After heavy rain rough idle and hesitation is back. - Mercedes-Benz Forum
 
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post #1 of 7 (permalink) Old 08-12-2019, 10:48 AM Thread Starter
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After heavy rain rough idle and hesitation is back.

As mentioned on title

This time the car was just parked there under the rain. It can't be something under the hood (can it?) as I didn't drive under the rain this time. Found the problem like six hours after the rain stopped. I made sure engine bay is well sealed with rubbers. Did humidity find a way inside, does it hurt in the first place? A day before the rain it drove like a dream.

My electrician once said it could be the ABS sensors so he wrapped them with electro-tape for maintenance and seated them well. Are there other sensors I should worry about under the car? Or any parts for the sake of it?

I noticed that in the first startup, the fuel pump hissed long and didn't stop until I started the engine. Usually it hisses just for a short while. It didn't happen again.

This is getting annoying. This always happens and clears after a couple of hot dry days on its own.
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post #2 of 7 (permalink) Old 08-13-2019, 01:43 AM
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Might be your crank position sensor get it checked with a multi meter for ohm.They can cut out with the change of heat
Or the fuel pump relay ,,to sort this, and to eliminate the relay pull the relay out [ difficult ] i use two plastic drifts the relay will have lugs on that clip in to the plastic type cradle at the rear of battery. Once you removed it you can then wire 7 and 8 pins of the relay together , and refit .
After you do this it will operate the pump all the time,, so only use it to rule out if the relay is the problem ,then remove the link from 7 and 8 pins right away . Dont use it with the link in place on the FPR .
And one more thing- if it is a m103 lump you can have condensation in the rear of the distributor cap the cap will need removing another fiddly job but they are on our type of engines, now at the rear is a disc with a seal that stops condensation from entering the distributor from the rear . Next time it is like this , open bonnet in the dark and look for blue flashing on the ht circuit that could be your fault . All this information is for the M103 12 valve .So may not be the same for your engine .Its best to point out in your post engine type .
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post #3 of 7 (permalink) Old 08-13-2019, 02:11 AM
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First and foremost check the crank sensor as mentioned. If in doubt, throw it out. Cheap part that can leave you stranded.
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post #4 of 7 (permalink) Old 08-13-2019, 05:55 AM Thread Starter
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Thanks for the heads up, guys. Good advises.

Yes, the engine is an 3L m103 12v as mentioned in my signature.

I have an update actually that could rule out some of the above, as per your further possible advice/diagnostics.

Leaving the car parked under direct sun light at ~35-44C for ~6 hours solved the problem, or I should say went around it. Now the car drives like that never happened.

Does that rule out (some) of the above? Maybe like the distributor cap and its attachments especially that the problem showed with a completely parked car?

Now that things are back to normal, I don't think I can look at anything since nothing will show up now on a perfectly working engine

Guess I'll just replace cheap parts and fine-seal the others because maybe water/humidity found a way inside.
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post #5 of 7 (permalink) Old 08-13-2019, 06:06 AM
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I am now thinking may be its this round disc i told you about with the seal on in the distributor .The damp or condensation would have dried out once you get it started . But you see how it goes from now on.
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post #6 of 7 (permalink) Old 08-13-2019, 06:56 AM
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My vote is for moisture getting into the ignition system. Remove the distributor cap, rotor, back plate, etc. and check that large o-ring others have mentioned. On every M103 I've worked on that o-ring is rock hard and not sealing anything. IIRC it's stupid expensive for an o-ring.
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post #7 of 7 (permalink) Old 08-13-2019, 08:33 AM Thread Starter
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I remember looking for that rubber ring and couldn't find it. I wonder if there's a way to counterfeit/retrofit/you-name-it a sealant for it. Maybe a good PVC plumbing tape or something? I don't think silicon glue works well there.

Time to improvise like with other parts, I believe. I did seal that external plastic cover in the sides after all.

Any ideas to do that are welcome.
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