External Thermostat Problem - Sanity Check

[cfleury14]

Hello all,

Having an overheating / running hot problem.

I have a '85 3.2L Carrera. Runs great, has been daily driver for past five months. Running 15w50 oil. No recent timing or mixture adjustments.

Throughout the hottest days of summer, the temperature never gets above the first white hash (around 200F). Sitting in traffic in 90 degree heat, 2 hour drives, etc and it stays low

However, last week, on a cool night after a 10 mile drive, I was sitting in the car idling and noticed the temp gauge hovering around the second hash. Thinking it was strange (and immediately suspecting a thermostat issue of some sort), I read literally every thread I could find on here about the oil cooling system and the thermostats. The problem continues - runs hot, and at idle continues to creep up (even above the second hash if i let it go).

I bought an infrared thermometer to check the thermostats and engine temps (to rule out a faulty gauge), and the diagram below shows the temperatures that I found, and the corresponding oil temp / pressure gauge readings. (PLEASE SEE DIAGRAM BELOW - ATTACHED AS PICTURE). For reference, this was on a 65F night, after a 7 mile drive and 10 minutes of idling. (Apologies for the anatomically-inaccurate sketch)

From the measurements, I am deducing several things, listed below:

1) The temperature gauge is relatively accurate

2) The external thermostat is opening, at least partially, because the lines to and from the front oil cooler are heating up

3) The front oil cooler is working, as evidenced by the drop in temperature across the cooler

4) There must be a problem in the thermostat somewhere, because there is no significant temperature difference between the line from the engine to the thermostat, and the line from the thermostat to the oil tank

Does this logic seem sound? I would think that if all were working correctly, there should be a drop in temperature between the line from the engine to the thermostat, and the line from the thermostat to the oil tank (especially given the drop in temp across the front oil cooler feed and return lines)

This leads me to suspect that either the external thermostat element is bad, or the pressure release valve is stuck (fully or partially) open (allowing some oil from the engine to go right back to the tank), or a combination of the two.

I am currently garage-less and lift-less, so removing the entire thermostat housing and dealing with those PITA fittings is beyond my scope at the moment. Given that, my plan of attack was to get an external thermostat element and replace only that element while leaving the thermostat housing physically connected to the car. If that doesn't fix the problem, then would get a replacement pressure release valve for the external thermostat housing and replace that (again while leaving external thermostat housing connected to car). Does this seem reasonable?

One other thing to note, and i'm not sure if its relevant or not: when i crouch near the passenger side front tire to measure temps of those lines, I hear a faint high pitched whining which I assume is coming from the oil flowing through the lines and cooler. Its not the classic "gurgling" that people describe, but more of a whine. Again, not sure its relevant, but it is somewhat interesting

Anyways, thank you all for reading and contributing your thoughts - looking forward to hearing them. Hope everyone is well!

Chris

[ed mayo]

I would say your problem is the engine thermostat, sounds like the front cooler is working.

[David Inc.]

That temperature gradient across the thermostat is sending out all sorts of alarm bells. I don't think that it would be able to go 100% flow through the thermostat, but it should have a hell of a lot more percentage of the flow than that.

[7783911]

removing the thermostat (fun job!) will allow you to test the unit for open/close based on the wax plug within. Yours looks like the flow is good but why is the OUT to tank higher then the in from front cooler? Another thing to look for is to ensure the pipes from the thermostat to the "rad" are not restricted, many get pinched/crushed when folks jack the car improperly. If you have a restriction then you need to remove those pipes and there are several good posts on a 5$ fix to bring those pipes back to round (i did it and it worked)

just my 2 cents

[David Inc.]

Originally Posted by 7783911

removing the thermostat (fun job!) will allow you to test the unit for open/close based on the wax plug within. Yours looks like the flow is good but why is the OUT to tank higher then the in from front cooler? Another thing to look for is to ensure the pipes from the thermostat to the "rad" are not restricted, many get pinched/crushed when folks jack the car improperly. If you have a restriction then you need to remove those pipes and there are several good posts on a 5$ fix to bring those pipes back to round (i did it and it worked)

just my 2 cents

If the out to tank is higher than the return from the cooler the answer has to be extremely restricted flow, either through the thermostat not opening fully or scrunched lines like you say. I wouldn't think it would be smooshed lines because it seemed to come on suddenly for the OP, but maybe something whacked a line and they didn't notice? Definitely another thing to check.

For sure flow is present but highly restricted.

[cfleury14]

Thank you all for replies

Oil lines from thermostat to front cooler have no damage, "scrunching", or external signs of restriction.

Glad to hear general agreement that there is an issue somewhere - it makes sense to me that in a normally-functioning thermostat the oil temperature in the line from the cooler back to thermostat should be the same (or at least similar) to the temperature in the line from the thermostat back to the oil tank. Clearly seems like the issue is in there somewhere, either restricting flow or allowing inappropriate mixing of oil.

