1992 YJ 4.0 overheating

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rct2000

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1992 YJ 4.0 engine.
Water pump bearing was worn out, allowing plenty of free play on the shaft. You could wiggle the fan blades sideways considerably. Engine was not overheating, though.
Replaced the water pump. Compared both new and old pumps side by side, and impellers looked identical. New pump was marked as "Reverse" as well.
Replaced the thermostat in the same job.
Started engine and everything worked fine while idling. Took it for a drive and the engine overheats before reaching one mile.
I though perhaps the new thermostat was the culprit (the new one didn't have the "jiggle" valve). Installed the old thermostat back. Still overheating.
This is what I know:
- I run vacuum in the cooling system before filling it out with 50/50 coolant.
- If you run the engine with the radiator cap off, there are no bubbles showing, and no oil in the coolant either.
- If you leave the engine idling, it doesn't overheat.
- If I connect a water hose down the radiator cap port, water comes out of the radiator lower port.
My reasoning is: the engine was not overheating before, and I replaced thermostat and water pump. Thermostat doesn't seem to be the issue. Before pulling the water pump out, I thought perhaps other members have experienced something similar. Any thoughts please?
 
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Years ago I was told by a Jeep dealership that it's critical to put the front end up on ramps to "Burp" the air out of the engine. There's an air pocket in the cylinder head that will cause overheating if it's not flushed out. When cool, raise the front end and remove the radiation cap. Start the engine and add coolant as necessary. The 4.0L had this problem more than the 2.5L.

Thus will be a place to start to resolve the overheating.
 
temp sensor at thermostat head. also check the temp head with a thermo hand held device, there might be some wrong with heat gauge. i also agree with the"BURP" style, it works every time. good luck and safe driving.
 
Put the front wheels up on ramps, removed radiator cap and installed a funnel (it is part of a kit that attaches to the radiator port). Topped off coolant and left about 2 inches of coolant above cap level in the funnel. Turned the AC system into heating. Started the engine and let it idle for a while. When I felt the radiator top hose getting hot (I assume thermostat was open by then), I increased the engine rpm's for a short while. Never saw much of a bubble coming up the funnel. Let the engine idle. After a long while, engine started overheating, coolant level goes high and started bursting out of the funnel, like the coolant was boiling.
Before all these issues, the coolant temp gauge would hardly reach the mark before 210. Now it goes past 210 and then boils. I have the same radiator, same fan clutch, same thermostat, same hoses, no coolant leaks. Only thing I know changed is the water pump. Wondering if I have inadvertently introduced some other changes in the process.
Something I noticed: the coolant coming up the funnel looks pretty dirty. It is not green/translucent any longer.
 
As I recall, when you start and run the engine with the radiator cap off you should see the coolant flowing at a good rate when the t-stat opens. Be careful because it will overflow as it heats up.

Low flow and dirty fluid makes me think there's a bunch of rust or sediment that collected in the radiator.

You can try using radiator flush and let it sit overnight to see what comes out. If it really bad I'd pull the radiator out and try flushing it with a garden hose. Rotate the radiator when flushing it to see if there's any more loose stuff coming out. I've done this several times with different cars and found lots of mud/rust inside. Just don't let the garden hose build up too much pressure, I had a garden hose get stuck in the top hose connection and it blew the top off the radiator. 60 psi water pressure will do that. I've also sprayed water in where the pater pump goes and flushed out a ton of mud from the block.

Is the YJ running smooth and ok after the pump change? Timing issues will cause it to overheat but if you didn't touch anything else or it's running smooth then that's probably not the cause of overheating.
 
I am glad you mentioned: I didn't play with timing as such, but I did replace the exhaust manifold, which had several cracks. Because I had to move the power steering pump out of the way in order to remove the intake manifold, I decided to replace the water pump (bearing was gone, even though it wasn't leaking). Jeep sounds a lot better now, and it runs ok except for the overheating issue.
Just replaced the fan clutch and radiator cap today. It helped a bit: ...you may now drive for about 3 miles before temperature starts rising. When the temperature increases, I stop right away and the radiator can be heard bubbling.
Planning on replacing the radiator tomorrow. Still wondering what on Earth changed: may be the cooling system was barely able to keep up and then the new exhaust manifold somehow makes the engine produce more heat, pushing the whole thing over the edge?
 
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Any chance you got one of the messed up water pumps with the backwards impellers? Happens sometimes on the Jeep inline motors, especially when buying from parts stores.
 
A few options. Usually not overheating at idle, but overheating at speed is a water flow issue. Could be

  1. The pump is bad or has the wrong impeller on it.
  2. Your belt is routed improperly running the pump the wrong direction
  3. Stoppage in the block or head from sediment/mud build up
  4. Radiator tubes are clogged with above mentioned sediment and mud.
Are you using distilled water with your coolant or mixing your coolant with mineral or tap water?
 
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