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Discussion Starter #1
My 91L has a sudden coolant malady that I can't quite figure out. When hot the reservoir overflows through the sensor cap. I'm not overfilling the reservoir; when cold it's at the minimum level.

I first noticed the problem a few days ago when I parked the car for a few hours after a long drive. I returned to the car, started her up and saw that the coolant warning light was on. I also noticed coolant on the street. When I opened the hood and checked the reservoir it was almost empty. There were no signs of leakage except around the top of the tank near the caps. This morning, for an experiment, I drove the car until hot, parked her, observed the coolant coming out through the sensor cap and then opened the filler cap (carefully, of course) where additional overflow emerged. I then milked the top radiator hose (leading to the thermostat). This sent up little puffs of steam from the reservoir. The coolant level declined with each squeeze of the hose until it was back down to a normal level.

There is no mayonnaise inside the oil filler cap or dipstick and none in the reservoir. I replaced the thermostat a few months ago.

Anybody have any idea what's going on?

Mac D
 

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Were you in stop and go traffic? What was the ambient temperature outside?

Could be your relief valve on the coolant tank not keeping the proper pressure i the system and thus lowering the boiling point and hense your overflow.
Could be a blown headgasket. Check the coolant for oil residue

Could be a tired old radiator, check the fins for flaking , if the flaking is greater than 20% of the fins then the radiator is probably not cooling down the engine very well, hense the boil over.
 

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Does your cooling fan run on low speed system or just high speed? What kind of reading are yoiu getting on temp gauge? Is car stock cooling configuration?

Cap can be bad and little coolant can bypass but if sensor float cap leaking sure it can be bad, too or have bad gasket but cap and sensor should not be bypassing if system is operating correctly.
 

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Discussion Starter #5
The thing is, the system is not boiling over. The fan is working properly -- low and high speeds. The temperature gauge shows everything within proper range. And the coolant is not steaming or boiling out when I open the cap.

I first noticed the low coolant level following a long freeway drive (no stop and go) and after the car had sat parked for a couple of hours. I now see the problem after driving around in regular traffic. There is no visible indication of a blown head gasket. No mayo in the oil; no oil scum in the reservoir. Nor do I see any bubbles in the reservoir when I idle with the cap off.

Could there be a blockage somewhere? Why would the coolant level go down when I milk the upper radiator hose? It's a mystery. Where's Agatha Christie when you need her?

Mac D
 

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Well maybe one of two things or both. Say system working as normal with good fan system, good engine thermostat and radiator OK.

Coolant cap relief valve stuck so thermal heat after shut down ends up forcing coolant by seal around sensor float shaft is original style with brass rod and "wine cork" style float or seal under retainer nut cracked or nut loose.

New style sensor floats a sensor float around a plastic support.

Inspect retainer nut gasket and if good try tightening sensor retainer nut on tank and if that doesn't stop it leak.

Coolant tank cap maybe OK if seals bad in sensor.
 

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Discussion Starter #7
Just for the heck of it, I replaced my fill cap with a used one I had on hand. Took the car for a spin -- same trouble: I got home and found coolant coming out through the sensor cap.

So I took off the fill cap in order to observe the coolant level as I milked the top radiator hose. I could see the level sink back down. I kept the cap off and ran the car at fast idle in the driveway until she was as hot as the fan would allow; the level stayed down. But when I shut off the car the level rose back to almost an overflow. I started the car again. The level dropped. I shut it off again. This time the level stayed down.

What's interesting is that the coolant reservoir was almost emptied the first time this problem occurred -- as though something had been pushing the coolant out. My first suspect was a leaking head gasket blowing pressure into the system. But as I noted before, there is no other evidence of a bad gasket. Also, it seems that the overflow happens after the engine is stopped -- just as it did the first time.

Yikes!
 

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try bleeding the tstat

Given all the symptoms I might suspect a big ole air pocket somewhere given that what youve already tried
 

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Discussion Starter #9
Goats:

Yeah, I was thinking of something like that. The air gets hot, expands and pushes the coolant out. But it would have to be a big pocket in order to almost displace the entire contents of the reservoir, and it would have to be trapped somewhere or else it would bubble out through the reservoir coolant. How do you get rid of such a gremlin if it is indeed the cause?

