Pontiac Grand Am 3.4 L engine bucking at high RPMs

[jpsantell]

I have a 2004 3.4 L engine that starts to buck and miss when you step on the gas hard like you are passing another car but if you drive and accelerate like an old lady it is perfectly fine. It seems to happen around 3000 rpms. I changed the fuel filter, threw in some injector cleaner in the tank, changed the spark plugs and wires , tested the ignition coils and they are fine. I even sent the car to the GM service center and all they could come up with is the crankshaft sensor was out of speck so I change that also. All of this has not caused any improvement . I am running out of ideas, anyone have any suggestions? By the way, no codes and the shop though it could just be a pcm problem but hey, they were wrong about the crank sensor too so I don't have a lot of faith in them.

[RedZMonte]

I would post in the Pushrod section, the 3.4L DOHC and the 3400 SFI are much different motors... the pushrod guys are very quick at responding, there are a few more of them hainging around then the DOHC guys these days... We are a dying breed these days

Does it feel like a Miss fire or something else? have you put a fuel pressure gauge on it? I would suspect something other then crank sensor as well from what ive allways seen it either causes it to run or not if its bad or going bad.. What Plugs/Wires are you running now? a really bad Missfire like that to make the vehicle buck really hard i would expect the ECU to click on a CEL and have a code if it was a miss. Ive had simmilar issues when my fuel pump was failing and never got a CEL.

Hope this helps,
Shane

S

[bszopi]

Moved to the appropriate section...

Which crank sensor did you replace?

[jpsantell]

The sensor located under and to the right of the harmonic balancer and boy was that fun getting off. The sensor is the type 24 shaped like a banana not the pencil kind.

[bszopi]

I would look at the 3x sensor on the back side of the block. The front crank sensor (24x) is only for better crank position resolution at low RPMs. Also look at the wiring from the rear crank sensor to the ICM, checking for any damage to the wires.

[jpsantell]

I looked through my Haynes manual and can only find a speed sensor in back, is that the one you're referring to?

[3400beretta]

I had what sounds to be exactly the same problem on my car. It turned out to be either a bad ignition module, or bad crank sensor wire (the one that goes from the crank sensor to the ignition module). I don't know for sure which was causing the problem since I changed both at the same time.

[bszopi]

I looked through my Haynes manual and can only find a speed sensor in back, is that the one you're referring to?

A speed sensor would be on the tranny, not on the engine. The 3x sensor will be on the engine, although very close to the tranny (they are a pain to remove). It will be in about the middle of the block.

[jpsantell]

Today I changed the throttle positioning sensor, it had a small dead spot but didn't take care of the problem. I'm still looking for the 3x crank sensor, there is soo much crap under this hood it is very hard to find anything. Also thinking about changing the ignition module. I'm tired of working on this car.

[jpsantell]

I just noticed that the car misses at high RPM even when in park. I was looking at the motor Saturday and moved the throttle up and found that the engine misses like every 2 or 3 seconds at a very regular pace. Like clockwork. Do you think this could be a ECU problem that doesn't pop a code or even an ignition module issue?