3.4 stumbles at 4000 RPM

[Rory Bellows]

My brother's 1995 Lumina w/3.4 is starting to crap out. After the intake gaskets were replaced it began to bog and shudder around 4000 rpm. The egr valve was leaking/venting so we replaced it. Still leaked and bogged at 3500 -4000 rpm.

Then we cut open the cat pulled out some broken honeycomb and welded it closed. It ran much better and did not leak at the egr but still missed/bogged/stumbled on acceleration at 4000 rpm.

I have replaced the wires and plugs and it idles pretty good as it always did but still has a problem around 4000 rpm. The car really can't be driven and I really don't know what to do.

I plan to take it to get the codes read because the engine come on again after the cleared cat and new wires and plugs. I do plan on putting on a new cat when and if we figure out the problem with the bogging/missing/stumbling at 4000 rpm.

specs 1995 Lumina, 3.4, 99,000 miles

I was thinking possible coil issues or even crankshaft sensor after some research here but I really don't have a clue.

[cadgear]

Check the FPR as well. At higher speeds, the FPR should close, restricting the amount of fuel flowing back to the tank, increasing rail pressure. If the regulator isn't dropping back down to block the return line, you'd have insufficient pressure at the higher RPM which could cause stumbling.

Just a thought.

[Schurkey]

I'm thinking fuel pressure, too, but on the other side of the fuel system. When was the last time you installed a fuel filter? The one I took off my '92 was so plugged you could hardly blow through it.

(That does NOT rule out coil/plug wires/plugs/etc. Wouldn't be a bad idea to look at the plugs and do a compression test.)

[Rory Bellows]

Well I went to autozone and they couldn't get any codes so I went for a drive and when I hit the hwy the light went out. It still stumbles a bit sometimes but it is drivable now. I still have to put a new cat on it and will replace the fuel filter at the same time. The car is still a POS but it drives much better now.

[richtazz]

AT 99k, you may be due for a new timing belt. As RPM increases, so does timing belt stretch. As the belt gets old, the tensioners and guides can no longer take up the slack, causing the timing to retard and power to drop off.