Hints of a runaway, revving on its own - PeachParts Mercedes-Benz Forum

MonsieurBon · #1 01-25-2008, 08:41 PM

Guys/gals,

I have an '85 300D running on vegetable oil. It's been running just fine for almost 2 1/2 years now on the veg.

Yesterday, after I got off the highway on the way home, I was at a stop sign and the engine started to rev a lot. I checked that I was not depressing the gas pedal. It revved twice, very loudly, then calmed down. Kinda freaked me out.

I watched all today for it to happen again, and it did, on an uphill about 2 miles after getting off the highway. I quickly popped it into neutral, and after about 2 seconds, it subsided.

I'm trying to figure out what could be causing this. I don't see anything out of place in my hoses and valves. However, the more I'm reading about runaway diesels, the more I'm wondering if it was possible that I popped the lift pump seal. It was really cold the other day and when I had to start the car on veggie after a one hour lunch (when I normally wouldn't need to purge), it took a couple turns to start up, and it was a little sluggish.

So, what do I need to check, and how can I be prepared for it potentially running away?

I am going to go check the oil level now. Oil pressure has been good, and I am due for an oil change, so the oil should at least be below the MAX mark.

Thanks in advance folks,

#2 01-25-2008, 10:14 PM

Might be the cruise control freaking out for some reason.

MonsieurBon · #3 01-25-2008, 10:54 PM

Quote:

Originally Posted by ForcedInduction

Might be the cruise control freaking out for some reason.

Yeah... I thought it was next to impossible for it to engage under 30MPH or so?

Cruise control usually works normally for me at highway speeds, except for those thrilling times when it decides that I actually want to be going 10 MPH faster than I am currently going.

I never use it.

MonsieurBon · #4 01-26-2008, 12:47 AM

OK, so I disconnected the cruise control linkage. I also checked oil level, which was exactly between the two markings on the stick, and did not appear to be contaminated.

toomany MBZ · #5 01-26-2008, 03:24 PM

Note if this happens when cold. How high do the revs go?

leathermang · #6 01-26-2008, 04:08 PM

Check all your ball joints in the fuel control between your foot and the fuel injector pump... they are supposed to be oiled with transmission fluid... but something light like Chrysler type A white lube should be fine... would not hurt to clean them first... and check that any springs supposed to be in the system are there..
Greg

MonsieurBon · #7 01-26-2008, 04:45 PM

Quote:

Originally Posted by toomany MBZ

Note if this happens when cold. How high do the revs go?

It has not happened when cold. Only after about a 15-mile highway drive.

Tach is dead, but the first time it happened it was so loud, it was definitely higher than I've ever revved it before.

MonsieurBon · #8 01-26-2008, 04:47 PM

Quote:

Originally Posted by leathermang

Check all your ball joints in the fuel control between your foot and the fuel injector pump... they are supposed to be oiled with transmission fluid... but something light like Chrysler type A white lube should be fine... would not hurt to clean them first... and check that any springs supposed to be in the system are there..
Greg

Can I use lithium grease? The weird thing is that it happens starting at an idle. It's not behaving like it's sticking open. But I shall certainly check all of those joints.

TheDon · #9 01-26-2008, 04:50 PM

I use lithium grease on the ball joints(linkages)

also check that the bushing on the firewall is still good and not flopping around

toomany MBZ · #10 01-27-2008, 05:57 AM

Quote:

Originally Posted by MonsieurBon

It has not happened when cold. Only after about a 15-mile highway drive.

Tach is dead, but the first time it happened it was so loud, it was definitely higher than I've ever revved it before.

OK, doubt it was I thought maybe, revs too high for a malfunctioning RD bolt.
Good luck.

MonsieurBon · #11 02-08-2008, 06:44 PM

Ahhh... It's been happening again, but this time I've detected a bit of a "thump" after it starts happening and I tap the accelerator. The sound comes from where all the pedal and linkage stuff is.

So it sounds like you folks might have been right. It's also been happening when I'm accelerating sometimes.

So I should use that spray-on lithium white grease on all those ball sockets?

Also, the accelerator only does anything if you press it at least 3 inches in. I'm not talking like a dude who's never driven a diesel before, I mean NOTHING happens until the top of the pedal is pushed about 3 inches in. Is that too much slack?

Also, the linkage that attaches to the IP seems very loose and shaky. The tightening nut on the rod is way down the shaft.

Any ideas how to clean this all up?

You guys/gals are invaluable.

2.5Turbo · #12 02-08-2008, 07:04 PM

Way too much slop in that pedal. In my car, even the slightest pressure from the toe affects the idle sound of the engine. A little more and the revs start to rise.

rrgrassi · #13 02-08-2008, 07:54 PM

Check all the linkage and connections.

MonsieurBon · #14 02-11-2008, 01:06 AM

OK, so I lubed all of the joints in the linkage. The rod that goes down to the IP linkage was unscrewed a good bit, and the ball joint was reaching the last few turns of the rod. I tightened that and its appropriate nut down.

As for the accelerator play, I noted that half of the linkage moves a good bit, then the little slidey-thing engages the ramp-shaped surround thing. I wish I had better ways to describe it, but I have included a handy-dandy picture.

Please take a look and see if I need to tighten something to make the accelerator do something sooner. Thanks!