help please - 328 catalytic converter

DannyW

DannyW

DannyW

Man of Honour
Joined
31 Dec 2005
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hi guys my 328 is given me a couple of problems and i could do with some opinions please. it has done 150,000 miles, fsh, well looked after car.

problems:

1: engine has a little wobble on idle i think its a misfire and you can hear it in the exhaust at the back

2: it has now stopping revving above 5400 rpm instead it gets there then theres like a rev limiter but it bounces up and down crazy.


-----

it has had new plugs, and i've swapped the coil packs from a low milage car, the engine has also been compression tested and its fine.

i have tried it without the maf sensor, just the same (prevous owner also also said its had a new maf)

it is going on a diagnostic machine on thursday/friday to see if out shows up


my mate is a mechanic and knows a fair bit about bmw's, he is convinced its the catalytic converter as there isnt much pressure coming out of the pipes at the rear of the car, and it also stinks (had 9 cars never had a smelly exhaust lol)


does anyone here have any ideas??

thanks, dan.
 
It could also be collapsed baffles in the exhaust. I had exact same symptoms a while ago and was convinced the Cat had gone, but it just needed a new back box.

I was quite lucky as the problem was found quite accidentally when an emissions probe was stuck up the exhaust for testing and actually hit the blockage..so the scrawny probe was swapped for a long steel bar and we managed to unblock it, but the back box needed replacing so I just put on a new system as the rest of it would'nt be far away from dying.
 
DannyW said:
hi guys my 328 is given me a couple of problems and i could do with some opinions please. it has done 150,000 miles, fsh, well looked after car.

problems:

1: engine has a little wobble on idle i think its a misfire and you can hear it in the exhaust at the back

2: it has now stopping revving above 5400 rpm instead it gets there then theres like a rev limiter but it bounces up and down crazy.


-----

it has had new plugs, and i've swapped the coil packs from a low milage car, the engine has also been compression tested and its fine.

i have tried it without the maf sensor, just the same (prevous owner also also said its had a new maf)

it is going on a diagnostic machine on thursday/friday to see if out shows up


my mate is a mechanic and knows a fair bit about bmw's, he is convinced its the catalytic converter as there isnt much pressure coming out of the pipes at the rear of the car, and it also stinks (had 9 cars never had a smelly exhaust lol)


does anyone here have any ideas??

thanks, dan.
Click to expand...

If it was one of the 4 cats, then you would hear them rattling before they collapsed. More likely the baffles have gone in the exhaust box.
 
The rattling is almost certainly a heat shield, well, I say that because similar rattles on both my previous e38 and my current e36 both turned out to be loose heat shields.

As far as the idle issue & reluctance to rev, that reminds me of my Dads e39 528i around 180k (Its done about 255k now) which turned out to be a new cat job, it cured both the idle & rev problems on his car, with a bit of luck I'd say this is probably it. :)
 
hi ladies

one of my mates wants to do a decat, will it fail its mot with no cat?

how much more power will it have?

mot no due until mid nov might be worth doing
 
When I had a problem with the car not revving, it was nothing more than a blocked air filter. For the few quid it costs, its worth looking at.
 
Fair enough... These things normally seem to be "breathing" related so chances are its something in the exhaust...
 
hi guys..

its deffo not the cat or back box, tryed a new one tonight even with the full exhaust system taking out it still limited at 5400

i have put the original coil packs in, taken out the air filter and left the battery disconected for an hour. made no difference what so ever

any more ideas guys? it's going on a diagnostic machine tomorrow hopefully.
 
DannyW said:
hi guys..

its deffo not the cat or back box, tryed a new one tonight even with the full exhaust system taking out it still limited at 5400

i have put the original coil packs in, taken out the air filter and left the battery disconected for an hour. made no difference what so ever

any more ideas guys? it's going on a diagnostic machine tomorrow hopefully.
Click to expand...

o2 sensors?
Multiple cat sensors?
Crank Case pressure
 
What year is the car Dan ?

When the car is idling, lift the bonnet. Spray some WD40 (or very similar) under the plenum/inlets just under where the inlet pipes go to the head. Spray it backward toward the bulkhead. If the idle momentarily clears up, let us know. And/or tell your mechanic. He should then point you int the right direction to a fix. Shouldn't cost the Earth if this is the case.

If it turns out to be the exhaust, and sorry if you see this as a cheap plug but FWIW I know where there is a perfectly mint used 328 system should you need it going for one fifty, but it's here in Telford. It was brought in for a customer who in the end went for a stainless system, so it's been sitting there up against the wall for about ten months now. It needs to be gone and is more use to you possibly (should the above test prove fruitless).

Good luck!
 
hi its been plugged in today at bmw

only fault to show up was vent valve on the petrol tank

anyone have any ideas? the guy who diagnosed it didnt have a clue:(


UPDATE:-

when he cleared the fault, the car was fine for a few miles, then went back to running crap and not revving above 5.4k

my mate was spraying brake cleaner around the inlet manifold and the revs were changing


i think the car may have 2 faults relating to my problem??
 
Last edited:
Ok, I could potentially be barking up the wrong tree but I have checked on the phone just now with Nigel Moseley (Moseley Motorsport) and the unit in question is called a cyclonic valve. There is a diaphragm within this that basically breaks down after a time. It can make the engine seem like ye olde Nikasil issue, but it isn't. As you say, everything else appears to check out.

This valve sits directly below the throttle body, hence my suggestion to spray under the inlets back toward the bulkhead. I'm not entirely clear whether by doing that you were saying that is cleared up for a few moments, but if it did, this is almost guaranteed to be the issue. The cyclonic oil seperator uses engine vacuum and hoses connected to the valve cover and engine dipstick/oil pan to recycle the oil vapors created by the engine. The seperator itself seperates the air from the oil, returning the oil to the engine pan. By all means if you want to call on the telephone Moseley's tommorrow, varify this with him, he will describe to you much better than I can the exact issue and perhaps how much a replacement unit is, possibly second hand as well as new.

I hope this helps, and wish you luck getting it sorted. The symptoms you have described do exactly mirror those of a customer who came in whilst I was there a couple of weeks ago. Hence this thread struck a chord with me. But I don't know or recall since then exactly how much the replacement cost myself. Your mechanic may well also be ale to help you on that. I don't know.

Anyway, good luck again.
 
hi Fido McNasty

just been speaking with my mate, and he agrees with what your saying, we are going to strip it down tomorrow

thanks for the advise, thats helped me a lot
 
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