Help with overheating troubleshooting appreciated - PeachParts Mercedes-Benz Forum

michaeld · #1 06-25-2006, 07:54 PM

I've got a 77 450SEL w/ 127,000 mi.

I started getting hot during a 100 mi freeway drive. By the time I noticed the issue and was able to find a place to get off the freeway, I was too close to redline for comfort

[So far, I haven't noticed any telltale signs of a cracked head gasket, i.e. white smoke/steam from exhaust, gunk on oil cap, or oil in coolant, thank God].

Basically, the car gets hot at 65-70 mph freeway speed (and probably at slower speed as well) about half an hour into a drive. And it gradually just gets hotter and hotter. I tried turning on the heater, but - while it possibly seems to help if it is turned on at the start of a drive - it does NOT seem to lower the temp once the car starts to overheat. That might indicate a bad water pump. My understanding is that a fan clutch should be suspected only if it is overheating at under 30 mph.

I pulled off the thermostat and did the boiling water/thermometer trick, and it begins opening at the 167 point as it should (it opens gradually, rather than "popping open," BTW).

When I first bought the car (7 mos and 3000 miles ago), I was gradually overheating, and bought a new radiator core. I note from the PO's records that he had the water pump and thermstat replaced w/ quality OEM components less than 4 years and 4000 miles ago. [In addition to the aforementioned replacement, the pump/thermostat were also replaced 9 years ago at 115000 miles (It has 127000 now)]. It is hard to believe that any of these components should be failing again so soon. Could another component be the culprit? If the water pump IS bad again, why is it failing so quickly?

BTW, I noticed a light-brownish, powdery residue at the top rad hose juncture w/ the water pump. It is in ball-shaped clumps that turn to powder at the touch. A parts store guy said this comes from using regular instead of distilled water. My fear was that someone put some kind of "stop leak" crap in the system that could have seized the pump, but the PO/original owner was a medical doctor who took the car to quality repair facilities and didn't monkey w/ it himself.

There's also some kind of temp sensor on the top of my water pump that has a "greenish" hue to it. That couldn't cause me to overheat at freeway speed, could it?

Any troubleshooting explanations would be greatly appreciated.

DonB · #2 06-25-2006, 08:16 PM

The problem you're describing sounds like a capacity problem. I had a Honda that did something very similar. If I kept the car under 65 mph it was fine, over 65 it would slowly heat up.

I flushed the cooling system, but eventually had to replace the radiator, which fixed the problem immediately. So, I assume the radiator was clogged, which reduced its cooling capacity. Good luck, I hope this helped,

Don

t walgamuth · #3 06-25-2006, 08:25 PM

have you checked the ignition timing?

if it is retarded it will act like that.

tom w

Brian Carlton · #4 06-25-2006, 08:52 PM

Presuming that this vehicle has a condenser, has anyone bothered to thoroughly clean it in the last 10 years? You'll need to remove the radiator to do a proper job from both sides using compressed air and a proper condenser cleaner.

You can change every component in the cooling system, but the engine will still overheat at highway speeds if the airflow through the radiator is not adequate. You changed the radiator core recently, so hopefully, this culprit is eliminated.

michaeld · #5 06-25-2006, 10:44 PM

Hi guys,
Thanks for your responses.

While it is certainly possible that my radiator could be clogged, I just find it incredible that such would happen in only half a year and 3000 miles. Given the fact that my overheating problem is actually far worse than when my radiator was (40%) blocked, it seems that it would have to be even MORE blocked now for this to be my problem now. When I had the blocked rad, it gradually got hot (to somewhere around 190F, but not much hotter than that, and stayed there. Now it is getting hotter faster, and there is no ceiling but redline. In my (limited) understanding, flushing the cooling system almost never "fixes" an overheating issue; it is merely routing maintenance rather than a panacea when one is overheating.

Now, the latter two suggestions - while quite amazing to my inexperienced troubleshooting ability - are interesting. I cannot tell you how frustrated I would be if I replaced the thermostat and water pump and then continued overheating!!! I had never heard that retarded timing could cause an engine to overheat. Coming from t walgumuth - whom I know is a mechanically smart feller from his posts - I have to consider this a serious suggestion. As for Brian's suggestion that a dirty condenser could be the culprit - again, news to me. I DID know that debris could block the airflow to the radiator and thus cause overheating. But no, I have not checked the condenser. I CAN tell you that - of all things, the PO (again, an M.D.) - was more concerned w/ his creature comforts than anything

During the 27 odd years he owned the car before me, he dumped silly money into his precious AC system!

