Random fluctuating idle on 1994 C280 M104

[anf6789]

So last week i noticed my idle would be wierd if the car was creeping, or sitting in the driveway, randomly. If i would give the car a little gas, it would drop below 500rpm, then go up to 1K. and back down and up. While moving the car seemed to be fine. Just when i came to a stop it would sometimes idle funky.

I pulled P0400, and EGR code. so my mechanic buddy thought the egr valve was getting stuck open, dumping exhaust into the intake when it shouldnt, thereby the rough idle. So i capped of the vacuum line that tells the EGR to open, figuring if it cant open it cant get STUCK open. The problem went away until tonight.

I started the car and my buddy and i noticed the car smelled really rich, even inside the car. It started doin its funky idle thing again, and I noticed a slight loss in power while on the road. Problem still intermittent. Went and got dinner. Started the car and the idle fluctuation was very fast like 500-750 back and forth, and if i gave it gas in park it took a good 3 seconds to get to the 4k rev limiter. Started driving and it seemed fine and hasnt done it again.

Im going to clean the throttle body tomorrow, as it could be messing with my idle. Also going to put the vacuum line to the egr back.

Anyone have any Ideas? I searched but nothing that came up was quite like this

[Dearlove]

my c36 is doing this as well

pretty sure its just the throttle body, give it a good clean and all should be well, will be doing mine this weekend if i dont have to work

[ypkx]

mine does this also, i have a c180. I think its just the throttle body also.

[Sulaco]

could also very well be a vacuum or fuel pressure problem. could even be air filter. regardless, typically a cheap/easy fix. check your air filter, check your vacuum lines (particularly the one that runs from changeover valve at front of engine over to underneath intake just above throttle body). clean your throttle body (be careful not to disturb its connector cable, could mess up old wiring and cause a huge electrical failure). unfortunately, only way to rule out fuel filter is to replace it. it made a WORLD of difference on mine, but I had 200k on the original fuel filter (that's miles, no kilometers, kids)

[anf6789]

new fuel filter/pump and lines (160 on original). no vacuum leaks will check again, fuel pressure regulator is functioning. Air filter is clean as well.

Just cleaned the TB thoroughly (not too dirty though) so i will post an update if problem persists,

[Sulaco]

if all that stuff is 100%, then you're probably looking at some kind of sensor malfunction, ie the MAF. a dying MAF can have lots of different symptoms. rough idle, bouncy idle, lack of engine power, stalling, etc. if you rule out all other possibilities, try the MAF. some people have luck cleaning it with simple green on a q-tip. DONT TOUCH IT WITH YOUR FINGERS

[anf6789]

It will happen when i start the car and wont stop for that drive. Also if I start the car and it DOESNT happen, it wont fluctuate for that entire drive. It is very sporadic. It happened tonight after i cleaned the TB, yet on the way home, the car was running perfectly fine.

Going to see if i can find a DataLogger and try to figure out whats going on. I will also clean the MAF. None of my mechanic buddies can point to any one thing.

[Sulaco]

If that's the case I'd say it's more than likely a sensor like the MAF

[strictlyspeakin]

Did you fix the EGR issue or no?

[anf6789]

not sure if there ever was one, just a code. going to put the vacuum line back to the valve, as i had blocked it off to see if it was causing the idle problem.

[strictlyspeakin]

Rough idle and poor acceleration can be caused by a faulty EGR valve.

If you have a hand vaccuum pump with a gauge, you can check to see if it holds vaccuum.