BJ74 occasionally overheating

[calsbeek]

1986 BJ74 with a 13-BT, 218,000Km.
I've been driving with no problems for over two years of owning this truck. Last week I had about a 15 second episode where the temp gauge went to 75% (not in the red) and then dropped immediately. During this brief episode, there was no heat from the blower but as soon as the temp gauge dropped, hot air blew immediately again. I figured the thermostat got very briefly stuck. This event repeated itself last night, overheating but just for a matter of 15-20 seconds, not all the way to red. I pulled over and checked under the hood, no steam or anything. Drove home.
I let the engine cool a bit and then pulled the rad cap (actually its on the thermostat housing, not the radiator) and the fluid was down. I think I added about 1/3 to 1/2 gallon of coolant (also the overflow reservoir is empty).

The rad cap is an aftermarket thing that was on the truck when I bought it. I just ordered a new Toyota version.

My Questions:
1. is this hopeful fix just me kidding myself about a blow head gasket?

2. I am slated to drive 5 hours on the highway in a week. Should I make this trip or will I do serious damage if its not just a bad cap?

Any way to tell if the HG is blown for certain? There is no sign of water in the oil (dipstick is black) but I understand that the original 13BT HG was flimsy and tends to break down over time.

 

[FJBen]

Well as you probably only know, there are really only a few places for coolant to go. A leak, into the oil, headgasket. You might lose some if you boiled over at some point, but 1/2 gallon seems like you would notice a leak somewhere.

As for the overheating, sounds like possibly stuck thermostat, air bubble, radiator cap and low coolant. All of those can add up to a blown headgasket if not fixed. Good call on the new radiator cap.

Personally, I would not drive a 5 hour trip if I were dealing with this issue. I know @woody had this similar issue here: 13BT Overheated His was just a thermostat and REALLY low on coolant, so that may be all yours is. I would throw a new cap and thermostat on and test again. Headgaskets usually let you know by bubbling over into overflow tank.

I can tell you if you do blow a head gasket and don't catch it to shut it down right away, you are looking at headgasket, probably a lifter, bent pushrod or 2 and possibly scorched cylinder wall....then while you are in there you might as well do new liners, pistons, grind crank, cam, bearings, rebuild head, on and on... Previous owner of my rig had that happen and I have a $$$ machine shop bill rebuilding it.

 

[calsbeek]

Thanks for the reply. I am starting to wonder if when I did the coolant flush this fall I didn't fill it up far enough. I put less than two gallons in before it seemed full but I have now read that the 13BT is notoriously difficult to fill all the way. THat combined with a bad cap might be the culprit but I don't want to screw anything up.

No bubbles in the overflow reservoir, no white in the oil, no white smoke.
Now that I refilled the coolant I've driven it hard on the highway and going up steep dirt hills. I can't get it to overheat again.
Can I test for a blown HG some other way?

 

[FJBen]

Aside from testing the coolant for combustion gasses, there aren't a lot of other tests to try. Usually it's oily coolant, or milky water along with bubbles in overflow.

At places like auto zone or other parts stores, you can rent a "block test" that basically tests the coolant for combustion gasses. You just need to buy fluid usually. Might be worth a try, but also sounds like maybe you fixed it?

 

[calsbeek]

unfortunately the block test doesnt' work on a diesel (at least I don't htink it does) becuase diesels produce next to no Carbon monoxide.

I'm hoping that the top off fixed it.....I may have to cross my fingers. I guess I could pressure test the radiator to double check that there isn't a leak.

 

[jblueridge]

You probably are already doing this but keep checking fluid level. It may change as bubbles work their way out.

I tried the block tester kit on mine not that long ago. Engine kept burping up coolant which messed up the test liquid.
I had to withdraw a lot of coolant to get the tester to sample correctly.

 

[calsbeek]

Here is the update: After driving around for about a week, monitoring fluids and seeing no loss, I decided to drive to NYC (5.5 hours) . I made the trip down, no issues. After the engine cooled I checked fluids and all looked good. Drove home the next day and again had no problems till about 20 minutes from home I looked down and both the temperature gauge and the fuel gauge were pegged! I immediately panicked but then the thought occurred to me that I had just shown 3/4 tank of fuel so I thought maybe an electrical issue? Pulled over and popped the hood and nothing seemed awry. A quick google suggests that indeed a bad ground in the dash cluster may cause this weird behavior.

Is this just a strange coincidence? Or could it be a sign of a deeper issue?

 

[OGBeno]

Known issue with these older gauges.

Guage is toast.

 

[calsbeek]

I've read its a grounding issue?
If its the gauge, is it both the fuel and temp gauges as one unit? Or the whole cluster?

 

[Gun Runner 5]

I had the same issue. I believe the fuel gauge and temp gauge share the same power source or at a minimum a common ground. In my case both gauges would peg at the same time and return to normal at the same time. I solved the issue by installing a high quality aftermarket temp gauge.

 

[Reinhardtius]

@calsbeek we could rig up an Auber temp gauge to verify safe temperatures and that the gauges are the issue.

Auber Instruments, Inc., Temperature control solutions for home and industry

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www.auberins.com

i've got the multimeter and some other electrical diagnostic equipment. let me know.