W124 - poor running when wet

[RacingTeatray]

Can anyone help?

My W124 E500 has developed an aversion to heavy rain and carwashes. If exposed to prolonged wet weather or a jet wash, it develops a misfire and runs very poorly (low hunting tickover and major flatspotting on acceleration) until it has dried out again. When dry it behaves immaculately.

Local (non-specialist) garage suggests it could be moisture getting into the ECU - if so, what can be done to stop this.

If it's not that, does anyone have any idea what it might be and how to cure it?

Thanks!

[Egbert Nobacon]

1. The multiconnectors on the ECU have waterproof rubber seals. These can split or are sometimes cut open by mechanics working on the engine loom. Water penetrates the connector and causes arcing.
It's worth checking before it corrodes.

2. Has the engine loom been replaced ? Heat causes the looms to become brittle and crack (aggravated by movement during servicing ) Damp could be penetrating a crack.

3. Coil

[tricky 100]

Have you tried giving the whole ECU/electronic ignition system a good dousing in WD40 ?

[Howitzer]

A common problem, remove each fuse in turn, clean up the contacts, then replace.

Previous owner of mine was told to do this each year as it can cause some strange faults.

Also, have you had your engine wiring harness checked? The Americans are quite adamant that this causes rough running, intermittent problems with the ASR light etc etc. Moisture would only accelerate dodgy running.

Dave!

[RacingTeatray]

Yes the wiring loom has been checked and re-checked - it's fine.

Apparently the wonks on the M-B club forum say it is a dizzy fault - apparently it's a well-known fault with the design of the dizzy cap which causes moisture to seep in.

[Howitzer]

RacingTeatray said:

Yes the wiring loom has been checked and re-checked - it's fine.

Apparently the wonks on the M-B club forum say it is a dizzy fault - apparently it's a well-known fault with the design of the dizzy cap which causes moisture to seep in.

You have only recently had the caps replaced haven't you?

Have you got the receipt for main dealer parts?

Dave!

[RacingTeatray]

I'd have to take a look. Unfortunately, I'm not due back in the UK again now for a month (was just over last w/end to collect the Alfa and take it back to Russia - considered taking the Merc instead but not keen on idea of setting off on a 3000km journey with a car that is running imperfectly, and in light of the major rain I drove through, this was undoubtedly a wise decision). It'll have to wait until then.