Oil in coolant but no coolant in oil

Asked by Aug 11, 2016 at 12:12 PM about the 2006 Buick Rendezvous CXL AWD

Question type: Maintenance & Repair

Vehicle is 2006 Buick Rendezvous, 3.5 V6. Trans
fluid is full and looks good, but was losing oil.
Vehicle never overheated. Pushrod engine so can't
be head gasket or cracked head. My guess is oil
cooler or cracked block? Engine is torn down to
block and head mating surfaces now - I'm
stumped. Any insight?

9 Answers

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16,155

AFAIK the oil cooler is a separate assembly not connected to the radiator and mounted to block with oil filter mounted to it. I would make absolutely certain it's not trans fluid in the coolant, as you have a trans cooler in the radiator. If coolant gets into the trans your looking at an expensive fix. As far as the oil in the coolant, you may have an upper plenum or lower intake gasket issue, too. I believe that has been a noted problem on 3.5Ls. That is usually how oil gets into the coolant in these engines, with a bad gasket you can have: oil in coolant, coolant in oil, an external coolant or oil leak, or an internal coolant or oil leak. (could also happen with a cracked head or block, in a worse case scenario.)

1 out of 1 people think this is helpful.

Thanks Ken. I removed the upper and lower intakes and gaskets looked good but I will be replying them upon reassembly. One of the aluminum heads failed the pressure test but this is not an OHC engine, it is a pushrod engine where oil flows through rods and falls back down into the case via gravity so head gasket and heads shouldn't cause oil in the coolant? Oil cooler is on the block but was checked by mechanic and passed. Maybe I will remove and have it tested. Getting a reman head for the failed one and putting back together at this point and then maybe pressure test again?

11,305

I had a similar issue with my car. I replaced the gaskets on the intake, and used the top grade expensive ones. $75.00. It stopped the oil leaks, and over heating. A tiny leak can push oil out of the engine, and head gaskets can push oil into the coolant system. This also can cause the engine to run hot as hot oily air hits the coolant and heats it up. My guess would be gaskets, but since you have the heads off, it would not hurt to have them checked for warping.

77,735

Your idea that a push rod engine cant blow head gaskets is false, they can and do blow them all the time, even back in the old days they blew and is more common now as the engines are mixed metals, cast blocks, cast aluminium heads, the two exchange heat at different rates so there is considerable stress on the head gaskets and cause of the heads to warp or even crack.

1 out of 1 people think this is helpful.

Sorry tennisshoes, maybe I misspoke. What I meant was that there is no oil pressure there because oil is moved up push rods via hydraulic lifters. If this is the case, how would pressure less oil creep into a pressurized coolant passage? Wouldn't it be the other way around (coolant in oil?). Not trying to flame the post, just doesn't make sense. I did check the gaskets and they looked good, no blown passages. Anyways, any insight is appreciated!

77,735

It has been my experance through the years, that if you have oil in the cooling system you generally find something wrong with an engine part, be it cracked or badly warped head, or a cracked or damaged block, you usually find something that forces compulsion gasses through an oil passage and it winds up in the coolant, normally if it is just a head gasket the coolant winds up in the oil. But it looks like you are on the right path. I just wanted to clarify the head gasket problems are just not confined to overhead cam engines.

Appreciate it. I did have a cracked head and bought a used one. Pressure testing it and resurfacing. Plan is to put it back together and see from there.

77,735

Sounds like your doing it right, hope all is good afterwards.

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