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My temp gauge is off the chart (Hot)...changed everything but the radiator, which looks fairly clean. I am suspecting now that it's the gauge/sender giving a bad reading, because the engine is running fine, and doesn't seem to be hot by feel? Anyone run into this, and if so, what's the fix. That sender looks hard to get at!!

1st off let me say welcome to the site..If you have replaced everything but the radiator I would have it flushed. Well if it is saying its hot but it doesn't feel hot..could be possible that the engine is putting compression in the coolant? or if it is the sending unit it should be a fairly easy fix. Then there is always the option of adding after market gauges. I am personally not to fimilar with the Isusu wiring.

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Sometime a sending unit can get coated with mineral scaling on the brass sensing end that goes into the block over time and give some real odd readings....or by the wire shorting out somewhere.

Try removing it and cleaning both ends of it off carefully with a brass brush, a fine scotch brite pad, or some fine steel wool....and also check to see if it looks defective or is leaking.

Then check your wiring to and from the sending unit and the guage for a shorted out or bad connection somewhere.

You might want to also check to make sure it's the right one for your truck...I've seen people put in any old sending unit they could find before because they were either too cheap, too lazy, or too stupid to get their hands on the right one and install it intact.

Also be careful when you are taking them out regardless of where they are or how difficult they are to get to...it's pretty easy to break one or ruin one by rounding the hex end off....since most of them are made of brass and plastic.

Hope this has helped ya.

Welcome! Follow the coolant hose that comes off the top of the radiator down to the engine. Underneath the place where it attaches to the engine is the coolant temperature sensor. It is a plug type thing and it has a small brass button that the wire attaches to (The wire snaps on/off the button for installation/removal). The sensor points directly downward. If you replaced your thermostat, you'll have a better idea where it is, it sticks up into the chamber directly under the thermostat. It may not be so hard to get to it to unscrew it. There was a thread maybe a year ago that describes the location better than I am doing here and has some pictures, but I can't seem to find it right now. Anyone remember where that thread is? I redid just about my entire cooling system last year and at some point wasn't sure if it was a bad sensor (turned out to be a blown head gasket!)

Thanks, guys.

I do know where it is, just seems impossible to get at with regular tools. I did clean the nub inside the t-stat housing. if you unscrew the sensor, will all the water come out??

What should the voltage be at the meter in the the cluster?

Just disconnect the wire at the sensor, if should now read cold with the key on. If so the sensor is bad. If it still reads hot then the wire is shorted to ground somewhere between it and the gauge.
Does the fuel gauge read normal? If it is also reading full all the time you have a defective regulator for the gauges.

if you unscrew the sensor, I believe any water/coolant that is above that height in the radiator/radiator hose and engine will flow out. It won't be everything in the system, but it will be some. I am pretty sure that sensor comes up right into the chamber below the thermostat which has the coolant running through it, and with it gone, there would be a hole there.

If the sensor is good, are you sure it is not a head gasket leak? It took awhile for me to diagnose mine. I replaced the thermostat, water pump, radiator, flushed the system, and still had the system overheating. It turned out that I had a pinhole leak of coolant into the oil that over time made the oil lose its ability to lubricate the engine. It took a little while before the oil looked like chocolate milk, but that was when I knew it was a leak.

Here is the thread about that. Maybe there is something helpful in there. Just so you know, I am not a mechanic like many of the guys on here, and know virtually nothing about the electric system.
http://forums.luvtruck.com/viewtopic.ph ... ght=dottie[/url][/code]

egg has a very good tip for your first step in diagnosing what is wrong. You also don't say if it starts out cold and them climbs up to way hot on the gauge as the motor warms up. If it seems to work normal at cold start and climbs, you may have bigger problems like a headgasket leak. Severely overheated these trucks start to chug and really lack power, so much so you would have noticed. Since you say it seems to run ok, I think you are on the right track to think it is just an instrumentation problem.

egg's point about the fuel gauge is good too, the two gauges use the same voltage reg (on the back of the gauge pod) for power so a problem there will show up in both.

In case you do start to suspect more serious problems, here are a couple of tips on head gasket diagnosis. Open the radiator (with truck cold) and see if there is any kind of oily film on top of the water. Even just a trace of a sheen could be a clue that you have a leak as it would get a trace of fuel in the water even if the leak does not mix the oil and water. A compression check is also a good idea.

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95 Isuzu Trooper Daily Driver
86 Isuzu Trooper reliable backup
77 LUV 2wd stock beltway blaster (resting)
79 4x4 LUV project: 2.6L, 5spd, 31s (eventually)

MEPR: Man, my 4x4 makes all other LUVs look good

Well, I think it may have been just the gauge after all, but in all the flushing/draining/filling I've done in the past three days I think I finally did bust the head gasket. There is now a bit of white smoke coming from the exhaust, and it's losing coolant. Not surprising as this truck has been ridden hard (it was my first car at 16...23 years ago) and it has been run in an overheated state several times over the years due to blocked radiators, broken pumps, hoses, etc. It's got 163,000 miles, and frankly I am surprised it lasted that long. Not sure if I am going to bother fixing it...might just sell it now. Not really wanting to do that job, and the truck is not worth the money to have it fixed. BTW...how hard is it to do the headgasket? Seems like access at least is pretty easy. Any super special tools needed? How much does it cost, and how much does it cost to have the head straightened/milled if needed? Also, would it be easy to remove the pistons to replace the rings? Lots of blowby right now too.

Thanks