The cap does not release due to heat, it releases due to pressure.
The pressure could be due to heat.
The pressure could be a failed head gasket.

__________________
Greg
2012 S350 BlueTEC 4Matic
2007 ML 320 CDI
2007 Leisure Travel Serenity
2006 Sprinter 432k
2005 E320 CDI
1998 SLK230 (teal)
1998 SLK230 (silver)
1996 E300D 99k, 30k on WVO
Previous:
1983 240D, on WVO
1982 300D, on WVO
1983 300CD, on WVO
1986 300SDL 237k, 25k on WVO (Deerslayer)
1991 350SDL 249k, 56k on WVO - Retired to a car spa in Phoenix
1983 380 SEC w/603 diesel, 8k on WVO
1996 E300D 351k, 177k on WVO

check the resistance of the temp sender when the engine is at 100, and the surrounding area besides the sensor actually reads 100C on the infrared, There is a spec of it on this forum and they can go out of sync, and our temp gauges are nearly "actual" temp gauges unlike new japanese cars whose temp gauge serves purpose of an idiot light.

__________________
2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

Heating to nearly-dangerous temperatures only during long uphill grades was one of the only two behaviors my 300SDL exhibited when the head was cracked.

The other was that the system held pressure overnight. You'd drive for a while, get everything hot and pressurized, park in the evening, and the next morning when it should have been stone cold, the hoses were still hard and when you opened the pressure cap, you got a very noticeable hiss as the pressure escaped.

That's an easy test to do. If it happens, head gasket and/or head is a more likely suspect.

__________________


~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL

Just making sure you know he means IP timing.

__________________
Chief

1991 350SD

My 86 603 really needs a good fan clutch to stay cool when summer comes. No problem below 90*F. But above that the 603 realy needs that fan yanking lots of air through the radiator or it will get in the +100C zone. By the way....needs it a highway speeds too cause there is too much front end (body & mechanical) deflection to pass the required air flow at high ambient temps.

They (fan clutch) seem to loose thier "grip" after a while and not rotate the fan as fast as needed, even after 5-10k post installation for aftermarket jobs.

Fooled before so now when summer comes I replace it, or as last week added silicone. Also wash the AC condensor & radiator fins.

Today (post fan clutch service & "fin" cleaning) I'm in the Phoenix area with ambient temps up in the 111 to 115 range. Had the AC cranked to max all day, temp guage never made it up over 95*C. Mixed driving with stop & go to 75mph.

Like others say your temps may be due to other componets / head condition but I for one don't trust those fan clutchs farther than I can throw them. Very sneaky b*stards

__________________

1986 300SDL 440,xxx

You're not alone. AFAIK Steve/swogee is on year 8 of the same problem - http://www.peachparts.com/shopforum/diesel-discussion/102579-1987-300tdt-overheats-long-hills.html

Sixto
87 300D^2

Do you have a 1.4 bar (140 marking) cap on the reservoir? How would you see it overflow? A hose goes from the neck to a bottle in the fender.

The aux fan trigger switch on the 124.133 is rated for 105*C. It's the 3-pin switch atop the turret to which the upper radiator hose attaches. I forget which pin pair to jump to test the aux fan. It won't hurt to jump pin pairs to test the fan. One pair triggers the fan (105*C), another pair cuts the compressor (IIRC 127*C in a 124.133, say what???), the third pair does nothing. An 86-87 switch has a lower AC cutout (120*C?), an early 300E switch has a lower still AC cutout (115*C) and a 190D (red top) switch is the lowest I've found rated at something like 95/110*C. Yes, the 190D switch has a lower aux fan trigger which helps cabin cooling on hot days. And when the temp gauge reads 110*C, I won't need AC since I'm not going to drive the car further.

Sixto
87 300D^2

The highest temperature with the infrared gun is 94C. The dash gauge is reading 110C.

Clearly, either the sending unit (most likely) or the gauge is in error by a serious amount.

Sure, it's "that hot" being 94C. But, it's nowhere near 110C.

You don't have any head gasket issues.

For the cost it would certainly be worth trying a new temperature sender.

Is it possible for a temperature sender to work correctly up to a certain temperature 80°C for example and then send an inaccurate signal as the temperature of the engine rises?

Hooverfull can you repeat the scans with the IR gun when the gauge in the car reads 80-85°C or whatever the gauge reads during normal conditions?

__________________
1987 300TD 309, xxx 2.8.2014 10,000 mile OCI


Be careful of the toes you step on today, as they may be connected to the ass you have to kiss tomorrow. anonymous

“Be who you are and say what you feel, because those who mind don’t matter, and those who matter won’t mind.” Dr. Seuss

OK, sorry for being out of touch for so long, but this, by no means is the problem solved.

To recap.

The car overheats on hills. Used to be it got hot on long freeway hills, but now it would just overheat just driving uphill on any load.

I have replaced:

Thermostat
Radiator Cap
Radiator
Coolant 50/50 mix
Fan Clutch
Fan
Shrouding
Water pump

I have not replaced the temp sender, but it seems to work since it seems to know when the car is going uphill. I do think the needle on the guage is a little jumpy. Also, the car has actually overheated, blown out of the cap when the temp was high.

Other than the overheating issue, the car runs very well.

People have mentioned the head, and I am starting to believe them.

Any ideas?

Did you read posts #23 and #24?

Did you understand them?

Without any more data, we have no confirmation that you even have an overheating problem.

The highest posted temperature with the IR gun is 94C.

There is no problem with anything other than the driver, and, possibly the sending unit.

Ignore these "people". They are usually factually incorrect.

I do understand them. I thought it could be my own paranoia. However, it did blow the cap and all the coolant out last week. My paranoia was confirmed. This prompted me to change out the water pump.

The old water pump seemed ok, temp still goes high on hills, I don't want to test it too much. Any help is appreciated on this. I am stumped.

Hoping the problem is the water pump and the temp sender. I will order the sender tomorrow.

It is noteworthy to mention I use all genuine MB parts

This data is is diametrically opposite to the data that you posted with the IR gun. It cannot blow it's cap and lose coolant unless the coolant boils, and that doesn't occur until the engine reaches 120°C. if the coolant is proper.

Therefore, with conflicting data, nobody can really help you with precision. You'll just get a bunch of WAG's..................

Sorry.

Well, situation has changed. It used to not register above the 110 mark or so, but one day, it actually did on a modest grade. The data from the IR gun was before the coolant boiled over. Now the coolant does boil over on hills.

I can drive on flat land forever. Car runs great. Once there is a hill though, the temp shoots straight up.