Odd RPM behavior ... tranny? throttle linkage? - PeachParts Mercedes-Benz Forum

BodhiBenz1987 · #1 11-03-2005, 01:10 AM

... my 87 300D did something funky today which concerned me ... oddly enough, my Jeep did something similar a few weeks ago, and I had forgotten about it.

It was chilly this evening, but not remarkably cold. I started up the car aftet it sat for oh, 17 hours or so, and let it warm up for about five minutes. Everything normal. It was not at operating temp but I pulled out and started to pull out of my little parking lot at about 1000-1500 rpm (I try not to let it over 2000 until completely warm) ... well I got about 200 yards out to where there is a stop sign, and when I put on the brakes and came to a complete stop, rather than dropping down to 700 rpm where it idles, it stayed at 1000 and sounded a little "frantic" (does that description make sense?) for a couple of seconds. Because I was a little too far out in the intersection and someone was coming, I put it in reverse to back up and collect my thoughts, and at that point the rpms went back down. Everything went normal for the remainder of my errands. I know sometimes if the car is going at higher speeds, it takes a while for rpms to drop, but I was going at snails pace and under these circumstances, stopping usually causes it to go right to idle. Instead it definatley hung at 1000 for a moment. Any ideas why? It's possible it was a tach readout error, but I felt the engine pulling a little more than it should have. I can't really get my brain around how that might be a transmission problem that would cause that behavior ... my dad thought maybe it was a stuck throttle linkage, but it didn't exhibit that behavior again during the drive.

Any ideas? It may never happen again, but I'm going to worry about it anyway, at least for now. Heaven knows what I would do with my spare time if I didn't have this car to worry about.

pawoSD · #2 11-03-2005, 01:25 AM

Since the 603's have an electronic idle speed control, maybe it "got confused" or "stuck" for a second at a high idle? Since it was cold it could have brought it on. Electronics that are old can always do random strange things, so I am thinking it had something to do with that. It could also be a sticky throttle linkage like you said. Try lubricating the system to make sure it moves freely and smoothly.

t walgamuth · #3 11-03-2005, 05:44 AM

if loose the engine will slide forward under braking and this might move the throttle link.

tom w

BodhiBenz1987 · #4 11-03-2005, 03:50 PM

Thanks guys ... I'll let you know what I can find out. What kind of lubrication is best to use? All I have is WD40 ... my big tub of shlock is at home in my dad's garage.
Also, how do I go about checking the motor mounts? This theory sounds interesting because I don't believe they've been checked or changed in the car's history, and the engine has been a tad more "rambunctious" on startup at idle than before (although still pretty smooth for an ol' diesel).

TimFreeh · #5 11-03-2005, 10:41 PM

It's almost certainly a throttle linkage lubrication problem - this is commonly first noticed in the late fall when temps fall and the old lubricant hardens up due to the lower temps. After a few minutes of operation the engine compartment warms up and things are back to normal.

The correct lubricant for throttle linkage pivots and balls is standard ATF. Unfortunately a couple of the lube points are very tough to get at since they are buried under the intake manifold. Lube up the ones you can get to and see if that does the trick.

BodhiBenz1987 · #6 11-04-2005, 12:37 AM

Thanks ...

I also realized another possibility ... it dawned on me today as I looked at the throttle linkage that I had very thoroughly vacuumed the inside of the car about 20 minutes before the incident ... including ramming around behind the accelerator pedal. I suppose it would be possible that I kind of jammed up something right behind that pedal in the process, and it corrected itself. Won't hurt to lube as much up as I can anyway.

Jim H · #7 11-04-2005, 06:52 AM

Quote:

Originally Posted by pawoSD

Since the 603's have an electronic idle speed control, maybe it "got confused" or "stuck" for a second at a high idle? Since it was cold it could have brought it on. Electronics that are old can always do random strange things, so I am thinking it had something to do with that...

My SDL did that, I figured it was an electronic hiccup, since the idle speed is set at around 750 rpm by the servo. The mechanical idle stop is 500 rpm.

BodhiBenz1987 · #8 11-07-2005, 11:23 PM

... redux on this ... it did it again today ... exact same place, behavior and roughly same ambient temps (not that cold, but colder than it's been recently). I did notice that it didn't seem to be "pulling" on me, although I didn't think to just let go of the brake and see what happened. I held down the break leaving it in drive and watched the rpms ... they very slowly dropped to normal ... I then proceded to drive on and everything was normal from there on. If the throttle linkage were stuck, wouldn't it try to fight me a bit? Maybe it was and I just didn't notice (1000 rpms is hardly blazing).

I'm going to check the mounts tomorrow, but I'm leaning toward it being a idle-speed error. My bottom-line question is this: is it something I can ignore as long as it doesn't get worse or more frequent, or is there any danger of this turning into a runaway situation?

I know that idling at 1000 rpms is hardly a runaway, but I fear runaways almost as much as I fear death. Is this kind of behavior in any way a pre-runaway antic? If so, I'll skip my DIY desires and take it to the mechanic immediately to get it a bill of health before I do anything else.

TimFreeh · #9 11-09-2005, 10:29 AM

You could have a fault in the idle speed circut but your symptoms still lead me to believe that its a throttle linkage lubrication issue - I've had multiple cases of first hand experience with the exact problems you are describing and all were caused by binding throttle linkage pivot points.

If you want to test your theory you can disconnect the ELR actuator on the IP, this will defeat the idle speed control circuts on the car and it should idle (roughly) at around 500-550 RPM.

If there is indeed an idle speed circut malfuction I don't think the amount of adjustment capable in the system could possibly result in a "runaway" condition and I've never heard of an OM603 suffering from this condition.

Even if your car did suffer a runaway condition the braking system has more than enough capacity to overcome the engine and stop the car. If you don't believe me find a nice empty parking lot and try to move the car with the engine while you stand on the brakes.

You may want to check the condition of your flex discs before your try this....