Help with Cooling Issue - NA FMII
 

Issue: Car overheats (110C+) even in ordinary (off-boost) driving


Possible Cause: Fans don't cycle on until about 110C, both before and after replacing fan switch with new unit


Setup: 1991 NA, 1.8L swap, lightly built, FMII turbo with "1.6L" Link ECU, 15psi
Custom coolant re-route with block-off plate in "old" thermostat location up front, with fan switch located there
Thermostat opens at 64C, as confirmed by watching engine temp on Link ECU keypad during warmup
FM radiator with high-flow SPAL fans and shroud from FM
Car and systems otherwise appear to be functioning properly


I was hopeful that changing the fan switch would solve the overheating issue, since I believe the fan switch is supposed to cycle the fans fans on closer to 100C (about 10C lower than I'm observing). But even after replacing the fan switch with a new unit, the fans don't cycle on until about 110C, which I suspect is too late to be helpful in effectively cooling the car. Curiously, when replacing the fan switch today with a new unit, I noticed that the old/suspect switch as mounted in the "old" thermostat location at the front of the engine was bone dry, and even had some oxidation on the tip, whereas the radiator and overflow tank both appeared to be at proper levels (car had, however, been sitting for several days).


Is it possible that the engine has some kind of air bubble in the cooling passages at the front of the engine (perhaps even isolated in the "old" thermostat neck at the front of the engine), and therefore the fan switch is not getting "hot" coolant passing over it, and therefore reading "colder" than it should?


Any help/theories you could provide would be very helpful. Thanks in advance!


PSICHO
Dallas, TX

If you temporarily bypass the fan switch and just have the fans on constantly does it still overheat?

Does it overheat only while stationary/slow, or does it overheat while cruising at speed?



The fans only really help with cooling at low speeds, at highway speeds there is way more air coming through the rad than the fans could ever do on their own.


Basically, if hardwiring the fans doesnt stop it, and if it's overheating while cruisng at speed, then the fans are likely not the problem.
If that's the case, you'll be wanting to look for things like a dead water pump, of a blockage somewhere in the system reducing flow.




Also, I cant remember the normal operating temp of the B6/BP, but from memory around the 100c mark isnt a concern.
The system is under pressure, and with coolant added, it shouldn't boil until way over 100c

Thanks - quick answers:

If you temporarily bypass the fan switch and just have the fans on constantly does it still overheat?

No, brings down temps pretty quickly -- these are super-high flow SPAL units

Does it overheat only while stationary/slow, or does it overheat while cruising at speed?

No, it seems to get hot (108C+) cruising at normal highway speeds also

if it's getting hot at highway speeds, but is ok at low speeds with the fans on, i'd suggest there's a blockage somewhere.

I'd start by giving the cooling system a damn good flush.
And replace the thermostat while you're at it (they're cheap, may as well)
If that doesn't help, might be worth getting a rad shop to properly clean the core (they remove the end tanks, hot tank the core and blast it clear, then put the tanks back on)



Out of curiosity.... have you confirmed the fans are spinning the correct way?
If they've been wired with the terminals flipped they may be trying to push air through the rad, rather than pulling.
Fine while stopped, but would cause issues when moving and they're trying to push the air back out the way it came

Is the entire bumper mouth opening sealed completely to the radiator? Does 100% of the air entering the bumper mouth go through the radiator? If not, start there.

Then consider placing the switch in a location where the hottest coolant is flowing across the element constantly. A blocked off area receives no flow, only stagnant conditions.

I think this is implied by Six's question, but do you have the under tray in place? A lot of people who have heat management issues don't. It's essential to creating the pressure delta needed to make air flow through the radiator.

If you don't go through the trouble of flushing the entire system I would check that your reroute doesnt create any high spots that are trapping air bubbles. A good bleed session and squeezing the coolant hoses (with a rag or glove of course) helps to get air out of weird pockets in the engine or lines.

Thanks again -- yes, fans spin correct way, once they come on. But they don't turn on until 110C, which I think is way too late to help. I had the thermostat replaced with a new one in connection with some recent head work, and I replaced the fan switch myself this week. Today I verified that coolant is running through the "old" thermostat neck where the fan switch is located.

Even in moderate driving the car still heats to 110C, which I find scary (it never used to stay north of 100C during ordinary driving). Maybe it is time to get the radiator cleaned inside (outside is fine), but I still think the fans are mostly the key here, since for some reason they don't come on until 110C. Can't figure that out.

Yes, entire radiator opening is shrouded, and undertray is installed. Ambient air is forced into mouth at speed, guided through intercooler and oil cooler (side by side), and then large core radiator with high-flow SPAL fans. May need to relocate the fan switch, but I'd rather not -- this is a new-ish problem on a long-lived system, so I'm not sure relocating the switch is the answer.

Thanks, yes, undertray is in place. And I have an extractor hood as well, which makes this even crazier. Large core rad, oil cooler, high-flow fans, extractor hood, undertray, shrouding, etc. Still nears meltdown in ordinary driving.

Thanks, I did that today to make sure no bubbles. Highest spot is the radiator fill neck.

The old Link systems had a setting for the fan on temperature. On my old unit, it was variable z9. Have you checked that the fan on temperature (in degC) is correct ?

Unfortunately for me, I believe that setting only exists on the 1.8L Link ECUs -- not the 1.6L Link ECUs.

The more I think about this, the more I believe the front of the engine runs much cooler than the back, even with my coolant reroute. That would explain why the fan switch (located in the front of the engine, nearer the water pump) doesn't come on until the engine coolant temps (transmitted via sensor on back of engine) start reading pretty high, at about 110C. Since the fans are supposed to switch on at well under 100C, there appears to be at least a 10C delta between the coolant in the front of the engine vs. the coolant in the back of engine. I wouldn't think this large of a delta would be normal, given the coolant reroute.

Silly question, but could you buy a fan switch that completes the circuit at a different temperature?

Even if this is the case, you shouldn't be overheating at 40 mph+ speeds. You shouldn't need fans at that speed, the normal airflow should provide for adequate cooling.

^
This.


If it overheats ONLY at low speeds, or when stopped. Look at the fans.

If it overheats while at speed, look at the cooling system itself (Either stuck thermostat, low coolant flow, pump not working properly, etc)



Does your thermostat have a jiggle pin, or a bleed hole?
If not it may be that the thermostat it taking far too long to open.