The infamous idle surge- a new clue?
 

I've been fiddling around with my '93 300TE this afternoon trying to get to the bottom of the idle surge problem. I've searched and read through many posts and I haven't tried everything yet but I did find that my spark plug wire harness is in perfect shape.

I haven't had any problems with cruise control so I'm hoping that means the OVP is good.

With or without the MAFS connected makes no difference at all.

But I did notice one thing that I did not see in any other post... the idle ONLY surges in park or neutral. Once the car is in drive the surge disappears with or without my foot on the brake.

There is a very very slight hesitation off the line but there is no lack of power or unusual performance with my foot on the floor.

I've checked for vacuum leaks and found nothing but I'm not very confident in my check so that could still be an issue.

So shifting the car into drive is the only clue I have, does anyone have any idea why the surge would be gone once the car is in drive?

Try cleaning the TPS with carb cleaner, then dousing it with WD40. It worked for me after spending hours looking for a vacuum leak that wasn't there - do a search on throttle position sensor and you'll find it.

Thanks, I'll check that next. Did your surge also disappear when you put the car in drive?

It disappeared completely. Only recently it started giving hints of this again, and I will just go in there with my cans of carb cleaner and WD40 and clean it up again.

When you do have the problem, is it present regardless of being in park or drive?

When the problem occurred, it was most noticeable in P or N, however it does occur in D. The only difference is that in D the engine RPM is not able to fluctuate much due to the load on it. I believe your problem may be the same - it is being masked when in D. Think about it - putting the tranny in D does nothing electrical or electronic; it only puts a load on the engine and likely dampens any RPM fluctuation which gives the appearance that the problem only occurs in P or N.

Pull the vac lne off the fuel pressure regulator and see if any gas is evident coming from that port while running. Also check for good vacuum at the line you took off.

Hmmm... mine seems slightly different. I don't notice anything when it's in drive, not even the slightest indication that the engine would like to surge despite the fact that it is loaded.

I'll just assume that I am not noticing it for the time being, this is most likely the same issue.

AD thanks for the fuel pressure regulator tip, I don't have that on my list.