3.4 lq1, rough idle, also stumbles at 1900 rpms, gassy exh.

[Necaras]

My car is a '95 Monte Carlo z34 with the 3.4 dohc, auto trans.

 

I've had this problem for quite some time now - but more noticeably for about 6 months.

 

Basically the symptoms are:

- rough idle, especially under load, like with a/c or rear defrost turned on. Sounds like it's missing. Much better with a/c or rear defrost turned off, but not perfect.

 

- hesitates slightly when cruising at about 1800 rpms (recent).

 

- exhaust wreaks of gas, and it smells like it's running rich.

 

- oil leak. probably not related, but I thought I'd mention it anyway. I have no idea where it's leaking, but there's always a spot left behind.

 

- otherwise runs like a dream, with good throttle response, no dead spots.

 

And this is what I've done so far to try and fix this:

 

- changed battery, spark plugs, plug wires, fuel filter, cleaned air filter (k&n), egr gasket, intake plenum gasket, lower intake gaskets, and injector o-rings.

 

- checked intake manifold vacuum - steady at 18 lbs and recovers quickly after open throttle, so I'm not thinking cat's clogged (but hey, I could be wrong)

 

- timing belt has roughly 2 yrs, but only 15,000 miles on it. Maybe the cam timing was not done right? It was done just before I bought the car as a preventative maintenance thing. The problem didn't exist right after this was done, and has crept up on me since.

 

I took it to the dealer for a diagnostic as this had me stumped. They quoted me 8 hours of labour to check the timing after already spending 2 hours doing a compression test (150 lbs each cylinder) and checking for fuel and spark. They told me that the plugs, wires, coil pack, and cylinder compression were all good, but I didn't go for the massive labor bill this time.

 

Any suggestions here?

 

Thanks,

 

Necaras

[Prospeeder]

o2 sensors would cause it to run rich

[Necaras]

Would they not throw a code if bad? I forgot to mention that no codes were thrown here, and the engine light is perfectly happy.

[19Cutlass94]

maybe a cat.?

[Necaras]

It's possibly a cat...I was thinking of having the exhaust redone anyways, but if the cat isn't the cause, then I'll just foul another cat if the problem's still there afterwards. Same with the o2 sensors. Any way of ruling them out without replacing them?

[19Cutlass94]

You could use a hack saw and cut the cat. off and run it, see how it runs. When you go to put a new cat. on just clamp it back on, find some otehr exhaust pipe, like 4 inches and clamp that over the ends of the cat./exhaust pipe and that would be fine. Or you can take some of the exhaust off and run it with open headers

[z34Phoenix]

im having similar problem to this on my 96 DOHC the stumbling at 1900. what am i looking for if i unbolt the cat?

[Necaras]

After doing a bit of homework, I've heard that plugged cat problems can be confirmed or ruled out by unplugging the upstream O2 sensor and allowing the exhaust gasses to escape thru the sensor hole. If it runs better when you do this, your cat's restricted.

 

I'm also going to try swapping the downstream O2 sensor with the upstream one, since they're identical part numbers, and see if that makes any noticeable difference. Otherwise, I'll probably yank the plugs again and post some pics if I can't figure this out.

 

Thanks for the suggestions, so far.