300TE Running rough.

[B.J.Keniger]

Hi,
After four years of owning a 1989 300TE estate I have experienced the following problem.
From the start I have experienced problems with the ABS light showing occaisionally and this was usually associated with the slow running being affected - leading to stalls at lights and making driving difficult to say the least.
The ABS would show at random but usually go off after some time - and I got used to the situation. About a month ago the ABS came on and refused to go off again. This made starting almost impossible - however once started and warmed up the car would run as normal at speed, the only problem being tickover speed which was affected.
After reading on the web about the over voltage protection (OVP - strangely not mentioned in my Haynes manual)) relay I bought a new one via e-bay from the USA and hey presto the car starts and runs again. However after about 5 miles or so (when it warms up) the engine starts to feel rough and feels very lumpy when reving. Accelarating becomes poor and the power drops. From looking at the plugs I would guess that the mixture is weak - whitish plug tips.
The OVP is an updated version of the original model - it has 2 fuses and more pins than the original but I assume this is not the problem because my local MB outlet was ready to sell me the same part (at twice the cost).
The rough running was not present before my old OVP gave up the ghost - I would appreciate any advice on what could be going wrong.

Thanks BevK

 

[Ian B Walker]

A flatish battery will cause the ABS light to light up. Is your battery fully charged?

 

[television]

Yes, the OVR with 2 fuses is now the only one supplied now. Re the running rough, A faulty OVR will cause poor idle and stalling.
The first thing is to check that you have 12volt on pin 30 (in) and the same on pins 87 (out)

malcolm

 

[B.J.Keniger]

The OVR (if that is the correct term) has cured the problem that I originally had eg very hard to start, ABS light staying on permanently and very poor tickover. The engine ran just fine above tickover.
Now with the new relay fitted the car starts fine and runs well at tickover, and the ABS light goes out normally however, once the engine gets warm it starts to get rough above tickover speeds and loses power on accelaration.
The rough running at higher revs was not part of the original problem before the new unit was installed.

BevK

 

[television]

The OVR (if that is the correct term) has cured the problem that I originally had eg very hard to start, ABS light staying on permanently and very poor tickover. The engine ran just fine above tickover.
Now with the new relay fitted the car starts fine and runs well at tickover, and the ABS light goes out normally however, once the engine gets warm it starts to get rough above tickover speeds and loses power on accelaration.
The rough running at higher revs was not part of the original problem before the new unit was installed.

BevK

Your car is controlled by a relay and an ECU. At idle the main ECU is out of action, it only switches on above idle. I think you need to check the ignition.

Malcolm

 

[B.J.Keniger]

Malcom,
Earlier this year I changed the ignition coil and leads. The rotar arm and distributor cap and plugs were new last year (I do not do much milage).
I have fitted a new fuel filter this year also. The car was running just fine apart from the slow running before the relay was renewed.

BevK

 

[B.J.Keniger]

I thought I should just end this thread by reporting what actually turned out to be the problem.
I changed the fuel pump relay, the OVR to no avail.
A new set of NGK spark plugs improved the running a little starting more readily but the engine still ran rough especially as it warmed up - at low revs, in drive, it would stall which made it undrivable.
The improved performance made me think it might be worth checking the distributor cap again despite being nearly new. This I found to be heavily corroded and burned at the pick up brass terminals. I cleaned these up and the engine is back to normal!
This car seems to get through distributor caps faster than any car I have had before - I have fitted two new caps and rotar arms,a new set of leads and an ignition coli in four years. The distributor cap is a pain to remove but next time I will start my checks there.

Bev K

 

[Bolide]

IIRC burnt distributor cap terminals are a symptom of a bad condensor

Nick Froome
www.w124.co.uk

 

[GEORGEROV]

Condensers went out with contact breakers. This is a breakerless system. If it was not a genuine Bosch dist cap then this can give said problem.

 

[B.J.Keniger]

Yes, there are no points on the 300 TE.
Perhaps this means that it is an engine management sytem problem - when running it all seems smooth enough.
In future I will just try to be more aware of the tell-tail signs when it starts to get lumpy.

Bev K

 

[television]

Malcom,
Earlier this year I changed the ignition coil and leads. The rotar arm and distributor cap and plugs were new last year (I do not do much milage).
I have fitted a new fuel filter this year also. The car was running just fine apart from the slow running before the relay was renewed.

BevK

The answer here is that when the OVP relay was faulty the High voltage was reduced, and restoring the 12volt to the ignition took the high voltage output to max,this will find any weakness in the insulation of the components.

Dizy caps are porus and not of a super quality moulding as used today, spraying them with a good sealer,including the rotor arm can stop this problem.

So pleased though its all sorted,I will never forget my 300TE.

malcolm