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Car Forum / BMW Cars / April 2004Attn Nick & Michael Low - cat problem?
Thanks for the info guys!! Science question: So, when revving hard, the car's producing a higher volume of exhaust gases which pass over the cat and cause it to heat up and expand and thus block off the cat and cause back pressure - the more gas, the hotter it gets and the greater the cat elements expand?. The engine management system detects this and shuts down a bank of cylinders to protect the engine? Is my thinking correct? When revving in idle, *sometimes* I hear a rattle from under the car - like pennies in a glass jar - I rev to eg 4000rpm and when the revs are falling to around 2000, I get the sound. This only happens now and again - could this be pieces of the cat rattling about? It not under my seat but sort of in the passenger side cheers Steve > Thanks for the info guys!! > [quoted text clipped - 13 lines] > cheers > Steve I'd bet my mother it's one (or both) of your Cats. IF you're not under warranty don't get BMW cats unless you have more money than sence. BMW wanted GBP2000 upwards for replacement. Got them replaced aftermarket for GBP500 (both cats). > Thanks for the info guys!! > > Science question: So, when revving hard, the car's producing a higher > volume of exhaust gases which pass over the cat and cause it to heat up > and expand and thus block off the cat and cause back pressure - the more > gas, the hotter it gets and the greater the cat elements expand?. No, that was not my experience. The normal result of a degraded a catalytic converter is that pieces of the catalyst break off and these "chunks" become lodged in the exhaust pathways. That creates back pressure against the engine and can actually eventually stall it if the blockage is severe. Low engine revs don't exhibit the engine "exhaling" problem as much as with high revs because you're creating a slower rate of exhaust with low revs. The > engine management system detects this and shuts down a bank of cylinders > to protect the engine? Not in my experience. One cylinder in my car did shut off because there was oil on its spark plug. It was this problem that probably hastened the demise of the cat. Apparently, the misfiring cylinder resulted in overly rich fuel (and oil) being ejected on to the 02 sensor as well as the catalyst in the exhaust - thus fouling them both gradually. A fouled 02 sensor is no longer able to meter fuel properly so that hastened the problem further. And why was there oil on the spark plug to start off with? The oil came from a leaky valve cover gasket, dripped into the sparkplug well, through the thread of the plug, into the plug tip in the combustion chamber - so you could claim that started it all. > Is my thinking correct? See above. > When revving in idle, *sometimes* I hear a rattle from under the car - > like pennies in a glass jar - The rattling is typically from below along the transmission tunnel and is transitory; more frequently heard in the midst of changing low gears than when cruising steadily or in high gears. I rev to eg 4000rpm and when the revs are > falling to around 2000, I get the sound. This only happens now and again > - could this be pieces of the cat rattling about? It not under my seat > but sort of in the passenger side > cheers > Steve Like Nick says - I'd wager (his) Mom your cat(s) is(are) dead. Don't waste money on other things till you sort that out first. Sometimes the dealers are way off the mark. You have to wonder. Michael Thanks for all your advice you guys - I've found a UK 3rd party specialist that'll do them both for under ?500!! I'm going to ask BMW which bank of cylinders is constantly affected - from this I should be able to deduce which cat is failing. Although, saying that, I may just replace both - if one's on it's way out then the other will probably go soon..... thanks again Steve |
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