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Slight idle problem above 500 RPM and sluggish accel at low speeds

2K views 16 replies 4 participants last post by  Jostep 
#1 ·
Hi everyone,

I did some searching on here, but I wanted some input before I try tracking down what the issue could be.

I have had idle issues with this car since I bought it. I could feel it needed mounts replaced because of it's rumble at idle. My buddy changed out the tranny mount because he said the pass and drivers don't have issues as he did the whole brake and acceleration test that he does with all cars to check mounts and said he couldn't see anything wrong with my pass/drivers but the tranny one was messed up so he replaced that.

Since then I've replaced the spark plugs and wires and the car ran better....but the idle was getting rougher and later down the road (now) the MPG dropped and car got sluggish at lower RPMs and from a stop. I feel like I really need to gas it at low RPMs to get the car moving aka pedal closer to floor.

At higher RPMS on highway the car is fine.

It threw a code on Saturday but since Sunday it went away. It ironically after throwing the code...starting running at least at idle, better.

Since it threw a code and disappeared if I take it to an autoparts store to use theeir reader, will it still tell me what that code was?

Any guesses out there? From searching I am thinking either MAF or Fuel Filter. Thoughts?
 
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#2 ·
I have very limited experience with MB, but currently am working on a C220 and learning.

You should have a code stored, a parts store can read it, I don't know what vehicle you have but MB had OBD 2 before 1996... my 1995 has the 16 pin connector under the dash to the rt of the steering wheel. This despite the claims from the parts guy that 1995 would not have obd2.... you may or may not run into this.

I would consider something other than fuel, stumble at idle, but running fine at highway speeds shows you have flow, it works the opposite if you have fuel restrictions. I would look at the MAF... but you're guessing until you can get a code out of memory.

So far the folks on the forum have been dead on with helping me bring back mine from the grave.... Hopefully someone will chip in soon with more info. Keep digging in the forum, and let us know the car in question...wiring issues were a problem in some years. (That was a huge part of my problem). Best of luck.
 
#4 ·
The code would likely be stored in memory, and it might point you in the right direction. For now this seems like it could be a bunch of issues. I suggest that when you have the codes read, they should also read the fuel trims, if they are way off, it could help you identify where the problem might be.

Does this only happen when the engine is warmed up? Or does it happen when cold also? The reason for asking is that not all sensors are operational when the engine is started from cold, the MAF needs to warm up (about 30 seconds) and the O2 sensors also need to warm up (about 60 seconds). If the engine idles fine right after starting from cold, this might help rule out a sensor issue. If it idles poorly when cold, you could be looking for a vacuum leak or a fuel delivery issue (if the trims are compensating for a lean condition), an excessive amount of fuel delivered type issue (if the trims are compensating for a rich condition), like a bad regulator or a stuck injector. If the trims seem fine then it could be a simple spark issue, in your case, since you already changed the plugs, it could be a bad coil. However, if your check engine light isn't on that's because the misfires aren't severe enough, so it could be a poor/loose connection to a fuel injector or a plethora of other problems.

Basically, with the information you have, it could be a billion different problems, fortunately it doesn't sound severe. Things I would:

1. Use a code reader, it will help for sure, get as much information as you can out of the code reader as you can, it will help immensely.
2. Do a vacuum test, check all your lines and connections, make sure nothing is loose. You can buy a can of starter fluid and spray at connections wit the engine running, if the engine revs up, you know you've got a leak there. Sometimes an old rubber connector can crack and let in lots of unmetered air.
3. Has the K40 relay been replaced? It's a relay that protects the critical electronic components from overload. If it fails, it can rob the ECM from the electricity it needs to run properly, and can cause a great deal of confusion (and money wasted replacing good parts) trying to find. I would put in a new K40 relay if it hasn't been replaced even if it is not the issue, there is a large peace of mind factor outweighing the cost here. In theory a bad K40 could screw with the on-board diagnostic system, leading to faults not being reported.
 
#5 ·
The code would likely be stored in memory, and it might point you in the right direction. For now this seems like it could be a bunch of issues. I suggest that when you have the codes read, they should also read the fuel trims, if they are way off, it could help you identify where the problem might be.

Does this only happen when the engine is warmed up? Or does it happen when cold also? The reason for asking is that not all sensors are operational when the engine is started from cold, the MAF needs to warm up (about 30 seconds) and the O2 sensors also need to warm up (about 60 seconds). If the engine idles fine right after starting from cold, this might help rule out a sensor issue. If it idles poorly when cold, you could be looking for a vacuum leak or a fuel delivery issue (if the trims are compensating for a lean condition), an excessive amount of fuel delivered type issue (if the trims are compensating for a rich condition), like a bad regulator or a stuck injector. If the trims seem fine then it could be a simple spark issue, in your case, since you already changed the plugs, it could be a bad coil. However, if your check engine light isn't on that's because the misfires aren't severe enough, so it could be a poor/loose connection to a fuel injector or a plethora of other problems.

