jp196400

yeah I know what you mean, you shouldn't even feel these cars idle.

jp196400

ok so I disconnected the MAF today and it ran smoother when Idling, but when I took it for a drive it stumbled a bit and didn't really have all that much power, then after driving when it was idling it was running kinda rough.
I would just attribute that to the MAF not being connected though right?
should I replace the MAF based on the fact that it ran smoother?

I don't have a fuel pressure gauge so I have to borrow one from a friend.
if it turns out to be the fuel pump are they hard to change?

Rudeney

If the hesitation happens typically in warm, humid weather, it's the Mercedes curse! They simply don't like warm, humid air. Some have speculated that it's something to do with the adjustable vanes in the manifold to vary the intake length, but that's just a suspicion. Unless it throws a code (other than misfire), the dealer won't be able to solve it. I used to run 87 octane in my ML when it did this. It helped somewhat. Knock on wood, neither my C240 or CLK550 have this problem.

jp196400

its happens all the time on my car, we haven't really seen any warm humid weather yet this year.

jp196400

ok so I checked my fuel pressure and its a steady 55 psi when cranking and running.

03renntechamg

Without a check enigne light its going to be hard to diagnois...

My guess is its a combination of your plugs, motor mounts and tranny fluid and/or a possible vaccum leak...

I would check for vaccum leaks first based on the shuttering upon acceleration. I had a similar problem and it turned out to be a combination of my motor mounts and my intake manifold, not the gaskets but the manifold itself, there was an obvious leak.

jp196400

its very hard to diagnose that's the problem I'm having lol
the plugs are in decent condition and from what I understand the motor mounts go bad if they leak and mine aren't, also if it were the mounts I don't think I would get the hesitation on acceleration like I have been getting.

I have a MAF on order so we'll see what happens when I replace that, and I'm going to check for vacuum leaks when I get a chance.
aside from the manifold are there any other places I should be looking for leaks?

03renntechamg

If you perform a "smoke test" to check for vacuum leaks the leak should be obvious.

jp196400

not familiar with a smoke test, how do you go about doing that?

jp196400

ok so I ordered a MAF from parts train and of course the one I got didn't work, it just caused the ca to run really rough.
I also checked for vacuum leaks by spaying all the vacuum lines, so far I have forund nothing.

I have been really hesitant to order a new MAF that going to cost around $250 here because I cant return it if isn't the MAF causing the problem.
anyone have any other suggestions?

Glyn M Ruck

The cheapest way:

https://mbworld.org/forums/c-class-w...af-insert.html

TheXenochrist

From the sounds of it, seems like I have experienced with the same problem. I have posted in this thread before. When in idle, my car would vibrate, almost felt like the engine was shaking. I heard all kinds of things, MAF sensor, Transmission flush, vacuum hoses, etc...

The other day I had an appointment to finally get the tips of my Stebro exhaust changed, and I got a call saying the connectors that hold my exhaust pipe in place (dont know proper term), have rusted out and needed replacement. After that, I havnt had the same vibrations. Give your exhaust a check?

jp196400

So I ended up ordering a new Bosch maf and the rough idle is still there, it's actually getting a bit worse.
I heard from a freind that you have to re program the computer when you replace the maf. Is there any truth to that?

I'm also getting a knockin noise at start up when cold, it sounds almost like a wood pecker pecking a tree and it does it when the car is reved and coming back down to idle.
Any ideas what it might be?
I know it's extremely hard to diagnose with out more indo and actually being there to see the car.

D-MAN

New member, I have been searching every where for a solution for low/rough idle.

I have the same problem, I have replaced plugs, wires and cleaned MAF. I have checked the air cleaner seal etc. It's not the motor mounts because I can see the tach bounce when it starts the low/rough idle. Some times it will idle smooth at the correct rpm but it seems to start running rough when I give it a good run. I have taken it to the dealer and no codes=no advice. Have you made any progress with diagnosing the issue?

jp196400

I wish I could say I have, the idle has gotten a bit worse and still no codes.
I was going to change the plugs but no one local had them in stock so I have to wait a bit for that.

also just an update from my last post, the knocking on start up is all gone, I did an oil change and after a few hundred km it went away.
I figured when I got the car the dealer (not Mercedes) might have put the wrong oil in.

D-MAN

Good news that the knock is gone. I have the knock as well. I think the knock is louder at low idle, I just did an oil change so we will see what happens. I am also wondering if this may be related to an egr or something like that. The reason I say this is because it happens randomly and other times it seems like it idles perfect and the exhaust note sounds good. I have unplugged the MAF and saw no difference. I guess we just continue the trial and error method until someone can come up with something that works!

jp196400

Lets hope the oil change works for you too.

what you just described is exactly whats happening to my car, sometimes it idles fine and other ties it runs rough.
let me know what you find out with the EGR
I'll keep everyone posted if I find out anything new.

D-MAN

I am no expert by any means but here are a few of my thoughts that may be a possibility that is causing the low/rough idle. Maybe someone will chime in with some advice:

1. Throttle Positioning Sensor (Is there a way to test TPS?)
2. Coil Packs
3. Fuel Pump
4. Fuel Filter
5. EGR (I'm not even sure if our cars have this but a lot of them do)
6. Throttle body
7. Catalyic Converter?

As I mentioned before the car sometimes idles perfect and others it idles rough. I have cleaned and unplugged the MAF and no change. I have heard of people going to the dealership and having the idle raised 500-600 RPM but I'm not sure if that is just a band aid? I have also replaced the plugs and wires ( noticeable difference in overall performance but didn't do anything for the idle). I have done concentrated cycles of Techroline and no difference. Still looking for the silver bullet

Rudeney

If you unplug the MAF and get no change, then that is a symptom of a bad MAF.

D-MAN

Thanks Rudeney, I'm going to try a new MAF in a few weeks. I think you mentioned on another thread or board to only go with a OE product?

D-MAN

I have been doing some searches and came across this about EGR valves on 2 separate sites:

http://www.car-stuff.com/mercedesegrvalve.htm

http://www.racepages.com/parts/egr_valve/clk500.html

D-MAN

I thought the second link describing the knock and rough idle fits in with my symptoms

Rudeney

Yes, definitely go with either a genuine MBZ or a Bosch MAF - the cheap aftermarket ones the sell for $100+/- on eBay are junk.

A bad EGR could be the problem. In the old days, we used to pull the vacuum hose and cap it off to test them, but if it's clogged with junk and staying open, that method won't tell you anything. I think for the price, I might just go ahead and change it.

D-MAN

Thanks for the info on the MAF, I am going to have the EGR replaced on Friday. Is the EGR a DIY, I'm not even sure where it is located?

Rudeney

It's not a bad job. It's on the rear of the engine on the right (US passenger) side. You have to remove the MAF sensor and the air intake "elbow" pipe below it. Sometimes that can be a challenge due to aged hoses and plastic connectors that break. You also have to remove the oil dipstick tube and bracket, and remove the fuel rail on that side. The valve is fairly close to the firewall, so it's tight working conditions, but there are no special tools or procedures necessary.