Engine runs but overheating...

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BonitaCS

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Well it's been about 6 years but I finally have the engine running and the coupe should be back on the road in a few months! I've done the Motronic 1.3 conversion with a 3.5L from a 90 535i completely rebuilt. I now have an overheating issues. I start the engine and let it run for about ten minutes and it slowly starts creeping up into the red; I shut it off before it gets into the red. The water pump and thermostat are both brand new. The radiator is what was in the car before the swap which I never had overheating problems with. I have a 9 bladed fan and viscous clutch from a 635i I found at the junk yard but they look brand new; the car I pulled them off of had just had a the top end done and had everything new, pump clutch, fan, radiator, so I think they are ok. I have the front end jacked up and have bled the system numerous times and still have the problem. How can you tell if the viscous clutch is working properly? Little resistance to rotation when cold and more when hot? I have borrowed a pyrometer from work and will check temperatures around the engine/radiator tonight. Any help/suggestions is appreciated.

Thanks!
 
Yep, should free-wheel when cold, and spin when hot.

The stock radiator will get hot at idle for the manual, non-AC coupe, even with the 9 blade-fan and newer viscous clutch. I've seen the gauge go up to right before red and hold there.

After I switched to an AC coupe radiator, it barely moves. It's about 1/2" thicker than a non-AC radiator.
 
Yes it is flowing to the expansion tank. I never let it get into the red so not sure if the needle stoped moving; maybe I'll be a bit more gutsy tonight.
 
It is possible to put the thermostat in reversed in which case it will never open. Worth checking if nothing else seems wrong.

On my list of dumb things I have done. We should start a new thread on that - confession is good for the soul.

Doug
 
Running

Hot!

Might try backing off the timing a bit. 2-4 degrees.

Running lean at idle? Fuel pressure?

Drop the front end and rebleed. Air pocket in the block?

Backflush the radiator- once it gets hot- scale releases.... very fine particles. Turn it upside down and flush.

A newly rebuilt engine can add a little heat till things get happy together... but near red.... something's not right.
 
It sounds like the cooling system isn't working at all

Fast go/nogo test: Remove thermostat and run. Also, be sure to bleed the system thoroughly.
 
Ok ran it for 15 minutes tonight with similar results. It never got into the red but was about 1/8” below. Had a pyrometer and measured temperatures all over. Looks like the thermostat is opening fine and coolant is circulating. The temperatures going into the radiator was right around 200F and coming out of the radiator was about 174F. Top left of radiator was around 190 and lower right in the 170s. I felt no change in the rotational resistance of the fan from before starting the engine and after shutting it off after 15 minutes; is this normal or should the fan fill stiffer when warm? Might just be the TS? Anybody know the approximate resistance of the TS should both hot & cold? There is a tiny bit of crud in the expansion tank, fine particles in the bottom but not a lot; will flush and add new coolant.

Scott
 
TS expected Ohms

A guideline to use for similar Bosch unit:

14F, 7 - 12 kOhm
68F, 2000 - 3000 Ohm
176F, 250 - 400 Ohm
 
Based on what you have - I'd suspect the fan clutch (as you had no change in rotational resistance).
 
Sounds suspiciously like a thermostat issue to me!

On my Land Rover Defender V8 (sorry to bring 'foreign' cars into this) the temperature gauge always goes all the way to the red field before the thermostat opens - and I tested this thermostat (82 deg. C) before I put it in. Once the thermostat is open, the temperature needle drops to where it should be and stays there.

The viscous unit will not function until the thermostat has opened and the radiator is hot. You should feel quite some resistance when the engine is hot - and the viscous unit engaged.

Have you tried driving the car? The temperature will rise if there is no wind flow!
 
I think it might be the clutch too, I was suspecting a lot more resistance when warm but it was the same as when cold. I'll put a heat gun on the clutch and see if the resistance changes before the fan melts... I'll re check the thermostat as well and, while I'm at it, I'll drill the 2mm hole at 12:00. I know the temp will drop when I drive it but sitting in traffic it will go up. The car is not ready to drive yet; have to do some minor things like bleed the brakes first and finish putting the underside of the dash back together and the seats back in. Thanks for the resistance values for the TS, I'll check those too.
 
Just a thought - are you sure you have the correct thermostat? There was a design change in the thermostat housings and thermostats sometime in the 80's. And original housings are NLA, so if yours is not original it may have been "updated". I'm not sure what the implications are...

Charlie
 
Maybe.

You might be on to something... I know the engine is from a 90 535i but I might have pulled the thermostat housing from a mid 80's engine; It was a few years ago and I don't remember what car I pulled it from. Any idea on how you would tell the two apart looking at them?
 
I think the main difference is in the depth of the thermostat - axial distance from the outer flange to the small plate that controls the coolant flow.

But if you want the specifics, call the guys at Mesa Performance. They know.

