Atelectatic

Hi all-
About 2k miles and 4 months into an engine rebuild (local, LN resleeve to 3.6, gaskets, bearings, water pump, low temp therm, IMS, JE pistons, etc) I saw a drop in my engine oil and maybe a bit rougher idle. I went to put a little more oil in and checked my coolant reservoir- intermix. I sent the car back to my mechanic who is working it up but thus far nothing has surfaced- block is at LN being checked but thus far they haven't found a fault. Oil coolers (above trans and above engine) and cylinder heads were checked professionally and all passed.

The car is a 99 996 tip 210k miles. Any missing culprits?

DBJoe996

AOS - air oil separator. The AOS is heated by coolant from the engine. But when it fails, usually coolant in oil instead of oil in coolant. The usual culprit for oil in coolant intermix is either a bad head gasket (which I've heard almost never fail) and D-chunk on cylinders 5 or 6....or the O-rings under the engine oil cooler failing to seal because the base of the oil cooler is not flat. I think it was Jake that said he has found even brand new oil coolers that are not flat on the bottom and thus won't seal.

Since you had the cylinders sleeved probably not a cracked cylinder wall. But what about the heads? Were they refreshed and checked for hairline cracks? Just throwing stuff out there....

Compression and leakdown test on all cylinders done? Maybe not now if the engine is disassembled and the block is at LN. That would have been a great baseline to have.

lowpue

Porous case....isn't that an issue with some engines?

Porschetech3

Get a second opinion on the heads. Send to another place and have them test. Your covering all the possible spots, the AOS does not have "pressurized " oil to it so the intermix is always coolant in the oil, not vice versa.

I'm probably the only person on here who has "heard" an M96 head crack. I was rebuilding an M96 engine about 10 years ago, the heads had been rebuilt and tested and known to be good, I was torquing the head bolts on the second "turn to degrees" step when I heard a "crack" sound, and thought "WTF was that?" Convincing myself it was a bolt or sleeve settling or the timing chain flopped over. I finished up and installed and fired up the engine, ran good but quickly realized oil in the coolant. Remembering the weird "crack" sound, removed the engine and bank 1 head and sent for testing. Yep was cracked.

Moral of the story, these heads crack easily and sometimes the crack is hard or impossible to see, get a second test.

Charles Navarro

I'll add some additional details about this since I know specifically about your block. We have fixtures to pressure test blocks and normally do so at 30-40 psi after sleeving to ensure there is no chance of intermix. We got this block back from the builder and re-checked it. It passed our test. We also put the block in the oven and heated it up to 175F and then re-tested it, to see if it would leak at temp. Still no leak. I originally thought it could have been the block as it was one that was previously sleeved with steel sleeves and I was concerned that perhaps we might have missed a stress crack somewhere from the interference fit required for steel sleeves (seen this before), but that was not the situation with your block.

Porschetech3

Any update on this intermix?

Noz1974

I had a loose core plug in one of my heads but surely that would have showed on the pressure test?

fpb111

Charles Navarro

There are various freeze plugs installed throughout the engine. If one of them fails, you get intermix. Pressure testing the block and/or heads will identify if any of those plugs are compromised.