KuHL 951

My 91 BMW 325 is having a problem where it wont rev past 3500 after it warms up. It's like it is getting starved of fuel, no miss, or the backpressure is too high. It idles well at any temps and It runs to well past 6K for about 10 minutes when firstr started in the morning or after sitting for a few hours. I think it's the original cat @ 205K miles, no new welds or pipes. Since it's Motronics and Bosch can you think of anything else that might cause the fuel to be reduced? O2, temp sensor? The TPS is adjusted spot on and the car was in a good state of tune just a month ago. Any ideas, I didn't have much luck asking on the BMW forum. I hate to throw on a cat and O2 if it might be something else.

Thanks

Don 944 LA

clogged cat will be just like a bannana in the tailpipe. The car will jump and surge.
(from experience)

Geo

When the cat collapsed in my SE-R it was similar to what you describe. It would not seem to rev past a certain point. What I found was that it would eventually rev higher, but only if I accelerated at part throttle and was patient as it needed quite a wile to rev higher.

I think you may be onto the right thing.

How quickly will it rev to 3500? If it revs quickly to that and stops dead it might be an ECU thing. I know Nissan ECUs will have a harsh low rpm cut-off when in limp mode. Don't know about Bimmers. I'd say if it revs slowly to 3500 and just seems to hit a wall that requires too much effort, more likely it's the cat.

KuHL 951

It will rev a bit quicker when cold and can be slowly feathered past 3500 under no-load conditions in the driveway; on the road it doesn't really hit a wall or anything. It just stops accelerating past a certain point of 3200-3500 rpm when completely warmed up. I'm looking into having the cat replaced today as well as the O2 sensor. The car ran very rich at cold start for more than a month or two with a bad idle control valve last winter. I'm thinking this probably started the cat on it's down spiral. Thanks for the response.

sca037

I think you're onto the correct diagnosis.
When the cat clogged on my GLH-S, it ran fine while cold but when the substrate heated up it could dislodge after a bump and the car would fall flat on its face.
Check for very high cat temperatures, as mine got to the point it was melting undercoating off the car before I figured it out!
I removed the exhaust, hollowed out the cat.....and it ran like a raped ape

Brian

Don 944 LA

Thats what I ended up doing, and later when I got another cat I REALLY noticed a difference.
Although I hollowed it out while on the car, couldn't get some bolts off
Rebar, and an old callaway golf shaft (pointed tip) helped.

It did run like a raped ape

Don

KuHL 951

Well the new cat is on and the problem is gone for good. The old one was rusted out bad with various plates shifting around inside. Even without a new O2 sensor the car has never ran this well since I bought it last Summer. I can't believe it passed smog in June, 05. I think the cat had been partially plugged since I bought it. The shop couldn't get the old rusty O2 sensor off and were afraid it would snap off. They tested it and said it was withing spec. That leaves me with the decision to return the new O2 sensor I bought or have to pull the complete headers/cat off the car and have it removed at a shop that can weld on a new bung if it strips out. I know it's a good idea and recommended that the O2 be replaced at the same time as a cat; is this rule set in stone? I can't really get to the headers for at least a month. Should I leave the O2 on until then if the car is running so well?