Oil in coolant. Need help ples!
 

Background: Block (94) was built about 9 years ago with ARP hardware (not sure if they were torqued to 40 or 60ft lbs as that was a huge arguement back when I built the motor), 84mm pistons, Mazda gaskets / seals and a 99 head with a minor p&p. The setup has about 9k on it and the majority of that is NA driving (it has been tuned like 4 times also). The car has seen a few trips up to 250 degrees about a year ago compliments of a bad tune and oil pressure is usually 60-90psi. Also I have drained the coolant numerous times since the intial build and have never seen oil in the coolant nor coolant in the oil.

I checked the coolant two weeks ago and found oil in the coolant but no coolant in the oil. We did a leakdown test with the engine warm at 100psi and had leakby of 3% - 7% on all cylinders. I tested the oil cooler at 100psi (via the coolant route) with no sign of failure. Yesterday we pulled and inspected the head, head gasket, and block. There are no obvious signs of head gasket failure or cracking. I am bringing the head to the machine shop today to have it checked out...is there anything else I should be looking at or for?

I have a cometic head gasket ready to go on it this time around and am planning on using the ARP hardware again.

Thoughts?

You sure it wasn't something as simple as a little oil spill?

I use 60 ft-lbs and the wider OEM washers instead of the ARPs to spread the load.

What do you mean an oil spill?

I will have to pick up those larger washers, akamai!

I just meant perhaps someone accidentally spilled some oil into the coolant when the radiator cap was off -- something like that. It seems like you are being methodical in eliminating the usual head gasket or block/head crack causes. Are we talking a lot of oil?

Yes, a lot. When I took the radiator cap off while pressure testing the cooling system (20psi) I had a chocolate malt pouring up out of the radiator.

Dropped the head off at the machine shop. They looked at it quick when I dropped it off and said it probably isnt the head (or headgasket) but they are going to pressure test it and check its trueness.

Have you tried jumping the starter?

I had to do that before, it was fun. But that has absolutely nothing to do with oil in the coolant at all, but thanks!

https://cimg9.ibsrv.net/gimg/www.mia...5fa8215ccf.jpg
https://cimg5.ibsrv.net/gimg/www.mia...20d99ab9a4.jpg

Looks like you've covered all the obvious culprits. Could be a trashed block. I hope you post what you find in the end.

I most definately will. If its a beat block its going to get the guts swapped to another block. I hope thats not the case though 😬

Got the head back, everything is good on it. Pressure test good to go and no warping. Any thoughts?
https://cimg6.ibsrv.net/gimg/www.mia...fbc04f46e8.jpg

Quite silly but...do you by any chance run an OEM oil/water heat exchanger?

I do and I pressurized the coolant path (of the cooler) to 100psi with no leakage. I had it in liquid to easily id if there was a leak. No leaks seen.

Assuming the deck surface of the block itself is flat, I'd say you've got a crack in that block somewhere.

They are both good to go so either a crack or the head gasket was just starting to go bad. I am hoping for the latter.

Anybody have any ideas on how to find a block crack?

Magnaflux.

I was referring to a way to check it myself (other than visual inspection) but thank you. I am going to put it together and hope for the best 🤷-♂️

Good luck. If it does the same thing, then I would remove the oil cooler and try again (in case your 100psi test at room temperature wasn't valid). If it's still a no go, then it's time for a replacement block.

BTW, been down this road myself. Had an engine that made tailpipe steam. Cracked block. Dangers of learning how to tune. Fortunately, Miata blocks aren't expensive (unless you live in Hawaii, that is).

You can pick up a dye penetrant/developer kit.