1988 240 Overheating. When it overheats, running the heater seems to cool it down. Overheat condition comes while sitting at idle. I have changed the water pump, flushed the entire cooling system, changed the temp sender, changed the fan and the clutch. Where do I go next? The next step as far as I know is to change the head gasket. Your inputs greatly appreciated.
Darkmon316
1988 240 DL 222,198 miles
1984 240 DL 198,135 miles

Any luck yet? I am going through the same situation right now and feel like I am going to go crazy! I first flushed the entire system and cleaned the radiator well....I then replaced the thermostat with still no luck. This weekend I replaced the radiator, hoses and waterpump. The aux fan runs with the AC on....and everything seems to be in working order.....but the damn thing still runs hot. If I sit in traffic without moving for more than 5 minutes, the temp starts to climb towards the red....the only way to cool it down is with the heater. I am now hearing here that I may need to change the fan clutch or temp sensor.

Please let me know if you have had any luck with anyting in particular.

Rob

I changed the temperature gauge and the chip underneath the gauge, and everything seems to be working fine, running good for day three.
Darkmon316

I also am having the same problem. I replaced the thermostat twice in 3 months.
Talked to my mechanic, immediately said bad thermostat.
My mechanic told me that he has had trouble with batches of thermostats for some reason, he once went thru 9 till a good one.

quick question? were the oem thermostats that the mechanic was using?
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never snort horseradish

I suspect, but don't know for sure, that they were not OEM.

Without any of the symptoms of a leaky head gasket, I wouldn't jump to that just yet. Since it only does it at idle you should make sure the fan clutch is working properly, the replacement might be faulty. And you should also check your timing to make sure it around 12 +/- 2 degrees. You might look at the O2 sensor output if you have a voltmeter handy, the mixture could be off due to a idle switch not engaging on the throttle body.

If all of that looks good then maybe you don't have very good cooling efficiency with your radiator. Use a high pressure hose, like the one at a do it yourself carwash, to blast the radiator from the engine side forward so that any debre that might be caught in the fins will be dislodged. No matter how well the fan clutch works, if the radiator fins are clogged then little air will be pulled through.

If that doesn't help, you might consider having your radiator acid dipped or replacing it.

You might also check for any leaks in the system. Check the radiator hoses, the hoses that run from the engine back to the heater core, and maybe replace the reservoir cap with a new one.

What's the condition of your radiator like?
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'91 944 SE 217K(now mine), '85 Mercedes-Benz 380SE & '95 Jeep Grand Cherokee I-6 Laredo 4x4 (parents)

I suspect you might not be overheating at all but have the twitchy temperature compensation board issue. The response to opening the heat valve is what you see when the board is acting up. Check it out. There are some repairs and replacement boards are abundant in salvage yards.

What about the thermostat? I didn't see that listed in your changeouts.

AndrewinNC
87 240GL 100K (this month)

I am sorry. Yes, I have changed the thermostat, twice.

is there any sign of, steam in the exaust,, oil in the water,,, water in the oil- water condensate under the oil cap or dipstick??
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never snort horseradish

No sign of oil in the water, water in the oil, steam out of the exhaust.
I was reading this article from the archives:

Need advice on overheating at speed (I'm losing my mind) 200 1987
posted by old paint on Sat Jun 14 17:45 EST 2003
Author Info: [PROFILE] [EVERY POST] [THREADS]
i have an 85 slant 6 cyl dodge van.....did the same thing for the longest time. started doing after i replaced a leaking water pump. it was fine untill i pushed it to 70-75 sustained speed. temp gauge would go up 2 marks above center....i would cut on heater, drop speed to 60 and the gauge would go back down. put up with this for a year then that water pump(AUTO ZONE ELCHEAPO)sprung a leak. went back to AUTO ZONE,(LIFETIME WARRENTY WOOPIE!!!!) told them i wanted a NEW WATER PUMP, not a rebuilt!!!! i would pay the diff.
i put on the new pump(the internal pump fins were larger then the one i took off)and i havent had an over heating problem since. i took the van to N.C., W.VA., PENNA. LAST SUMMER IN AUG. and never got hot....on all those mountains.
also only other thing will cause this type of overheating is TIMMING, BAD TRANNY(uses radiator to cool tranny oil)put a seperate tranny cooler on it if you live in a hilly area.

My transmission lines going to and coming from the radiator are very hot. Would this be part of my problem?. No issues with the transmission.

you dodge problem did jog my memory,, i did at one time own a 70's vintage 244 with a b20f, had an overheat problem took the thermostat out and at speed it would stay cool but at rest the temp would climb, the theory was that at speed the water was of course being cooled down by the additional air thru the radiator but the water was flowing too swiftly thru the engine block to sufficiently carry away the heat, i dont know what temp. rating the thermostat is in yours but you might need to go to a higher temp rating thermostat to bring the overall temp. down. a lot of people may go out and purchase a lower temp. stat. only to find that it actually runs hotter. give me feed back if you can. / mike
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never snort horseradish

p.s. the initial reason i took the thermostat out was because it was sticking. restricting flow too much.
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never snort horseradish