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'87 overheating, boiling radiator, no hot coolant getting to heater core.


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I've owned my '87 for 30,000 miles or so and it's always been a great truck. That all changed on my last road trip, though. I'll try to give some background of the sequence of events without rambling on forever. It was hot out, I was pulling a small trailer, and running the AC on the interstate. Maybe asking a bit much of the poor truck but I've done it a lot in the past without any trouble. The gauge always shows the truck running pretty hot under these conditions but it's never gotten into the danger are of the gauge and never used any coolant. In fact, the cooling system has always been perfect in the past.

Anyway, we're cruising down the road and the gauge starts to rise into the danger zone. I pull it over and it's boiling over into the catch tank. I let it cool down a bit and find that the radiator is really low on coolant. I look at the radiator cap and the gasket is rotted and parts are missing. I figure that the system wasn't fully pressurized and it just boiled off the coolant. I fill up the radiator with water and go down the road to Autozone and buy a new cap, and top up the system again. I hit the road and about 10 miles later the top tank of the radiator blows apart. I think that it was the original '87 radiator so I figure it was maybe just brittle and didn't like the full pressure that a new rad cap provided.

At this point I have the truck towed to my destination. While I'm there I install a new radiator and thermostat. I fill it up but I now notice that the engine is running a bit funny. The "cadence" is off while cranking and the idle is just a slight bit rough. I drive the truck around and top off the system a few times. It's now consistently running hotter than usual, though. Also, I notice that it's having oil blowby into the air filter that it didn't used to. But other than the slightly rough idle the truck runs great.

On the way home it starts running in the danger zone after about 30 minutes. I pull over and it's boiling over again. I let it cool a bit and add more coolant. Back on the road, things are looking alright for another 30 minutes or so. At this point I try to turn on the heater and it's blowing cool air even though the engine is piping hot. Every once in a while the heater would blow a gust of hot air and the temp gauge would dip down a bit at the same time. I pull it over again and go through the same thing, adding more water. Back on the road this time, the gauge stays right in the middle and the heater is blowing hot. I begin thinking that everything is cool but after maybe an hour this time it starts acting funny and running hot again. From this point on I have to pull over every 20 minutes or so and fill it up. It's constantly boiling over, running hot, and won't blow hot air.

Anyone have any ideas? I'm sure that it could be a blown head gasket but I don't know how that could prevent the coolant from getting to the heater core. And it's odd to me that for about an hour of the trip the heater worked, the engine ran cool, and it didn't seem to lose coolant or boil over. I'm kind of at a loss with this one.

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Did you bleed the air from the system after changing the radiator? Trapped air can cause weird symptoms including no hot air from heater core.

 

Pull the dipstick and check for coolant in the oil. Also look on the underside of the oil fill cap for a whitish substance, also a sign of coolant in the oil. See if there is white smoke out of the tailpipe. All of that coolant has to be going somewhere, and since you aren't seeing it drip, it must be going into the engine.

 

If it was a severe overheat, then it is possible that it warped a head. Should be able to verify/pinpoint that with a leak down test.

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I'm pretty sure that I bled out all the air. I loosened the little cap on the upper radiator hose and filled it through that port until everything was full. Do these motors have more of a bleeding procedure than that?

And as far as the head gasket goes, it's not showing any of the classic signs. There's no evidence of coolant in the oil, no steam or coolant smell from the exhaust, no sign of oil getting into the cooling system.

I think the coolant loss could just be that it was all boiling off through the catch tank. Every time I stopped the tank would be boiling away and dumping coolant all over the ground from its overflow tube.

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If the heater's not blowing hot, it was definitely low on coolant. I don't know the TBI intake, but I know the MPFI has a bleed screw on top you have to pull out and squeeze the hose to fill it up properly. I don't know how low coolant has to get in a VG30 to not circulate properly, but if it's too low to circulate, it's not doing much of anything for the engine.

 

Hopefully the overheating didn't damage anything else, though the rough idle and increased blow-by aren't promising on that front. I have heard of head gaskets failing just enough to put combustion gas into the coolant (though not necessarily on a VG). Remove the radiator cap, start the engine, and see if bubbles start pouring out.

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