Is this consistent with a failing coolant temperature sensor?

is $30 too much?

it's a cool tool, I was just looking for mine this past weekend (trying to calibrate temp gauge on my '55 Stude)

nate

Right. I meant if one deliberately lowers the engine temperature in an attempt to reduce the tendency to knock it would not be cost effective.

-jim

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On second look, the repair didn't bring the temp up as much as I expected. As I start the cold engine and take to the highway, the temp rises steadily to something over 200, then drops pretty quickly to maybe

160, then rises again. After awhile cruising at 60 mph, the gauge stays pretty near 180F (compared to 160F before the repair).

I tested the removed thermostat in boiling water with a candy thermometer. I guess it works about as it should. There were some limitations in my test method, but the valve was certainly closed below

190F and at least partly open above 195.

Is there a designed-in coolant flow channel that bypasses the thermostat so as to prevent deadheading the pump? If there were, an increase in the capacity of some other part of the cooling system might prevent the engine from heating to the specified operating temp even though the thermostat is closed. A more powerful pump or radiator might cause overcooling. Fans coming on when they shouldn't would be able to cool too much. Also it would take longer every time for the engine to warm up.

I'll throw in a few more facts: the water pump was replaced sometime before the heater-not-working complaint---I don't know how long before. The coolant low light comes on briefly sometimes when the engine is warmed up, but I claim the coolant isn't low.

Did you get all the air out of the system? A small air pocket could get compressed down to nearly nothing once the max pressure is reached but at lower pressure it can interfere with circulation and cause something like you describe. I assume it has done this more than once. if it did it only the first time you drove it that might mean it worked the air out on its own.

That is good way to test it assuming the thermometer is accurate it sounds like that one was working correctly. Is there a possibility that the old thermostat was crooked or didn't fit well allowing flow to go past it?

The pump doesn't mind being completely shut off. It is a centrifugal pump and there is actually less load on the pump when it has no flow. But to answer your question yes sometimes there is a bypass circuit. But usually the heater circuit acts as a bypass allowing some flow to occur even with the thermostat shut. This should be enough to keep the coolant mixed in the engine so that you shouldn't have the type of fluctuation you are seeing. Sometimes the thermostat has a tiny hole that allows a small flow. The purpose is to make sure the thermostat doesn't open late and overshoot the target temp. So is your heater working now? You might check to see if the hoses get warm pretty soon after you start the engine cold.

Fans are pretty much irrelevant at highway speed. The thermostat controls the temp. Your radiator will get more powerful in winter. The radiator can be very cold when the engine is fully warmed up when the temp is below zero outside.

Well it sounds like something is amiss and they were throwing parts at the problem. The sensor for the coolant level should be measuring the overflow tank. Did you flush the system and change the anti-freeze when you changed the thermostat? Was the coolant clean?

-jim

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I don't know much about clearing an air bubble. I parked the car pointing uphill with the engine warm and opened the bleeder screw on the thermostat housing a little, and coolant but no air squirted out. I guess that that wouldn't do much to remove a bubble in the radiator.

Maybe if I park with the radiator cap higher than the rest of the cooling system while the car is warming up, air in the radiator would be pushed out through the overflow tube?

I don't think an air bubble would change size that much. If you happen to know chemistry:

PV/T is constant.

P1*V1/T1 = P2*V2/T2

V2 = V1*(P1/P2)*(T2/T1)

(P1/P2) = 15 psi / 30 psi // assumes 15 psig radiator cap (T2/T1) = (460+195 degrees)/(460+80 degrees) = 655/540 = 1.2

V2 = V1 * 0.60

So a bubble at operating temp and pressure is only squashed to something more than half its volume at ambient temp and pressure. Not to say that the underheating problem isn't caused by an air bubble.

I don't think so. It comes with a fitted rubber gasket, and there hasn't been any leakage around the housing. The removed gasket didn't look new, but I don't think I'd be afraid to use it again. It seemed to fit about like the new one.