Another development: since changing filling caps it appears that the high-speed fan only is coming on. No low-speed. The temperature gauge is averaging higher than usual and when the fan finally does come on there's a big jump in the ammeter and a sensation that the fan is lurching into action.

Does that add anything to the equation?

Mac D
 

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yeah you might have found it!

does the fan spin freely by hand? Even though your fan may be coming on, I wonder if its spinning at the designed RPM range- if its binding up at all, that might explain this whole thing

OR, you could have a bad/dirty connection to the fan that is showing up as a high resistance to the fan motor, which in turn will make the fan run slower. Fan relays are a suspect, but first I would check fan mechanical condition and connection to the fan motor
 

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Goats:

Yeah, I was thinking of something like that. The air gets hot, expands and pushes the coolant out. But it would have to be a big pocket in order to almost displace the entire contents of the reservoir, and it would have to be trapped somewhere or else it would bubble out through the reservoir coolant. How do you get rid of such a gremlin if it is indeed the cause?

Another development: since changing filling caps it appears that the high-speed fan only is coming on. No low-speed. The temperature gauge is averaging higher than usual and when the fan finally does come on there's a big jump in the ammeter and a sensation that the fan is lurching into action.

Does that add anything to the equation?

Mac D
Yes it does. Check fan resistor in radiator fan shroud it maybe bad. Try bypassing it by hooking dark blue and brown wire to it together and see if you now get a higher fan speed at a lower temperature.
 

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Discussion Starter #12
Thanks, guys. I'll follow your suggestions in the next couple of days and post the results.

Mac D
 

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Discussion Starter #13
I'm baaaack...

Okay, I replaced the coolant level sensor with a unit from an old reservoir. So far, no overflow.

But I'm having trouble with the low-fan speed by-pass. I don't have wires that match your description, Steve. I have red, white and black wires going into the thermal switch. At one time, I knew which was the power supply wire and which belonged to the low and high-speed systems. If I still knew, which I don't, I figure I'd be able to switch the positions of the low and high-speed wires, allowing the high-speed system to activate at the lower system temperature. Am I right? And if so, which wire is the power supply wire?

Mac D
 

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Discussion Starter #14
Oh wait, Steve... You're talking about the wires going into the resistor, not the thermal switch. However, wouldn't it be simpler to reposition the switch wires to test the system? There's less to remove, etc...

Mac D
 

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Steve wants to you to bypass the low speed resistor to be able to see if it is faulty. If the rest of the circuit is ok you will get a high speed fan when the thermal switch trips at the lower temperature setting, and of course, you will still get the high speed fan at the high temperature setting (of the thermal switch).

If the low speed fan thermal switch instead is faulty, you will get nothing for the low speed setting, and the high speed fan setting will work, or nothing will work as well at the high temp setting if that part of the switch is faulty as well.

Nothing to remove, just jumper wire the two wires together when they are removed from the resistor. Easy to reach.
 

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Discussion Starter #16
Sorry for the dumb thinking. I cut to the chase and pulled the resistor. It took all of three minutes... The flat resistance wire that coils around the core was obviously broken, so I cut off the two wires leading to the resistor and soldered them together. Then I tested everything, now jumping the wires at the thermal switch to make sure the fan was activated through both the low and high-speed circuits. It worked. Next, I'll order up a new resistor and, until it arrives, drive with the old one out of the loop.

Meanwhile, I'll keep my eye on the original overflow problem. It seems that after replacing the sensor/cap the reservoir is holding pressure -- which it wasn't doing before. The big test: I've restored the coolant level to about midway between minimum and max (it was below minimum when cooled down after the last long run). Hopefully I'll find no overflow or coolant loss after the next trial run.

Meanwhile, thanks everybody.

Mac D
 

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Discussion Starter #17
Oh, oh... Too soon to say "solved!" Who sells replacement resistor units? I can't find any in the Alfissimo, Centerline or International Auto Parts catalogues. Is there a parts number I can use for inquiring at Difatta Brothers or other sources?

Mac D
 

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