1984 300d Turbo 135k
Air In System Remove The Top Radiator Hose Fill With Water

As a damn good M/B tech, I think you have a combustion leak into the system. Maybe head gasket, maybe cracked head. I would monitor the cooling system pressure closely. Release the pressure off of a hot engine and hook up a gauge to it. Rev the snot out of it for a bit (2000 to 4000 RPM) and observe if the pressure rises. A hot engine wouldn't have much thermal expansion so a rise in pressure on a hot engine indicate a combustion leak. That means the head comes off.

This thread is from 2001. It was a cracked head as I recall.

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

I went through all of this and replaced everything (raditor, temp sensor, water pump, fan clutch) and still the temp would go up over 100c. Than I ran into a mb tech on the golf course and he had me check the wiring from temp sensor to the cluster. I soldered a new end on the temp sensor AND ran a new ground from the cluster (back of temp guage) to the body. Now the car runs at constant 90 t 95c and I live in south florida. The ground was the last thing that I did and that solved the problem.

1984 300d Turbo 135k
Take The Top Radaitor Hose Off Fill To Top, You Have Air In The System Had The Same After I Change My Radiator And Replace Or Clean The Bypass Small Line On Front Use New Copper Seal Washers, A Kit From Jc Whitney.com Also Rubber O Ring Metic Kit One Fit The Leaking Alda

First, you need to relax. 100 Deg-C on an 84 300D is really not that hot. I have owned several of these models and over 250k miles or so they tend to run around 100 Deg-C. The issue is probably not with the coolant system anyway. In these vehilces, unless you have an obvious problem with the cooling system, running too hot is usually a sympton of poor combustion. I recommend adjusting your valves and see what happens. This is a fairly easy job for a DIY-er, but you will need the special wrenches and a good set of feeler gauges. Replace the valve cover gasket as part of the job. The next thing I would try is running some "Diesel Purge" through the fuel system, or better yet, have your injectors professionally cleaned (this is a lot more $$$). I bet you engine smokes a little on acceleration. That's another indication of poor combustion. But don't worry about 100 Deg-C. Until you start to get up to about 120, you will not damage the engine.

Check out the diesel forum for guys who know more than I about this.

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KCScout
98 S420 My Wife's Baby
97 S320 My road car
83 300D My old road car (love that diesel)
81 300SD Wife's former Baby
81 240D A fun stick shifter
71 220D My first one

Hi RTH:

I don't know if your car has a catalytic converter or not but if you do, you might want to check that to see if it is clogged.

Aloha,
Eric

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1987 260E Charcoal Grey 157,000 Miles
1987 260E Forest Green 120,000 Miles

A couple of ideas: check condition of water hose on suction side of water pump. perhaps it is collapsing when the car is running, but appears ok when you look at it; are you certain you are actually getting all the air out of the system? The dealer had a heck of a time getting all the air out of my w124 after a radiator waterpump replacement under warranty. The car ran warmer than usual but not hot until they got it out. They had to raise the front of the car on rampstands and warm it up, then turn the heater to full hot.
One would think that this wouldn't be necessary, but it was.
Also, maybe I read too fast, but where are you? In Fort Worth it is 102deg right now and there is no way my car is going to run at its "normal" operating temp.
As far as the hose idea goes I may be all wet since I don't know the config of the 300 cooling system.
Let us know what happens
Steve

RTH,
I looked in Glenns 'Mercedes-Benz Repair and Tune Up Guide'
for additional ideas that have not been posted yet, and he came up with a couple. For overheating:
1. Fan belt slipping
2. Rust and scale in cooling system
3. Exhaust leak into cooling system
You stated that you flushed the system, but the water jackets may be clogged in the engine.
A pump-type pressure test on the cooling system will detect a exhaust leak by fluctuating dial at about 7psi.
Also check to see what is between your condenser and the radiator. On an Isuzu diesel I owned I found a piece of cardboard had been put in for severe cold operation.
Good luck
Steve

First, I want to thank everyone for their input on this matter. I really do appreciate it, plus all of these suggestions will serve as a guide for other people in the future when they search the archives.

Mike Murrel: the radiator is the EXACT same as those from the dealer with one exception - there is no ‘Star’ on the upper tank. Behr makes these radiators for MBZ. Oops, make that two differences, the price premium for the ‘Star’ isn’t included on the Behr OEM unit. Good try though.

Smalltime: Nope. I’ve seen that before myself. Why do people use sealants as though they need a bead the size of that which they squeeze out of their toothpaste tube? Unbelievable isn’t it? BTW, this engine is a virgin. But then again, they all say that.

Etsa: There is no converter or trap oxidizer on this model.

Steve Hutson: The hoses are brand new & O.K. Plus the pressure in the system tends to ‘balloon’ them out which further keeps it from collapsing. You are quite correct as to the ambient temperature: it is about 90 here today, and I don’t expect the car to run really cool, but the temps I described occur even at ambient temps in the 60s. There is no rust or scale in the system, nothing between the condenser & the radiator, and the belts are also new & not slipping. As far as the exhaust leaking into the system, that seems to be the only thing left that I can think of. However, if the head gasket were bad, I would have other symptoms such as *high* pressure in the cooling system which would then blow out through the pressure cap. Plus I am not loosing any coolant, and the system holds pressure in it overnight.

