, and only if it doesn't pass.

Why? Its $85 for a new one!
72k miles and no doubt the throttle body has never been cleaned. I do it on my vehicles every 10-15k miles. The theory is that the butterfly valve and body get a bit sooted with blow by and the butterfly isn't seating fully as a result (at idle the butterfly is fully closed and there is a tiny idle air bypass passage around the butterfly), to idle correctly the ECU is looking for this assumed amount of idle air through the by-pass. Here's how I clean:

1. Only use Throttle Body cleaner NOT Carb Cleaner. The throttle body in many cases has a thin coating of plastic. carb cleaner will destroy this and may ruin your O2 sensors as well.

2. Buy some super soft shop towels, the yellow ones, with a soft, fleecy side.

3. Pull the air intake hose off the throttle body. I do all this engine not running and cold.

4. Spay the throttle body real good with the cleaner. Let it soak a bit. Spray some on the clean shop towel (no grit!). Gently wipe the throttle body and the circumference of the butterfly opening the butterfly by hand. The idea is to not use mechanical rubbing and scarring the thin plastic layer in the throttle body, but let the cleaner dissolve the old soot and have the clean shop towel absorb it.

5. Keep repeating the above using different parts of the towel. Eventually it will all come clean and the butterfly will seat. You should also be able to see the small hole in the throttle body which is you idle air bypass.

I keep my throttle body cleaning towels and cleaner in a sealed plastic bag so it will not inadvertently accumulate any shop grit.
Interesting stuff. Not sure if this is related but I had these same symptoms years ago on a vehicle. I had to double foot it to keep it running at stop lights. The dealer messed with it for two hours while I waited. It turned out to be a faulty aftermarket PCV valve.
Your intake gasket is sucking in air, try pouring water on both sides of your intake manifold and i bet it will die.


PS, I work at a GM dealership

Originally posted by toneloc12345:
Your intake gasket is sucking in air, try pouring water on both sides of your intake manifold and i bet it will die.

How can I fix it if this turns out to be the problem???
AstroVic, your 5.3 does have an idle air control valve (motor). It's located on the throttle body and held with two screws. Take the screws off, pull the motor off and genlty clean it. It can and does get "gummed up". I do this about every 10k on my LS1 and I can usually notice a smoother idle. This is very easy to do and should be done regularly. Do not twist the shaft as you can damage the IAC.
Great ideas, thanks!

I'll try cleaning the throttle body first as MGB suggested. It's (basically) free.

Then I'll go with the fuel pressure regulator next if the throttle body cleaning doesn't work.

Thanks to everyone for their comments!
it could also be the TPS. have heard dodge trucks having a bad idle prob.

but clean the TB. know alot of people dont do this but take the TB completly off beings you have that many miles on it. this way you can get everything out of it.
ex_MGB had it right!

I cleaned the throttle body thoroughly and the problem is now completely gone! I plan on doing a second and third cleaning on the throttle body in another few weeks just to make sure I get it really good.

The interesting thing to note is this: The throttle body *looked* clean from the outside looking in. But once I opened the butterfly valve and got to work, it was evident how dirty it was on the inside.

Total cost: about $5 at Walmart for throttle body cleaner and a really soft terry cloth.

Thanks to everyone for their replies, especially to ex_MGB for saving me time and money!
I love this site.. I just cleaned the TB and IAC on my ranger and it idles ALOT better. Seemed peppier but that might just be imaginary...
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