SlowPokeEg

Long story , so i'll try and summarize.

Z6 With a t25
Mishimoto Radiator

1. Car runs great all summer. ZERO problems
2. Do a couple pulls 3 weeks ago...come home, engine is overheating (in 40deg. weather)
3. Coolant level is high in the overflow and the cap is popped.
4. Change thermostat to Failsafe (stays OPEN when it fails)
Car runs great
5. I DO get a CEL once every 5 days i'd say. Only on the freeway....turn the car off it goes away
6. Drive 2hrs last night at 80mph'ish (3500rpm or so). Car runs great
7. Drive to work this morning.....Car heats up, i turn the heat on. Air is cold. Turn the heater off......Car starts to overheat. I crank the heat to high and go into work.
8. Leaving work, i check the rad. The neck is dry, and the overflow is FULL. I pour the overflow into the radiator and head to lunch.
9. Coming back from lunch, it overheats a little until i turn the heater on high


OK, so i've ruled out the thermostat, my compression is gravy. I would think my Coolant Temp Sensor, but the fan DOES come on? (takes forever though).

Can the radiator cap be doing this?! I'm at a loss!

HELLLLLLLLLLLLLP. Probobly something stupid.....but it seems to be both mechanical and electronic (CEL).

Probot

hmm, have you changed the waterpump?? and is it possible that its your radiator?

SlowPokeEg

We changed the waterpump say.....30k mi. ago?

Am i wrong to think that the waterpump would still spin regardless? How exactly does a waterpump fail?! I've only experienced leaks coming from waterpump housings, that's when i knew to change it.

And the radiator is only a few months old. Actually cools much better than stock. I didn't spring any leaks in it, which would be the only way, other than a clog somewhere, that i think a radiator can fail.

That's why i considered maybe the cap is junk for some reason?

+ + + lol

Probot

im pretty sure the waterpump impeller spins on a bearing and when that bearing is shot..you get over heats and leaks - it could be the cap, try that first and see if it helps

instructor74

Did you run the car up to operating temp with the cap off to allow air out of the system? You want the heater on full hot fan off rad cap off start the car run until the cooling fan comes on. This could take some time esp in cold weather. If you have done this then you might have a leak in the headgasket. What are you compression numbers? you said gravy but even 5-10 psi low esp on # 2 and # 3 could be signs of a leaking head gasket. If all else fails take it to a shop and have them test for exhaust gasses in the coolant

muasboi

it could be something as simple as your radiator cap (cheap fix)

my second guess is headlift. i had the same problem too, and i had to get arp head studs and a new head gasket

SlowPokeEg

Thanks all.

Got to thinking it SHOULD be the cap.......here's why.

After doing those few mentioned pulls. I parked the car, and heard POP from the motor, checked it out, and the cap had popped (relief). Since the overflow bottle is full while the radiator is near empty, i figure the cap HAS to be letting the coolant bypass into the reservoir, correct?

Now..........where to find a radiator cap that is compatible with the Mishimoto neck.

Froth

Hit up a Mishimoto Dealer, they should be able to get you just the cap. I forget exactly which cap works but there is one out there you can buy at a simple auto parts store that will work. Take your cap in and match it up, make sure you check the depth of the seal/spring and the diameter of the cap/seal as well as the hold down prong location/depth.

-Froth

instructor74

you could always pressure test the cap before you replace it. Man by the sounds od what you are describing I am going to bet that its not the rad cap and you have a leaking head gasket.