I began to overheat (in my 77 450SEL) during a 100 freeway drive. By the time I was able to get off the freeway and shut the car down, I was nearing redline. The system held pressure; no leaks that I could see. The overflow tank was completely full. After half an hour (after the level had gone down) I opened the overflow cap, w/ barely a hiss. The fluid was hot. I gave it another half hour to cool, then started her up again. The fluid level in the overflow tank was fine. I have been paranoid that I could have cracked my head gasket, but no telltale signs (no white smoke/steam from exhaust; no oil in coolant or vice versa; no yellow/white gunk in oil cap; and I seem to have full power).

Pertinent service history: currently car has 127000 mi. Radiator core replaced 7 mos ago at 124k mi, and had run fine since. PO replaced the water pump and thermostat w/ quality OEM parts at 123k mi less than 3 yrs ago. The car was driven only 1300 mi between when the PO replaced thermo/pump in Oct '03 and when I took over the car in Sep '06.

I'm actually having a hard time deciding if turning on the heater lowers the temp. If I allow the car to get hot, turning on the heater does NOT seem to bring the temp down. However, if I turn it on when I start to drive, it may help keep the temp down. The car seems to overheat at any speed (but my in-town drives have been pretty short since the issue). On the freeway, it takes about half an hour for the temp to get to about 180-190F at 65-70mph, and then climbs from there (My thermo is 167F/75C). Also, when I felt the upper and lower hoses, I couldn't feel pressure; but when I took the thermostat out and put it in hot water, it opened as it was supposed to. I have a new Behr 75C thermostat and lower hose, and will replace old w/ new. The system is currently drained of coolant w/ the thermostat housing off.

It is very difficult for me to imagine my new core failing after only 7 mos and 3000 mi. It is also hard for me to believe that my water pump or thermostat failed after less than 3 yrs and 4000 mi. But obviously something is wrong! Why am I having overheating issues after such a short time between repairs?

In any event, I am generally able to take off part x and bolt on a new part x in its place, but I am not a mechanic, and stink at diagnostic. I would appreciate all the troubleshooting help I can get!

Someone suggested that retarded timing could be the cause of overheating - and I admit I haven't checked it myself - but it was professionally set w/ new quality OEM parts in '04 at 124k.

 

I think the car is running too lean. Or the timing is not correct or not advancing. or the exhaust is restricted. Timing easily checked with a timing light, exhaust easily checked at a muffler shop. Too lean would have to be checked on a smog machine. bottom line I think you've covered all the cooling system bases.

 

Carnut,
Thanks for your response.

I have something of an update: I replaced the thermostat (the old one did seem to open as spec); however I continued having troubles. I further found that the car overheats both at idle and on the road. I drove the car from Orange County to Palm Springs yesterday to get it (and me) back home, pulling off for an hour after 60 miles of driving before I got to 212F. I pulled off again for half an hour, but didn't need to.

I was VERY annoyed to realize that - as I was replacing the thermostat - that I could have tested the water pump by disabling the ignition and cranking the car w/ the housing off (if I got a stream of water, the pump was working). However, the Tech Companion says you can check the pump by feeling the pump outlet hose to the rad after the thermostat opens, and that you should feel pulsation if the pump is working. I performed that test, and felt no pressure.

I also used my non-contact thermometer to "shoot" the radiator. Because of the AC condenser, I "shot" the inside w/ the engine running (is that the correct procedure?); and I seemed to be getting pretty consistent readings at several points. Unless I'm using the equipment incorrectly, the radiator seems to be okay.

That leaves the water pump, one of the things that carnut is suggesting, or something really nefarious (like a blown head gasket, altho there are no telltale signs of that). Because of the cost and effort of replacing a pump, I would like to verify that my timing is okay, and the exhaust is okay. I intended to check the timing, but thanks for the idea of taking the car to a muffler shop for an inspection.

In a way, I actually hope its one of those latter issues - even though a cat converter or muffler would be more expensive than a pump - because these are things that are expected to fail over time. If it IS the pump (and it very well could be), I'm left worrying over why a pump would fail after 3 years and 4000 miles.

What issues would cause a car to overheat at both idle and freeway? Could it be as simple as the pressure cap on the overflow?

 

Everything that you've written describes an engine running warmer than normal, not in fact, overheating. Your description says the temperature is not stable. The thermal test you did on the radiator confirms no blockage there. I still think the cooling system is probably not the culprit here. I'm interested to find out the verification of timing and is it in fact advancing with engine rpm? In re reading your original post you say it takes 1/2 hr to get to temperature. Thats way too long. Assuming your thermostat is closing all the way cold I would suspect the temperature sending unit is not giving you a true temp reading. Wouldn't it be great if it was just an inaccurate temp sender! 212 degrees is not overheated. most aux fans dont even come on till around 215 to 220 degrees.

 

Carnut,
I may not have been clear in my description.

