Hello there-

The patient is an '86 560SEC. It had the dreaded "intermittent rough running and no power at all" problem for a few days now and it is getting worse. I checked and resoldered the terminals on the fuel pump relay, no help. I then did the same to the OVP relay. That is when I discovered a new thing. If I pull the OVP while the car is having the rough idle and no power situation it suddenly hits on all cylinders and idles at 1500rpm! Drives with full power as well.

Is is the OVP itself, or is it whatever the OVP controls that is the culprit here?? What does the OVP do? I hate just changing parts for nothing. Any help would be appreciated.

Thanks,

Alex

Bump to top. Ideas anyone?? I did a lot of searches but can't seem to find the solution.

It does not sound like your OVP relay. If you leave the OVP relay plugged in and Rev-up the engine to 1500 RPM, I believe that you will get the same effect as if you removed your OVP relay, i.e.: the engine will smooth-out.

I think you need to look elsewhere, but not sure where. Lots more diagnostic effort is required.

The logical place to START would be Spark-Plugs, Ignition System, Fuel Injectors, and other fuel delivery related systems.


Phil

__________________
'95 E300 Diesel, 264,000 Miles. [Sold it]

Thanks for the reply. I forgot to mention that I recently replaced the plugwires and injectors while trying to track down a rough idle issue. When the car is working it runs very well. Intermittent problems suck.

The new syptom is a complete lack of power followed by a sudden "healing" where it returns to normal. It will them be fine for a part of a day or so only to run very poorly again usually after a hot start up.

Check your vacuum hoses, especially for the idle control valve, as well as the valve itself. This is often the hidden culprit behind a poor idle and hard start. The oxygen sensor could also need replacing. Good luck and let us know what it was.

As for what the OVP does, click on blue and yellow D.I.Y. button at the top of this page and scoll down until you see the section on "Evaluating Electronic Engine Contols". About midway through that artcile is an explanation of the role and purpose of the OVP. Hope this helps.

Thanks for info. Here is some more from my end. My O2 sensor is new and I have no visible vacum leaks. Some of my symptoms match a bad OVP as outlined in that article but not all. The sudden super low idle sounds like a failing OVP relay indeed but when the problem is happening the car also has nearly no power even off idle. I press the gas and it crawls forward as if runnign on 4 cylinders. Then after a few moments if "catches" suddenly roares to life and runs great! Does the OVP also control other functions that could cause a big loss of power when it fails?

By the way this seems to happen more when starting an already warm engine.

Thanks for the help,

Alex

K- jetronic is basically mechanical injection with electronic fuel trimming, the reason it revs up when you pull the relay is that it runs at full mechanical injection with no fuel trimming. The Over voltage protection relay powers the fuel trim control unit. A good starting point with your concern would be to check fuel pressure at upper and lower chambers of the fuel divider. My guess would be you have a bad fuel divider.

Actually the ovp powers a number of systems. The two that are involved with running are mixture control and idle air control.

One thing to always remember, engine speed is determined by the amount of air not the amount of fuel. The various idle systems MB uses vary in their response to NO control. The V8s go to a very large airflow. The 4 and 6 cylinders go to a fixed middle psoition slightly larger than controlled no load condition. They do that as the system is a newer generation than the v8 system and the system was designed to "limp home" better with every version.


The way your car responds to no ovp is totally normal in the idle speed part. The fact that you are getting more power from it in the ovp missing condition is quite unusual. My first suggestion would be to try it a little differently. instead of pulling the ovp pull the EHA connection. Rememebr that ovp is powering two systems: idle control and mixture. The end result of all input is the signal to the idle valve in the idle contro case and the EHA in the mixture case. Your system if adjusted properly will not drive noticably different when warm with the EHA disconnected.

By disconnecting the EHA and idle valve separately you will accomplished a segmented reproduction of the act of pulling the ovp. It is hard to imagine power under acceleration being affected by disconnecting the idle valve, so do the test separately and verify.

If you find the EHA connection to have effect on the power, then you will need to learn how to monitor it. If you wish to learn how to view "cONTROL" on your motor and how to adjust it, monitoring EHA is imperative.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Thanks for the great info Steve!! My next question is where is the EHA connector?

Now that I think about it the car has never crapped out when hot. It has the issue mostly when started up after sitting far a couple of hours. Then is runs poorly as I said before. I will try these new tests and see what's up.

More later...

Alex