Gradual overheating while driving - PeachParts Mercedes-Benz Forum

michaeld · #1 05-20-2007, 02:33 AM

This for a 77 450SEL 4.5L CIS injection w/ 129k.

I've had an ongoing problem w/ overheating that has become worse.

- The car had a new radiator core put in a couple years ago.
- The thermostat and radiator hoses were replaced. The old thermostat tested fine.
- The pressure cap was replaced.
- The cooling system was professionally backflushed and refilled.
- The car has been seen by three different mechanics, last by MotorWerks in Palm Springs. They pressure tested the system, and let it run for two hours and never saw an issue. They assure me that the radiator, water pump, thermostat, fan clutch, and auxilliary fan work fine (as did the previous mechanics). They told me I did NOT have a bad head gasket; nor have there ever been any symptoms of a head gasket issue (the oil and coolant look great; no white smoke, etc).
- The car does not overheat at idle; ONLY while being driven. But it is overheating now faster. What used to take about an hour now takes about 20 minutes: If I don't pull off the road and stop the car, it will gradually boil over.

The car just passed its last smog test with flying colors. Since the Palm Springs area is "enhanced" for smog requirements, I had to do a dyno test along with the idle test. Here are the results:

Test M1: 15 mph
RPM: 1691
%CO2: 12.6
%O2: 2.9
HC ppm: 17 (with 184 allowable)
%CO: 0.00 (with 1.21 allowable)
NOx ppm: 258 (with 1248 allowable)

Test M2: 25 mph
RPM: 1671
%CO2: 12.5
%O2: 3.0
HC ppm: 17 (with 151 allowable)
%CO: 0.00 (with 1.01 allowable)
NOx ppm: 216 (with 1108 allowable).

Is it possible that I have a problem with my fuel injection? Is 0.00 %CO normal? I replaced the fuel filter recently, but if anything experienced even more problems.

P.S. I replaced my spark plugs. The insulators and electrodes looked quite good; all I saw was moderate carbon deposits particularly on the end of the threads. When I replaced the plugs, I did a compression test: 160 psi for all but the #6 and #8 cylinders, which tested at 165 and 167 psi, respectively.

What's my problem?

ozawa · #2 05-20-2007, 03:25 AM

a few things things come to mind:

you're running too lean, (lean mixtures run hotter.)

perhaps your timing is out of whack? overly advanced timing will cause your car too run hot too.

fan belts tight enough?

belt driven fan facing the right way, ( i've seen this before where someone put the fan on backwards.)

50/50 coolant to water ratio

air pockets in the cooling system?

michaeld · #3 05-20-2007, 04:44 AM

Thanks for the post, Ozawa.

I actually retarded my timing 2 degrees for the smog (in fact, I'd intended to set it to TDC [as per spec] and forgot to do so). Prior to resetting, it was about 2 degrees advanced.

I am running about 35% antifreeze with Water Wetter, which should give me better cooling than 50/50.

If the fan is on backwards, it took several years for it to become a problem, so I doubt if that's it. And the belt tensions seem fine (and the car's been seen by several mechanics).

You raise two issues that seem like they could be the case: air bubble and fuel mixture.

I've had the system backflushed and refilled, and changed the coolant and refilled, more than once without result. Is it possible that I have an air bubble that has been resistant to flushing and draining/refilling? I've drained and refilled the coolant a good 5 times since I started overheating (and burped the system by a) filling the system; b) starting the car on an incline; c) waiting for the thermostat to open and then adding more coolant to fill the system). If so, how on earth do I get rid of the dang thing? If I've had an air bubble all this time, the thing is dug in like the proverbial Alabama tick!

If I have a fuel mixture or fuel injection issue, would there be any other signs of it besides overheating? My acceleration seems fine. The car idles smoothly. I'm not sure about whether my fuel mileage is down or not; I'll have to check (I haven't been driving the car a lot due to the overheating thing).

nhdoc · #4 05-20-2007, 08:50 AM

I'd bet you a quarter it is the thermostat. You should have replaced it instead of testing it. It's a cheap part and prone to failure. That would be my next try if I were you.

