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For a while now I have noticed oil in the anit-freeze overflow tank. Will now its almost black. i have notice when the car is warmed up the oil presure drops to 40psi. No antifreeze in the oil, but have oil in the antifreeze. This all started after I replace the water pump and flush the system when I change it. The LIM/UIM has not been changed. Do you think its time and what would cause oil in the antifreeze and not antifreeze in the oil.

Did you remove the timing chain cover or loosen any of it's bolts other than those that are shared by the water pump when you changed it? That's alot more likely to put oil in your coolant than the UIM/LIM.

HOWEVER, if you haven't ever done the LIM/UIM, do it ASAP. Or your car will suffer an ABRUPT death.

Ultimately, I'd drain the overflow tank and fill it with only water to the cold line, drive for two days normally, then check the tank and see how much you collect over that time. Drain it into a clear jar at that time, let it seperate, and take a pic for us.

I just removed the usual bolts for the water pump and didn't mess with the timing chain cover. I replaced the water pump about a year ago. At that time I also got rid of the Z-antifreeze and change to just regular antifreeze. I'll drain the overflow tank tonight and post pictures of it for all to view. There is quit a bit of slug around the radiator cap spring. This is looking really bad.

If you left ANY DEX in there by not doing a full flush using a commercial kit, it will sludge up on you, and look like oil. Make sure it's what you think. Oil won't sludge up for long, and will seperate into what you'd expect if you dropped oil into water.

The radiator cap has this oily slug on top of it. I am also losing a little of antifreeze and have to add some water to the overflow bottle about every two months.

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Looks more like DEX sludge to me than oil.

I strongly suggest a Prestone flush/fill T kit from your parts store, flush the sucker out very well with the thermostat removed. You may want to try some kind of flush product to break up sludge first.

I suspect you didn't fully flush the system (including block, heater core, and radiator) when you ditched the DEX.

Cool I'll give that another try before I tare into the LIM/UIM project. I thought for sure that I flush it out pretty good when I did the water pump job.

Well I finaly flush the system out again last night. I also back flush the radiator. One thing I did notice was the T-stat was burned looking. I have never seen this before. It looks like someone thru it in a fire. I went ahead and drill the t-stat out. Now I am just playing the waiting game to see if the slug comes back. Oh don't buy Peak Anti Flush fuild. It dosen't work and the directions tell you to drive the car for 3-5hrs with the stuff in the car. I used Prestone flush the last time and it only took about 10 minutes and it work.

Well it has been awhile since I did the flush. I have no more oil flim in the Radiator. System is working just fine.

Gremlins, Jarhead. Gremlins. At this point, I'd go with some peace of mind on it.

Semper Fi.

A friend of mine once looked at a 2002 Bonneville for sale near my house, with a "cracked block or something". Had the worst case of Dex-sludge Ive ever seen.

He ended up buying it for a song. Took all of a weekend to flush it, got almost a coffee can of sludge out of it. We also did the intake gasket work while it was down.

Found out later they drained out a gallon of Dex-cool, then added some Peak green stuff to top it off for winter "it was on a shelf in the garage", we were told.

One mans trash, another mans Bonnie!!!

I still can't believe that Dex-cool stuff would do that much damage to a block. You would think engineers would have enough since. Specialy when I know how much testing goes into the cars before they are release to the public for sale. GM Junk.

Any one have any advice on replacing the water pump? Things to look for?

I noticed the overflow res is low on my 1996 Bonneville SE (3.8 vin K), can see antifreeze on the passenger side of the engine bay (on firewall, front of engine, radiator, etc). I can not see the leak but suspect the water pump (155,000 miles).

I'll change the rad hoses and thermostate while I'm at it. Thanks