Is this consistent with a failing coolant temperature sensor?

Maybe. Maybe they used a cheap one that broke open. Maybe they used the wrong one, that is set for too low a tempeature. The first thing I would do is to measure the temperature at the radiator and compare it to what the shop manual says it should be. The second thing I would do would be to get a known-good thermostat (NOT a cheapie from a chain store) and replace it.

--scott

I was thinking that myself. But the past owner was not a technical guy, and 160 F seems rather extreme. When I look for thermostats for my make and model at advanceautoparts.com, they offer only 180 F and 195 F. The owner's manual lists 195 F for the stock thermostat.

Maybe the shop (Car-X) had a 160 F thermostat laying around and didn't notice or just wanted to get rid of it. Or would there be some reason to put in such a cool one? The repair was done in Minnesota. The problem was that the heater wasn't working. Could the choice of 160 F be related to the coolant leaks that this engine is prone to? There was a class action settlement lately for that problem. Maybe they promote cool thermostats so the customer's engine will be cool sooner when they need to work on it so they can get you in and out sooner.

Aha. Maybe I'll drop by some service station and ask them to check it. I would buy an IR thermometer if they're $20.

no

I don't know which loop you mean. Explain?

The other running-too-cool symptoms I hinted at were:

1) the oil looks brown and not too clear, even though it was changed only about 300 miles ago, and 2) the oil level is about 2/3 qt. too high on the dipstick.

I expect most of the high level is simply from overfilling, but could the oil pick up that much blow-by in 300 miles at 160 F? Maybe I should check the PCV valve too?

My impulse is to put in a stock (195 F) thermostat, but if there is a reason to go with 180 F instead, I would be happy to listen. I don't know much of the pros and cons about using a non-stock thermostat. The car has 160K miles in case that matters.

The computer relies on the info from the coolant temperature sensor. That is different from the sending unit for the dash gauge. Does the coolant fan come on in stop and go traffic on a hot day? If it does that would be a good indication your gauge is off.

This could be related to the engine not being hot enough but it is much more likely you need to change the oil a little more frequently to get rid of accumulated grime that is probably simply due to not changing the oil often. After a couple of frequent changes it will probably not get dirty so fast.

If the oil is overfilled and stays overfilled I don't understand how you conclude that means there is excess blow-by?

Checking the PCV and the hose is certainly worth doing. Is the oil milky brown or just brown? The oil will pick up whatever excess grime is inside the engine. In theory the oil keeps the grime suspended and gets eliminated with an oil change, but if you don't change the oil often enough then it will start to deposit the grime inside the engine and that can be why fresh oil gets dirty fast.

If the reason the t-stat was replaced was to get the heater working it probably already is 195° t-stat. If that is the case, that would suggest the temp gauge just reads low. If your cooling fan comes on, it is unlikely that it is running too cold. Get the temperature checked. If the temperature gauge is not reading accurately I wouldn't worry too much. It should still do it's job. It will tell you how long it takes to warm up and if the engine starts to overheat you should see the needle move upwards.

-jim

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I wasn't clear. I mean maybe the full condition is all or partly due to blow-by condensing in the oil rather than being overfilled at the last oil change. Could the oil level rise 2/3 qt in 300 miles assuming 160 F t-stat and a PCV problem?

I'll try again.

Could the overfull condition be all or partly due to blow-by condensing in the oil rather than due to being overfilled at the last oil change? Could the oil level rise 2/3 qt in 300 miles assuming 160 F t-stat and a PCV problem?

It sounded pretty clear to me. You said "I expect most of the high level is simply from overfilling". Obviously you don't know exactly where the oil level was after the last oil change. Trying to reconstruct where the level might have been by speculating on engine temp and pressure at this point is pure foolishness.

I don't think the oil is going to rise due to condensing blow-by. Worn rings will show up as missing oil. Plugged PCV can also cause you to lose oil, but you would probably be able to see where it is going. If the engine is running extremely rich it could cause raw gas to get into the crankcase that would make the level rise. But if that was what was happening I would think you talking about other symptoms than those you have mentioned. Do you get terrible gas mileage? Is there black smoke out the exhaust? Does the engine run rough? If the only real problem you have is the oil is a little dirty then change it. Do the job yourself or check the level after someone else does it.

-jim

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I agree... the oil lever won't rise, unless maybe there's water leaking into the oil. Are any water globules visible on the dip stick? Does the bottom of the oil filler cap have what appears to be frothy dirty looking half and half creamer all over it? Another test for water is to (carefully) put a sample drop on a hot exhaust manifold. It should just lay down flat and smoke. Sizzling is indicative of water content. (Oil that hasn't been changed in a long time, and used for many short trips 'might' sizzle a little.)

Also, do you always check the oil with the car sitting level, and after giving the oil a few minutes to drain back into the pan? The car not sitting level can induce apparent oil level fluctuations.

(Way back when in the old mechanical fuel pump 'era', some pump failures could result in fuel leaking into the oil. On rare occasion you would find quite a bit.. as in many quarts.)

It is conceivable a 160?F thermostat was installed in an attempt to mask some other issue/s.

Good luck! Let us know what you find.

