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2.9/I4 Part Throttle Miss, Getting Worse

1724 Views 7 Replies 4 Participants Last post by  coletrain373
I bought my '08 2.9/5spd last November as a dealer trade-in with only 38,000 miles on it. Since then (less than a year) I've put almost another 40k on it myself (up to just shy of 75k). For the most part it's been a good truck. I've always been a little disappointed with the idle quality, but from what I've read that's par for the course.

I have an increasingly worsening part-throttle miss. The truck has always knocked a little when leaving a dead stop if I let the engine revs get below about 1500, but it's been consistent for most of the time I've owned the truck and hasn't bothered me. Starting about a month ago I began getting an intermittent jerky miss at low throttle, generally when cruising in 3rd or 4th at 35-40 mph. If I let off to idle the miss goes away, and if I floor it the miss goes away. I initially chalked it up to maybe bad gas, but I've put another 2500+ miles on it since then (8 more tank fulls) and it has gotten steadily worse. I can now duplicate it in any gear except first. The miss is still not constant, but now quite frequent. Still most noticeable when cruising lightly, but also now present during moderate acceleration. The truck still cruises fine down the highway, I've noticed no loss of fuel mileage either. The miss can occur whether the truck is cold, warm, or hot. The miss is often accompanied by knocking, so I'm pretty sure it's a lean condition.

No CEL. The only CEL I've had on the truck in the last year was for an O2 sensor, which I changed. I've scanned it and there are no pending or stored codes. The truck has had an air filter every 15k miles, and has also had an induction service every 15k miles. The dealership warranties the engine for life if I follow their schedule for this stuff, I'm not usually the kind of guy to pay for stuff like an induction service. I also physically removed and cleaned the TB 10 or 15 thousand miles ago, but it was pretty clean.

So what can cause a miss, that's getting worse, without a CEL? At first it was just annoying, but now the driveability is suffering.

Also, this may or may not be related, but occasionally the truck is hard to start. This has also been going on since I bought it. Once in a while the truck will crank but not start. Usually giving it gas the second time I crank it will start it. It seems unrelated to any weather condition or anything I do, just something that happens once a week or so. It's not a passlock problem, there are no dash lights or anything associated with it. It seems to be worse if I turn the key on and don't start the truck immediately.

I'm a bit at a loss here. If it was a fuel pump I'd expect the problem to be worse if I give it more gas, but it gets better. I'm pretty sure it's going lean because of the knock, but apparently not lean enough to throw a code? Is there a code for lean condition like there is for rich condition? Where should I start, TPS, MAF?
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Cleaning the maf sensor would be a good start if you haven't yet

make sure to use maf sensor cleaner.

are you sure its a knock and not an exhaust leak ?
Thanks for the tip. I'll clean it tonight and see if it makes it any better.

Definitely a knock. There's also a heat-shield rattle somewhere that comes and goes around 2k rpms, but the noise I hear associated with the miss is unmistakeable as engine knock. It only happens when the truck is missing, or occasionally under heavy load/low throttle (like leaving a stop in 2nd).
I would bet money that the heat shield rattle is the heat shield on the elbow of the manifold. I spent 2 years chasing one simular because that shield. Couldn't tell from looking at it but once it was gone it was quiet again
I will also second the maf clean and seafoam. There is a how to on the seafoam. Have you taken a look at the spark plugs? Just a thought.
This evening I did the SeaFoam treatment through the TB vacuum line. Lots of smoke, both during the treatment and the drive following. I also removed the MAF and cleaned it with the CRC cleaner and let it dry for about 20 minutes before reinstalling and going for a drive. I put about 10 miles on the truck after this, and frankly the miss is worse. The idle seems a bit better, but the part-throttle miss is worse.

I did the plugs at 50k (about 25k ago). Replaced them with AC Delco as I recall, but I can double check the receipt. Used anti-seize and dielectric grease like I always do. Last air filter was about 12k ago (almost time for another one). Always used stock airfilters to date. The whole truck is bone stock.

Do I need to reset the computer before the MAF cleaning and SeaFoam take effect? I'll try it tomorrow anyway.
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I had the same exact thing. It happened when I was in like 4th gear and what I did was take it to PCM4Less down in the Carolinas when I still lived on the East Coast; you can take it to any tuner though. What they did was adjust the knock retard (not quite sure exactly what that means) but it went away completely. Not sure what the exact problem was but thats how I fixed it. Good Luck!

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