At idle I am actually overheating very quickly (w/in a few minutes). What I was trying to describe was the fact that on the highway it takes me about half an hour before I begin to get to that point (near 212F) where I am considering myself as begining to overheat (although when I made the drive to Palm Springs the other day, I actually got 60 miles before reaching 212F and promptly pulled off the freeway and shut her down for an hour.

I agree w/ you: 212F is not overheated in a pressurized system w/ a 50/50 mix of coolant. But the car shouldn't get that hot w/ a 167F thermostat under normal driving conditions, and I DON'T want to blow a head gasket. I know I've got some kind of problem, and will not push a system I know to be deficient.

I am not feeling "pulsation" from my water pump, and think it might be the culprit. But I'd rather rule out other stuff before ordering a pump. I will be replacing plugs (I figure if I take them out to inspect them, I might as well just go ahead and replace them) to see if I'm running lean and I will aslo check my timing. As the car actually seems to be running rather well (other than the overheating thing) it's hard for me to get my hopes up that my timing is off badly enough to cause an overheat condition. I'll be sure to post my results.

What I'm looking for is something that will create an overheat at both idle and on the freeway; and in particular something that will create an overheat rather quickly at idle and much more slowly on the open road. That would seem to rule the fan clutch and auxilliary fan out. So I'm considering the water pump, the exhaust, the timing, and the radiator overflow cap (is there anything else?). I'm frankly not sure if any or all of them could create the overheat condition I'm experiencing. Diagnostic can be a real bugger.

I appreciate your insights,
Michaeld

 

I'm a definate novice when it comes to working on cars, so perhaps my answer is something that was already looked at...is your aux. fan working? I had the same issue with my car and when I looked at things I never even paid attention to the fan, loandbehold, it wasn't working and was a simple fix and now my engine runs at a nice temp. I found that at speed, the fan would be working just because air was being forced over it, making the temp lower at highway speeds than during idling and when I was idling the temp shot up like it was on speed, once I got moving again the temp lowered

 

Thanks for the better description. One rather remote possibilty is the fan clutch. They work primarily at idle but they will cycle engaged/disengaged at freeway speeds. My wifes car had the original clutch on it from 1981. She started to notice a 10 to 20 degree temperature rise one summer a few years ago. Eventually it was verified the fan clutch was never fully engaging and was replaced. Within 2 hours the temp stabilized! One way to confirm it works, is to blow compressed air against normal rotation with the engine at idle. Disengaged, the fan will be held stopped by the air against it, but once engaged, the clutch will over power the air and start spinning. That way no FINGERS! get removed.

 

Carnut, Brian,

Just a note to let you know where I'm at;

I was just about to start her up and check the timing and test the fan clutch (per carnut's method) when I noticed my overflow tank level was down. Then I saw that coolant was dripping. I drained the system and discovered that the upper hose had torn at the hose clip. That put the kibosh on my efforts to check things out until I get a new hose (special order only at every part store in town).

BUT... I DID call Mercedes-Benz at Laguna Nigel - and they were extremely courteous to me. I talked to a mechanic named Chris, who advised that he thought the most likely culprit - based on my description - was... the fan clutch. You guys win the cupie doll!

I looked back over the PO's service records, and the fan clutch was last replaced 16 years and nearly 45000 miles ago. I will test the thing before replacing it (which means replacing the hose and filling up w/ coolant), but if it IS the fan clutch, I suppose it lasted about as long as I had a right to expect.

I read a good dozen troubleshooting articles on overheating, and several of them ruled out a bad fan clutch if one was overheating at speeds of over 30mph. I had therefore pretty much dismissed the fan clutch as a real possibility in my head (although I had every intent of checking it, just to be safe). Don't believe everything you read, I guess.

As a P.S. I also asked the mechanic where the coolant drain plugs on the cylinder heads were located - in order to properly drain and flush the system as per the MBz manual - and he told me NOT to use those, as they could too easily corrode and break. He advised flushing the system w/ the front end downslope to maximize draining.

Thanks for your advice, guys. I will post what turns out to be the problem. A fan clutch would certainly be easier to replace than a water pump!

 

I repaired my upper hose enough to continue on diagnostic.

First, I checked the timing: I am w/in 2 degrees of TDC (my 'mark'); I can't imagine bad timing is my problem when I'm that close to spec. I didn't adjust it to TDC because I've got other worries right now.

Second, the "fan clutch test": I used both forced air (I have a powerful leaf blower, but not an air compressor) and the 'rag test' to check my fan clutch; it seems to be engaging fine. It spun through both resistances. [If anything, it's not disengaging, as I was not able to get it to 'slip' or spin freely. It's possible I did not perform the test correctly. Obviously, I can't do more w/ the air blower than I did. With the rag, I balled it up, and lowered it so that it was in constant contact w/ the fan clutch; it continued to spin w/ force through the pressure of the rag.].

I'm still thinking water pump; but I'd sure like to be more sure that is my problem before going through w/ what appears to be a VERY big job. The mechanic quoted 4 1/2 hours labor (for them, not me!); that aint no walk in the park!