Rapidly fluctuating oil pressure and rough idle

[Brian300E]

Well this problem has been driving me crazy, and my mechanic can't seem to figure it out either. When the car is warmed up, in drive, and stopped somewhere such as a stop light, it has an extremely rough idle. The oil pressure fluctuates rapidly between the 2 and 3 bar, and the car has a rumble-like shake. This problem still occurs despite the fact that I have just had a valve job done and replaced the injectors (o-ring, guide seal, etc.). I have absolutely no idea what it could be, and I want to suggest something to my mechanic so he doesn't blindly guess at what the problem is.

I have checked a previous post 300E rough rough rough idle and it suggests that the EGR may need to be tested. Even though some of the symptoms are similar, it doesn't address the issue of the fluctuating oil pressure, which makes me think the problem with my car is different. The car idles fine when cold, but when it warms up I have the problems described above. I would appreciated any help, because I am totally lost on this one.

Regards,

Brian

[Brian300E]

TTT

Does anybody have any ideas about this? Any type of help would be greatly appreciated.

Brian

[Markjw]

My 1983 126 had a similar problem it turned out to be the accumulator which keeps pressure in your fuel line. Not sure if your model has this but its worth a look at.
Mark

[Brian300E]

Thanks for the reply Mark. I'm not quite sure if my car has one, but I was thinking that the fuel pressure regulator is the same thing. Hopefully one of the mechanics will stumble upon this post and give us a definitive answer. Thanks again.

EDIT: I just found that FastLane has the fuel accumulator (http://catalog.eautopartscatalog.com/mercedesshop/quote.jsp?partner=mercedesshop&year=1987&product=E4000-33285&application=000465844) I wonder if this is the same thing as the fuel pressure regulator?


Brian

[bobbyv]

on my 190e2.6, that surging happened for a while, especially in winter. Then it got worse, with the engine stalling. This was traced to a bad O2 sensor.

the surging of the oil pressure is just the effect of surging engine rpm.

[MikeTangas]

A good sized vacuum leak will also cause surging idle. Before I fixed all the vacuum leaks on the 560 it too had a surging idle accompanied by the fluctuating oil pressure - as idle went up so did pressure. Having head/valve work done recently leads me to think there is a vacuum line or two dis/misconnected.

[Brian300E]

Thanks Mike and Bobby. The only thing is the RPM's don't fluctuate; they sit at 500 when it is ideling. However, the oil pressure fluctuates sharply and the shaking begins. The oil pressure will sometimes drop a bar or so to 2 or a little less, but it pegs when I accelerate. Do you still think this could be a vacuum leak or maybe even the O2 sensor like Bobby recommended? The O2 sensor light hasn't come on so I am not sure if that is the problem. Also, I had this same problem before I had the valve work done so I am inclined to think that it is not something that is interrelated. How can I have my mechanic test for the problems you guys recommended? Thanks so much.

Brian

[HJAMES608]

I had almost exactly the same symptoms: fluctuating oil pressure and very rough idle when warmed up. This followed head work and gasket replacement after an overheating episode.
My mechanic suspected the O2 sensors, but it turned out to be the idle speed regulator. Specifically, the insulation on some of the wires in the connecting harness had become brittle and was allowing sporadic contact. My mechanic discovered this by noting that the idle was affected whenever the harness was shaken. Opening the covering of the harness revealed the shorting wires!Really sharp work by my mechanic.
The solenoid for the EGR valve was also replaced.

We found that the oil pressure problem disappeared when the oil was changed and 20W50 added.

Hope this is helpful. Best of luck.

James

1995 S-420

[Brian300E]

Thanks for the advice James. I am currently running 20W50 oil. When you fixed the idle speed regulator did it just fixe your rough idle and not the oil fluctuation? Also, was your idle speed fluctuating or was it steady and just rough? Thanks in advance.

Brian

[HJAMES608]

If I recall correctly, the idle was essentially stable, just very rough. The revs weren't fluctuating widely.

The idle problem was addressed first, and then the oil was changed. I don't think the idle speed regulator was much of a factor in the low oil pressure observed. The pressure was high- normal when cold but as the engine warmed up, it trended down even at normal cruising speed. At idle it was around the "1" mark.

You might want to have the oil pressure checked with an external guage to verify the observed readings.

Good Luck,

James.

[Brian300E]

Mike -

Will a compression test reveal the vacuum leak or is it done some other way? Also what is the order that I should have my mechanic check this stuff? Vacuum leak first, fuel accumulator, idle speed regulator, EGR, then 02 sensor? Thanks in advance for your help and everybody elses.

Regards,

Brian

[MikeTangas]

Vacuum leaks can be located a number of ways, using water mist or carb cleaner around the vacuum connectors and lines will affect idle when sprayed near the leak. The newer the car the greater number of vacuum lines/connections mean longer to locate. Easiest way to tell if it is a vacuum leak is with a vacuum gauge, should see somewhere around 16-17 inches HG at idle (with ACC off), anything less and you have a leak.

I'd say have your mechanic check vacuum at idle, then the O2 sensor, EGR and idle speed regulator.

If you aren't having hard starts after sitting overnight I wouldn't suspect the fuel accumulator.

[Brian300E]

Thanks Mike. I do have hard starts in the mornings and when the car is cold. Could this be indicative of a bad fuel accumulator that would be causing the hard cold starts, and the rough idle with oil fluctation? Thanks again.

Brian