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Strange engine temperature issues, sticking t-stat? 700

posted by  Rtilghman on  Sat Oct 16 07:05 CST 2004
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1985 745t

The other night I was running some errands and I noticed that my temperature guage was a little high. Normally it hits 12pm and drops to 11pm, where its stabilizes (with slight variances for heavy load, boost, hills, etc.). However, this time it was sitting up just shy of 1pm and hanging there, as though this was a new stable point.

Obviously nervous I checked everything out. I replaced the cooling system about 6 months back (everything except the pump), and I've run it through two 2400+ mile trips since, so I figured its probably in pretty good shape:

- Hoses checked out fine
- pump pulley seemed tight and pump looked fine (no leaks)
- radiator looked good
- fluid level checked out

The hose from the rad to the head wass hotter than the exit hose down below OR the radiator itself, but I figured it could just have been nothing worth noting (normally temp. conductivity to the hose, etc).

As I was driving the temperature would increase when I turned off teh car and ran an errand. At one point when I came back the temp was up around 2pm (quarter inch below red), and after I started running it STAYED at that level. However, a few minuites later it suddenly dropped back to the normall 11pm position (I think after I hit some rough road), and ran like that for awhile with forays into this same problem periodically since.

My question is what the hell is going on with my brick? I'm inclined to conclude its a sticking t-stat that isn't opening all the way or not as its supposed to anyway. The rough road and temp drop was what really led me to this conclusion (from a hypothesis) in the first place since I thought maybe the stat had just been shaken loose by the jolt.

Obviously a t-stat is an easy replace (I've got a spare), but I'd rather not deal with slightly draining the coolant to avoid massive spillage, pulling the housing, and replacing it unless it'll solve the problem. Does anyone know of something else that could be wrong? Common faults that could cause the kind of erratic behavior I'm seeing? Faulty temp. sensor? I would think a temp sensor wouldn't respond like I'm seeing, but I don't really know.

Any opinions or insight appreciated. Thanks in advance.

Best,
Rick







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    Reply to all... 700

    posted by  Rtilghman on  Sun Oct 17 10:23 CST 2004
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    I pulled the stat and the ring loked pinched and damaged. I' yhinking it might have been heat or position based damage, but whatever it was the stat wasn't performing right. The new one I swapped in has been rock steady all day, so I figure the stat was the right call.

    As for a compression check, I don't think that would be merited given my symptoms. The temperature is, despite the various positions, fairly stable, and if the gasket had failed I would expect more variation as air entered the cooling system, caused local overheating, and got burped. I would also expect my coolant to be overflowing constantly with the 75kpa cap I have in place, and it hasn't dripped a bit. Going the other way I would only be loosing coolant and seeing white exshaust, neither one of which is the case.

    Thanks again to everyone for the input, preciate the help.

    Best,
    rt








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    Strange engine temperature issues, sticking t-stat? 700

    posted by  Patrick of Mtl on  Sat Oct 16 09:15 CST 2004
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    last visit: Tue Oct 12 23:46 CST 2010
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    Sounds like a sticking t stat to me. It is also the easiest to check compared to timing/fuel issues that could cause overheating. BTW I no longer use after market t stats on my cars, as too many have started sticking toO soon as compared to original dealer parts ( they are pretty much the only part I can get elsewhere that i buy from the dealer).








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      Strange engine temperature issues, sticking t-stat? 700

      posted by  Rtilghman on  Sun Oct 17 10:19 CST 2004
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      I tend to agree, but I'll buy aftermarket if the part is equal to or better than the one supplied by Volvo. This extends to all manner of parts, from brake pads to plug wires. The t-stat I used when I swapped out the old coolant system is a brass 87c Wahler, which as far as I know is tops for the part at present.

      I replaced it anyway... with a scan-tech t-stat I had kicking around (88c). I guess we'll see if it lasts longer than my 6 month Wahler, eh? ;)

      Thanks,
      -rt








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    Strange engine temperature issues, sticking t-stat? 700

    posted by  ndesouza on  Sat Oct 16 07:25 CST 2004
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    last visit: Sun Mar 18 15:14 CST 2012
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    One easy way I've found to do work on any of the upper coolant-system components (like gauge-sensor, coolant-sensor, thermostat) without draining the coolant is by unhooking the coolant-overflow tank and lowering it down below the exhaust-manifold level and catching the overflow in a container. Of course, if you have a turbo then there's not so much space in that area - in which case disconnecting the lower-hose from the overflow-tank might be an option.

    Noel








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      Strange engine temperature issues, sticking t-stat? 700

      posted by  someone claiming to be swede4 on  Sat Oct 16 12:21 CST 2004 [RELATED]

      Do a compression check to make sure you don't have a leaking headgasket.








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        Strange engine temperature issues, sticking t-stat? 700

        posted by  VolvoSavvy on  Sun Oct 17 00:52 CST 2004
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        If all of the above recommendations fail to fix, I would check grounding connections for the guage first. Also you could have a faulty temp sensor too!

        Steve........







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