Basically, with the information you have, it could be a billion different problems, fortunately it doesn't sound severe. Things I would:

1. Use a code reader, it will help for sure, get as much information as you can out of the code reader as you can, it will help immensely.
2. Do a vacuum test, check all your lines and connections, make sure nothing is loose. You can buy a can of starter fluid and spray at connections wit the engine running, if the engine revs up, you know you've got a leak there. Sometimes an old rubber connector can crack and let in lots of unmetered air.
3. Has the K40 relay been replaced? It's a relay that protects the critical electronic components from overload. If it fails, it can rob the ECM from the electricity it needs to run properly, and can cause a great deal of confusion (and money wasted replacing good parts) trying to find. I would put in a new K40 relay if it hasn't been replaced even if it is not the issue, there is a large peace of mind factor outweighing the cost here. In theory a bad K40 could screw with the on-board diagnostic system, leading to faults not being reported.
 
#6 ·
Thanks for the detailed response, Patman!

It is fine until car does warm up. I feel slight mis-fires but nothing crazy at idle.

The car runs great though when driving, just trying to get my MPGs down. Did the tranny comp reset and I think that's helping some to erase my crazy driving pattern. :p

I think it may be the MAF.....but I am going to find out today. Bringing it to my mechanic buddy who will scan. I may have him change in the future the tranny fluid/filter/gasket too just in case.

How could I tell if my relay has gone bad? Where exactly is it located? Is that the same relay that controls the rear defrost? Mine hasn't worked since I owned this car and I remember doing some searching a while back and finding that the rear defrot is tied to a relay....but forget which one.
 
#7 ·
It's not the same relay, before you replace the defroster relay, I suggest you check the fuse for the defroster first. The fuse will be located in the engine compartment fuse box. If the fuse checks out, you can remove some screws on the fuse box to uncover the relays behind it, one of the relays there will be for the defroster. The K40 relay is located in the passenger foot well I believe, do some searches on this topic, it's a recurring issue, and it might explain the symptoms your car is having. A visual inspection of the K40 usually reveals nothing, even if it is faulty, often the cracks in the soldering joints are microscopic, but they are enough to screw with the relay. I'd replace more as a peace of mind item, it's not expensive.
 
#8 ·
Gotcha! Where's the best place to buy the K40 relay? Direct from Dealer or online?

I will check that fuse this afternoon. Would that N10 relay also cause one of the side mirrors defrost function to not work? I only ask because my drivers side mirror doesn't defrost at all but my passenger one does. Strange....

Thanks again, Patman!
 
#9 ·
I would try online, but check with your local dealership also, sometimes their prices are very competitive. If the driver's side mirror doesn't heat but the passenger does, it's because the heating element inside the glass has failed. The only solution is to replace the glass, you probably have the dimming driver's side outside mirror, so the glass will be a little bit more expensive, but it's very easy to replace yourself, there are some guides on the internet.
 
#11 ·
Sooooo......turns out it was the P0400 code. EGR valve. My buddy took out the main line to see if there was a clog, flushed it out with chemicals they use in his garage, dried it up--and it reinstalled it. Next I had him check the little rubber hoses on the EGR....that was the culprit! Brittle and old...he had a line that fit it perfectly and trimmed it to the right length.





Cleared codes, started car, instantly just driving out and sitting at light, the car was purring like a kitten. I could see the needle STILL blip ever so slightly....but I only drove a couple of miles home. I hope with more driving it will calm even more down.

I've been driving wtih this crappy idle so long I forgot how smooth this baby COULD idle!! :)

Thank you everyone for helping me figure things out.
 
#12 ·
Wellllllllll of course I had to speak too soon!


Having similar if not A TAD BIT more rougher idle than before...but with the replacement of that rotten tube the car while driving/acceleration is smooth as can be! I think my MPG has improved some...but I can't really tell as I feel the 'pep' back I've been driving a bit more aggressively lately. ;)


I noticed the idle got REAL rough yesterday as I decided to go to the cleaners to drop off some clothes but went during a down pour. The car once it warmed a bit got really lumpy. I wonder if it was the pressure/moisture or something? It happened within 2 mins of full-cold to semi warm.

Could that be the MAP?
 
#13 ·
Given that you found one hose in that condition, it is a safe bet that others look that way as well. I'd start with a thorough inspection of all the rubber hoses, and replaces anything that has cracks in it, even if its not cracked all the way through. Also I think a vacuum check was suggested. That's a good idea. I would do that also, probably pulling the lines off at the intake manifold and pulling a vac on them and seeing if it holds. I'd also check for codes again and see if anything has come back.
 
#14 ·
Which specific hoses should I check for? My buddy checked all the rubber hoses he could when he changed the plugs and wires for me.

He also checked the EGR since that was the code it was throwing the most recent time. Is there any other hoses that are hidden that I should check for? I think I read about one that was around teh tranny? I don't think I checked that one with him.
 
#15 ·
Looks like I'm not the only one persuing the elusive perfect idle.

On my car, I've replaced almost everything, of course it had 7 codes stored from previous owners neglect, running on 2 cylinders so... not the same problem. I am chasing a suspected vacuum leak and everything checks out.

Solution, go get 20 bucks worth of vacuum tubing, swap them all out. I've discovered that the tiniest crack causes a very slight leak, multiply that x every damn hose connected to intake vacuum and you get a nice volume in total... then the other sensors try to get that magic 14.7:1 ratio and you get crappy idle, running rich etc.... spend a couple bucks and just replace them all, it isn't hard to do and you get piece of mind.

Then look at the MAF afterwards... you've already replaced all the potential vacuum issues.
 
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