Over heating SL500?

[xtriple]

Okay, slightly misleading as it's not actually overheating... yet! Bit of history: bought this R129 500 February last year and it was soon apparent that there was a problem with the temperature readings. It wouldn't warm up for 15 miles and then the temp would not remain stable, dropping on a fast cruise and going up to a 100 degrees in town.

A dodgy thermostat was diagnosed and was replaced with a (non genuine) part. Bit expensive as it is a complete alluminium housing... Anyway, this got the engine to warm up in a sharpish fashion, but after 15 - 20 miles and stuck in traffic, the temp would still climb to a 100 degrees.

So, the head gaskets were "sniff" tested (fine - thankfully) but the viscous coupling on the fan was dodgy so replaced - no difference. Then I had the water pump replaced - no difference. Then I had the radiator rodded and chemically cleaned inside (though it was clean apparently) and this made... no difference!

Anyway, fast forward to this weekend and it's still doing it. It climbed to well on 100 degrees and was not dropping even after clearing traffic (and I watch the temp gauge like a flaming hawk!) and I was getting a tad niggled with it, had the opportunity to overtake a camper van and hoofed it, dropped a couple of gears and overtook. The temperature dropped like a stone!

Seriously, it was just like the thermostat had opened and the temp dropped to about 85 degrees and remained at this level for the rest of the (short) journey home. The thermostat fitted is a 90 degree one and as stated is not a genuine Mercedes one.

Does this sound to you intelligent people like I bought another dodgy thermostat? Only this one doesn't seem to open thoroughly? It seemed like the sudden rush of water through the thermostat as I accelerated caused it to suddenly open properly.

Dodgy thermostat? Plausible?

Thanks for your help.

 

[Will]

Well, I would probably start by checking the thermostat as you have suggested.

I wouldn't say ~ 100c is actually 'overheating' though - if the thermostat is rated to not be fully open until 90c anyway.

If it were me, I would flush the cooling system first, the drain and check/replace the stat, new coolant too. There's a small chance that the temperature reading could be inaccurate too (clutching at straws, I know..)

What's the hottest it gets too if you leave it idling? Does the viscous lock up and spin strongly? Does the radiator feel properly hot (ie - 100c?)

 

[xtriple]

Thanks for the reply. It has never (so far) got over 100 degrees when idling but then I've fitted twin Kenloe fans switched to come on at that temp! I changed the coolant and flushed the system last year and I also changed the three temp sensors on the the engine at the same time.

I think the rad gets properly hot as a mechanic burnt his arm on it last year when havng a look.

I know it's not"Properly overheating" it just feels like something is wrong. Even when the kenloes cut in, they will not bring the temp back down to a sensible level ( i.e. 90 degrees) until the car starts moving again.

The more I think about it the more inclined I am that this new thermostat is not opening fully.