New owner of a 2002 Z06. 67k miles. Love the car. Appears to be in top shape minus a few quirks. One of them being a slight buck at low rpms. It tends to occur in 1st and 2nd gear and it is most pronounced at very light throttle. What is odd about this issue is the fact that it occurs only occasionally. For example: I leave for work in the morning and it is smooth. I commute on the highway for about 25 miles miles. As soon as I exit and reach residential roads (stop and go) I begin to notice it. I have ruled out the transmission as the clutch does not slip and, like I mentioned, it does not always buck. So far I have:

-Changed oil, diff, and trans fluid
-Checked MAF sensor connections
-Ran a Seafoam treatment through the brake booster vacuum line

Spark plugs and a throttle body clean are next on my list. I also believe it could be a tuning issue. The car has an aftermarket intake and long tubes. Am I on the right track here? Is this just a normal thing? Another quirk that I am trying to work out is the fact that the cooling fan comes on within a few minutes of startup. Even on a cold start. Oil and coolant temps are rock solid. I don't think I have cracked 220 degrees on the oil yet. Could this be related to the bucking? Leaning towards no but I am by no means a mechanic, so any sort of input would be greatly appreciated. Thanks to all who take the time to respond.

 

Sure it doesn't have a MS3 cam in it?


Jk. If the car has long tubes and intake, coupled with the fans coming on early, it definitely has been tuned. It might be a tuning related issue.

On a second note have you driven a stock C5? And how does the car feel with headers?

 

If the engine has been modified and tuned, It could be a WIDE number of things that can cause it.
Remember... The TUNE will not change. Its the wear items in the engine that change. Like plugs, wires, O2 sensors, dirty MAF, vacuum leaks ect...

You already have the right track on new plugs and wires. MAF cleaning is also an excellent idea along with throttle body/blade cleaning.

Once you do that, if it does not solve the issue, find some one to data log the PCM.. You may have an air issue or an O2 sensor issue. The O2 sensor data, Long Term Fuel Trims and Short Term Fuel Trims will tell us a lot about the state of the engine.

If you don't already have one, Get a good scan tool that can read PCM LIVE DATA and you can see ALL the sensor data and insure that they are working correctly. That way you can give us the data that we need to direct you further.

Bill

 

I agree that the car appears to have been modified, so all bets are off, but for what it's worth, my '99 would buck and surge at light throttle in 1st gear when it was totally stock. It never bothered me much, seemed to just be a quirk of the car. Since then I've done a bunch of mods and tuned it myself and it no longer has the issue, but I can see the AFRs swing back and forth at light throttle a little more than they should, so I know the computer isn't totally happy, but unless you are staring at a wideband gauge you would not know it was there.

So, I think if you can get a little time with a good tuner they should be able to straighten it out for you. While it's always a good idea, I am not sure any amount of preventative maintenance will fix this. Good luck!

 

Is it bucking under constant light throttle? Or is it bucking when you just barely touch the throttle and go from idling to moving?

Probably set up that way by the tuner. Really easy to change. I think my fans come on at, I dunno, 160F coolant or so, but that's because of the tune for the SC unit.

 

I am not sure, but it ain't choppy enough to make me think its cammed! I have driven a stock C5 and another '02 Z as well. I did not notice the bucking on those vehicles but then again, I did not spend much time with them.

 

Thank you for the thorough response. I will go ahead with my current plan and see if it resolves the issue. If not, I'll go the route you suggested. Hopefully you won't be hearing back from me lol.

 

Thanks. I was leaning towards a tuning issue but like I said, I am a complete amateur. I just want it resolved.

 

The bucking happens at constant light throttle. I do my best to keep the throttle steady but it doesn't completely alleviate the issue. In regards to the fans, they come on almost immediately. I don't think the coolant even gets to 160F. Would changing out the thermostat to a lower temperature one make a difference?

 

Don't worry about the fans, unless it's bothering you, it's probably harmless.

 

The problem you describe does sound like a cam issue, but a big enough cam to buck would be a big enough cam to hear.

Look up up how to read your codes from the DIC and post them here. Even if the tuner disabled the codes and there’s no CEL, you’ll still be able to see if there are codes stored in the computer.

I think I may know what it is. Do you have headers?