I have a race setup cooling system that is pushing into the overflow bottle.
It has a re-route kit on it, and an all aluminum radiator.

The coolant is filling up the overflow bottle, and seems to be related to high rpm.

Have tried multiple radiator caps, from stock to 20lbs.
Have also tried backing off of the timing.
and I have also noticed that the coolant does not go back into the engine.

When checking the radiator, it usually will take 1/2 gallon of water to top it off.

any idea what is at root cause?

Wingnut

 

Blown headgasket, the engine is pushing combustion gas into the coolant system.

 

Check the mouth of the radiator, see if there's a low spot / dent that might be letting higher pressure seep past the radiator cap seal.

 

Cap seal or headgasket as mentioned. Pressure testing is in order to determine.

 

Ok, the head gasket will be replaced.

Previously, the head was decked, the block was cleaned with square razor blades, and then went back in with ARP studs.
Maybe it needs MORE to get a good seal ?

Can anyone recommend a sealing spray?

Wingnut

 

If it was the radiator cap seal then the overflow would be higher but the radiator would still be full. How is air getting inside?

It could be that the head is cracked. Or I have seen on Hondas where the head would lift or the bores would sink when the engine got hot letting air into the coolant jacket. I don't think that is as common of an issue on miatas. What power level is this car?

 

You can use a little fine scotch brite on the block, but nothing more. I wouldn't use the scotch brite on the head though. Just make sure both are flat and clean and torque them down. Don't forget both dowel pins, and make sure there isn't any oil or other chemicals present.

 

I did some more tests last night. I was able to get a radiator pressure tester. I filled up the coolant system and then applied the pressure.
I can see the pressure bleed down, and no coolant on the ground.

Performed a leak down and compression test.
1 - 6% 180psi
2 - 4% 165psi
3 - 7% 140psi
4 - 18% 160psi

When I released cylinder #3 compression test, I saw a mist. Could be the coolant/combustion is crossing over with cylinder #3.

I removed the cylinder head and sent it to my local machine show for a pressure test.

The head was good by the pressure test.

2 issues:
Cylinder #3 is venting between the combustion chamber and the coolant system
Cylinder #4 has high leakdown.

I may just put on a new head gasket and see if that fixes the issue with pressurizing the coolant system.

any other thoughts?

Wingnut

 

Also, my leakdown tester wouldn't push more than 12psi :(
This may not be enough to push through a small leak to produce high % leak down. (?)

My leak that is pressurizing the coolant system, is now isolated to the head gasket or the block.

Maybe there are other possibilities for pressurizing the coolant system?

Wingnut

 

Using another new Cometic 4568-040 head gasket with BP05 cylinder head.

Wingnut

 

I would check the head with a straight edge.

 

Head looks true to a straight edge across corners.

Wingnut

 

Was the block decked for the Cometic MLS gasket. It requires a very smooth finish to work. Original gasket is a composite that is supposed to bite into the rough finish of the block and let the head float ever so slightly. The 2 types of gasket are not directly interchangeable. Also do you have a coolant flow restrictor on the outlet of some sort. You need to keep the head under pressure to make the coolant wet out against the cylinder head to keep down hot spots that will boil and cause the problem you describe.

 

The block was not decked for Cometic gaskets.

I don't have a coolant restrictor, other than the thermostat.

I have the heater core circuit plumbed directly from the rear coolant adapter(M-tuned) to the back of the water inlet adapter(Hawley).

Wingnut

 

​​​​​​A stock miata BP head gasket is MLS. You might be thinking of the B6.

 

I cleaned the decks of both the block and head with a razor blade, then a couple wipes with brake cleaner.

Torqued the head studs in 3 passes to 71 ftlbs.

Filled the coolant, attached the pressure tool to the radiator and the pressure bleeds down slowly. No water on the ground.
The water level dropped a little in the radiator.

Just disappointed. I'll replace the block between now and November.
I may option the Ecotec conversion.

Wingnut

 

If the deck of the block and the face of the head are verified true by a machine shop then the block or head could be cracked. But usually it is a HG area problem. And it is solvable.

 

Spray both sides of HG with permatex copper head gasket spray [ 2 coats] and if it is the head gasket that well solve it. I have been doing this for years and it works. I know you shouldn't have to, but I have never had a issue doing this on my engines over the years nor on others that I have done.

 

Wondering if you got to the bottom of this?

It's relevant to my interests as I am seeing CLT spikes on WOT that seems to trend with MAP.

Block and head were also flat (0.002" was the largest measurement I took with a straight edge and feelers).

 

I removed the head and inspected both the block and head mating surfaces, Saw no issues.

I put on a new Cometic head gasket, and the issue was resolved.

Using a head gasket sealing spray *May* have benefitted the sealing, but I have never used it.

Wingnut