Love this forum and looking for some good advice on my overheating problem. I am just finishing an amateur restoration of a 1978 FJ40 I purchased in pretty rough condition. The only thing that seemed in decent shape was the engine. Compression was good with all cylinders in the 140-150# range. Did the first start-up last week. Idled nice and smooth but overheated within 5-10 minutes idling in my shop. Put in new fan clutch (fan is in good condition), new water pump, new thermostat, new radiator. Belt is new and tightened properly. Flushed block. Still overheating. No white smoke out exhaust, no mayonnaise at oil cap or dipstick, no loss of coolant (currently using just water). Today I will be doing test for combustion gasses in coolant to hopefully rule-out blown head gasket. I am hoping with the wealth of knowledge and experience of this forum that you can give me ideas of additional things to check to overcome this issue. I am also pulling my hair out (what's left of it) bleeding the all-new brake system for a good pedal, but I will save that for another discussion. Thanks in advance for any advice you might have to offer on my hot FJ!
FJ40 Overheating Mystery (3 Viewers)
- Thread starter 4ZKRVS
- Start date
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Get a IR heat detector and look at every thing that has coolant. Find the spot(s) that heats up first.
Retarded timing will run hot, way to far advanced will run hot. Too lean on the carb will make them run hot.
Vac leaks can make them run hot. Bad brake booster will burn up cylinder #5
Radiator is plugged in side or on the outside in the fins.
Block passages clogged with crud.
Both can be victims of way to many stop leak treatments.
You didn't burp out the air in the system.
Retarded timing will run hot, way to far advanced will run hot. Too lean on the carb will make them run hot.
Vac leaks can make them run hot. Bad brake booster will burn up cylinder #5
Radiator is plugged in side or on the outside in the fins.
Block passages clogged with crud.
Both can be victims of way to many stop leak treatments.
You didn't burp out the air in the system.
- Thread starter
- #3
Thanks 007. Some good ideas to double-check!
Keep us updated.....I have almost the exact same issue
As you describe it with all new cooling system parts you are likely looking at tuning. Charlie hit on it, timing (fairly easy to confirm) then air fuel mixture (more factors and harder to confirm). I would look/listen hard for a vacuum leak; spraying flammables around everything on the intake side and/or listening to every seam and orifice around the intake and carburetor with a piece of tube with the other end held up to your ear. Temporarily plug the brake booster vacuum line as well to eliminate that. Good luck.
In my experience they can be hard to purge all of the air out of the system. Do you have a burp funnel? Open the heater valves and use the funnel. Watch your temp gauge and squeeze the upper radiator hose, be ready to add coolant if needed as it’ll make a huge mess if it runs dry.
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Exhaust restriction will also cause overheating. Ensure you don't have mouse houses, or something else blocking the exhaust system.
My technique for burping the system is find a steep ramp. Make sure the radiator cap is the way highest point in the system. Chalk a wheel, Remove cap, start engine. Let run until you see the level of coolant surge up and down in the rad - thermostat is cycling. Make sure heater valve is full open. Hemostat off one hose to rear heater - forces full flow to front heater; let thermostat cycle a few times. Move clamp to a front heater hose, let cycle a few times.
- Thread starter
- #10
Overheating Mystery Solved!!!
First - thanks to all of you who offered advice and ideas to try! This forum is the BEST. You won't believe what I found... At the base of the thermostat housing where it bolts to the head there is a gasket. We were going to run a test with the thermostat out to see if it made a difference. While we had the upper half of the housing off, we looked with a flashlight into the tunnel that goes into the head and saw an obstruction. We unbolted the lower housing from the head to find a solid gasket (no hole for water to flow through). Needless to say, we put a new gasket (with a hole) in and re-assembled everything. On our next test drive, the engine temp barely climbed into the operating range and stayed there! I am leaving soon on a long trip in the FJ, so although I have several new parts I may not have needed - it will be good to have spares along! Thanks again for all of your suggestions - what a great resource for those of us who are operating beyond our bandwidth in trying to get these great old trucks back on the road!
First - thanks to all of you who offered advice and ideas to try! This forum is the BEST. You won't believe what I found... At the base of the thermostat housing where it bolts to the head there is a gasket. We were going to run a test with the thermostat out to see if it made a difference. While we had the upper half of the housing off, we looked with a flashlight into the tunnel that goes into the head and saw an obstruction. We unbolted the lower housing from the head to find a solid gasket (no hole for water to flow through). Needless to say, we put a new gasket (with a hole) in and re-assembled everything. On our next test drive, the engine temp barely climbed into the operating range and stayed there! I am leaving soon on a long trip in the FJ, so although I have several new parts I may not have needed - it will be good to have spares along! Thanks again for all of your suggestions - what a great resource for those of us who are operating beyond our bandwidth in trying to get these great old trucks back on the road!
