Hey folks, first post on these forums although Ive been following for a while.

Ive run into an issue with my Jimmy thats got me stumped. Ive found a similar thread to the issue Im having but thats almost 3 years old now and has no useful posts on it.

Here is the issue. About a year ago my jimmy started chugging around 2k RPM in what I assume is second gear. Its not a large chug, if you watch the RPMs, they dont jump more than a hundred or so. I was stumped then and I had no idea what the issue was so I just went on with it as is because I didnt have time to really dive into the issue. I did have to change the cat because it was plugged solid, but after a few weeks the problem just disappeared.

Well it just came back. Every time I get into my truck after sitting overnight and I accelerate it chugs. Same thing as last time, not a huge chug but only about a hundred RPM. Its like it takes two steps forward and one backwards. Always accelerating but having a hard time getting there smoothly and after like 5 min of driving it goes away completely. Thats the part thats got me stumped. Why would it chug when cold but then run smooth as glass when its warmed up?

I thought it was bad gas so I ran my fuel tank down as far as I dared to on the "Low Fuel" light (about 40km). I filled up with a full tank of shell gasoline and ran some injector cleaner with it. Im still having the same problem. Just for curiosity, I booted it way past 2k RPM to around 4k to see if it would chug while under alot of power. Didnt have an issue with accelerating through 2nd gear and 2k RPM when i booted it. It only seems to do it when i accelerate gradually.

Ive ruled out bad gas. It got a brand new cat, back in June ish of last year so I doubt very much that is plugged. Did the oil less than 1000km ago, did the plugs/wires/distributor around last April.

I did have an issue with my 4x4 not fully locking the front dif due to a bad vacuum lines, causing it to have low suction. It was like that for over a year and WAS still an issue when this chugging started last year. And it still wasnt fixed when this chugging started again. About a week after it started chugging again I replaced all my vacuum lines that were leaking and my 4x4 is now working 100%. Im not sure if this is related to my issue or whether or not having a vacuum leak for a year had any sort of long term effects on my engine or not.

All I know is that its become a real annoyance to me and now that I have some time to fix it once and for all, it will be fixed. Any info or tips you guys may have will really help me out.

Thanks alot!

 

There are a few things I would recommend that may or may not be related. If you haven't done so already, I would clean the MAF, throttle body, and EGR valve. All are relatively easy and cheap, just buy a new EGR gasket before removing the EGR valve.

More to the meat of your problem, here are some things that could cause the symptoms:

• Harmonics from the balance shaft affecting the knock sensors which retard spark timing, hindering performance - GM has a TSB on this issue. - I say this because of the RPM which you state you have this problem.
• Vacuum leak - you have already ruled out the lines, but there could still be a leak at the lower intake manifold gaskets, EGR, etc that could be affecting performance.

The easiest way to rule out some of these things and to get a more honed direction for troubleshooting is to monitor engine parameters with a scan tool.

• The knock sensor count would spike at or near the RPM you speak if something was activating the knock sensors. Knock Retard could also be used to show this phenomenon.
• Vacuum leaks that allow fresh air into the engine not sensed by the MAF sensor should result in higher than normal fuel trims since the PCM has to add more fuel to bring the O2 sensor readings back in spec.
• A leaking EGR valve would tend to result in low fuel trims since the EGR valve would be allowing spent exhaust gases back into the intake, displacing combustion supporting fresh air.

 

Thanks for the quick response!

The MAF, throttle body and EGR are definitely top of the list. Ive never cleaned any of the 3 in the 3 years ive owned it. There does appear to be alot of carbon build up in the throttle body so Ill have to clean that up.

Ill tackle those 3 hopefully tomorrow and hook up my code reader to it to check the knock sensor. Ill check back in with my results in the next day or so.

Thanks alot for your help.