Eric_Oz_S2

Car overheated while cruising on the highway, quite suddenly. The radiator is cold so nothing getting past the pump. Small hose from pump is very hot, so either pump not working or thermostat stuck. What's strange is I was driving for 45mins before it suddenly overheated. Lots of steam in the head when bleed valve was opened.

Is there a way to diagnose whether this is a thermostat failure or pump failure. Pump makes no noise so my guess is thermostat, but why would it fail suddenly?

I don't really want to remove the pump if I don't have too, but I have heard the stat is hard to remove as well.

Any advice?

PaulD_944S2

Eric, check the large hose going from the radiator to the water pump where the thermostat is. If its hot, like the upper hose, then chances are the thermostat is working ok.

Chances are its air in the system. My experience is 16 valvers seem to be sensitive to water level.

This is from a post earlier this year when I was having an overheating problem:

I first tested the cooling system by pressurizing it to 15 lbs and then monitoring how quickly it lost pressure. After 30 minutes it had only lost a half-pound, so I considered the system tight.

The fix was to pull vacuum on the bleeder port. I removed the bleed screw and replaced it with an 8 mm banjo bolt and banjo with a hose barb. I disconnected and pinched the expansion tank hose from the rear of the tank. I then attached a small 12V vacuum pump (could use a Mighty-Vac) to pull air from the bleed port through a hose and fluid cup (to keep coolant from going into the pump). It took several minutes since my pump wasn't that strong, but eventually I got coolant into the cup. The expansion tank level was lower. I pushed the overflow hose back onto the expansion tank port, added about 1/3 gallon distilled water to the expansion tank, and replaced the radiator cap.

I turned the heater on and set the fan to #2. When I started the engine it was several minutes before the thermostat opened. I could feel heat coming on before the thermostat opened. Keeping a close eye on the dash water temperature gauge and using a Harbor Freight IR thermometer on the front hoses and bleed port, the temperature went up to about 180 deg F. and stabilized.

It worked! The radiator fans came on just slightly above the first white mark. I let it idle for about 20 minutes and then drove it. Temperatures remained stable in all types of traffic for 45 minutes.

When I got back I let the engine cool and repeated the process. I was able to add about a pint of coolant. An hour's worth of driving was the same experience and temperatures remained pretty constant.

So, here's the take-away:
1. If your heater output is weak, you are low on coolant.
2. If you hear ANY gurgling noises when operating the heater, you are LOW on coolant.
3. If you are low on coolant, the fan thermo switch will not operate correctly since it will not have a good flow of hot coolant across it to turn on the fan(s).
4. S2 engines are difficult to get all of the air out of the coolant system.
5. You could add coolant by gravity feed trough the bleed port with the radiator cap off; it will be slow going but will work.

BTW, I tried all of the usual methods that this forum has suggested. I worked on this without success until I tried doing it this way. This car runs cooler than it ever did. YMMV.

One more trick you could try is squeezing the lower and upper hoses while supplying coolant to the banjo bolt in place of the bleed screw. You should hear air being pushed through the system. Eventually the air gurgling will stop and only coolant will come out the bleed port.

Tom M'Guinn

You might start by pressure testing the system to see if you sprung an external leak and went dry? You shouldn't get much steam from the vent absent air in the system from poor venting initially or an external leak (or internal leak like HG failure). Wherever coolant gets out, air gets in.... If that checks out you can always pull the t-stat and test it in boiling water. It should open up fully in boiling water.

Eric_Oz_S2

Thanks for the replies.

Ok, I pulled the thermostat and it works fine.

Bottom hose was hot near tstat but cold at radiator end. So tstat opening but water not flowing.

Water level was full when I left home. It all escaped through the overflow tube. There are no leaks, no steam from exhaust and car was running fine.

I'm leaning towards a pump failure. It is the original pump with 100,000 miles on it. I actually only recently removed it when I rebuilt my engine and it looked perfect. Do these pumps fail catestrophically without any noises?

I have bled the system before and never had a problem. Seems like pump failed, water boiled, water escaped, air drawn in through empty tank hence lots of steam at bleed valve.

spanky

This sounds like trapped air in the coolant system. for there to be 0 water flow with a good thermostat the pump impeller would have to become detached from its shaft or there is trapped air blocking the coolant flow. I always put my car on ramps with the front end elevated as high as possible to bleed ar, using the heater in the on/ high posiion to help bleed the air out.
I always disconnect the top radiator hose at the engine and fill / flood the engine with water to the top, reconnect this hiose then fill the expandson tank . it can take 30/45 min to complete removing all air from the engine.

jmj951

I think you found it. If the coolant escaped through the overflow tube, you very likely have a bad coolant reservoir cap. This is allowing coolant out and sucking air in when it cools off. Replace the cap, bleed the system, and you should be good.

mikey_audiogeek

My money's on plastic impeller loose on pump shaft. Happened to me. Same symptoms.

Cheers,
Mike

PEvans

I would have a low "replace it" threshold on a 100k water pump, particularly if it has been disturbed. Not saying necessarily that that is the culprit here, but IMO these are not a thing to take chances with.

Eric_Oz_S2

The impeller is steel (I have removed and replaced it previously). I have a '91 engine pump as well and it also had a steel impeller. Plastic must be aftermarket.(?)

Radiator cap is fine - it shouldn't have got anywhere near hot enough on a cold morning on the highway to start generating enough pressure to tax the cap.

I can't see how the tstat could work intermittently, but bought a new one anyway as they are cheap.

The only other thing is a blocked radiator - but that is unlikely to occur instantly as there was no previous issues.

Compression test done - all good so head gasket OK (no fuel / exhaust smell in coolant either).

No steam from exhaust - so unlikely anything has cracked.

I have ordered a new thermostat and Laso brand pump anyway (not cheap!!!) - hope this is a metal impeller. Will strip down and report back when I know for sure if pump is the culprit.

JET951

Hi Eric , with the upper "plastic " timing case outer cover removed , start the engine & look to see if the water pump pulley spins ( driven by the back of the cam belt ) , it sounds ( the way you described it ) a bit like a seized water pump

Eric_Oz_S2

Sean

The pump spins, slight wobble, hard to tell if it (or something else like belt rollers) is making a noise. Doesn't seem to slip.

Have you seen any loose shafts on pumps?

Because the radiator wasn't hot (and fans were off) but engine temp was very hot, I can only surmise water isn't getting past the pump.

JET951

I would remove the pump to inspect , because you have already changed the thermostat , I must admit , it does sound like the plastic impeller has fallen off

Regards
Bruce Buchanan
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jerome951

Definitely possible. I bought a rebuilt pump many moons ago where the impeller would slip on the shaft under load, so at idle the car cooled fine, but heated up when driven. Took me a while to figure that one out...

reno808

It could also be a collapse hose. i seen that before. i would take off the water pump its easy. Do it just to make sure

Eric_Oz_S2

Three hours of dismantling later, I got the pump off. The impeller is spinning freely on the shaft (completely loose). Contrary to my earlier post regarding the impeller being metal, I stand corrected, it is black plastic. The impeller is attached to the shaft by a brass (?) bushing that has serrated edges that key into the plastic impeller bore.

The impeller has its own part number 944 106 036 00. It would seem that if one had a new impeller, it would be possible to push in a new impeller. This then presents a question, how do you rebuild a pump if the impeller is not separately available?

Too late for me now as I have ordered a new pump, but an interesting question nonetheless.