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Leaking water coolant

I have a 1988 GMC Seirra extended cab p/u with a 350 engine. For the past two or three years, I cannot keep coolant in the system, as it reaches about halfway up/down radiator. From this point it stays. I do not get any signal that the system is overheating but during the colder months, the heater has a hard time warming up the cab if I am at a stop light. After going, it heats up. My mechanic says it is a radiator lek but there is no evidence of this and all the hoses have been replaced. There is no sign of coolant on or around the engine joints where you would expect it. What can I look for now?

My '95 Suburban had a leak at the back of the intake manifold, twice over 12 years, I think it was a common problem. Some odd connection back there, I think. Also, the temperature sensor joint on the driver’s side radiator tank leaked, but you would see that.

The 4.3 V6, 5.0 and 5.7 (350) V8 were prone to intake manifold leaks. The intake gasket on my 2000 Blazer developed a leak in 2004 not long after I bought it. My regular mechanic suspected a leak when he found a few drops of coolant in the drained oil. He suggested I take it to the dealer to confirm the leak.

Two months ago, I replaced the radiator on the Blazer. The coolant was a little low in the radiator. There was a crack in the side tank near the top. It took a while, but I found the crack after I removed the fan shroud. Perhap there is a crack near where the coolant level stays constant on your truck.

Good luck,

Ed B.

I’m not sold on the intake manifold leaks. If so, it would not stop at a certain spot once the radiator runs half down. I suspect a bad radiator cap, or a warped cap sealing surface. This is even more probable if the coolant recovery tank is full. The coolant is running out past the cap, and into the recovery. The heat problem sounds like a bad thermostat. Try a flush and fill with a new cap and thermostat, and see if that clears it up.

You could be right - OP needs to get a pressure test, that should show where it is.

If it leaks down to a certain point in the radiator and then stays there, my bet would be a leaky radiator. The plastic tanks can crack and leak, and the seams between the plastic tank and radiator are good leak points. The coolant would flow down the seam and out the bottom and wouldn’t show much. That would account for low heat also. If it continues to go down, I’d suspect something else.

You at least know how high the leak is. That should help you track it down. How about a pressure check? You can just remove the intake manifold and look for a broken gasket. Someone has to do it.

I have an '86 F350 with a leak about 1/3 down on the plastic “chamber” on the side of the radiator. Do you know of any product that works for “patching” a crack in the plastic?