Please read, need help, crappy running LQ4 in 03 Silverado.

[andrewb70]

You said the misfire is on #1 and #6. I would swap coils around and see if the misfire follows the coils.

Andrew

[Samckitt]

Dann bud, that sucks. And not trying to ask too stupid of a question, but did they do another smoke test after replacing the gaskets?. I ask cause I did the very same thing, replaced all the gaskets and hoses then smoked it and found that cracked fitting. It was such a tiny leak , I couldn't believe it caused all my issues.

I don't know, I will have to ask. Coil is not firing at all on #1 until something gets warm. If intake leak, wouldn't coil at least fire?

Find someone near you with a similar truck and swap MAF sensors. Not sure why everyone was saying a MAP. Odd that you only have a misfire usually when a MAF goes, it will give you a lean or rich dtc.

Has anyone taken either of the valve covers off and inspected for a broken valve spring on the misfiring cylinders? Usually the exhaust will go but either could break seeing as they are the same spring. If you have the covers off run it and see if you see either valves on 1 or 6 not moving like the others. Could be a collapsed lifter or a rounded lobe.

It really could be anything but at least you have a pretty good place so far.

I wish you were in IL I would come look at it just to try and beat the truck lol.

I just had a G8 the other day with a L99 with a collapsed lifter. I know different gen engine but still

I have changed the MAP & tried a different MAF, no difference. Have not checked the springs. Once something gets hot it runs normal. #1 coil does not fire when cold. I have tried a diffferent coil, coil harness & ECM, no difference. If collapsed lifter, wouldnt it still run the same once something gets hot? I keep saying "something" , because it is not once the water temp gets warm. But once something gets hot it isnt as noticable. When cold, it will not go 5mph with it floored.

My buddy went through about the same thing a few months back. It ended up being a bad lifter. His had the NON-ACTIVE FUEL MANAGEMENT system. I guess there are some people having problems with these lifters.

I am on the second air filter & third fuel filter & second fuel pump since this started. No difference.

You said the misfire is on #1 and #6. I would swap coils around and see if the misfire follows the coils.

Andrew

I have. With brand new set of coils, & different coil harness.

[jlcustomz]

Obviously sounds like you've had a misdiagnosis with something there.
From what I've usually seen, coils are more likely to fail when hot, not cold.

Not sure about the possible lifter issue. Could try a compression test. Definitely time to change directions in diagnostics. There always has to be a difficult one , sorry it has to be you.

[Poncho72]

subscribing, i have read the whole thread so as not to be redundant. i would really like to know whats going on too, but as yet i have no suggestions. i will be thinking about this "situation" and revisit often to see the outcome and hopefully contribute.

[GBNL]

I had a similar problem on a customers 03 Silverado , i replaced the engine harness and the problem disappeared . I tried pretty much everything before swapping harness with no luck.

[Bonehead]

This thread is two years old. Outstanding!

[Z06vet]

it is 2yrs old, but as of a month ago the original poster replied above and the issue is still unsolved. I've been checking this thread occasionally because I have an 03 2500 that does the same thing. I forgot about it because when its warm outside, the miss goes away. Last month we dipped into the upper 20s and the miss was back. I have narrowed it down to a range of 38-40 degrees or less, the miss will occur. The colder it is, the longer it will miss. Within 2-3 minutes its usually gone. Very strange but interesting. Scott

[Samckitt]

Yes this is an old thread, in messed with it here and there trying to figure it out. It wasn't my daily driver so wasn't a priority. But today I got it back from its 4th to the shop (different ones, and not a dealership). But its fixed. Problem was rust inside the hole of the crank sensor, on the mounting face. It caused a bigger gap between the reluctor wheel and the sensor, throwing off the signal.

[LXCam]

Wow, chalk that up in the memory bank. Glad to hear you finally got it fixed!.

[Samckitt]

Wow, chalk that up in the memory bank. Glad to hear you finally got it fixed!.

No kidding. Me too!

[armourmark]

Wow. that's a new one.