Strange overheating problem on 1996 S500:
 

I have a '96 S500 w140 that has a blockage somewhere in the coolant circulation that causes it to run hot. A shop of ill repute did a head gasket job on it before I got it, (~one year ago), and told the previous owner that the heat/climate control was somehow causing the car to run hot. The shop bypassed the heater hoses on the back of the engine at that time, (connected the L/R together from rear of engine), and the car did not overheat but ran hotter than normal still.

When I got it, I suspected a bad radiator and changed it. No real difference. Items that have been changed or otherwise ruled-out are the thermostat, water pump, fan clutch and electric fans. The fans all come on when they should depending on temp and/or A/C activated.

It is 100% sure a circulation issue. I have the heat connected again and it works as it should. No leaks, L/R heat operable and aux. pump as well as monovalves are good. The car will overheat on a cool day w/ heat connected but shut-off and it will not OH if heat is on inside car. (But it will still eventually climb higher than it should).

I am at my wits end. I know that the heat is a bypass system on all liquid-cooled engines, i.e. it should have nothing to do w/ coolant circulation even if it's completely plugged. I am pretty sure that the reason the car runs cooler w/ heat on is because there is a blockage somewhere between the left and right side of engine on the back and the heater cores are allowing the flow when on, even though it's still not a 100% flow rate. The main radiator and fan are working efficiently, when the car overheats you can put it in Park and rev the engine and watch the temp gauge come down pretty quickly.

HELP! :)

Looks like you have throughly and methodically gone through the car's cooling system.

Have you replaced the "cap"?? What's the operating temp while driving??

Have you checked the af/water ratio YOURSELF?? If you're in a warm climate, try a 40/60% ratio if you haven't already done so.

Thanks for responding but it's 100% a circulation problem. That much we know for sure. I'm hoping that someone reads this who really knows the Byzantine plumbing of the w140 climate control/bypass system.

<<Thanks for responding but it's 100% a circulation problem.>>

What do you think 70/30% ratio does to "circulation"??

You didn't answer any my questions . . . on this forum, we use "numbers" not just rhetoric.

If you are sure the temp sensor is ok it's probably a block loaded to the gills with stop leak sludge- and it's all caked up somewhere between the block and a cylinder head..

Seen that before. Ugly.

rjp

maybe someone has the left intake gasket on the right side and right on the left? i think thats possible,

That is entirely possible considering the shop that did the work. Would that cause a partial blockage? Slow down the flow enough to make it run hot or overheat?

:cool:Thx.

I have no idea what you are talking about. What "rhetoric"? :confused:

The car has had various combinations of coolant/H2o including straight water at the moment, for testing and constant opening/closing/draining. An engine will run coolest with straight H2o. You know this, right?

My preferred mix is approx. 30% MB factory coolant to 70% distilled H2o normally. I am in SoCal and do not need the freeze protection. In MN., where I am from originally, it is obviously different.

Randy you nut, it's speeder. ;)

The temp gauge is accurate. I wish it wasn't.

This intake gasket suggestion has my mind buzzing. Taking off the intake manifold is a piece of cake on this car, the heads are a little more work.

DUDE. What up?

Why aren't u speeder?

Anyhow I think it's Bush's fault ;)

rjp


PS- People Denis / treetrimmer isn't a MBZ rookie, if he says he's got a problem it's probably a serious one he knows what he's doing.

The reason i mentioned intake gaskets is one side is open the other closed
on a 129 sl that was worked on by a mechanic , the car started overheating after , and we found out he mistakenly installed the intake gaskets on wrong sides, they bolt up the same but a passage that should be open was closed , that might be worth investigating.

I will pull the intake manifold tomorrow and check. The head gasket job was done by a shop that I don't trust at all, (not MB people), before I got the car. They charged my buddy $3600 to replace the head gaskets w/o a valve job or other head work and used the cheapest parts available in every case.

I'm pretty sure that they then test drove the car and it overheated right away. Rather than tear it back down and figure out what went wrong, they somehow determined that it would run cooler with the heat comepletely bypassed and told him that he needed his heater cores replaced for some crazy price. (That is about a 25 hour book job). He drove w/o heat from then on until he basically gave me the car when it needed another pricey repair later.

The car does run cooler with the heat bypassed but it will still overheat on a hot day w/ A/C on. Something weird is going on and I've been chasing my tail going through the normal cooling components. It's a really nice car, (70k miles), I can't wait to solve this problem. (And one other but that's another thread).

Thx again for all replies.

Thanks, bud. I'm treetrimmer on most boards, it's funny but Pelican was the first BBS I ever joined back in the old days and I could not think of a user name. I've never really liked that name but of course I have 14k posts or something w/ it so it's a little late to change it.

For everyone else, you have a great community here and I really appreciate the help. Randy and I are on Pelican forever but I do not even own a Porsche at the moment. I go quite aways back owning and repairing them.

I must chime in here, JimF is correct I had a similar problem with my 96S500, I followed Jimf tread on installing a resistor over the fan temp switch, sure this made the fans kick in earlier but I still had a engine running to hot for my liking. I had the fan clutch replaced with Jim's mod to it, still ran hot in traffic and town, like about 105 degrees celsuis .

Then I changed to a 50/50 coolant water mixture and replaced the rad cap with a OEM cap from the dealer. What a diffrence car now runs at 90 degrees in traffic and electric fans does not even get triggered by the tempreture switch.