Another female desperately needs help with..... ML320
 

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?

Crawl under the vehicle and put your ear under the catalytic converter. If that is what is rattling, you will be able to tell. Just don't touch; it gets real hot, real fast.

Hey. Best thing you could do it take it to a Mercedes specialist independent mechanic. Those guesses being made are really expensive (and a real pain). There are experts who can test to figure out exactly what's wrong and replace only the necessary parts. Good luck and enjoy.

Yes, it is sometimes difficult to diagnose a check engine light over the internet. Do you know exactly what the codes that were pulled are?

Whats with the multiple postings?

Find a boyfriend who's handy around the car!

That's who I take the car to. Which is why it is so frustrating to hear him say that now all he can do is guess!

No, he did not tell me the codes. He just said that the codes showed that the engine was misfiring. It did not show problems with any specific parts. I am planning to take it to Autozone and see if they can give me the codes.

I was not sure which forum to post to and did not think that ANYONE would have an issue with two postings.

As for your advice, all I can say is............Thank-you to all of you who took the time to be helpful!

From what I hear ML's are not Mercedes finest effort. With 100K on the clock it might be a good idea to start shopping around for a replacement. Your probably just getting a taste of what's to come....

I wouldn't sell off the car just yet at 100k miles. Sure there were teething problems, but I wouldn't necessarily put off the vehicle solely because it has 100k miles on it. If you could ask the mechanic about the specific codes he pulled it may be easier for someone here with more knowledge on your specific problem to narrow down the possibilities.

Since the "expert" MB mechanic can't seem to tell what the problem is, I can't do much worse by guessing.

Things that are in need of replacement at 100K...spark plugs at least. You may also have a bad coil or two. The tech can check these easily.

Also, a bad MAS and an ailing fuel pump can set off misfire codes (as was in my case).

Bottom line is this could set you into just under $2K if you let the tech just start replacing all of what I mentioned. For starters, if the plugs and MAS are original, I would get those done first.

Did these symptoms just start out of the blue, or was it a gradual degeneration in performance?
How did the plugs look when he pulled them?
What made him think that one O2 sensor was bad?
Are you using the correct grade of fuel/using a quality fuel?
You say it idles rough...what about when you accellerate/drive at a steady speed?

Only one person has hinted at what seems fairly obvious to me, that her mechanic lacks necessary skill. A trained Mercedes mechanic should be able to provide a correct diagnosis, or at least much better than what she got from this mechanic. I admit that it can sometimes be difficult to find a mechanic both skilled and honest. Also, I wouldn't be surprised if G-benz's diagnosis is correct.

Mike

I talked to my mechanic and he said that they did not note down all the codes because there were so many and they all basically were that cylinders 4 and 6 were misfiring. So they changed the spark plugs, thinking that would fix the problem.The car was due for a service anyway and it needed to be done.
Since the CEL is still on, I took the car to Autozone and when they read it the only code that came up was 'P0306, Cylinder 6 misfire detected'.
I talked to the mechanic again and he thinks that the catalytic converter on the driver's side might be causing the problem and should be replaced.

The check engine light came on a couple of weeks ago, till then the car was running fine. The O2 sensor was changed a little over a year ago when the CEL light had come on then. I have only used premium fuel in the car. The car runs fine at a steady speed but occassionally I can feel it "skip a beat" (probably when one of the cylinder misfires!)

Right on!

Looking at the first post, this mechanic says a rattling under the car could either be the cat or the fuel pump. Seems to me that a good mechanic should be able to differentiate those two -- via fuel pressure test, maybe?

One thing I've learned, from this and other lists as well as my own personal experience, is that different mechanics have different aptitudes and interests. A mantra we hear on this site, from Gilly and other pros, is that proper diagnosis preceeds parts replacement. Some mechanics are just plain better than others at thinking through the logic of tracking down a problem, esp. if it involves control circuits, is intermittant, causes malfunction indicator lights, etc.

Find another mechanic. Folks on this site can help with that. If not, contact a local MBCA officer and ask him/her who local club members take THEIR cars to! I can just about guarantee they'll know who's good.

