Rough Idle & Running Hot

[timm9]

I have used the search ‘button’ and don’t see much that I can relate to my issue. Last year I had some problems that were eventually resolved as outlined in this post (thanks Chuck!) http://www.peachparts.com/shopforum/showthread.php?t=221171 but not completely.

After replacing the Air Guide Housing and ‘getting close’ to the correct fuel mixture, my mileage has improved greatly (from 11 to 19 mpg) but I’m still experiencing intermittent rough idle and the car is running hotter than it should. Even after a short trip and parking in the garage I can smell the heat long after the car is shut off. Last week driving home from a dinner party (12 miles up hill) the temp ran up to the point the Aux Fan kicked on and the outside temp was approximately 67 degrees. At this point I have replaced the O2 Sensor, Aux Fan Sensor, Plugs, Wires, Radiator Cap, Rotor & Cap, Vacuum Lines, and all Fluids. At one point the WUR was suspect, but both sides hold vacuum after the air guide repair. Also the injectors have been tested. I have inspected the water pump and do not see any evidence of a leak nor does it sound like it is failing at idle. I have also tested the Clutch Fan after running up to temp and it appears to have resistance, although I’m not sure how much resistance there should be. As of this writing the Car has 32,500 miles.

In the post referenced above I had noted that with the huge vacuum leak the fuel mixture was very rich and it appears that the cracked air guide housing had been failing for a very long time. I also suspect that at purchase the car did not properly pass smog considering the huge vacuum leak and rich fuel settings it caused. Is it possible that the grossly rich mixture has damaged the Converter? Would/Could that explain the intermitted rough idle and why I can’t get the duty cycle adjusted correctly without the car running poorly? Currently, the duty cycle fluctuates between 44 and 62 percent at 80 degrees C and above. Any attempted adjustments in mixture in either direction results in the idle deteriorating further. I ‘searched’ but did not read anything about testing the CAT. Sorry for the long post but I figured some of these items would come up as suggestions.

As always, I greatly appreciate you thoughts, suggestions, and ideas!

[ctaylor738]

I think you have two unrelated problems. On the running hot. Are you sure that the thermostat is functioning properly? Do you have around a 50-50 mix of coolant and water? Is your radiator functioning properly? Most shops have an infrared sensor they aim at the radiator to check for flow.

Is the timing to spec, not over-advanced?

The mixture on your car was probably set rich to compensate for the extra air from the vacuum leak. So the net was probably not an over-rich mixture in the combustion chambers. If the car passed smog and has decent power, I wouldn't worry about the cat.

Can we do the mixture drill one more time, though? Disconnect the oxygen sensor, and hook your meter between the sensor and ground. Set the mixture to where the the car idles well - maybe go lean until it stumbles and them back off. Give the sensor a minute to adjust and see what the voltage reading is. See if you can get it to stabilize around .45 volts. Then plug the sensor back in and see how the idle does.

[timm9]

Chuck, once again thank you for your wisdom and willingness to help!

I think you are absolutely correct in your statement regarding two unrelated problems. I also discovered that the WUR is holding vacuum after the Air Housing Guide repair. In my previous post I think I also mentioned that I have replaced the vacuum lines with the exception of the round plastic vacuum check valves which are next on the hit list.

The Thermostat was changed out approximately 3000 miles ago. I have checked the antifreeze mixture which registers in the norm. This week the Thermostat will be changed as well as flushing the Radiator and checking on the flow. Additionally I have also replaced the cap on the reservoir and checked the timing which is dead on. The Car goes to the shop this coming Tuesday.

I have made several attempts to adjust the mixture based on your previous guidance, but the idle will not smooth out in either direction and actually becomes much worse. That was what was making me suspicious of the Cat as I don’t believe there were any other components we had not tested. As soon as I have the Car back from the shop I will follow your instructions and DC the O2 sensor and try adjusting the mixture as per your post.

Thank you again! Hope all is well with you and yours!

[neds380sl]

Timm- I'm certainly no expert on M-B's, but I would check the radiator closely! My old fuel injected Land Cruiser consistently overheated, even after replacing the water pump, thermostat, hoses, everything except the radiator. It turned out the radiator was the villain. You might remove it and have it boiled out, or rodded out (whichever they do these days). I had mine rebuilt for $350 (!) but it no longer leaks and cools just fine. Also, as stated earlier by others, an engine running rich will tend to overheat. If nothing else seems to work, including the above, have you performed a compression check? Hopefully, you don't, and I wouldn't worry except as a last resort, but you might have either a bad head gasket or cracked head? The mileage on your car seems low, so it must have done a lot of sitting, that equals rust and corrosion and junk in the radiator and rest of the system. You have my sympathy; these issues are frustrating!
Just my $02. Good luck, Ned.

[Richard Wooldridge]

Timm, on my '82 380SL the engine fan thermocoupler failed about a month ago, and caused the car to run very hot and the aux electric fan to come on when it really shouldn't have needed to. You might want to check yours out, or just replace it. After replacing mine the car runs right at the bottom of the range unless it is REALLY hot and the A/C is working hard.
As regards the WUR, holding a vacuum doesn't remove the WUR from the suspect list, as what usually goes wrong is that a spring inside the WUR loses it's tension and allows the control pressure to go off spec, usually causing the engine to run rich. The only real valid test on the WUR is to check the control pressure. A quick and dirty test is to very lightly push down on the air plate and see if the engine runs smoother. If it does, it is a sure sign that the control pressure is off.

[timm9]

Thanks to all for the responses.

The car was dropped off at an indy shop that I trust last Tuesday. I told them to keep it as long as they need to diagnosis the cooling system. As soon as it is finished there I will start in on the 'other' issues. Thanks again for the suggestions. I will update my post as the 'culprits' are hopefully identified...

Tim