'92 Jimmy Overheating - Please help!

[old_master]

Coolant is drawn into the engine through the lower radiator hose; it circulates through the engine and into the intake manifold where the thermostat is located. Once the coolant has achieved whatever temperature the thermostat is designed for, it opens and allows coolant back into the radiator to be cooled and the process starts all over again. In your case, (I believe) coolant is unable to reach the thermostat sufficiently, so by bypassing the heater core you are providing an additional way for the coolant to get back to the radiator where it can be cooled. Take into consideration the diameter of the heater hose as compared with the diameter of the upper radiator hose and it’s easy to understand that you’re still running a little warm.

I would bet you'll find the wrong gasket, however, pay close attention to the coolant passages in the cylinder heads and intake manifold for obstructions. It is possible that "the last guy that worked on it" may have mixed Dexcool and green anti-freeze, a big no no, it makes mud and can restrict flow also.

Another little tid bit of information. To efficiently burn gasoline in an engine, there are three things that need to be done to it: First it needs to be atomized. That is done by the injector spraying it. Next it needs to be vaporized. This is accomplished either in the intake manifold, or on top of the valves depending on the design of the fuel injection system. Last it needs to be compressed, then you light the fire! Point being, the vaporization process is accomplished in part by heat transfer from warm coolant in the intake manifold. This in mind, an engine running cooler than designed will not get the fuel economy that it should.

[old_master]

I have disassembled engines and found core plugs in the block that the "last guy" couldn't get out so he knocked them inside the engine. Even with these restricting the coolant flow, it doesn't cause an overheating concern. Core plugs, sometimes referred to as "freeze" plugs, carry a myth that they will "pop out" if an engine block freezes. In over 30 years as a tech I have NEVER seen one "pop out". They are generally installed with a glue or sealer on them to prevent leaking. The reason they are there? When an engine block is cast, sand is injected through these openings and removed after the block has cooled leaving a void called, yup, you guessed it, a coolant passage. Gotta be something serious to restrict flow the way you are experiencing.

[BlueMonster]

Well your name "old master" is definitely accurate. Well I am not sure how old you are, but you are the master for sure. : I just got in from removing the intake manifold and what did I find? Two ports, one on each side were blocked off with little metal snap out plates. It looks like someone changed the gaskets but didn't bother to bust out the plates to allow the coolant to flow through the manifold. As a result it looks like some coolant was blasting by and made a mustard sort of looking stuff out of the oil and coolant mixing together. So I have to finish putting the new gaskets in, then I guess an oil change is an order.

Thanks so much for the help. I will post the final results when I get it all back together. At least I feel hopeful now.

[blazes9395]

Originally Posted by BlueMonster

Two ports, one on each side were blocked off with little metal snap out plates.

As a result it looks like some coolant was blasting by and made a mustard sort of looking stuff out of the oil and coolant mixing together. So I have to finish putting the new gaskets in, then I guess an oil change is an order.

O.K. if its a TBI engine, "Z" engine code, then you need those plates there, if its a vortec CPI engine "W" engine code, then you have a little different type of gasket. You have to be very specific at the parts counter, especially if you go to a place like Canadian Tire.

Do you have a lot of coolant in the oil? You mentioned compression checks out good. Also, when refilling the coolant system, leave the ran cap off while filling the system, even while its running, this will make the air purge out much faster. Good Luck!

[BlueMonster]

Well I ripped it all apart and sure enough there were coolant ports being blocked, including the port that circulates the antifreeze for the heating system. So I replaced the gaskets with the correct ones. VIN code "Z", but it has 6 cooling ports instead of 4. Anyway, it made not one damn bit of difference. It still overheats, 230 degrees. If I touch the upper rad hose right where it connects to the thermostat housing, it's hot. But as I move my hand closer to the rad, it is totally cold. So I am still stuck in the same boat. Any other suggestions?