Overheats only at idle

[evannice]

Hi Paul,

Well I thought that the coolant header tank neck modification was the fix, but the 1300/124 still overheats. But only after driving for a while and then stopping to idle for a few minutes. The car warms up rapidly until the T-stat opens at about 1/3 gauge and then stays right there as I drive many miles at varying speeds. But stop to idle for a few minutes and the gauge goes high and coolant boils out the (13 PSI) cap vent tube and I have to shut down quick. I notice that the (front) radiator gets warm and the fan comes on and goes off periodically if it is a warm day or if I am driving hard, but the fan does not come on when it overheats at idle.

I am thinking that the water pump is not getting the job done at idle RPMs, and I do not know if the water pump in the car is a reverse-rotation one or not. I have been told that it doesn't matter. What do you think?

Eric Van Nice

[pheyden]

Evan,

This is often a vexing problem.

Obviously the thermostat is working. It would be nice to have a REAL indication on the guage of what the temperature actually is, but that is a different story.

The reason for the fan not coming on in the "idle overheat" situation is that the sensor for the fan and the sensor for the water temperature gauge are in different locations in the system. In order for the temperature gauge to register the higher temperatures, it must be in cooling fluid, as if the sensor itself were in a high temperature gas pocket, then it would show either lower temperatures. Again, it requires fluid for these sensors to work.

The first thing that I would recommend is that you find a way to hook up a temperature guage that gives a real temperature reading, instead of some that reads "good" or "bad". The sender for this should be in the head, preferably as close to the thermostat housing as possible.

Next look at the pulley setup. If the water pump pulley is appreciably larger in diameter than the crankshaft pulley, then the speed of the pump at idle will be lower than the engine RPM. If there is a localized hot spot around the waterpump, and the local coolant there turns to steam, then the pump will be very inefficient and perhaps even non operational. After a long, hard run, there is a lot of latent heat that is being absorbed by a small amount of coolant in a short period of time.

Are you running coolant of any kind?

Regards,
Paul

[evannice]

Hi Paul,

The water pump pulley is slightly smaller in diameter than the crank pulley, and I am running Prestone II (green) at 2 parts water to 1 part antifreeze. I'll stop by the auto parts store when I get a chance and buy a temperature gauge. I am still thinking about the reverse impeller water pump idea, but I am enjoying driving the car around (finally) almost every day. It sure likes to be used, just runs better and better all the time. Unless I let it sit and idle for a while!

Eric

[pheyden]

Hello Evan,

Here is a test you can do quickly. Wire the fan on the front radiator to come on full time with the ignition switch. Just unplug the wires from the thermostatic switch and hook them directly to the fan.

Do a test drive/idle sequence and see what happens.

If the coolant that you bought was already diluted 50/50 (it comes that way now) and you diluted it 1:3 then you have only a 16% ethylene glycol mixture. Not enough. If it was full strength, then up it to 50/50.

If you get a chance give me a call on 919 757 6888 and I will discuss some other alternatives with you. They are somewhat complicated, but I think you will get the idea.

Paul

[evannice]

Okay, I kept chasing this problem until I eventually replaced the water pump, and the problem is gone . The pump on the car was a relatively modern GMB style with the bent sheet metal vanes on the impeller. The one that works is the older style with the cast disc impeller with straight flat vanes radiating outwards. It appears to me that the GMB works well CW but not well enough CCW, and that the earlier disc style is omnidirectional. I have been told by those that made their own reverse-124 in 850 cars back in the day that they did not run into this problem. I bet they were using the omnidirectional pumps.

Now it is quick and easy to bleed air out of the system, it warms up faster, the heater works quicker and is noticeably hotter, the radiator fan comes on when the car is pushed hard and stays on for a long time. I just put 100 miles on the car today with fairly high freeway speeds followed by periods of idling in traffic. The temp gauge stays between 1/3 and 1/2 operating range and stays there no matter what.

Hooray, on to the next problem!

Eric

[pheyden]

I am pleased that you found the problem. Just for giggles is there any sort of brand on the one with the straight impeller vanes? It would be nice to find a source for them and a name helps.

Regards,
Paul

[pheyden]

Evan,

I had another look at the water pump and I think I can machine an impeller with straight vanes. I will give a try next week. The pressed steel impeller should press off without too much hassle.

Paul

[evannice]

Hi Paul,

There are no identifying markings at all. The guy who put this together for me chose a casing and an impeller from his stash of odds and ends and then put them together with modern bearings and seals. He said he chose this particular casing because it looked to him like it would flow better and said that before he disassembled it, it had an electromagnetic fan setup, so I presume it must have been from an early 1438 TC. The impeller could have come from anything, he said it was just one from a pile that looked like the best match for our purposes.

That would be cool if you could make some more, maybe there are a few folks who would like one.

Best regards,

Eric

[pheyden]

Evan,

So you surmise that this was a pump from a 1438 Fiat pushrod OHV motor? Early generation made of cast iron?

I have a similar problem with the GMB pumps for the smaller motors, and I was looking at actually casting impellers for those in the future.

Regards,
Paul

[evannice]

Hi Paul, I believe the housing is from an early 1438 DOHC just because that's what I recall having the electromagnetic fan "clutch" operation, and I think the housing is cast aluminum. The impeller appears to be cast iron.

Eric

[gil]

Hi Paul,

well I for one would be interested if you can come up with an impeller that has "straight" vanes (ie., not the stock angled ones). I took the pump out of my 1300/124 which came with a 1438cc motor in it. I also just removed the pump from a 1585cc (Fiat 131) motor and a new "crate" 1438cc motor. I also have a new replacement stock pump. All 4 pumps have cast impellers with angled vanes in them. My 1300/124 had some kind of racing history. Might have been local club racing or autocross, I just don't know. If the previous owner ran the 1438cc hard under racing conditions, it must have worked okay. It sounds from Eric's experiences that maybe the stock pump doesn't work as well at idle.

I'd sure like to get a straight-vaned impeller, or modified pump to get optimal function from my engine. Please keep us posted.

Thanks,
Gil Nicholls

[pheyden]

Gil,

SInce you have an "abundance" of pumps, can you send me one and I will use it as the "workhorse" for developing a straight vaned impeller.

Regards,
Paul