'92 Jimmy Overheating - Please help!

[BlueMonster]

Hello everyone. I have a '92 Jimmy with a 4.3 that is overheating.
Here is the way things happened. I bought the Jimmy from a guy in town. After the safety inspection and what not, I noticed that it was overheating. I made sure that the fan was coming on and decided that maybe it was the temperature sending unit or the gauge itself since the rad was COLD. So I installed a mechanical gauge with it's little sensor. Well the mechanical gauge showed that it was overheating as well. So I changed the thermostat.....3 times. The rad still wasn't heating up, and neither was the top rad hose. So I double checked to make sure the thermostat was in the right way (it was). I read about a service bulletin from GM where the engine would overheat because of an incorrect water pump. So I decided to replace it. Same problem. Everyone told me it was the clutch for the fan, even though the fan was turning like mad. So I replaced that and both rad hoses. Same problem, but now the rad heats up. So I flushed the entire system for 30 minutes. Same problem. I did a compression test to see if it might be a head gasket problem, but the compression came back good. I don't seem to be losing any coolant, and if I take the thermostat out complelety, it stops overheating but then I have no heat/defrost. So I have no idea what to do now. I am thinking that the rad needs to be changed, but everyone I talk to is saying something different.

Can somebody please help me? I am losing it.

Thanks to anyone who can offer some insight into what's happening here.

[DetroitMuscle]

Is it really overheating or is that just what the gauge says?

My gauge either overheats or stays cold, are you sure the thermostat is good? any coolant bypasses on there that might be clogged or pinched?

[Smoke666]

As you say that it stops heating up when you remove the termostat there's chance that it's the termostat. Did you used GM thermostats or cheaper ones? Often, cheaper thermostats does not work well, they frequently open at to high temperature causing heating problem. I've already had this problem with and 87 Ford Ranger and the first thermostat I've put was cheap and the truck was still heating. When I've put a Ford thermostat, the problem where solved, it never heated after. Also, chek your radiator grid, it might be to dirty not allowing the air to flows though, if so, cleans it with compress air.
Hope that helps!

[BlueMonster]

Well I know it's not the gauge/sending unit, because I used a mechanical gauge that I bought at the store, and it also indicates that the engine is overheating.

I have tried 4 different thermostats in total. 2 GM and 2 from the local parts store, including a 160 degree thermostat. I know it's not supposed to use a 160, but I tried it just to see what would happen. I overheated, but not as badly.

My Jimmy doesn't have any bypasses except for the overflow bottle, which is in good shape.

I also did check the radiator fins to make sure that they were all good and not obstructed. I even did a test. I started it up and when the fan came on, I put stuff in front of the rad behind the grille. The air moving through the rad sucked the stuff up against the rad. So I know that air is flowing really well through the rad, because it was able to even suck my hand up against it when the fan was on.

[94 Jimmy]

I had a similar problem several years ago with another brand of car. It would overheat even though everything in the cooling system seemed to be OK.
I found that most of the tubes in the radiator were totally of partially plugged. I was using a garden hose to cool the radiator and noticed that with the engine running a section of the core dried while some of it remained wet.
I unsoldered the radiator and rodded it out, problem solved. I would not try this repair again, first it's a pain secondly most radiators are plastic now.
You could try a junk yard for a cheap radiator or buy a new one $120-150 depending on if you have an oil or AT cooler. Or you could take it to a radiator shop and have a high pressure flush, costs about the same, but it's a lot easier.
Good luck, let us know what you find.
See ya
94

[BlueMonster]

Thanks for the answer 94 Jimmy. I was actually looking at it last night, and I found that half of the rad gets warm, but the other half stays cold even though the engine is cookin'. I was going to do the replacement last night, but there was no stock on the rad. So tonight, I am going to pick up a rad after work and replace it.

I'll let you all know what I find.

Thanks again for the help!

[BlueMonster]

Well talk about dumb. I replaced the rad and it is still overheating. So I did some experimenting and found that if I keep the upper rad hose disconnected from the thermostat housing, no coolant flows out of there when it's supposed to. The thermostat wasn't open. I pushed on the top of it with a screwdriver and coolant squirted out. So I took the thermostat, along with the other 3 thermostats that I tried and put them in a pot of water on the stove with a thermometer. At 180 degrees, the thermostats opened like they were supposed to. So I know the thermostats are good. I put the thermostat back in the engine and started it up. At 230 degrees, the thermostat opened a bit and coolant started "trickling" out. I think that means, I have a problem maintaining pressure in the system, right? Everything is new including the rad cap, so I am not sure what the heck is going on. The compression checked out ok. So what now?

[blazes9395]

Has anyone done intake gaskets recently on this truck? If the wrong gaskets were used, especially if its a TBI engine, it will do exactly what you say.

[BlueMonster]

I have no idea. I have only had it for a month. I am going to pressure test the system tomorrow. I am really hoping that the pressure test will reveal where the pressure loss is. I don't want to have to take the intake apart, change the gasket and then find out that the loss of pressure is still somewhere else.

Aside from pressure testing the system, is there any other way I can find out where the pressure is being lost? (Especially if it is the intake manifold gasket?)

[old_master]

Originally Posted by blazes9395

Has anyone done intake gaskets recently on this truck? If the wrong gaskets were used, especially if its a TBI engine, it will do exactly what you say.

