strange cooling problem - PeachParts Mercedes-Benz Forum

Bruce Kennedy · #1 11-29-2006, 10:32 PM

I recently changed my belts, hoses,thermostat, and changed the coolant on my 85 td. This was in preparation for an almost 700 mile road trip. The car ran at the proper temperature for the trip untill I was about 1 mile from dropping of Andy at his house. After pulling over and running the heater the temp came back down. I replaced the clutch fan thinking this was the problem. Two days ago I drove to Columbus (two hours away), no problem untill the return trip. The temp went up to about 110. I Ran the heater and it dropped back to 90, but then slowly went back up to 110. This was with the heater running full blast. It stayed at 110 for about twenty minutes and then returned back to 90. The weird thing is when I check under the hood when this is happening, the coolant in the reservoir is cold. Yesterday this happened on a short drive so today I swapped the radiator from another car that is running at the correct temperature and it made no differance. I replaced the new thermostat and it made no difference. This problem seems to have no pattern and has me stumped. I wonder is it possible that there is something floating around in the engine that restricts flow randomly?

The mileage is only 50k on this engine. The cam timing and IP timing are perfect. Valves are adjusted properly. When I start this one on a cold morning I never have to even touch the accelerator.

Sorry this went on so long. I needed to vent.

bgkast · #2 11-30-2006, 12:26 AM

Perhaps your gauge is in error?

Brian Carlton · #3 11-30-2006, 01:03 AM

The SD does similar behavior but it's climb is limited to about 99C. On random occasions, usually when the engine is running slower......say 2000 rpm or so.........it climbs up near 100C. and then, over the course of 15 minutes, it slowly drops back to 90C.

I can't discern a pattern to this behavior and it has nothing to do with the ambient temperature and it has nothing to do with the thermostat (replaced).

I need to perform some additional checks with the infrared temp sensor after I get a good baseline of the cylinder head temperature with the gauge reading 90C.

I also have a suspicion that it's gauge related somehow.

300SDog · #4 11-30-2006, 01:05 AM

Meanwhile water pump life expenctancy is typically 60-80k miles..... can be insidious too with cheap cast-iron blades on the pump corroding down to nubs while bearings are solid and no leaks, noticed this when replacing the pump on my 240 - now at 190k miles with 2 WP replacements making 3 WP's total in its lifetime.

And combined with what Brian just said about temp being rpm related worn-out WP blades make sense to me.

sailor15015 · #5 11-30-2006, 01:42 AM

Are you sure you got all the air out of the system? When I was having cooling problems, it would randomly shoot up to 110 then settle back down, then back up again for about a week while my coolant level would randomly drop. I keep adding and feeling the upper hose. Eventually it felt full of water and my coolant level has been steady for three weeks now so I think I finally got it all out.

hey_allen · #6 11-30-2006, 02:12 AM

I haven't experienced this with the MB, but on a previous car that also had the thermostat near the top of the engine, trapped air would settle around the thermostat, preventing the hot water from opening it.

On that car, I finally ended up putting a drain valve up on top of the thermostat housing to aid in bleeding the system, and had no more problems.

I'm not saying that you need to drill and tap a hole for a valve, but you might want to loosen the hose at the housing and see if there is some air you can burp out of your cooling system.

Bruce Kennedy · #7 11-30-2006, 09:41 AM

I dont think the gauge is bad. I can feel the difference between 90 and 110 on the gauge when I lay my hand on the rocker cover. If it was trapped air or a wp would it run 700 miles without over heating? My problem is also not rpm related.
Some of the marine diesels I've owned had bleeder valves in the system but on these cars I would think the little hose coming off the radiator going to the expansion would bleed any air, since this seems to be the high point of the system. Also after I drained the system it did take the correct amount to fill it.
I will pull the wp tonight but if it shows no wear I am thinking of plugging off coolant lines to the heater and flush the block with a garden hose. Maybe if there is a floating obstruction I can flush it out.

Thanks for all the input gentleman.
Bruce

rrgrassi · #8 11-30-2006, 09:44 AM

Even though the thermostat is new does not mean that it is working properly...

Bruce Kennedy · #9 11-30-2006, 09:51 AM

I swapped the new thermostat with another new one. I also checked it in a pot of water. Worked perfectly, but I can appreciate your input. I've had many hard to solve problems over the years that were due to new parts.

rrgrassi · #10 11-30-2006, 10:13 AM

Do you still have your old thermostat? The pot of water tests the opening of the t-stat. What happens if you leave it in the hot water for an extended period of time?

Are you getting combustion gas bubbles in the tank? The tank staying cold seems strange. The water in my tank gets pretty warm.

Does the system hold pressure when hot? How about when cold?

How is the air flow over the fins? I have seen plasic bags and newspaper get caught between a/c condensors and radiators.

Could be trapped air or head gasket getting iffy.

If you noticed by feel the temp being higher and the guage reading higher. I doubt the guage is bad.

It really is a Hmmmm type problem.

Bruce Kennedy · #11 11-30-2006, 10:54 AM

The fins are clean, good air flow. Not sure what you mean about combustion air bubbles. Ill check the system for pressure when hot and cold and get back to you.

tobybul · #12 11-30-2006, 11:27 AM

I vote for air in the tank. Not an MB but I rplaced stat of my kid's Taurus this weekend and the stat was doing the same thing.... I realized later on that I did not put enough coolant and it was probably cavitating inside. Added more coolant, still did it after driving it for a few minutes then settled down.

Did you loose coolant during the trip?

Bruce Kennedy · #13 11-30-2006, 12:00 PM

I just checked for pressure in the system. None when it was cold and very little when it was showing 90 on the gauge. Also the coolant in the expansion tank was luke warm, not cold like before. I could see flow coming out of the little hose going into the expansion tank when I would rev the engine. I did not loose any coolant on either trip.
I read that DieselGiant recomends drilling holes in the thermostat but wouldnt that make it run too cool some of the time?

JimSmith · #14 11-30-2006, 12:28 PM

Getting all the air out can be a chore. I usually run the car up a hill in my yard at the end of the paved parking area to get the radiator significantly higher than the front of the engine, especially on my 240Ds. I believe you also have a climate control system that needs to be turned on full heat and run in this position so the new coolant flushes out any accumulated air in the heater cores. Once I do this for a while and the engine is fully warmed up, and topped off, the coolant system is usually completely full.

Newer models (W124 and later) I believe have the drilled hole in the thermostat already, and include a vent line from the thermostat housing to the expansion tank. This hole may already be in your thermostat as well, you just have to make sure you orient the hole so it is at the highpoint during normal, level operation.

Good luck, and hope this helps. Jim

rrgrassi · #15 11-30-2006, 04:44 PM

How long did you let the car run to get to 90c?

It's below freezing here in the DFW area, and I drove home and back for lunch (20 mile round trip, all highway) and the pressure on the upper hose was normal, or hard to squeeze. When its cold, is easy to squeeze.

When I changed my t-stat, I noticed a by-pass hole already in the casting of the t-stat housing. Why would you need to drill another hole?