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I recently had a major internal coolant leak which dumped alot of antifreeze into the crankcase. Thinking I had a headgasket leak or internal crack I changed the oil twice and refilled the radiator. I drove around for about 30 mins., no overheating, no contaminanted oil, no drop in coolant level--with the heater turned off. As soon as I started using the heater the coolant finds its way into the crankcase again. I can't find a good diagram of the coolant flow to help isolate the problem. Is there a way for the coolant returning from the heater core to find its way into the crankcase only when there is flow through the heater? Any ideas would be appreciated before I have to drive all winter here in Ohio without heat or spend a weekend tearing down the engine looking.
 

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My Thought Is That The Coolant Crossover Passage At The Rear Of The Intake Manifold Is Corroded Thru The Gasket.. Returning Coolant Could Under Pressure, Be Squirting Into The Lifter Valley, And Draining Down Into The Crankcase. Please Pull The Intake Manifold And Carefully Inspect The Gasket And The Crossover Passages. Very Common On That Engine That The Intake Gaskets Fail Much More Often Than Head Gaskets. Coolant Can Damage The Camshaft Lobes And Crankshaft Bearings If Not Corrected Quickly. Good Luck, Bill. This Engine "must" Have Coolant Changed Every 2 Years And Sealant Pellets Added. Without Fail! The Pellets Are Mentioned On A White Advise Sticker On The Radiator Support.
 

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Thanks Bill,

I figured there was a passage somewhere that was leaking returning heater coolant but I didn't have a good starting point. I'll get to the intake manifold this next week and let you know what I find. Hopefully the leak was recent and didn't corrode anything. By the way I didn't mention the car is an '87 coupe, I have changed the coolant every two years but didn't add any pellets.--John
 

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The Pellets Can Be Purchased At Any Gm Parts Dept. Or "barsleak Gold" Is The Same Product. Barsleak Had The Contract To Sell It To G.m. My Feeling On Adding It After Your Repair Is To Have The Heater Off. Set The Climate System To 60 Degrees. No Use In Possibly Restricting The Heater Core Circulation. Pay Close Attention To The Intake Manifold Surfaces. They Will Sometimes Be Very Corroded And Will Need Re Surfacing. Good Luck.
 

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Personally, I've had nothing but coolent problems with 4.1's, gone thru 3 of em before I said I'd never touch em again. One of em just stoped flowing coolent altogether, one would overheat with more than a 180 thermostat and one just leaked like a cheescloth. Mind you, these are all in the same car with the same rest of the cooling system. Get a 4.5 or 4.9, and forget the 4.1's.
D.
 

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Discussion Starter #7
Ok, I finally go a warm enough weekend to take the engine apart. Exactly what you said, there was a gap in the intake manifold gasket at the rear/ left(firewall). I was surprised to find very little corrosion on the machined surface of the intake manifold and very little on the head side. I did send a lot of time cleaning the surfaces anyway. Not the easiest backyard mechanic job to tackle in January in northern Ohio. Hints for anyone wanting to try it: Lable everything or take lots of pictures of hose/electical plugs etc... Replace spark plugs and wires if they are old(You'll never have better access). Also be aware that it may take a few attempts to start after the fuel lines are reconnected, I was getting a little worried then it started up and ran smoothly. Replaced oil 3 times and replaced the antifreeze with Bar's additive. Thanks for your ideas :worship: ,great diagnosis! Now I'am thinking about tackling the air compressor pump, but I'll wait till spring!:coolgleam
 

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Glad I remembered that diagnosis. I had the same scenario with my 84 Eldo years ago. First sign of coolant mixing with oil is a light brown milky substance on the underside of the oil fill cap. do not mistake normal condensation which is water droplets on the cap underside. Glad I could help, Bill.
 
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