Looking over the doctors sercice records (he was maximally anal in record-keeping, which I consider to be a compliment. I have every single service record for the entire life of the car

), I can tell you that he had a brand new condenser installed in 95' w/ 108000 miles on the car (it has 127000 now, w/ 3500 put on by myself in the last 7 mos). As for the ignition timing, he had that professionally set in Mar 2004, at 123,000 miles. I have not checked the timing - or replaced any ignition components - as I was "saving" that maintenance item for the next pre-smog test.

Does this sound more likely than a bad water pump? My problem now is I've got the cooling system partially dissasembled - and the car "down" - for the moment, and can't check the timing. I'd hate to put everything back together again, only to have to tear it back apart.

I'm not pathetic, mechanically speaking, as I generally CAN take off part x and install a new part x in its place; but - in the words of Clint Eastwood as Dirty Harry - "a man's got to know his limitations." I am NOT a diagnostic troubleshooter. I need help in this department. Would that there were a blue-suited, red caped do-gooder flying from car to car, and fixing it. Now THAT would be a super-hero!

DonB · #6 06-26-2006, 10:28 PM

Yea, I missed the part about the radiator being replaced. In the spirit of stretching-it, what about the oil. Many years ago I had a Pontiac (Grand Prix) that would slowly overheat during highway speeds.

A guy at the local parts house suggested that the 10w30 oil was too thin. We added a can of STP oil treatment

and it solved the problem. I later switched to 10W40 oil which had a similar (thickening) effect.

Don

michaeld · #7 06-26-2006, 11:15 PM

Donb,
Well, ya got me there; I HAVE been running Pennzoil 10/30 (a quality Pennsylvania grade crude, unlike the garbage that comes from the West coast). Unlike quite a few other members, I use oil rather than synthetics, but religiously change oil every 6 mos (and rarely ever go over 3000 mi in that period).

I have to agree that a lighter oil certainly could lead to greater chance of heat build up, but my overheating issue was far beyond anything minor; it screamed "something is drastically wrong!" Lighter weight oil could lead to minor increases in temp, but not what I saw.

The thing is, I don't have a mechanic I trust to accurately diagnose my problem. There are some gifted mechs out there; and then there are a lot of guys who earn money working on cars. Too many mechanics do the same thing that I would do; try one thing, and if that doesn't work, try something else. It's possible that engine build-up got into the new radiator core; but I could see this problem being the thermostat, the water pump, or something (worse) else.

The heater doesn't seem to lower the overheating temp as it should, so that might point to a bad pump; but then I felt the hoses and didn't feel pressure after the car got to operating temp, and that sounds like a thermostat. But when I took the thermostat out, it opened at 167F (but maybe not all the way? I don't know...). I do know that I hate randomly replacing parts; but at least if I'm the idiot doing it, and I don't have to pay myself labor, and can thereby afford to buy more parts, and increase the chance that I'll replace the right thing. It's such times that one realizes that skilled mechanics are truly God's gift to car owners.

I've heard that a blown head gasket can cause overheating (and vice versa); that a slipping trans can create overheating; retarded timing; air pockets in the coolant from improper burping; and several others. I had no idea that so many things could cause overheating! I've been hoping to come across something that said, "when you experience x set of symptoms, y is the cause." But so far, no avail.

I'm out of town, and therefore away from my tools. I will replace the thermostat, and hope that takes care of it. If not, I will limp it 100 mi home (pulling over twice for an hour break) and start eliminating candidates.

I still hope that someone has had a similar problem, and has "the fix."

Tomguy · #8 06-26-2006, 11:19 PM

DRASTIC overheating on your m117 is the result of 3 possible things:

1) Not enough coolant
2) Blocked passageway to the rad (collapsed/kinked hose, bad t-stat)
3) Bad water pump

If your system is circulating an ample amount of coolant, no matter how hard your M117 is working or revving (within reason), it won't overheat if it's moving. If you overheat rapidly even going downhill with your foot off the gas, you need to check these 3. From the sound of it, #3 is most likely, especially if a low-quality pump was used.

pcmaher · #9 06-26-2006, 11:24 PM

Pardon my ignorance guys, but does this car have a viscous fan clutch? That was the cause of my hot-running problem. While my article relates to a 95 W140, some of the info may help.

http://v12uberalles.com/fan_clutch.htm