I could be wrong, but I don't expect the water pump was replaced to fix the heater problem, if that's what you mean. I mean that if the replacement pump develops more pressure than the stock pump, the hypothetical bypass would let coolant through the radiator too fast when the thermostat was shut, the result being overcooling.

It's inserted into a recess in the cool-side radiator tank. I am guessing it's a sonic detector. Could the coolant-low light be explained by air in the system?

No, the coolant was supposedly replaced when the thermostat was changed six months back.

It looks pretty clean to me, but I can't say I really know what to look for. No brown chunks or oil droplets or foam or scum or major oil slicks. It was a little strong (maybe 55%), so I took out a quart and replaced it with distilled water. It is rather close to 50/50 now.

I would worry more about air in the engine coolant passages. That could cause hotspots. Try bleeding with the engine cold and rock the car a little. Air in the4 radiator can't reall get back into the engine and should get out on its own. A new rad cap wouldn't hurt.

The issue isn't the volume. The amount of surface area that is being removed from contact with anti-freeze is what cause problems. And no your underheating problem (if you even have one) isn't caused by an air bubble. Air that displaces coolant in the engine can only cause overheating. The overheating spike as the thermostat first opens is sounds like what happens when you have an air-pocket in the engine. What it sounds to me is that you have a gauge that reads a little low (not unusual) and now everything is as it was before you changed the thermostat except for an air pocket which causes an initial high temp condition due to the insulating effect of the air. Assuming this happens regularly that the gauge goes way up and then back to its steady normal level, that indicates to me the air pocket only disrupts the normal cooling initially.

The bypass (which is probably your heater) does send coolant to the radiator. It just sends it in a circle so that the temperature stays more or less even in the engine coolant passages. That includes in the neighborhood of the thermostat. The problem is when the thermostat is closed if there is zero forced circulation in the engine, part of the engine can get very hot before the coolant near the thermostat gets hot enough to cause it to start opening. Once it opens then that hot coolant gets to the thermostat and it opens wide which over cools the engine. So it goes too high and then too low and takes a while to stabilize. This assumes the sending unit for the gauge is also in the vicinity of the thermostat so you are seeing more or less what the thermostat sees. The bypass or a small weep hole in the thermostat prevents this from happening.

Yes that is what it is supposed to indicate. You either have coolant or air at the sensor. Or it could be intermittently faulty.

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I find that tonight Walmart is selling 91 octane (the minimum recommended for my engine) for ten cents per gallon more than 89 octane---about a 2.7% premium. I'm inclined to use premium (i.e. 91 octane) under those conditions. I'm not confident I can measure my gas mileage within 2.7%.

I'm thinking of using premium mainly as an antiknock measure, also that it will have more antiknock effect in town than on the interstate. What do you think of using premium in town and regular on the highway?

BTW my temp gauge is hanging around 190 pretty much now, after going to the OEM thermostat. Thanks for the suggestion.

We were making jelly today, and I read that at my altitude water should be boiled away from the jelly mixture until the mixture boils at 219 F or 8 degrees above the boiling point of water. It said that you should check your thermometer in boiling water. I did that and the thermometer read 217 F in boiling water whereas it should have read 211 F for my altitude.

Therefore my test of the removed thermostat has to be reinterpreted. Dumbly subtracting 6 degrees from the temperatures in the text of my previous result gives: "There were some limitations in my test method, but the valve was certainly closed below 184 and at least partly open above 189."

Another condition of using the candy thermometer is to immerse the lower

2 1/2 inches in the liquid. That condition may not have held when I was testing the thermostat. It's also possible I didn't let the thermometer reach equilibrium. That is an issue because the thermometer is enclosed in a glass jacket so that it is surrounded by air and therefore responds slowly.

It doesn't seem that these points explain the whole 20 degree gain in observed temp that I got by changing the thermostat, Not that I am worrying about it much, but I think the in-dash temp gauge is reading too low.