I have a 1998 ML320 with 100,000 miles on it. The check engine light came on recently. I took it to a mechanic and he read the code and said there are several instances of engine misfires howver he was not sure if that was causing the check engine light to come on.
He changed the spark plugs and air filter. I also recently replaced one o2 sensor.
Now my truck is running badly, when idling it is really rough, sometimes there is a rattling sound that seems to come from under the car and the check engine light is still on. The mechanic thinks the rattling might be because of the catalytic converter or fuel pressure?? He says at this point he is just guessing because he cannot tell for sure what the problem is. I don't want to keep paying for useless repairs. It is getting really expensive. Any suggestions?

Spark plugs should be changed every 4 years or 100,000 miles (whichever is sooner). So, if they had not been changed, they were ready.

Misfires could be caused by spark plug wires. These are expensive to replace.


The MAF sensor often goes bad and can cause reduced mileage and cause idle problems. This also causes the CEL to turn on.

The rattling could be the catalytic converter. These are a known problem on early model years. They crack at the weld. Cheapest fix is to have a muffler shop weld over the crack.

Fuel pumps are also a known problem on early model years. An updated fuel pump is available. Also, the fuel filter should be changed every 60,000 miles.

In summary, based on what you have indicated, your mechanic seems to be checking the correct items that are known problems. If you need to take the ML to the dealer and have them:
- troubleshoot the misfires and replace sparkplug wires as necessary;
- replace the MAF
- replace the catalytic converter(s)
- replace the fuel pump
you are looking at about $3,000 - $4,000. All of this can be done by a local shop (not the dealer for less). In general you should plan on spending an average of $1,000 per year to repair defective items, plus normal maintanence depending on how many miles you drive.

If you do a search you can learn much more about each of these issues, as all of them are known problems.

OK, I understand avoiding the dealer's expensive prices when you can do a repair yourself. But, when you can't diagnose the problem, that's when (I believe) the dealer is worth the price. Be specific, say you want to know what's wrong and get an estimate before they do the repair. Then, armed with the information, you can decide if you want to look for aftermarket parts and/or do the repair yourself.

The mechanic I go to was recommended by a couple of different Mercedes/BMW dealers in my area. Apparently, he specializes in these cars. So while his prices are slightly cheaper than the dealership, they are still quite steep!

I think that it's good that the mechanic is admitting that he'd just be guessing but, I always feel like the dealer has the know-how & the special equipment required to determine what the problem really is. You can't just start replacing parts based on a guess, you need to determine what the problem is. Good luck.

www.Getaboyfriend.com

Just to give you an idea of some of the prices, I had my Mass Airflow sensor replaced at a cost of $534, fuel pump replaced for $400 and fuel lines replaced for $150 over the last couple of years.

Truthfully, the cost of operation over the 6 or 7 years I've had the car has not been bad. It probably averages $300-$400 per year(excluding fuel costs), but often comes in $1000+ bursts.

Sorry, i just receive a call from the dealer because of the same problem that you're having. My pastor car is having the same problem. The car started moving very slow and the mileage is @ 98,000. I was told that the catalytic converter is bad. The quote i got is $2000 just for the CC not including everything else that i need to do on the car. The total cost to bring the car back to life is $3900. The car is also a 1998 ML320..




Tony

Has the FUEL filter ever been changed? That definately needs done. I assume the dealer did a fuel pressure test during the diagnosis.

My 2000 ML430 had an updated design, elongated filter with a pressure regulator. This may apply to your model. I changed around 50 k. It was causing major sluggishness when accelerating. Does yours just idle rough?