i have a 1993 del sol SI with 300k miles on it D16Z6 auto.recently had new water pump,thermostat,sensor replaced due to overheating.it runs fine around town with no apparent leaks but on long trips/highway speeds(60-75) the temp needle goes between 1/2 way and 3/4 up the guage.when i checked it this morning the radiator was about a quart low but overflow bottle was full.i suspect a small leak...but where?any ideas?

 

Head gasket, typically if coolant is being pushed into reservoir and rad is low, it means combustion gasses are being pushed into the cooling system.

 

borrowed a pressure checker from discount.it held 15 pounds of pressure for 2 hours,no problems.looked at the replacement radiator cap my son bought,and one of the tangs that holds the cap to the radiator didnt look like its machined right so we re used the old cap and drove for about an hour,highway and traffic with no problems.parked it in gear with the e brake on and verified proper cooling fan operation...all good and right in the middle of the guage the whole time.waiting to see if the radiator is full after the car cools off.is there something i havent thought about here?

 

the radiator cap is often overlooked. they are designed to hold a set pressure, if they go bad or are not correct, they will allow the coolant to push out into the overflow and cause air pockets in the system. its is also not uncommon on a honda for a slighly blown headgasket to push air into the cooling system and coolant out into the overflow. sometimes they will do that but still hold pressure with a pressure tester. what i like to do to check for that is use a combustion gas leak detector to see if its pushing out any combustion gas into the coolant. a spill free funnel also works because you can see air bubbles being pushed out once the system is bled.

 

I would try a new radiator cap.

Btw:The radiator cap can be pressure tested.

 

his old cap appears to be working better than the new one.at least the radiator was full once the car had cooled off so it was allowing the coolant back into the radiator.the combustion gas leak detector that you mentioned.is this something that can be borrowed from an auto parts store or will it need to go to a garage for that?im hoping it was a new,defective cap but would like to rule out the head gasket if possible.

 

Testing of the cap is generally with the same pressure tester as used for the radiator/cooling system.

 

i should have thought to do that before returning the tester.think i will wait and see if its cured or needs more work since the radiator isnt losing coolant right now.maybe a trip to daytona next weekend to test is required...it only seems to act up on the way home from daytona.

 

You most likely nailed it with the cap. totally missed that scenario, glad others didn't.

Some auto part stores will let you borrow/rent the block tester. But if you change the cap and the situation doesn't repeat, you've found your issue.