Arthur Dalton · #5 05-20-2007, 10:08 AM

Quote:

Originally Posted by nhdoc

I'd bet you a quarter it is the thermostat. You should have replaced it instead of testing it. It's a cheap part and prone to failure. That would be my next try if I were you.

I agree..and Behr only.

Tomguy · #6 05-20-2007, 10:42 AM

He said it was replaced. Of course, it's possible it was replaced with a bad one. It also should go in with the vent hole facing up, so air can bleed from the system easier when it's put in.

You are way too lean. You're supposed to have at least 0.05% CO on that engine. You're in the danger area where valves can burn. Mercedes calls for 0.05-2.00% CO.

Arthur Dalton · #7 05-20-2007, 10:45 AM

<>

...yeah, or a Wahler ..I see them bad all the time .
Behr is the only way to go on Benz..

nhdoc · #8 05-20-2007, 11:33 AM

Given your description of the problem the only cause I could think of is a restricted flow which is most likely caused by the thermostat not opening completely enough. Like others have said, use only OE or Behr brands in these. While some brands open when tested they may not open wide enough for this particular cooling system. Assuming the T-stat was replaced with an OE one then I'd be stumped too since it sounds like you have addressed all of the usual suspects.

Chas H · #9 05-20-2007, 12:27 PM

Was the thermostat removed for testing?
The symptoms you describe are of a impaired radiator. That it was recored is not proof that is trustworthy. Get it tested with an infrared thermometer while the engine is running and headed for overheating.
And BTW, a fan installed backwards will still move air in the correct direction.

ozawa · #10 05-20-2007, 03:41 PM

you could be running too lean yet not really notice it until you get to the point of a lean misfire. by your HC readings, i'd say you haven't reached the point of a lean misfire but i would richen up the mixture. like tomguy said, you don't want to be burning valves.

to bleed these cooling systems, make sure you turn the heat on high and i always refill these with the ignition on so that the auxillary coolant pump is pumping, (provided your car has one.) let it run until the thermostat opens then shut it down but don't cap it yet. let the car cool down then top off the coolant and cap it.

this is rare but i've seen it on cars whose cooling system has been neglected. i've seen cars do what you car is doing and the culprit has been the waterpump itself. the actual impeller on the water pump can corrode away to the point where it can pump enough volume at idle but not enough once out on the road. i've also seen the impeller slip on it's mounting hub causing the same problem. the bad part about all of this is that it's no simple task to pull the waterpump to take a look at the impeller. i'd consider this step a last resort after making sure everything else in the cooling system is working as it should.

michaeld · #11 05-20-2007, 09:27 PM

As Tomguy pointed out, I DID replace the thermostat - with a new Behr unit - after testing the old one and then testing the new one to compare to the old one. Both the old and the new thermsotat tested fine. And I did in fact install facing just as the picture in the manual indicated. My thermostat is not my problem. It opens at the correct temperature.

Chas said I should test my radiator with an infrared thermometer. I bought one (also have one with a thermocouple). And I put it on the radiator. I could try it again, but I posted the readings taken at various points way back when and that didn't seem to be my problem. If I DO have a blockage in my radiator, what would I expect to see when I test it?

I've been assured repeatedly by every one who has seen the car that the waterpump is working fine. It is not leaking or squealing. I myself have wondered how they know that with such certainty, but I really don't want to just start replacing one part after another. I only have so much money to put into the car, and want to have a more targeted approach.

I can't be positive, but the car has been seen by mechanics from 2 radiator shops, and it was diagnosed by MotorWerks. All assured me that the basic items such as radiator, water pump, thermostat, fan clutch, and aux fan were fine. And I would reasonably expect that they would catch such problems (well, maybe not the mechanics who installed the recore and didn't want to have to replace it!).

