A customer (in this case a friend, too, so I won't be making any money!) brings me an 87 190E 8v stick shift with an odd propensity for EXTREMELY poor running in wet weather... a real problem here near Seattle. The car seams to do OK after a day or two of dry weather, but after a day in the rain, the car will start but stumble at idle, rev but not pull load, and go out like a light when the throttle is held down, with an occasional spurt of acceleration. A prolonged warm up makes the car driveable, but the tendencies do not disappear in the rain. I've already put new cap,rotor,wires & fuel filter on the car, and checked the fuel pump relay (OK, and appears to have been recently replaced), even bypassed the relay (hot wired the pump) with no success. The OVP seems good (no other symptoms of over-voltage, but I haven't done the full diagnostic set on that), and the deceleration switch can be disconnected to no avail. The diaphragm pressure regulator, over pressure return valve, and pump (mechanical) shouldn't be sensitive to weather(?). I am going to look at (and maybe hose down) the pump tonight and see if the motor is water tight, but the day is dry so the car is running. Is one of the "brains" a likely culprit (i.e. ignition control module or injection control module)? Any notorious switches (i.e. throttle position, temperature, etc)? The weather dependency points towards electrical, but the behavior seems like cross-over. Any ideas?

Aha!

I have the same problem on my 92 400E, the rain finally stopped last night, today is very dry, it becomes such a joy to drive, idles like silk, I couldn't feel the car running without looking at the tachometer.

But until last night, when the humidity was high, it was idling very rough, shaking and humming, when I shifted it to P or N, it got better, it always does this when it rains.

I have also replaced distributor caps&rotors, plugs and fuel pressure regulator, did help the overall performance, but didn't help out on this.

I am considering to do some serious cleaning on the Mass air flow sensor, any comments?

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99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.

Check the ignition coils...especially at the spark plug wire connections.

I know it's a stretch, but my old VW had the same problems whenever the environment was moist...I finally traced the problem to the ignition coil...corrosion was discovered at the main plug lead.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car