Hi, my turn:
Let's see if I have this right: Truck ran fine, but check engine light was on (I am assuming it was running OK), OK, tech sez misfire codes, lets replace plugs and air filter (good maintenance, I like that, worries me a little (just a little) when maintenance is recommended to repair the vehicle tho. OK, so plugs and airfilter are replaced, THEN the truck runs badly? And check engine light is still on. AND now it rattled underneath, too. Yeah, great. WHAT do I tell you.
Not sure, this is I think referred to as "FUBAR". My thought would be a botched spark plug install somehow, but how? Broke off a sparkplug and he's hiding something? Wrong wire on a couple plugs? Ruined a plug wire? I dunno. Should run better not worse, and now it rattles underneath. Maybe due to a dead misfire?
At this point you may want to consult a different MB mechanic, dealer or a good indy. I guess my first step, well, if it's really just #6 misfire, start there, check connections are correct, plugs are tight, maybe inspect the plugs, who knows what happened. I'd discount the rattling as a "cause" of the problem, maybe an effect not a cause.
The fuel pump I've never heard of one that "rattles", plus it's mounted in the tank.
Gilly

I took the car into the dealer , they said both catalytic converters need to be replaced and they recommended all 4 oxygen sensors be replaced at the same time.
All of it adds up to some serious $$$$. I am considering ordering parts over the internet but have never done so before. There is such a big difference in price, does anyone know if these parts/places are reliable. Any suggestions?

Silent Mode
 

Silent Mode,

She's a NEW MEMBER, let ask her questions. She came to this forum expecting to be HELPED along with some courtesy from ALL members.

I just bought one made by DEC. I have not put it in yet (just got it Monday) but I have looked it over. The dimensions are very similar. The "output" flange is mis-aligned about 1 degree which I hope I can deal with. Probably, but it would be nicer if it was straight. The quality of construction is not as good as Mercedes but it was 1/4th the price, literally. I am curious also what people have to say about the different brands. I never heard of catalytic converters "rattling apart" inside until I got this Mercedes and started using this site. My original Toyata Supra cat is still going strong.

Mike

It's not an uncommon problem. Certainly not limited to Mercedes.
I would not replace both converters if only one was rattling. As far as ordering parts online, you can certainly save money. Just be sure you are ordering the correct parts. Returns are sometimes problematic. Many of the online vendors don't actually handle the parts, they just forward orders to distributors. So they like to charge restocking fees on returns. Also, if you are not going to install the parts yourself, make sure that you have a mechanic that will install owner-supplied parts. Many will not.

Thank-you!

While only the driver's side converter was rattling, the technician told me that both converters were bad..the driver's side was in worse shape!!
What's confusing is that if both were in such bad shape, why didn't my check engine light come on sooner? Why didn't the oxygen sensors detect any problems?
I was also told that while they are pretty sure this is what is causing the problems with the car, it is not a guarantee. So after I change the catalytic converters and the oxygen sensors, they might still find additional problems!!

In my case my cats still work perfectly well, they just rattle under any load. And I also have just one side (I think) making the noise, but mine is all one welded assembly so I have no choice. I think you have multiple unrelated problems. A good diagnostician can sort through all of this. Did the dealer claim that the cats are responsible for all of the symptoms?

God, I hope not!!!

[/QUOTE]A good diagnostician can sort through all of this. Did the dealer claim that the cats are responsible for all of the symptoms?[/QUOTE]

He suspects that is the case, but will not offer a guarantee.

Well, I suppose if it is clogged up. I would think there is a way to test this (other then installa new cat). On the other hand, if the cat is bad then it needs to be replaced in any case. Hope they are correct!

I'm just really skeptical of the diagnosis, doesn't seem right to me, especially given that first the problem (I assume) was the check engine light, THEN after the plugs, etc were replaced, it ran badly. Shouldn't run worse after maintenance was done. Or was it running poorly before the work was done?

Gilly

Before laying big bucks for catalytic converters, have the exhaust back-pressure checked! VERY simple operation for any MUFFLER shop & OR repair shop that does emissions repairs. IF the back-pressure is high then have THAT catalyst replaced. WHY replace parts that TESY good?

If you're anywhere near NE Indiana I know of a place that installs owner's parts dirt cheap. They run their business out of a little old gas station, but they get the job done. They're good with exhaust stuff. They'll weld pipe, etc. to save from buying a whole new exhaust or even a muffler assembly.

Thanks
David

When I took the car in for the CEL, I told them that there was also a rattling noise coming from under the car. So they did the back-pressure test and that's how they decided that both cats needed to be replaced. I guess what worries me is that no one knows for sure what made them go bad. They keep telling me that the engine misfires would have caused it...but what caused the engine to misfire?? Their answer to that is that if the cats are bad it will cause the engine to misfire!!!
What came first the chicken or the egg??????

