b25 - loosing coolant but no visible leaks or overheating.

[robo_trigger]

So the motor is a m20b25. sometime in the past 10k (before purchase) it had the head replaced due to a tb failure.

Now on to the issue, Under normal driving conditions it recently has begun to run a little hotter. (It was running cold when I first bought it due to a thermostat stuck open, replaced that last fall with a lower temp tstat. after that the temp would never rise above 1/4, functioned perfectly.)

Now after I pulled it out of storage this winter it started running a little hotter from time to time (especially when I would pull off the highway). It would sometimes climb to the one third mark. I chalked this up to a fan clutch so bought that (havent installed) but I've also noticed I've topped off the coolant more than I'm comfortable with. It never empties the reservoir but every couple of weeks it will get low enough to trip the light.

No visible leaks, never even comes close to overheating.

So I tested the head with a block tester (sucks air in from the overflow tank through a fluid to test for presence of exhaust gases) which came up negative. Is it possible because of the slow rate of coolant loss that the exhaust gases are only entering coolant under certain conditions?

So basically to round it up, heres the issue
car runs hotter than I would think it should, creeps above 1/4 sometimes to 1/3, especially when coming off the highway. Also Ive noticed that if I warm the car up, turn it off, and back on the temp is high, it will come back down to just above 1/4, but is it possible this is when I'm loosing coolant out the overflow hose (ive noticed its a little damp at the end once or twice). I'm a little stumped here.

Theoretically new
water pump
h/g

on the newer side
thermostat
radiator

[SmokeE30]

So have u pressure tested for leaks? If its vehicle speed dependent its probably an air flow problem but vanishing coolant could be leaking from Any number of places mine leaked from the spider hose connection to the thermostat housing and washard to spot

[jlevie]

As suggested, pressure test to cooling system, cold and hot. If the system holds pressure for an extended period of time, there are no leaks.

I've only found the chemical test to work in the case of gross head gasket leaks. But an easy test is to remove the cap to vent any pressure when the engine is completely cold (like after sitting overnight). With the cap back on the hoses will be soft. Drive the car for a bit and let it completely cool down again. If the hoses are hard then, air is being pumped into the cooling system. An alternative test is to fully bleed the system, drive the car, and re-bleed. Release of a significant amount of air on a re-bleed is evidence of a head gasket leak.

The definitive test is to pressurize each cylinder for several minutes with 175-180psi while monitoring cooling system pressure with a sensitive gauge. A negative result on a cold requires the test to be repeated on a hot engine.

[robo_trigger]

Thanks Jim, I honestly figured that the tester would only be effective on a really huge leak, which if this is a leak, would not be a huge one. I'll have to run it through the tests you described.

[robo_trigger]

Is there anything else aside from a cracked head that could cause it to overflow without overheating? I inspected the system more thoroughly. The only leak seems to be from right before the heater core on the valve inside the drivers footwell. But thats barely a dribble. it seems to be soaking into the foam on the knee bolster cover. The car is definitely overflowing though as the tip of the overflow hose is wet. I left a water bottle on the end of the hose to try and discover how much and when but it fell and got chewed up by the fan so that test failed.

Next step is to do a compression test, is it necessary to monitor the cooling system pressure at the same time? Won't I have poor compression in a cylinder if theres a crack?