Sudden Idle problems...and panic.
 

Yesteday evening I pulled up to a stop light and suddenly felt an irregular, fairly hard shudder at idle. This morning it was there as well. The bad idle seems a little more pronounced this morning.
It seems fine at all speeds but idle. If I shift to Neutral at rest, the RPM's climb slightly and things are better. It may stumble when I pull off, but that could be my imagination as well. It also could be the engine is rougher at speed, but I really don't think so..

I purchased the car 3 weeks ago. 10 days ago I had the dist. cap., rotor, fuel filter, oil/oil filter, transmission fluid/filter and plugs all changed just becuase (not becuase of any poor performance). It has been perfectly fine until last night.

The panic? First thing I thought of was the timing chain. Why? Because my car has 110,000 and I know I need to get it done as a maintenance item. And, that would be the worst case scenario.

Seriously though, could it just be a bad (or loose) plug wire that affects it only when the RPM's are low and spark weaker?

Please help! The good MB garage near here is backed-up and I don't trust anyone else!

Thanks in advance,

Joe

Are any idiot lights illuminated when the poor idle manifests itself, perhaps the ABS light? Check the fuse on the OVP, it may be bad or even the OVP may be going south./

Nope, no idiot lights. OVP?

OVP (Over Voltage Protection) There should be a unit with a fuse in it...depending on your model where it is located. The OVP causes hard starting, rough idle etc. I had the problem once with my 190E hard starting and the OVP was the problem. Another thing to consider would be check the brushes on the alternator, if they are getting bad the output might be falling under 10 volts at idle which might be causing a bad spark. Do a search for "OVP" on this forum and I'm sure you find lots of info in the OVP.

-Brad

Brad,

Thanks. I've done OVP searches and could not find the definition (was still looking when you posted).

I get radio interference coming on and off when I go over bumps. Truthfully, I can't tell if it is one speaker or both. If it is one, then it is a stereo issue, if both I was thinking maybe a bad plug wire. However, to your thought on the alterantor, a mechanic told me the radio noise (which was occuring when he changed some parts for me) was possibly the alternator. hmm...


Mike, if the SRS light not on does that rule out the OVP or not?

Thanks,

Joe

one more thing, it starts just fine...

last addition...it idles for 3-6 seconds just perfectly and then the shudder hits..boom..then goes...it is not like the steady rough idle or stumbling I've experienced in the past..just a brief, failry harsh shake...at first I thought a heavy truck was hitting the over pass seams behind me.

just trying to be descriptive...

The OVP is very simple to check, it’s a little silver box next to your firewall. There will be a 10 Amp fuse in it. If it is blown then replace it, if not...rule it out. I don’t know where it is on your model but on my 190E if I recall correctly it was in the back of the front driver’s side fuse box. There was a flip up cover over the fuse and the fuse pulled out horizontally. I'd have to look again to be completely sure as it was almost 2 years ago but again, it’s very simple to check.

Have you tried some fuel system cleaner? maybe just as sample as that.

Before I had the same sudden shudders plus rough idle when cold, they all went away after I used one bottle of fuel system cleaner. My car is a 92 400E, I spent $1000 for parts with no luck, finally $10 cleaner solved the problem.

The OVP on a 88 560 SL in behind the kick panal to the right of the pasenger side foot well.

Thanks all.

I will check the fuse after work tonight and if it is not blown, I'll buy the fuel system cleaner. Cant eh OVP be bad but the fuse still be good?

Sitting at my desk, I lean towards the OVP and/or associated fuse being the likey cause just because it came on so suddenly. The idle has been smooth as glass until just last night (and is smooth as glass between the intermitent shudders).

Will report back...

The OVP may trigger the ABS light not the SRS. SRS will trigger if a fault in the SRS system or voltage drops below 10v. If an intermittent alternator problem, SRS light should go out once the system is above 10.1v.

Mike, I meant ABS, sorry...

Okay, so last night I put in some fuel system cleaner and got in the car. It was running worse than before...The intermitent boom turned into a more generally rough idle. I tried to locate the OVP. I see several small silver squares above the fusebox on the passenger kickpanel, but don't know which one it is. Any help?

Today I took it over to the local independant (Roxbury Foreign Car Repair in Northern NJ) to have look at the fuse. In the way there the problem worsened and the engine was rough accelerating up to about 2500 rpm.

I got there, the whole problem went away, we did not look at the OVP fuse and I drove off (I did however schedule to have the timing chain replaced as a maintenenace item).
He said maybe a fuel injector is sticking or there is some other fuel management issue. He basically said it could be a hundred things...

Two miles later the problem resurfaced and now the car just idels rough, but at the right speed. The engine remains rough up to about 2000 RPM, but still puills fine without any hesitation. I don't think it's misfiring. It starts up just fine.

Anyway, neither he nor the dealer can look at it for a couple of weeks so I'[ll just drive it until I take it in for the timing cahin on 5/21 - unless I break down or it gets much worse. I'm not really a do it yourselfer excpet for minor, minor, minor things (and interior, trim stuff).

Any thoughts/ideas would be great, otherwise I'll report back later.

you are in the right area for the OVP. it is the silver box with a red plastic cap on it that covers the fuse. hope this helps.

Checked OVP fuse (thanks Jerry and Mike) and it is fine...

Seems like it could be many things and I just need to get time at the mechanics shop...if anyone has any ideas on some things a simpleton like myself can check (like the OVP) any help is appreciated. It drives me crazy not to have it running right...

Mike,

Guess what? The ABS light went on today. I'm going to go ahead and replace the OVP. It seems like too much of a concidence. Will the ABS light need to be reset or anything or will it just go off if this is th issue?

No need to reset, it should go off. Now that the light is on check the fuse again. Also, the newer/newest version of the OVP has two blade type fuses and should be a better unit. And yes, the OVP can be bad even if the fuse is intact.

WAIT!!!
 

First Try taking the OVP out and seeing if it makes a difference..

My ABS recently went mental, and to disable it I ran the car without the OVP relay in. (Discovered now this isn't a good idea for long periods). It was hard to start when cold, and would stall until warm, then ran fine. I assume these are the symptoms of a broken OVP, and they don't really sound like yours..
If your car runs fine for 3-6 seconds, and then *suddenly* switches to rough idle, it's likely to be electrics/spark rather than fuel.. I would get a tech to diagnose what it may be.. and then decide how you wanna go about fixing it, rather than replacing random bits & bobs ;-) .


later

Russ

Russ,

Great point. I must admit I come here looking for easy and quick answers. As a non-diy'er I hope I can avoid spending tech dollars and find the easy fix. However great the advice here is (and it is great), without real diagnostic equipment and expertise on my part. People can only respond based on what I tell them.

I'll pull the ovp and see what happens first.

Thanks,

Joe

Update
 

Okay, I replaced the OVP and the ABS light went out. The really bad idle, the high idle and the worst symptoms went away.

What I'm left with is this:
- Slightly rough idle
- Rough accelaeration to 2000RPM,

Therse are the same symtoms I reported on 5-1, pre-OVP check and replacement.

Could this be that the engine managment system needs time to readjust or did I just have two problems?

Thought I'd cap this post. I took it in yesterday. It was indeed the plug wires - my original uneducated guess. I'm assuming when the cap, rotor and plugs were changed the wires got bent and a couple went bad. It idid work fine for a couple of days but I'm sure if they were near the edge a few dozen bumps over Manhattan's finest pavement did them in. I should hav ethem installed by this afternoon. Hopefully you won't hear from me on this thread again because that will mean I finally solved the problem.

Thank for all the help.