Thank-you. But unfortunately I am a little far from there.

Well, the spark plugs being worn out could have caused the misfires, it would have been pretty obvious to you that the engine was misfiring if it was bad enough to ruin a cat though. The info you gave is that you had a check engine light, and AFTER the plugs were replaced THEN the engine ran badly, not BEFORE, or did it run badly before replacing the plugs?
The problem as I see it isn't that you had a check engine light which they blamed on misfires and they replaced the plugs, what bothers me is that they are saying the cats are now ruined, but the truck only ran rough after they replaced the plugs.

Gilly

No, it did not run badly before. Anyway, I went ahead and replaced both cats and all four oxygen sensors yesterday. Now, I am just keeping my fingers crossed. I hope this fixes everything. So far the CEL has not come back on!

cats
 

Most oxygen sensors last about 60000 miles before needing replacement.
Replacing the sensors is a good thing ...... Not one but all four is best.
The sensors are part of the circuit that controls the fuel mixture. This could definitely foul a plug or cause a misfire due to fuel mixture. Too rich a mixture will cause a buildup of carbon in the cats and to lean a mixture will cause them to run hot and possibly burn the honeycomb inside. Not to mention fouled plugs.
Needless to say fuel mixture is critical. Does this Vehicle have plug wires? It may be time for a new set.
On some newer vehicles there is a coil pack (module) for each cylinder. In some cases one coil pack for two cylinders. I don’t know your vehicle. Have the hi-voltage ignition leads / coil packs checked. Some times you can see the ignition leads arcing at night. A sure sign that the insulators are dry rotted.
I recently replaced the oxygen sensor and complete exhaust system on my 1990 300SE. Including the cat. What a differance!
I did this after a tune up with plugs wires cap and rotor and still failing the emmissions test.....
When I put the car on the Dyno I found the power curve droping off over 3300 RPM. I cut the cat open and the honey comb was melted ....and broken-up in pieces. The car has 135000 miles on it. The new exhaust made all the differance.
Did they check the emmissions before and after? Was it Dyno tested for performance?
-A

WE find many times catalytic converter failure is from a lean mixture causing extreme heat inside of the exhaust. MOST of the time that lean mixture is caused by carbon build-up on the "film" of the air mass sensor!

Just curious about how carbon builds up on the air mass sensor if the air mass sensor is in the intake area, upstream of any combustion process. Or are you talking about the O2 sensor?

mass air
 

Correct! Another good point.
I have found this to be a problem as well. More like a slight coating of dirt
not really carbon. (Looks like carbon build up on the film). I have found that removing and misting the MAF with alcohol will clean the deposits. I will dry it with a hair dryer set on low warm. A dirty MAF will falsely indicate less airflow causing the Computer to lean out the mixture.
A lean mixture will make the cats glow bright red and actually melt the Honeycomb inside. You really must be through when diagnosing a problem.
$$$$$$$
-A

I'm curious, is the MAF sensor upstream or downstream of the EGR? If it is downstream, there's your source for carbon. If it is upstream, how does the EFI know total MAF and how to set the injectors?

Best Regards,
Jim

I could not agree more! I know very little about cars and have to depend on what the mechanic says, so it gets a little frustrating a times. After the regular mechanic I take the car to for regular maintenance was not sure what the problem might be, I took the car to the dealer. When I spoke to them and explained the problem with the CEL, they said the first thing that they would do would be to check the coils (since the spark plugs had just been replaced). And then proceed on down the line till they found the problem. After checking the car, they told me the cats were bad. I asked them if they had checked for other problems too, like something that would be causing the cats to go bad. They said no, as soon as they found the cats were bad, they did'nt look any further. Did not check the coils, etc. When I asked why, they said what they do is fix a problem when they find it and then wait to see if anything else shows up! So I might just have to pay to get another diagnosis done later for the same problem.
Anyway, now I have had both cats replaced (just two days ago) and everytime I drive the car, I keep waiting for the CEL light to go on!!
It may be that the problem is fixed but I don't know for sure. And what if the CEL light does come on, I might have done more damage to the car in the mean time and made these cats go bad too!

where are you located? Perhaps somoene can help with a recommendation to a good mechanic in your area.

If in the Los Angeles Area, I can give you the number of my mechanic

Alon