I think you hit the nail on the head here. One passage on each bank is blocked off in the "wrong" intake gasket not allowing coolant to flow through the cylinder head.

The pressure test will only show if a leak exists. Excessive pressure in the radiator would indicate a head gasket leak into a combustion chamber most likely accompanied by thicker than normal, white exhaust "smoke", bubbles in the radiator, and a misfire in the engine. Coolant in the oil would indicate an internal leak to an oil passage most likely accompanied by a tan "milk shake" substance on the under side of the oil fill cap, and possibly an engine misfire. A restricted coolant passage, such as the use of an incorrect intake gasket, can only be detected by removing the intake and visually inspecting it.

[BlueMonster]

I think you hit the nail on the head here. One passage on each bank is blocked off in the "wrong" intake gasket not allowing coolant to flow through the cylinder head.

The pressure test will only show if a leak exists. Excessive pressure in the radiator will indicate a head gasket leak into a combustion chamber most likely accompanied by thicker than normal, white exhaust "smoke" and bubbles in the radiator. Coolant in the oil will indicate an internal leak to an oil passage most likely accompanied by a tan "milk shake" substance on the under side of the oil fill cap. A restricted coolant passage, such as the use of an incorrect intake gasket, can only be detected by removing the intake and visually inspecting it.

I am confused. Don't I have the opposite problem? I thought my problem here is "lack of pressure" instead of "too much pressure". If the engine is overheating, I can open the rad and nothing happens (I wore insulated gloves and a full face shield for the sake of safety when doing things). I would expect that it would have spewed coolant everywhere due to the pressure. But nothing happened. There is definitely no coolant in the oil and the exhaust is pretty normal. I even went as far as to put plastic in front of the tail pipe to collect water droplets and steam. After 2 minutes, there was still no moisture on the plastic. So I don't think that my coolant is making it's way into the engine anywhere. And the coolant level stays consistently full.

So am I misunderstanding something here?

Thanks again, by the way.

[old_master]

From what you have posted here, it appears that you have no leaks, either internal or external. Let me attempt to clarify your confusion. Increasing engine temperature is what causes coolant pressure to increase.

The fact that you just purchased this vehicle leaves a lot to the unknown. You have no idea of the history of this vehicle. Perhaps the last owner sold it because of this problem. Maybe because the last person that worked on it was just a mechanic, not a technician. Maybe the intake gaskets were leaking oil and after installing the incorrect gaskets, the engine started overheating and he gave up. Who knows? If the incorrect gaskets were installed, this would create a restriction in the flow of coolant through the entire cooling system therefore causing the overheating. The passages in the cylinder heads and intake manifold are part of the main coolant flow. The fact that the radiator is cool and the engine is hot confirms this theory of restricted flow, (provided the thermostat is working correctly). The only way to know for certain is to remove the intake manifold, carefully remove the gaskets and take them to a GOOD auto parts store and compare them to the proper ones. Pay close attention to the positioning and diameter of the coolant passages in both sets of gaskets. Before reassembling, make sure the coolant passages in the heads and intake are clear. This scenario most closely fits your set of symptoms, restricted coolant flow through the engine. Like mentioned earlier, you don’t know the history, therefore you have got to start with the basics. RULE OF THUMB: Always keep in mind that the guy that worked it before you didn’t know what he was doing.

[BlueMonster]

Right ok. I see where you are coming from now: The thermostat housing is attached to the intake manifold, and when I remove the hose, the coolant just trickles out of the thermostat indicating some kind of blockage in the intake manifold (assuming of course that the thermostat is good, which I believe I have proven).

So now, that being said, here is another interesting thing that I found out today. If I bypass the heater core, it runs much cooler. What I did was disconnect the heater hoses and connect the heater hose directly from the intake manifold to the rad, bypassing the heater core completely. Now it no longer goes all the way up to 230 degrees and beyond. Instead, it fluctuates back and forth between 205 and 210 degrees. So it's no longer overheating, just running hot (but of course now I have no heat). The top rad hose gets hot now, and so does the rad itself. So anyway, I have a few ideas about what this might mean, but I am really not sure. Any tips?

I also love that rule of thumb, and I am definitely going to live by that theory.

[old_master]

You're on the right track. The heater core alone being restricted cannot cause overheating. The reason is that the heater core coolant circuit is "tapped" off of the main flow of coolant, kind of a side track route, so to speak. Bypassing the heater core should make very little, if any, difference in engine operating temperature. The key here is when you say that you have very little coolant flow from the thermostat. The coolant that flows through the intake manifold is heated by the coolant passing through the block and cylinder heads. There are two coolant passages in the intake that connect the cylinder heads together, (one at each end). The only way you won't have flow at the thermostat, is if coolant is not flowing sufficiently through the intake manifold. Like we've been suspecting all along, if the improper gaskets were installed, they will restrict the flow. Usually they have a hole the same size as the ports to allow full flow of coolant, however, some intake gaskets have a very small hole not allowing coolant to flow as much through the intake because they incorporate other passages for the coolant to flow through. The original thermostat is a 195F. What this means is the thermostat will begin to open at 195F, so if you're running 205 - 210 that is just a little high. Another clue that it may have wrong intake gaskets.

[BlueMonster]

That's cool. But why would bypassing the heater core cause such a dramatic change in behavior then?