I REALLY wish that someone like Stevebfl would chime in on this thread.

THANKS EVERYONE!
RTH

Really hard to get involved with seamingly such a straight forward problem. The answer is often in some assumption associated with casual observation.

Does the aux fan turn the right direction? Seen it twice that the polarity was reversed.

After being proded I read all the good suggestions and have been waiting for the results of the new pump. The depth and design of impellers are very critical and I don't think MB reuses the impellers on their rebuilts.

You are within ten degrees or so of normal operation, could your gauge be off.

Timing does make a difference with diesels. I hate to send you on a wild goose chase but timing would also include cam timing. Retarded cam timing will overheat a diesel quickly.

Anything that reduces power will increase heat. Do you get good boost? The car should be running at a few pounds of boost at constant speed above 50mph. Check the boost at the aneroid because lack of the signal here causes reduced fuel and less performance. You should have 9-10psi boost at full throttle.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Steve,

I know that it is hard to make a judgement from afar, but I am wondering if this is how they behave when the head gasket *first* starts to go bad. That’s the only thing that I can think of other than the IP timing. I know that assumption is the kiss of death, but I WAS assuming that the pump timing is OK because the cam timing is Perfect – I checked (visually, not with gauges) when I did the valve adjustment about 1 month ago (When I first bought this car). But having said that, I will check to see if it has drifted (or IF the timing chain was replaced at some time, & perhaps the pump timing was incorrectly set).

Fan polarity is correct. The new MBZ pump made NO difference at all. In fact I saw 107-108 on the gauge today, although it was 95 degrees outside today. The temp gauge is Right-On-The-Money. Checked & verified by using an infrared non-contact thermometer on the engine right where the temp sender is located. I am getting boost at the aneroid, and my car peaks at a little over 600 mbar. I haven’t adjusted the wastegate yet, until I get the heat problem under control. (Plus I have a Garret turbo – major pain to adjust. I am thinking about an external pressure regulator instead.) There is constant (low) boost at cruising speeds of 40-50 mph & up.

By the way, I think that MBZ does reuse the impellers, as the one I got does not look like a new casting.

If there is anything else, please let me know. I will try to check the IP timing this weekend, but unfortunately, now *I* am running too hot – coming down with a nasty cold.

A debt of gratitude to you Steve!

RTH

Runningtoohot,
When you first said you installed a new Behr radiator, Iwas going to suggest you shut the engine off after a long test drive,and check the entire surface of the radiator with your hand to see if it was equally uncomfortable to touch. I have gotten bad Behr radiators,but you dispelled that thought with the non contact infrared tester. I caution you to bear in mind that the field increases with distance. The only way to get accurate readings with that unit is to be within about 4 inches.The red dot you see is only a laser pointer:the unit actually operates on infrared. I now have to agree with Stevehutson:you need to add more water.I had a simmular experience recently with a 300SD.I had to keep adding water even though the expansion tank appeared full. Good luck.

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Auto Zentral Ltd.

RTH,
The cooling systems on the 5 cyl diesel does not bleed itself out that well. When I fill a cooling system on one of these engines I remove one of the coolant sensors on top of the thermostat housing and pour the coolant mix in (the reservoir) until I have coolant coming out of the hole. You will be amazed at how much the coolant temp will be affected because of an air pocket in the system.

Also if my memory serves me correct the single pole coolant sensor on the thermostat housing is in fact the 212F switch for the aux fan.

HTH

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Scott Diener
89 300E
93 300E
92 Volvo 740 Wagon aka "Mutt mover"

Autozen: could you please tell me more about the “bad” radiators that you have gotten from Behr in the past? What was wrong with them? (This won’t matter in my case, because the radiator is functioning properly, but I’m curious.) I am aware of the distance/field of view aspects of the infrared thermometer. This one has a distance to spot ratio of 8:1, and I *do* use it close up. There is also parallax error to deal with if the laser is used for aiming.

As far as adding more water? I’m not sure what you mean – the system is full. If you are referring to the antifreeze/water ratio, it is 40% antifreeze and 60% water, so I am not going to dilute if further. Perhaps you can elaborate further on your 300SD example. Why would you need to keep adding water unless you were losing it somehow?


Chowpit: When I fill the system, I use the upper radiator hose – I disconnect the radiator end of it and fill up the block & head this way. It is the same thing – it is the high point in the system. I can see on the service CD that the earlier diesels had a different configuration. Plus, this later system is supposed to be self-venting.

The thermo switch in the top does NOT control the auxiliary fan on this model (1984 300D California version). I know that every other MBZ I have (and had) did have a 212 switch where you mentioned. The 1979 240D wiring diagram that I just looked at has a 212 switch also. I don’t know what they were thinking at the factory when they decided to eliminate this from the system.

Thanks Guys

RTH