In the absence of restricted lines, it seems to me that either

1) return flow is restricted when it passes from cooler return line, through thermostat, to oil tank (perhaps by a thermostat element that is not fully opening)

or

2) there is a lower-resistance pathway through the external thermostat housing that is allowing oil from the engine to go directly into the line to the oil reservoir tank. (this would result in some oil going through the cooler, but a much larger percentage of flow bypassing the cooler, and would manifest with the temperature patterns that I am seeing, at least I think)

I know that there is a pressure relief valve/plunger that resides in the external thermostat housing. From what I have read, its function is to open if the pressure exceeds a critical threshold, and when it opens it allows direct flow from the engine oil line to the line going to the reserve tank. (This is to prevent excessive pressure from building up in the oil cooler and causing cooler failure).

It seems to me that the pressure relief valve could also be stuck open and that would manifest with the same temperature pattern that I am seeing (consistent with idea # 2 above). But again, I may be thinking about this all wrong, so please let me know your thoughts

Thanks,

Chris

[David Inc.]

Scrunching is just 4-D crunching ignoring quantum effects. It's a scientific term. I saw it in a book.

Could be something about the pressure bypass as well, or everything could be working fine but a chunk of something made it down the cooler and is now obstructing flow. Might explain the whining you hear at the cooler.

Just going to have to pull the t-stat apart to check.

[GH85Carrera]

Originally Posted by David Inc.

If the out to tank is higher than the return from the cooler the answer has to be extremely restricted flow, either through the thermostat not opening fully or scrunched lines like you say. I wouldn't think it would be smooshed lines because it seemed to come on suddenly for the OP, but maybe something whacked a line and they didn't notice? Definitely another thing to check.

For sure flow is present but highly restricted.

The purpose of the cooler is to cool the oil, so there should be some drop in the temperature on the return side of the lines. When the system is running, use the thermometer to check the temp of the oil to the thermostat, and returning from the cooler.

Has your car sat for a while? Is it possible a critter like a mouse got up into the engine and statrted building a nest on the cylinders? Use a bore-scope to look under the fan shroud and check for debris.

[boyt911sc]

Chris,

You need to test both the engine and auxiliary thermostats to determine the culprit for your high operating oil temperature. Without physically testing them you are just guessing. You need to test and verify that the slide valve are fully opening/closing during operation.







Place the thermostats in a water bath and heat up. By 185°F, the valve (inlet/outlet) will start to close/open and the warm water will begin to form tiny bubbles. By 200°F, it would be partially closed/opened and totally closed/opened before the water boils @ 212°F. We are talking 2 different valves working hand in hand. One is closing (inlet) and one is opening (outlet) to main uniform flow.

When the return is fully closed, the inlet is fully open. When the return is 20% open, the inlet is 80% open. Or if the return is 50% open, the inlet is also 50% open. And so on.

Tony

[cfleury14]

Glen,

In my original post I included a picture of a diagram I drew with all of the temperatures of the lines running to and from the thermostat (measured w IR thermometer) - please take a look at that and see if it gives you any ideas. That was while the system was running

They are summarized below but picture makes it a bit easier to visualize perhaps. I have included picture below as well - same as picture from initial post (except for some reason it is turned sideways - doing this from my phone , sorry for the necessary head tilt)

Line from engine to Thermostat: 229F
Line from thermostat to oil cooler: 220F
Line from oil cooler back to thermostat: 170F
Line from thermostat to oil reservoir tank: 223F

(It is that last line that makes me think there is a problem with the thermostat - seems to me that temperature should be much lower)

Car has not been sitting - I used a small fiber optic camera to snake through shroud and don’t see any evidence of critters or nests or debris

Thanks,

Chris

[7783911]

remove and do the thermostat test described above...at worse your eliminating one possible culprit. you state the lines to the fron are intact but didnt state the return was..bui i would guess its the thermostat and your reading statis oil temp instead of flowing

[chrisk70]

Chris,

I have an 86, and had very similar symptoms when I bought it in 2019. The front oil cooler was getting warm, indicating some flow, but it wasn't doing much for the overall temperature of the oil returning. I pulled the thermostat with the forward lines still attached as I while I could get the two rear nuts off, I couldn't get the bottom (hard line) nuts to budge. When I got the thermostat canister out, it was stuck about 10% open...hence the reduced flow to the cooler. Replaced that, and all was well again.

Getting the cover off the housing to get to the thermostat was an entirely different story. There are a couple of threads on it. I tried to fabricate a half-moon shaped tool welded to a large nut, but it didn't work for me. In the end I carefully cut the aluminum washer away with a small drill bit and lots of patience. Once that pressure was relieved, the cap came right off.

- Chris