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Throttle box? Air intake? This car is a mystery

Car info:
1997 Medcedes E420, ~186,000 miles
Check engine electronics light always on because of unrelated evaporation censor issue.
Having issues with my car, and my mechanic is a little stumped. Anyone here have advice?

Maybe in the last week or two, noticed the faintest flutter of the engine when stopped at red lights. Was faint, seemed like no big deal.
This last Friday, had been driving 45mph for 3-5 minutes (cool day). Stopped at red light, engine freaks. Got intermittent revving and lurching of whole car. “Acc.skid control” warning light comes off. I pull over in a fast food parking lot, turn car off, calm down cuz I was freaked out by aggressive lurching. Pulled over maybe 20 minutes. I decide to drive to my mechanic who happened to be nearby. Turn car on, and it stalls four times, in the time it took me to back out of the parking space I was in and get to the driveway.

Where things get interesting:
Mechanic replaced “failing mass air intake sensor”. Car seems 95% fixed…
I go to pick t up today, and the same issue seems to be happening, but way less dramatic. Did a ride-along to show mechanic the issue (because they couldn’t replicate it), and he said:
Seems like the engine doesn’t know what level to idle at, and hesitates to shift gears. Issues appear more when driving slow. This has only been an issue since Friday. Never noticed ANY other signs of transmission issues. He says he’s not sure what to do, and will try checking the transmission fluid and clean the throttle body.

I’m hoping for a confirmation or additional suggestions to get this fixed! Anyone have any suggestions? I REALLY am broke and just got this car six months ago!

Cold recently where you are? You can try a treatment of sea foam, cross your fingers and pray for luck.

Kinda, but my very. It was in the mid 30s on Friday. I do happen to have a can of Seafoam, but wasn’t sure how safe it actually is to use. I once had a car where some idiot put Stop Leak/Engine Block in the radiator and ruined it before I got it, so now I’m paranoid. If it’s safe, I know you can put it in a few places. Would I put it in my gas tank?

Gas tank per directions has been safe in my experience.

I’ve used Sea Foam before with good results. Yes, put it in your nearly empty tank, then fill it to mix it up with the gas. Might help, can’t hurt.

Unless things have changed the Seafoam can should have directions on it.

Some what off topic but OP says they are broke so what makes people with limited funds purchase a used high cost maintenance vehicle.

VOLVO V70, good question!
We live in eastern Kentucky, and it belonged to my parents mechanic, who said it was the car his wife drove. He sold it to us for just $1700, and it was better than the other options we could find at the time. The info we had about this car made it seem like it would be solid enough to offset the more expensive repairs for such a model. We figured it would also be useful to have access to the person who knows that car’s history. Lesson learned in some respects, but it was a weighed decision.

I can see why you have it, get it running well enough to trade and you should break even.

I never thought of trading. Interesting idea, but hopefully I won’t have to. It’s been running great up to this point. We’ve hoped that if we take good care of it it could last another decade. I’m mostly just hoping to gain some understanding of what might be wrong with it, in the hopes it could at least last another five years when I finish my degree. Which is clearly not in engineering. :wink:

The early electronic throttle control equipped cars can be challenging to diagnose and vary expensive to repair. A new throttle body assembly is $2200. I’m not suggesting that is the problem, rather you need to find someone with Mercedes diagnostic abilities and experience or you might spend a lot of money on unnecessary parts.

The first step is to check the fault codes, there could be other faults besides the evaropative emissions faults. You won’t know when new faults are detected if the engine light is always on.

I’ll double-check with my mechanic. They made it seem like the evap issue was the only code (especially when I brought it back and they couldn’t find any evidence of a continuing issue until I took a mechanic for a ride-along). But maybe I’m just assuming; I’ll ask if they’ve done a full check on the computer. If it needs a whole new assembly at the price, I guess I’m out a car - definitely can’t afford that much. It’d suck to have to scrap a car I’m still paying off!

This sounds like unmetered air getting into the engine to me. It’s odd that changing to a new MAF fixed it at first, then it reverted to its old ways. One idea, the problems wasn’t the MAF, but something else. And switching to a new MAF caused the computer to re-initialize, but when it re-learned the MAF the original problem of unmetered air re-surfaced. If you have a rubber boot involved with the air intake, this problem will often occur if that boots becomes cracked or split. The engine has to be air-tight from the air filter to the end of the exhaust pipe to work correctly. If you have one, suggest to have your shop completely remove that intake boot and check to make sure it isn’t leaking.

I had this problem occur one time on my Corolla, and it was unmetered air bypassing the throttle plate via the gadget that increases the rpm when the engine is cold. That gadget failed and the rpm was wandering all over the place, making the car un-drivable. My solution was to remove that gadget and throw it in the garbage with a big pleasing “plunk”. Now when the engine is cold I have to hold my foot on the gas pedal a little until it warms up.

I think your mechanic was right to replace the MAF sensor. After you replace a MAF sensor, the computer has to learn the new MAF sensor. It helps to disconnect the battery for at least 10 minutes to reset the computer to the factory defaults, it will learn the new sensor faster this way. Then it takes at least 10 minutes of idling to smooth out the idle.

BUT, the 97 Mercedes has a history with vacuum leaks, mostly vacuum lines. Your mechanic needs to go over these carefully and also look for any other leaks in the intake duct after the MAF sensor.

Thank you both for these suggestions! The sites I had seen didn’t mention the ‘97 Mercedes’ vacuum line issue before, and my mechanic does so many models, maybe they don’t know this very particular issue. They told be that they cleaned the throttle box today and the issue isn’t fixed, so the mystery continues! They also are stumped because the computer is giving no error codes (except for an unrelated evap code). Here’s hoping!

@Calenator You stated that you still owed on this vehicle. Any chance they take it back and you get to call it quits. I don’t feel they know what they are doing. .

I doubt I could return the vehicle, as we bought it from my parents mechanic (who, I should add, is NOT the mechanic that has it now). The guy that sold it to us somehow neglected to mention it had a broken spring mount strut - which my mechanic found and fixed. I don’t owe much at this point, given how little we paid for it initially.

Just got to ask, when you fill the gas tank, do you stop at the first click or do you “load up”?

The evap code could be the source of the unmetered air entering the system, I think this should be fixed next and stop ignoring it.

Interesting question! Both. Sometimes I’m in a hurry and stop at the click. Sometimes I try to get to the top of a dollar (of less than 50 cents remains)! What are you thinking of?

Chronic overfilling can lead to evap issues, always stop at the first click. Note, chronic overfilling, occasional overfilling should not be a problem. I used to fill to the dollar but eventually it lead to a CEL and evap code. I stopped at the first click after that and eventually the evap code went away and never returned.

Huh, good to know. The evap code has been there since I got the car, but this is still good to know! I never knew if it would make a difference! With such a huge tank, (18gal), I doubt and extra 59 cents of gas will get me farther, and if stopping at the click prevents bad things, the choice is clear!