overheating creep - PeachParts Mercedes-Benz Forum

biobenz617 · #1 08-14-2007, 10:43 AM

Hi,

I've had this problem for a while now and am wondering what is the main cause. The car regularly runs between 90 and 95 C on the highway; hotter the faster I drive. Yesterday I pushed it up to 100 C going up a hill at 70mph

I drove my wife's 300D-T up the mountains a month ago going about 70mph and that car stays around 85 to 90 C no matter what.

I changed the coolant (zerex) and replaced the thermostat 10k ago, but no improvement. Now that I want to increase the power, I really need to fix the problem.

Is it fouling of the radiator with deposits, or could it be an inferior water pump? The P/O replaced the water pump before I bought it and the receipt was from Checker...probably some cheap knockoff part. I also was going to try the true flush method (degrease, citric acid wash, rinse, M-B coolant) but was wondering if it was the water pump. Anybody have ideas??

blindwolf · #2 08-14-2007, 11:33 AM

Changing the water pump would not be my first choice. It is a mechanical unit and either works or dosen't. If it is not working, you will know pretty quickly. It is pumping what ever is availible to it. I would look at the fans or external blockage of the radiator cooling fins first.

MBeige · #3 08-14-2007, 11:40 AM

I recall mine became hotter as I sped up due to a bad thermostat, just like the symptoms your car is showing. Replacing that fixed the problem.

biobenz617 · #4 08-14-2007, 11:45 AM

Interesting. I've already replaced the thermostat due to my first suspicion. No change at all though.

dmorrison · #5 08-14-2007, 12:41 PM

It's either the water pump of the radiator. Go to Harbor Freight and by the cheapo non contact thermometer. ( I think it's on sale now 14Aug07 $9.99) Scan the radiator and see if you have consistent temps over the entire radiator. If the radiator is collapsing, the radiator unit will not be able to cool sufficiently and the temp will creep up, especially under a load.
If the radiator is fine then it is possible that it could be the water pump.

Dave

C Sean Watts · #6 08-14-2007, 12:52 PM

Do you know how many miles are on the radiator in the car now?

One thing to note - Even if you use the correct coolant and flush/change it on time, the inside of the radiator will develop a ceramic like lining of mineral and metal ion deposits. EVEN IF passage tubes are open, fractions of a mm are enough to slow heat transfer. Regardless of aluminum, brass or cast iron (heating radiators in old buildings) it does happen even if you use 100% water. Some small cars are bad in under 100K miles and some commercial vehicles are good for well over twice that much.

Douglas.Sherida · #7 08-14-2007, 06:26 PM

As always with engine temp issues, you should verify that the temp gauge is reading true. Measure it independently, replace it, or swap it out with the one from your wife's car.

Assuming that the gauge temp is correct, the fact that it gets hotter as you drive faster suggest insufficient coolant flow.

Checking the water pump is fairly easy, you just need to pull it (it can easily be done in an afternoon). It costs less than $5 (for a new water pump to housing gasket), drain all the coolant (radiator and block), remove the radiator, remove the fan/fan clutch, loosen the belts, then remove the water pump. When you discover that the waterpump is fine (as it probably will be), you can look for constrictions in coolant flow (scale buildup or crappy thermostat).]

Scale buildup can only be addressed by citric acid flush and/or aggressive flushing of the block.

Thermostats seem to be a crap shoot lately. Put in a new one and it may or may not work correctly. But you are lucky. You have a known good thermostat in your wifes car. Use that sucker for troubleshooting.

biobenz617 · #8 08-16-2007, 08:47 PM

Many variables here

I'm going to first try the real flushing method out of the manual and see what happens. Just picked up 1 lb of citric acid today. Any suggestions for the degreaser? I have some biodegradeable, nontoxic degreaser (Krud Cutter) that I believe is a "neutral" degreaser - only surfactant, not NaOH which is verboten by the aluminum rad.

biobenz617 · #9 08-16-2007, 08:51 PM

Quote:

Originally Posted by Douglas.Sherida

As always with engine temp issues, you should verify that the temp gauge is reading true. Measure it independently, replace it, or swap it out with the one from your wife's car.

Can I test the temp sensor by measuring resistance at known temperature? What are those values?

Thanks,

bellyboy · #10 08-16-2007, 09:54 PM

dont know if you can test them-you can check the wiring they are about $30 at the dealer

i would recommend as others have to get a digi thermometer

bgkast · #11 08-17-2007, 12:38 AM

Quote:

Originally Posted by dmorrison

Go to Harbor Freight and by the cheapo non contact thermometer. ( I think it's on sale now 14Aug07 $9.99)

I just looked for one and could only find a $60 one.

biobenz617 · #12 08-26-2007, 02:09 PM

While driving up a canyon with the 10% citric acid flush and no thermostat, I got the coolant again up to about 95-100C under sustained hard throttle in 3rd

Driving down the hill, I only dropped to about 75C (normally would stay at 90C).

I think I can scratch the thermostat and possibly the temp sensor off the list because I'd expect the coolant temp to be noticeably lower without the thermostat during similar driving conditions. Even if the temp sensor is off calibration, it would still read a lower value at lower temperature.

I have noticed that there is about 200 rpm difference between full and no throttle at highway speeds and been wondering if my torque converter has excessive slip. My wife's 300DT has no noticeable drop in engine speed when on/off the throttle at same speeds. Would excessive transmission heat rejection inside the radiator be enough to keep the engine temps this high

#13 08-26-2007, 02:13 PM

Driving without a t-stat will make overheating worse. It NEEDS a t-stat to flow the coolant in the right directions.

CamelotShadow · #14 08-26-2007, 06:59 PM

I'm far from an expert but the way I understand it
If he's running wo thermo wouldn;t it be an open system with engine water continually flowing through radiator & some also flowing back into engine so it would be a bit warmer than a stuck open thermostat.
Perhaps that is what what meant.

Isn't it sort of a temperature controlled floodgate?
When temp reaches temp of the rated thermostat it opens pushing a plunger down into the retuen port & allowing hot water into radiator allowing it to cool down. Now I am not quite sure what process controls the shutting of the thermostat but I do know if it gets stuck closed or sticks the water will not be sufficently cooled by the radiator leading to higher & higher temps.

Just took minein for this problem & it was a sticking thermostat.

Could have gotten a defective thermostat. Its such a easy fix.
If I recall you can test the thermostat.
put it in hotter & hotter water & see what temp it opens at.
This way you can rule that out before you start all that other repair.

#15 08-26-2007, 07:22 PM

No, this isn't a 350 Chevy engine. Its a mixing valve, coolant will not flow directly to the radiator without it.