backfiring when warm

Not if it runs ok cold...

Marty

More likely something to do with the air / fuel ratio being too lean. Either a significant vacuum leak - air entering the engine without being metered by the MAS - or, more likely, a defective MAS - mass air sensor.

A vacuum leak can occur at the juncture of the intake manifold and cylinderhead, also a split or disconnected vacuum hose. Look for that if this came on suddenly rather than gradually.

I have just had a MAS fitted, and I had a similar problem before it being fitted, except that it run poor at low temp, and not too bad at high temp. Just seems to have flipped. It cost me nearly £300 just before christmas, and after an expensive christmas, I cant afford to have it checked out again without really knowing whats wrong with it. The merc garage wants £80 hour just to find out whats wrong, nevermind any more repairs. Could it be a faulty MAS fitted ?

paul

Don't pay the ridiculous amount the Mercedes dealer wants! Use a good independent. Where are you, as I keep an up-to-date list of these here, so could point you in the right direction? Independents typically charge 30-40 pounds per hour and are often far more competent as well...

Al Bolton Caversham, UK

Of course the new MAS COULD be defective but since it's new let's assume for the moment that it's OK.

I'm sure you will scream, I would in your position of now being an unwilling captive of the "let's try this" prison, but has anyone checked the crank position sensor?

If it gives the computer the wrong crank position the spark will not be correctly timed. Badly retarded the engine will start easily, run smoothly but lack power and acceleration, too far advanced it will buck and may backfire as yours does.

You wanted ideas, that's mine.

Hi Al

I live in Worcester, and would apreiciate any pointers in the right direction. The Merc garage over here doesn't give me any confidence.

paul

In the old days you could make just about any car backfire by turning the key off while moving letting gasoline to get into the exhaust system and muffler and then turning the iginiton back on., boom. I the muffler was weak you ended up with repair job. So if your car is backfiring, it must be getting fuel inot the exhast system and being iginite. Now to find out how?

If you don't mind travelling a bit, I would suggest Andy Gayle in Smethwick, not desperately far from you. He is excellent and should be able to help you. Check out his web site, which is at:

Let me know how you get on. Call Andy and he can give you an idea what needs doing.