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Strange Sound on Hill

I was driving today and as I descended a steep hill the car made a horrible banging sound and the check engine light came on. I got home and the light has since gone out. What happened and is this bad?

Any ‘horrible banging noise’ could be bad. Have you checked your oil and transmission fluid levels? Where did the noise come from?

The noise came from under the hood. I checked all the fluids because I was wondering if the banging could have been from the transmission, but everything’s alright.

I hate it when that happens. Most of the tools I left under the hood have fallen out that way. I don’t believe that was the problem but I’m stuck here, miles away and just can’t tell.

Sometimes things do fall off, like big chunks of ice in the winter. A check engine light would seem to be an ignition problem which could cause a backfire. Anything like that could be bad or get bad after a while. There may be a code stored by the computer which “could” help. At this point, nobody knows what happened.

Would the hill have anything to do with it? It happened right after I’d released the accelerator to coast, but it might just be a coincidence.

Make/model/year/miles/engine/transmission type?

Mercedez E320 '99 300K Miles Automatic Transmission

Bring it to the shop and have them check your ECU for stored failure codes. ASAP.

I stopped by the dealership and they didn’t see anything wrong with it. I’m going on a trip this weekend so I’ll keep track of it.

@threephi how could the dealership not see anything wrong?

The fact that the check engine light came on means there should have been a stored fault code.

I suspect they didn’t actually hook up their scanner.

I agree in full! They drove the car around a little and said the light wasn’t on and suggested that I’d left the gas cap loose after filling. I don’t think they hooked it up either.

@threephi a loose gas cap doesn’t explaining the noise you heard.

Well, nothing seems amiss now.They’re making me second guess what I heard. The guy who works on my car normally wasn’t in today, so someone else looked at it and I’m not sure he really cared too much.

I really like being up on all the repairs and to keep everything in working order. I’m taking a trip this weekend and this is bothering me because I’ll be driving over 500 miles.

I drove the same hill again three times, but couldn’t re-create the sound. Is it possible it was just a fluke?

@threephi how/why would you imagine a loud noise like that?

You have better things to do than go to the shop just to have them tell you you’re imagining things.

It seems to me if you are certain the check engine light came on, the proper course of action is to review the status of all codes, stored, and pending. That’s why Mercedes engineers designed the car to diagnose itself, so you can read the codes and see what is wrong. If all you had was a noise, then it could be you just ran over some road debris and didn’t realize it. But if the CEL comes on, then the codes must be reviewed. That’s simply professional due diligence in my opinion.

The car started making an odd sound when cutting of the engine. It makes a couple extra chugs before it stops running. The check engine light is now on again… And I am 300 miles away from home. What do I do?

@threephi don’t start the car again. Have it towed to a shop. It’s obviously your call if you want to bring it to the same dealer again. Now that it’s acting up, they really can’t claim to have not noticed anything wrong.
A Benz dealer would be the logical choice, however, because they have the expertise, training, and technical information at their hands.
Perhaps another Benz dealer, the closest one to where you’re currently at . . .
Call your Auto club and arrange for a tow.
Again, don’t drive that car. You may make whatever’s going on worse.
That is my advice.

I had it towed to the dealership in this area.

The tech met me today and checked the computer. She said the problem is that its running rich. Is that plausible?

@threephi can you post the codes, please?
Are your codes P0172 and P0175, by the way?

What is causing the noise?
The noise is more worrisome at this point, I would say.
Did you tell the dealership that the engine was making noises and that this is worrying you?

Does the banging sound like gunshots or firecrackers? If so, this is backfire from the unburned fuel combusting in the exhaust. This sometimes sounds like a series of pops like a string of fire crackers but can also sound like a large shotgun blast or similar. Those loud blasts risk blowing something in the exhaust apart.

I hear this in old carburated pickup trucks but wouldn’t expect it in a Mercedes! Something is definitely wrong. This could be a clogged injector, bad O2 sensor, or other.