Charlie
 
BonitaCS said:
You might be on to something... I know the engine is from a 90 535i but I might have pulled the thermostat housing from a mid 80's engine; It was a few years ago and I don't remember what car I pulled it from. Any idea on how you would tell the two apart looking at them?
Click to expand...

hi

i simply hate the words “cooling system”, it is a pain, it starts giving you problems and never ends,

I agree with some of the presented suspictions and I will add some new:

thermostat

1- the thermostat. yes indeed you should check that, I don´t know your engine, because my CS is original, but check it, they gave me an incorrect thermostat and fortunately I realised it was wrong before installing it,

2- the problem of the thermostat (in my case) is that it opens the front side while it is closing the rear one. let me explain, the thermostat has a double effect, first of all (effect 1) it will open a circular ring opening to let the water flow from the radiator to the pump, and simmultaneously (effect 2) it will close a cylindric opening of the water flow from the top engine to the water pump, this will cancell the radiator by-pass in warming up stage, giving more speed to the heat interchange to air

3- so, if your thermostat is too short for it´s compartment case, it will not close the second opening, and this way the heat interchange will perform in a less efficient way, that will result in higher temp in the circuit


note: this also tells us that deleting the thermostat for a trial (go/nongo) can be not so clear (I have doubts here, because for sure you will know the thermostat is open, but the performance will be not good due to not cancelling the back by-pass ??? don´t know

in my opinion, if the thermostat is not working you will overheat the engine, for sure, inside the red area, and not below it, so I think the thermostat is working somehow

fan

unless the belt is loosen the fan and water pump will be working ok, I am assuming no big errors in instaling them had occurred, again I do not know your engine upgrade details, sorry

clutch

someone told me that is not a fact of a loosen or strong feeling of engage-connection, to check it, is more simple than that subjective appreciations: with running engine, switch off the contact while looking at the fan, the fan must stop before 2 turns,

please do not try the hot air gun again !:???:


bleeding

this is critical, although you said you did it well and thoroughly,

I read also you made a tiny hole in the tstat, well done !

radiator

without more information I can not tell.
an obstruction will cause problems for sure, but…


temperature sender

this was MY problem,

please take extra care in installing this device, I mean, clean the thread of both the hole and the part itself, do NOT add any thread adhesive or whatever can make the metal-metal contact worse, it is critical to have a good earth in the sender otherwise the reading in the gauge needle will be bad

other electrical problems can affect to a bad reading in the gauge, basically make sure that the temperature in the engine is good, (a secondary set of sender and gauge, not influenced by the rest of electrical connections in the dash, will help)

no more ideas by now, if you check and give feedback, I will try to help

regards
 
Last edited:
Two things can happen if you use the wrong t-stat.

case #1 the engine will overheat within minutes because you are using a t-stat from BEFORE 1985 in a housing that has an "A" stamped on it. The t-stat cannot open and therefore you will hit the red zone in mere minutes.

case #2 the engine will never warm up because the t-stat is never closed - you have combined a late t-stat with an early housing.

early t-stat

http://www.bavauto.com/lview.asp?imgfile=assets/imglib500/11_531254065_80.jpg

late t-stat

http://www.bavauto.com/lview.asp?imgfile=assets/imglib500/11_531466174_71.jpg


yeah- I may have the cases backwards but the facts are correct- you MUST match the t-stat with the housing... look for a changeover date of October 1985....

09 Thermostat 80CEL 1 09/1985 11531254065 $62.16

http://www.realoem.com/bmw/showparts.do?model=6874&mospid=47192&btnr=11_0691&hg=11&fg=35
 
sfdon said:
Two things can happen if you use the wrong t-stat.

case #1 the engine will overheat within minutes because you are using a t-stat from BEFORE 1985 in a housing that has an "A" stamped on it. The t-stat cannot open and therefore you will hit the red zone in mere minutes.

case #2 the engine will never warm up because the t-stat is never closed - you have combined a late t-stat with an early housing.
Click to expand...


you see? this is the accademic approach, full of detailed data ! :-D

thanks sfdon
 
One thing I left out of the equation... During the restoration I sent all of the instruments up to North Hollywood Speedometer and had them cleaned; they did an excellent job by the way! Perhaps during the cleaning somehow the calibration of the temperature gage was changed? Remote possibility but none the less a possibility. As I stated in an earlier post the temperature measured on the external surfaces didn't seem to be too hot; inlet neck to the radiator was right around 200F. So I'm wondering if it is really overheating. Also, I did clean the TS real well to the point it looks new. If someone had the time to measure the resistance of their TS both cold and at normal operation it would be greatly appreciated! This weekend I'll tear into the thermostat housing and check to see I have the correct components. I never hit the viscous clutch with the hot air gun but it sounds like DeQuency (sorry don't know your name) thinks that would be a bad idea?
 
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