1987 300D is running too hot! Where do I look first? - Page 3 - PeachParts Mercedes-Benz Forum

firemediceric · #31 10-11-2006, 09:38 AM

Well, I guess I'll be scheduling with my mechanic to have him do the compression and leak down test.

It may not be related at all, but my oil pressure is pegged to the top when I'm driving. I guess that doesn't indicate that the engine is healthy and has good compression?

It's really puzzling, as the car runs fantastic, once it warms up.

I wish I had a good mechanic that knew these cars, but no one has been recommended to me in this area

gsxr · #32 10-11-2006, 10:42 AM

A compression & leakdown test likely won't show much of anything. The oil pressure should be pegged anywhere above idle, that's normal, but only indicates that the main bearings are not worn. If you are continually losing coolant internally, my first guess is a head gasket, as the 617 isn't known for cracked heads. If it was a one-time problem, forget it. I assume you have a new or recent radiator cap?

BTW, the electric fan on the W126 (and W123, 124, 201, etc) does very little for engine cooling, it is there almost exclusively for the A/C system (to provide extra airflow at idle, when the main fan isn't doing a whole lot at low RPM). The main mechanical fan provides 90%+ of engine cooling. For some years/models, the electric fan will NEVER turn on based on engine temperature (for example, the 82-85 300D), only based on a signal from the A/C system.

You should probably start a new thread under the topic 1981 300SD, since your problems are in no way related to the 1987 300D...

firemediceric · #33 10-11-2006, 11:41 AM

Thank you all for the input. All of you are a lot more knowledgeable than myself in this area, yet there seems to be some difference of opinion as to the best way to diagnose the problem.

I think I will definitly send the oil off for analysis, but not rush right out to drain the fresh oil to do so.

I will take rrgrassi's suggestion of looking for bubbles in the reservoir as the engine comes up to temp.

Some feel that a compression and leak down test will tell the tale, others are not so sure. I will ask my mechanic (who is great on my F-350, but has little experience on these old MBs) his opinion, but once again I may not call him today, as I want to see how much coolant is forced out of the reservoir, through the overflow hose and into the little catch bottle I made.

I may pick up a new radiator cap some time this week, just to rule it out. If the cap were the problem, wouldn't I see moisture on the reservoir around the cap? Also, wouldn't the cap not hold pressure?

I'm glad gsxr straightened me out on the aux. fan, before I pursued that further with wrong expectations in mind.

I had hoped to avoid starting a new thread and cluttering up the forum, as I thought I had a simple issue that I was just too ignorant to recognize.

rrgrassi · #34 10-11-2006, 01:59 PM

Quote:

Originally Posted by firemediceric

I may pick up a new radiator cap some time this week, just to rule it out. If the cap were the problem, wouldn't I see moisture on the reservoir around the cap? Also, wouldn't the cap not hold pressure?

The cap holds pressures to 18psi IIRC, Mine is by JOST and I do know what it's rating is. If the pressure exceeds what the cap will hold, then the spring will be forced open and excess coolant or pressure should be vented via the vent tube that is attached to the filler neck of the resivoir tank. You might not see coolant on top of the tank it self, and since it is in a wet environment, you will more likely yhan not see coolant around the cap seal and filler neck.

firemediceric · #35 10-11-2006, 04:14 PM

rrgrassi,
You educating me on something so simple may have just solved the problem.

If I am understanding you correctly, a bad/worn out radiator cap would allow the coolant to exit the overflow hose, as I am seeing idications of. Am I correct?

Why would I be feeling and hearing the slight "POP" when removing the cap, even when the car has sat overnight, if the cap is bad and not holding the pressure it should?

rrgrassi · #36 10-11-2006, 04:45 PM

Quote:

Originally Posted by firemediceric

rrgrassi,
You educating me on something so simple may have just solved the problem.

If I am understanding you correctly, a bad/worn out radiator cap would allow the coolant to exit the overflow hose, as I am seeing idications of. Am I correct?

Why would I be feeling and hearing the slight "POP" when removing the cap, even when the car has sat overnight, if the cap is bad and not holding the pressure it should?

Glad I can help. I get so much info from this forum, it's good to able to give back.

Yes, you are correct, but if pressure exceeds, then the cap is supposed to allow escaping, but I have seen scale and rust buildup on the cap that does not allow pressure to escape resulting in other part failures like W/P seals, etc, as the pressure cannot escape, except from the next weakest link.