Erik

No black smoke. Runs well---no complaint there. It pings at idle when cold, probably using regular. I haven't fueled it or checked the gas mileage yet.

I changed the oil and filter. The owner's manual says it takes 5.0 quarts, so I just dumped in a whole 5 qt. jug. Now I find that the oil level is again about 2/3 qt. high, so it would seem the manual is wrong. The oil on the dipstick looks a lot cleaner now.

That is surprising---not that I would know.

In the intake manifold I found two devices other than the injectors. One is toward the rear of the car, (on the side with the even-numbered bank of cylinders). When I unplug it, the gauge stops working. The other is on the end of the manifold beneath the throttle body. When I unplug that one, the gauge is unaffected. Your post seems to imply that that one gives input to the computer. Which one controls the warning light?

I'm thinking that if one sending unit is bad, maybe the other is too. Mileage is 160K.

Maybe I should get somebody to hook the car to a computer for general diagnostics. Presumably a bad temperature-sending unit would be detectable. Should I go to the GM dealer for the diagnostic---or would a large repair shop have the right equipment?

I doubt that it pings at idle when cold. Probably you hear a little lifter noise.

I'd be real concerned if it didn't look clean right after an oil change:} Keep checking as you drive see how fast it gets dirty. I wouldn't worry about the slight overfill if you added the right amount.

-jim

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I dropped by the repair shop today and had the temp checked with an IR thermometer at the thermostat housing. It pretty much matched the gauge reading. Also the temp made it up to nearly 200F after idling for 5-10 minutes. I guess maybe I'd never let it idle that long. Back up at 60 mph, the temp dropped again to 160F. Maybe that's okay, but it makes me worry about sludge.

As for the oil level, it seems the owner's manual is just wrong about the engine oil capacity. It seems closer to 4.3 quarts than to 5.0 quarts. Hmmm ... now that I think about it, the bottom of the oil pan is dented in noticeably---pretty sure the dent isn't 2/3 quart, though.

While he had the IR thermometer out, did he check the radiator? If one side of the radiator is substantially hotter than the other side, you have a radiator problem. It only takes a minute when you have the tool out already.

--scott

That is a good indication that the radiator is doing it's job and the thermostat is not doing it's job (I'm assuming you are someplace in the world where it is the middle of summer). When you let it idle a long time and it gets to around 200F does the electric fan come on like it should? I would change the thermostat. At the least, you should deal with the thermostat before winter sets in. When the weather is cooler running that cool is definitely not good for it.

-jim

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I know he put his hand on the lower radiator hose. Not sure he knew what he was doing though. I told him the thermostat was put in last winter and asked if maybe it was for a cooler temp like 160 or 180. I said I'd read that sometimes people put in cooler thermostats as an antiknock measure, and he seemed to have never heard of that. BTW the car is rated for 91 octane minimum, but I think the previous owners have only run it on regular. I'm planning to continue using regular (89 octane).

I guess it should come on due to absence of draft through the grille when the car is not moving. I'll check that.

Thanks. Planning to change to a stock (195F) AC Delco tomorrow.

A related question: I've never had a car with an air bleeder valve in the cooling system. This car has what I guess is a bleeder on the thermostat housing. I've never used it. It makes me wonder whether any of this might be related to an air bubble in the system. I want to suppose that air is pretty soon forced out through the overflow tube if not let out through the bleeder valve.

When I take the radiator cap off, the radiator is pretty near full, but no coolant spills out. The pressure relief valve on the radiator cap does work, as I see the level go up and down in the tank as it should when the engine warms and cools.

I guess it should come on due to absence of draft through the grille when the car is not moving. I'll check that.

Thanks. Planning to change to a stock (195F) AC Delco tomorrow.

A related question: I've never had a car with an air bleeder valve in the cooling system. This car has what I guess is a bleeder on the thermostat housing. I've never used it. It makes me wonder whether any of this might be related to an air bubble in the system. I want to suppose that air is pretty soon forced out through the overflow tube if not let out through the bleeder valve.

When I take the radiator cap off, the radiator is pretty near full, but no coolant spills out. The pressure relief valve on the radiator cap does work, as I see the level go up and down in the tank as it should when the engine warms and cools.

Yes, both electric fans come on.

I swapped the thermostat with a stock (195F) AC Delco today, and the gauge is now staying close to 190F after ten miles at 60 mph. The one I removed is marked 195, but its structural details are a little different from those of the AC Delco.

Mystery apparently solved. I guess I'll blame the problem on a crummy aftermarket thermostat.

Thanks, all.

Well, the whole thing seems a little odd. Supposedly, the thermostat was changed last winter because it wasn't running hot enough. Kind of makes you wonder if someone didn't just sell them an invoice without doing the work. You also have to wonder what role that failed repair played in their decision to sell the vehicle.

Also you mentioned that the recommended fuel is premium. You might want to reconsider your choice of using regular. It is quite possible that it would get better mileage on premium. It might even be cheaper or at least the same cost to run on premium. It might pay to investigate the possibility with a couple of tankfllls to find out. And you were correct - a hotter engine would tend to increase the tendency to knock. But it also will tend to lower gas mileage and shorten engine life so it isn't going to be cost effective.

-jim

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