I'm in the process of purchasing a used '00 M5 in CA with 68K miles, but it failed the required smog test with high hydrocarbon output. It was run for at least 10 minutes before the test was done, although I read here (after failing the smog test) that it should be driven for as much as half an hour first. I've never had this problem with my current car, a 96 Mustang Cobra, which has always passed by far.
I took it to a BMW dealer and they said the spark plugs were fouled and needed replacing, which was causing misfiring, thus the unburned fuel and high hydrocarbons. The engine seems to run pretty smooth however. Does this sound like a reasonalbe explanation? They said the plugs had never been replaced, and would normally be replaced at about 57K miles.
They want over $300 parts and labor to replace the plugs, in addition to about $200 to diagnose the problem. Sounds pretty steep to me.
Is there a good chance it's really the spark plugs, or that driving it around for half an hour would fix it?
I know that the M5 plugs can become fouled - especialy after 68,000 miles - and this could cause the engine to run poorly. I'm sorry I don't know the answer to your smog question though. Let's hope one of the more mechanically knowledgeable among us reads this thread. Hang in there for a little longer and let more people read what you've posted.
If the plugs are way past due. Why would you want a bad running M5? The plugs should be replaced now under any circumstances. Hopefully this will cure the problem.
If one or more plugs is fouled and misfiring due to higher miles on them, this motor has probably been running rough for months. Not only will this increase emissions, but this will most likely cause the catalytic converters to burn up from raw gas being ignited inside them. This is very serious because converter meltdown can cause excessive back pressure and ruin a motor (rare). Dealers may have an adapter that screws in the exhaust system to measure back pressure. If a plug is, in fact, bad, I'd say there's a better than 50/50 chance you will need $1200 catalytic converter assembly and not just $50 worth of plugs. If the misfire subsides when the car gets good and hot after being driven for 30 minutes or more, the converters may still be alive.
Cat failures on the sister 540i are common due to fouled plugs, so I'd be prepared. I think the assembly is shared between the two models, much like the factory headers.
You might get lucky and find a dealer to replace the converters for free. I can't remember how many miles they are warranteed for, maybe 7yr & 70K. It's a long time.
I had the dealership hold off on changing the spark plugs, but they still charged about $200 to diagnose it. When I picked the car up they gave me paperwork showing that they re-smogged the car (without changing anything) and it had passed the 15 mph test (barely) but still failed the 25 mph test, and that was after warming it up for quite a while. They said they were aware that these cars need to be warmed up well before being smogged, but that this one must have fouled plugs since it still failed. They indicated that the car computer didn't show any fault codes. The service manager said fault codes would only occur for "hard misfires", and the fouled plugs were causing softer ones which weren't obvious but didn't burn completely. Does this make sense?
The seller asked me to bring it to the dealership he used where he knows the service tech and can get much better deals on the service (he's paying for this since it's the sellers responsibility to have the car smogged). He said the spark plugs were replaced during the 60,000 mile inspection 2 service, and it's strange that they would have fouled so quickly. He also said he suspected the O2 sensors on one side may be bad and causing the problem, since he had the other ones replaced a while ago after failing. I'll ask about possible damage to the cats from running with high hydrocarbon emissions. I'll report back what they found after I get back tomorrow.
they said the spark plugs were fouled and needed replacing, which was causing misfiring, thus the unburned fuel and high hydrocarbons. The engine seems to run pretty smooth however. Does this sound like a reasonalbe explanation? They said the plugs had never been replaced, and would normally be replaced at about 57K miles.
- Dave
I am under the impression that the ECU would detect a bad combustion and misfires to then cut the fuel suuply to the cylinder(s) with the fouled plug(s) when under load (> 3'500 rpm): Limp mode when flooring it.
Have you tried that ?
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Marc
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99 Carbon Black ///M5
BMW K1200R
I'll ask about possible damage to the cats from running with high hydrocarbon emissions. I'll report back what they found after I get back tomorrow.
- Dave
To clarify, misfires allow unignited raw gas to exit the exhaust port and find it's way down to the super hot cat's. When it ignites inside the cat, it's guts melt into a blob. It has little to do with hydrocarbon emissions.
Most shops have a back pressure sensor that screws into an O2 port in the exhaust to detect excessive backpressure.
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