Symptoms:
Drive for several minutes and as the car warms up it seems to get to normal op temp then slowly creep up to the third line. Then it will creep down almost to operating temp and then climb back up abd beyond if i let it, never got much past third line ever because i Stop driving.
I noticed about two months ago that it was starting to run slightly hotter than usual on a highway trip, maybe a bit below the third line where normally it runs about halfway down to the second line. Ran this way for a while untill recently when it started getting hotter.

Makes no difference if im stopped or driving it seems.

Attempts:
Just now i replaced the thermostat and rear seal. The rear seal was toast. No change.

Bought IR pyrometer a while back and confirmed that the temp was high. I measured at the passenger side rad hose and turned the car off at 210 ish. The radiator was only hot for maybe 6 in along the top then quickly dropped to ambient. Yes i know, the fan, but that fast? To me it suggested poor coolant flow which is why i replaced the thermostat. Also given the previous behavior i thought it might be sticking.

 

Forgot to post a question... What could be causing this?
Water pump doesnt leak coolant is topped of and burped.

 

It has a stock temp tstat but a 7lb cap as opposed to the stock 13. It has had this for a couple months before the issue started.

 

Year and model?

Generally if overheats at cruising speed radiator or flow through it is problem.

Do you have any leaks in the system?

Good cap on the plastic tank?

Used the correct thermostat and seals and put in right direction?

 

Why the wrong cap, won't support as high of a boiling point? Start by getting right one maybe.

 

If onset was sudden, I'd be somewhat concerned about water pump slip and possibly block interference, and therefore would almost immediately pull it down to inspect.

What can you tell us about the last waterpump and timing belt service for this car?

 

IIRC the 3rd white line on the gauge is 190 deg. F. That is what the stock thermostat is too, so it may be normal. Also the gauges are historically very inaccurate. The IR gun is a better check. I would be concerned with the readings on the radiator. To me it seems to indicate a poor flow problem. Could the water pump shaft be slipping when it gets to temp but okay when cold? Put the correct cap on and re-test.

 

How hot is the lower rad hose?

How old is the antifreeze and what type?

How old is the waterpump and who built it?

 

Its an 86

Tb was done over the summer. Wp was removed and inspected but reinstalled since it didnt even have 10k miles. It has a plastic impellar fyi. If the shaft was slipping i would imagine i would hear something right?

Onset was not sudden it happened ovrr a couple of months.

 

Lower rad hose was about 160-170 when upper was 210 at idle.

 

When i was pulling back into my apt the temp
Dropped back to normal in the parking lot. So it doeant get hot and stay there forever it floats around.

 

There's a...17d difference in boiling points of the coolant between those two caps..thats enough to create pockets of vapor in the system, which -is- just like a trapped air pocket.

 

By all accounts I've read, the slippage wouldn't be audible.

10K miles; what length of time on the waterpump? The ones I've brought home that sat a while had failed almost immediately.

 

water pump can slip and can be totally random. I would over heat, turn off the car, start it back up and things were fine.. things like that

air in the system, so you are not getting flow.

how does it run after the engine and t-stat are heat soaked? still a problem

definitely feel the face of the radiator when it starts to initially overheat. if its cold, its a circulation/t-stat problem.

 

Why did you use a non OEM cap?

The coolant system self bleeds air but to be sure remove the cap and start the car idling so it gets hot enough to engage the fans. Fill the system to the correct level, install a correct cap and take it for a drive.

Reoprt back when done.