[skyway2k]

There seems to be a lot of smart people here willing to help so I hope you can give me some suggestions or direction.

The car is a 1996 Monte Carlo LS, 3.1L. It has approx. 108,000 miles.

What I am experiencing is an intermittent hesitation during normal acceleration. Sometimes I can go a whole 20 minute trip with no problem, other times it happens right after it warms up. There is also sometimes a "bucking" feeling during highway driving when the car is on cruise or just at highway speed that goes on for a few seconds and then clears up on its own.

These problems only seem to happen after the car is warmed up and has been driven for more than just a few miles. The idle is pretty smooth, probably at little high at 950 - 1000 RPM. When the hesitation happens, the RPMs do not spike up like the trans would be slipping, they just flatten out and the acceleration stops. The engine sounds like it lays down and sounds different than normal. The sound difference is very pronounced with the windows down. What I have found is that sometimes when I go to take off from a stop and the hesitation begins, if I release the accelerator and go back in it at the same level, nine times out of ten the hesitation will disappear and I can go on like normal. Or if it doesn't, I can repeat the "let off of it and go back on" thing a few times until it clears up. Also, if I go to WOT it pauses for a second to grab the lower gear and then it goes like nothing is wrong and pulls through the gears fine.

The SES light is currently not showing. However, last summer when it was doing this same thing it was on for misfire in cylinder 3, Evap during non-purge condition, and an internal transaxle slippage code. I changed plugs (to Bosch platinum - I know, I'm going back to AC Delco 41-940 today) and wires and the misfire code went away, but the intermittent hesitation remained. I thought it was transmission related then at that point. I was only using it for short trips to work and back so I kept driving it. Then the tranny started leaking last July so I parked it and started using my truck for all trips.

I want to start using the car again so last week I took it to a reputable transmission shop to fix the leak, change the fluid and filter, and diagnose the problems I was having, thinking it was still transmission related and expecting to have to do an overhaul. They said they performed driving tests and scans and couldn't find anything wrong with the transmission but that there was a "horrendous" misfire.

So here is what I can tell you that I have done -

-All three coil packs are new as of last week
-Plugs (Bosch) and wires (Duralast) and air filter are new as of last summer. I am going to change back to AC plugs today
-Fuel filter is new as of last week
-I ran a bottle of the Lucas Oil fuel system cleaner though
-I performed the "suck test" on the fuel pressure regulator using a clear plastic hose - it holds the vacuum, however I seem to get the "taste" of fuel on my tongue even though there is no liquid fuel coming out of it.
-I changed out the EVAP solenoid last week and now that code has disappeared but the hesitation still happens intermittently.
-The water pump failed on a highway drive in Sept 08 and one of the head gaskets blew. So as a result, all intake gaskets are the new type to fix the IM gasket problems.


Of the things I have been reading on the net for the same problem, some people say clogged cat, MAP sensor, EGR, etc... so I'm hoping that maybe some of the symptoms I have described will help me get pointed in the right direction.


Thanks for time!!!

[ghettoandy101]

uhh, just looking thru the things u changes etc...i didn't see a fuel filter in that list. if i were u, i'd change that lil guy...it's <%15 at the auto parts store...

edit: i see u did change the fuel filter..hhmm, maybe a vacuum leak. since u say it usually happens after the car's warmed up. could be a rubber hose or something expanding when it gets warm, just enough to cause a leaky

This post has been edited by ghettoandy101: 20 June 2010 - 03:08 AM

[96mcls]

does the "bucking" happen when there is a load on the engine in 4th gear? like your trying too go up a small hill or slowly accelerating a few mph

swap plugs and make sure all your plug wires are on tightly, and clean the MAF sensor and see what that does

[poncho1982]

I'd check you Throttle Postion Sensor. They are notorius for wearing a flat spot right off idle, or at speed, wherever your foot is most of the time. Its just a brass wire brush, riding on a resistive surface, to give readings to the ECM on position. It also kinda acts as sandpaper over time, and loses resistance. The ECM can't figure out where your foot is and what ammount of fuel to allow the injectors to spray. Result, hesitations, bucking, etc. And heat can change the symptoms, because metals expand and contract with heat and cold. So it could be fine sometimes, and not others, depending on temperature.

Not that expensive either. Like $35 or so. You can use the Haynes manual, and get the Ohms basline, and hook an Ohm meter up to it, move the throttle through its range, and look for the resistance to go flat instead of steadily increasing or decreaseing. That saves you from buying one and it not being it....

This post has been edited by poncho1982: 06 November 2010 - 08:51 AM

[Adam]

Just a thought, I had some similar symptoms you described,  I had a faulty fuel injector on cylinder 4. It caused the car to misfire a few times and would as you put it "buck", usually during highway driving or when I was around 50-55 mph. I replaced it about it month ago and haven't had a problem since.

[RICH17]

Im having the same issue but it only happens about once a week. it feels to me like a bad MAF but i scanned and it was reading okay. Even when it was bucking. It kinda feels like an overheating ICM but i have no codes. I did all the same tune up and stuff and even did the LIM. I plan on switching the ICM from another car to see if that solves the problem. Il keep ya posted