Tomguy (who is the "Bruce Lee" of the vintage forum, IMHO

), says that - based on my dyno smog test that I am "way too lean" in terms of my CO%. And now I see Ozawa has seconded the motion. Hey, that's getting somewhere. It sounds like I should go get the fuel system professionally adjusted and see if that fixes my problem? Or should I just turn the CO adustment screw a little and see if that helps first? (I don't have anything to measure my fuel emissions).

Thanks for the help. I would really like to get to the bottom of this.

Arthur Dalton · #12 05-20-2007, 09:46 PM

Just as a side note , I would verify the thermo temp rating b/c that chassis calls for a low temp thermo [ 75C]..so you may want to check that is what is in there , if you have not already done so... and I always burb the system for air b/c the air cavity always winds up on top of the thermo, thereby effecting it's sensing abilty..that does not show up on a thermo/water test, of course..
Burbing can be done with gloved hands ..slowing squeezing the top radiator hose and slowly releasing., 5/6 times is sufficient....not too fast or you just push the air cavity around. You want the water entering the hose to pull the air from the thermo housing as you release the hose...nice and slow, let the coolant work by itself.....

michaeld · #13 05-20-2007, 10:11 PM

Hi, Arthur,

Y'know, I'm pretty sure my thermo IS a 75C (danged metric!); I know it is on the low range of what was listed when I checked for my year/app.

Interesting you bring that up, because I mentioned I was overheating to the guy who did my smog (AFTER I passed, mind you

), and he suggested running a HIGHER temp thermostat to cool the car (e.g., not 75C, but 90C). I would have thought lower was better myself.

In any event, this problem is so bad that I can't imagine that a thermostat that is within the spectrum of correct application for a given car can create this much of an issue.

Arthur Dalton · #14 05-20-2007, 10:27 PM

Yes.
Well. I do not know the/your exact problem cause , but I thought I would mention this b/c sometimes these simple things get over-looked as people are looking beyond the basics. I have had many a car that came back after repairs that did in fact need nothing more than a burb and I have had many a car that was remidied by replacing a thermostat that would check fine in a pot of water...
I am sure you will find your problem and I do hope you don't regard any of the post as having no value to the possible remedy.

michaeld · #15 05-20-2007, 11:29 PM

I definitely do not want to imply that any posts from people trying to help me are not wanted. I NEED help, as I'm at a point where I don't know what to do. Obviously, when a number of people are posting possible solutions, only one answer is likely to be "IT." I definitely need help in finding what "IT" is.

There are - as we know - a number of basic "culprits" that are renowned to cause certain problems. I replaced my thermostat when I first experienced overheating because bad thermostats are the primary cause of overheating.

The thermo that was in the car (I just went out and looked in my "Old car parts' box) was a Wahler 75C; I replaced it with a Behr 75C unit. I tested both units in boiling water, and found both to be working fine. I have to move on and figure that something else is probably causing my issue.

It's been very frustrating to take the car to mechanics, and have them tell me everything is fine, and then continue to overheat! I'm willing to entertain anything, I assure you.

I re-read your previous post, and saw your description of burping (I presume). I'd never heard of squeezing the hoses to alleviate air pockets. The MBz Tech Companion talks about removing the bottom hose at the thermostat and filling it there, and I haven't done that. So your idea of squeezing the hoses would seem to make sense in light of that.

FYI, my thermostat is on the lower hose. You mention squeezing the upper. Would I still squeeze the upper hose? And you mention gloves, which makes me infer the engine is running. Is that the case?

As a PS, my heater works fine, so I wouldn't think I had a bubble there. And I've drained and refilled the system several times, always with the same (bad) result. If I had a bubble, would one expect it to work it's way out of the system, or would it just move in and haunt me forever and ever? I've drained/flushed/refilled quite a few cars, and never had a problem. Are these Benzes that much more finicky than others?

But I also keep coming back to my 0.00 carbon monoxide percentage, and wonder whether there's such a thing as emissions being TOO low - especially in a 30 year old car.