The popping should not occur at cap removal, and if the pressure is below the cap's rating, it will not escape. Is the upper hose hard to squeeze cold like it is after a good hard run on the highway? The upper hose should be easy to squeeze when the engine is cold, and cap on the tank. If the hose is easy to squeeze, then odds are in favor of not having combustion gasses pressurizing your cooling system.

If pressure is there, you should get a hissing at the first indent at cap removal, and even more hissing, or nothing at the final indent, when you pull the cap off the tank, like when you remove it when hot. The first indent is designed as a pressure release so you do not spray hot coolant all over you.

Where does the pop originate? It just be the rubber seal flexing on cap removal, kind of like a cork on a wine bottle. No pressure, but still a pop.

firemediceric · #37 10-11-2006, 05:14 PM

rrgrassi,
I checked what you suggested and came back with what I think are mixed results.

The car has been sitting for the past 9 hours, so the engine is cold. The top radiator hose is soft and pliable. I can squeeze it easily. Good?

When I went to remove the radiator cap, it hissed and spurted some coolant, spraying it across the top of the reservoir. Bad?

Since some coolant has been seen dripping out of the over flow tube, and at the same time the cap is holding pressure, doesn't that mean the cap is good?

On a positive note, the coolant level did stay where it was in the reservoir on the 40 minute drive to work early this morning. I hadn't lost a drop to the catch bottle I set up at the end of the overflow tube or any where else, as near as I can tell. The car also never got much above 80*

rrgrassi · #38 10-11-2006, 05:32 PM

Quote:

Originally Posted by firemediceric

rrgrassi,
I checked what you suggested and came back with what I think are mixed results.

The car has been sitting for the past 9 hours, so the engine is cold. The top radiator hose is soft and pliable. I can squeeze it easily. Good?

When I went to remove the radiator cap, it hissed and spurted some coolant, spraying it across the top of the reservoir. Bad?

Since some coolant has been seen dripping out of the over flow tube, and at the same time the cap is holding pressure, doesn't that mean the cap is good?

On a positive note, the coolant level did stay where it was in the reservoir on the 40 minute drive to work early this morning. I hadn't lost a drop to the catch bottle I set up at the end of the overflow tube or any where else, as near as I can tell. The car also never got much above 80*

Top hose soft and pliable cold is a good thing. That is how it should be. Your cap could be good, but if it is weak, then is is not letting the system pressurize normally. That is bad, as the pressure helps keep the coolant from boiling, which results in air pockets.

You can also get overflow like that if your tank is too full. No room for expansion will results in higher pressure, so the cap the acts as a pressure relief. It is possible to have a little pressure that can cause hissing and spraying of coolant when cold. That is also caused by the cap and the rubber seal holding onto some coolant that did not drain back into the tank, or the drain tube. If the drain tube is restricted, that can give the same results, but it is caused by a vacuum instead of pressure.

Try replacing the cap, and check your drain tube for a restriction. Keep an eye one the coolant level.

Diesel Dan · #39 02-16-2007, 10:56 PM

I just noticed that I never did resolve this thread.

Boy do I feel like a dumb-ass! My fan blade was lodged against the radiator. As I tried to move it, a chunk of one of the plastic blades broke off, and the fan was freed. Now I wonder why my fan was in contact with the radiator all of a sudden, when it was spinning freely before?

gsxr · #40 02-17-2007, 12:42 AM

Quote:

Originally Posted by Diesel Dan

My fan blade was lodged against the radiator. As I tried to move it, a chunk of one of the plastic blades broke off, and the fan was freed. Now I wonder why my fan was in contact with the radiator all of a sudden, when it was spinning freely before?

The front radiator support on these cars bends inward VERY easily. If you tapped a curb with the front of the car when parking, this pushes the bottom of the radiator into the fan blade. With the metal fan blades, this destroys the radiator. To be fixed properly it really needs to go to a body shop to have the support pulled out. The lower engine shield also won't fit properly, as the holes won't line up anymore.

If this is what happened, you need to fix it ASAP. You can attempt a partial fix using a bottle jack against the support, braced against the sway bar, with a wood block between the sway bar and oil pan. But it won't fully correct it, and it's not the right way to fix it. I'd be surprised if this wasn't the cause... I've had this on both of my '87s.