[Garytx]

Car runs fine when dry, but when I run thru car wash with under body spray or when road is wet it runs rough until things dry out. Car is new to me and I discovered bottom engine cover missing. Ordered and installed one and this didn't fix.
I suspect an electrical issue.
Suggestions welcomed.

 

[sbaert]

Let's address the elephant in the room first, i.e. eco junk wiring.

Has main engine harness and throttle body been replaced with the later updated non self destructive equivalents? If not, then all bets are off and may very well be your problem. See link for more details.

PeachPartsWiki: W124 Engine Wiring Harness Replacement

PeachParts Wiki contains Do-It-Yourself DIY articles relating to the service, repair and maintenance of Mercedes Automobiles

www.peachparts.com

 

[Garytx]

Yes, harness has been changed.
I am leaning towards changing CPS. Car always runs like a swiss watch until is rains or I run it thru a car wash.
Thanks for reply.

 

[Jayare]

When were the plugs, coils and wires last replaced?

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[Garytx]

Within last 6 months. Car runs fine when dry. Wet issue. I thought adding lower engine cover would fix, but didn't.
Next I will inspect CPS, probably change. Will post result after order and change.
Thanks for reply.

 

[Jayare]

Within last 6 months. Car runs fine when dry. Wet issue. I thought adding lower engine cover would fix, but didn't.
Next I will inspect CPS, probably change. Will post result after order and change.
Thanks for reply.

Test the CPS before spending any money.

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[Garytx]

OK, but I suspect it will read fine as car runs good when dry.
Interesting that when wet it stumbles at low rpm but if I floor it it runs smooth.
Thanks again.

 

[Jayare]

Remember, the CPS is a good/bad scenario. If it's good, the car runs.

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[sbaert]

Is your battery splash shield installed properly? If not, it could allow water to get into the OVP relay and other critical electronics.

 

[Garytx]

Is your battery splash shield installed properly? If not, it could allow water to get into the OVP relay and other critical electronics.

I will check. Didn't know there was one. I will research to find a pic.

 

[sbaert]

I think the official name is "battery partition cover". MB p/n#1246261630

 

[Garytx]

I looked and mine is in place. The mystery continues.
Thanks for pic

 

[N3T]

Car washes spray water at everything from all sorts of crazy angles. Check the grounding cables next to the steering gear and transmission. Also your starter is probably getting wet which means the wires leading to it are as well. The starter & throttle body wires are also made from the eco safe stuff. If they are original then they have lost their insulation a long time ago & will short when wet.


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[Garytx]

Good suggestion.
I now need to find time in my busy schedule to fix this problem.
As I've said, car runs fine ... when dry.

 

[tuttebenne]

Next step is to lower the top and disconnect the hydraulic pump motor. This isn't going to solve your problem but it will ensure you don't take it to car washes...

Seriously, your engine still has the cast metal cover over the coils and plugs right? If the car is so predictable, next time you go to the car wash, cover the engine compartment from the top with a heavy plastic sheet (drop cloth you can buy at lowes or home depot). Then run the car through and remove the plastic after the car is out of the carwash, being careful to not let the pooled water run onto the electronics. This might give you a clue as to whether or not water coming from above or below is causing the problem.

 

[Garytx]

Original post stated "or when road is wet" not just a car wash issue.

 

[tuttebenne]

Original post stated "or when road is wet" not just a car wash issue.

Got it. Okay, then let's go in a different direction. Considering the "lion's share" of water is now being repelled by the underbody cover the entire engine compartment is probably not getting wet. When the car has been dry but run through a car wash, or driven on wet roads, what exactly is wet in the engine compartment? How quickly does the performance degrade?

 

[LeftCoastGeek]

ultimate test. when car is dry and running fine, dribble water from a hose on various things under the hood til it misbehaves just avoid the exhaust headers, they can crack.

 

[Garytx]

Thanks to all for suggestions, especially 'elephant in room".
A few days ago the AC stopped working. Time to get serious. In my shop it goes and guess what ? Found a problem with the wiring harness. When I bought car I asked and was told the harness had been changed. It looked fine to me with no obvious problems noted. The AC clutch wasn't engaging, no voltage, and when I cut back harness wrapping....groan !
I have ordered a rebuilt harness and expect in a week.
I hope after change, wet and AC, problems will be fixed. I will post result
Again, thanks for support.
Regards, Gary in Texas