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I bought an '81 4x4 Luv a few days ago and I've been having some overheating issues. Just recently I took out my thermostat as my engine was getting hot on street driving. The thermostat was in bad condition and the housing gasket was broken, so I took out the old thermostat, ordered a new one and took a city drive. It run's below middle on the temp gauge, but on the highway it starts to get really hot. My coolant isn't milky white, there's no cream color or milky color in the oil, my engine runs like normal, I just invested about $300 into the truck since. The engine is the standard 1.8L Isuzu I4, there's no smoke that comes out of the tailpipe, it doesn't burn oil, could it just be my radiator? When I went to go check the coolant to top it off, it started to come out all foamy, but then sorted out to normal.

edit: checked the engine oil, still no sign of coolant/water getting into it. Still nearly hits the red mark on the temp gauge. I don't smell any sweet coolant burning from the exhaust either. Also if it might make sense, there's no fan shroud either. Just the radiator and the cooling fan. I was told some factory Luvs without a/c never came with one.

Luvs do have head gasket issues. I think the aluminum head with a iron block is part of it. Keeping the headbolts properly torqued and re checking a few months after replacing the head gasket is advised.
In your case the pressurized coolant sounds to me like the clue that says the most. Fortunately doing a head gasket is not difficult.
You can do it in half a day.

Alright, I'll try to do a headgasket job. The thing I'm freaked out about the most is if I screwed over the cylinder head. It still starts, runs, and drives fine, guess I'll garage it and do the headgasket job. Is there anything I can also put on the headgasket to prolong its life so I won't have to do this again for quiet a while?

Do yourself a favor and have the head surfaced while you have it out.

Before doing a head gasket job I'd make sure that's gonna fix the issue. Lots of things can cause high operating temps. If coolant is getting into a combustion chamber you'll usually see steam out the exhaust. At the least, I'd do a coolant pressure test first.

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Tom
'81 Luv diesel 4X4 - daily driver

Yeah, I just did a compression check for the cylinders. Each of them checked out fine. Since there's no leaks/no mixing of fluids/no smoke/no power-loss or anything, I'm starting to believe the headgasket is the less likely subject. So I'm just going to take it to a radiator shop to get the radiator tested. Gotta wait for my new thermostat and gasket to come in.

When it overheats on the highway with good airflow and such, the culprit is usually a plugged radiator. Not enough square inches of open tubes left to transfer the hot water heat to the air. A temporary solution is to turn on the heater full blast which will help the engine but not help you.

Issue resolved. Turns out my temperate gauge and my gas gauge are complete garbage and shows very inaccurate readings. Mechanic shop did a bunch of tests, radiator is fine, head-gasket is fine, engine block has no cracks/leaks, so I'm just going to have to go get an aftermarket gauge set.

Try replacing the temp sending unit first. It's cheap and easy and worth a shot. You might be able to clean up the gas sending unit too.

Don't get in a hurry yet to replace the gauges. Both temp and gas gauge operate from a voltage regulator unit. A little metal box thing looking like a horn relay, usually plugs into the back of the instrument cluster circuit board.
If both gauges are bad it is usually the regulator.

And if you do need gauges, I think we could find you a replacement cluster.

Jack

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'83 Isuzu P'up Diesel LS - Original Owner
JoeIsuzu@IsuzuPup.com
IsuzuPup.com