SSilver

Ok the vehicle is a 1978 Jeep CJ7 with a 5.3 It's basically stock everything as far as the cooling system goes except it is a 2 core aluminum radiator and electric fan and I have an autometer water temp gauge and the sending unit is tapped in the passenger side head.

Everything was working great, you could let it idle all day long with the fan off in the summer and the temp wouldn't break 180* on the gauge.

Now the temp. gauge says 150* right when you start it and climbs up to 250* and makes no difference when the fan is on. There are no other signs the engine is running hot. I assume the computer would throw a code if it was actually 250*?

What could it be? bad thermostat? bad sending unit?

i r teh noobz

iTrader: (4)

If it starts out reading 150 from a cold startup, I'd suspect the gauge, sender, or wiring. They're usually some sort of thermal resistor, and I'd guess that yours reads high resistance as a higher temperature, so check any wire connections first.

You can also check the sensor readings with a non-contact thermometer. Read the temp of the water pump casting or the head. It won't be exactly the same, but you'll be able to tell if its at least close.

msydow

check the temp with a temp gun or something similar and compare it to what the gauge reads. i would guess if it says 150 as soon as you start it you have and electrical problem; gauge, sending unit, increase in resistance on your signal wire.

SSilver

I'll trace out the wire for the gauge and see what I find. I've been screwing with the wiring a lot and could have messed something up. If that turns out to be ok should I buy a new sending unit?

Is it possible the sending unit could get gunked up from dirt/ rusty coolant?

gofastwclass

iTrader: (2)

Always check the wiring first.

Yes, but the sending units are usually sold and calibrated for the gauges so it may be difficult or expensive to buy one without the other. If you can get a sending unit, it may be cheaper to get the matched set. Besides this removes all doubt of one or the other being faulty.

Possible, yes but probable... doubt it. Unless you have been driving your swap for years and years without changing coolant or use unfiltered street drain water (seen it) I would say no.

If you have access to a scan tool that reads live data you can see what the PCM thinks the temp of the engine is. I trust this more than anything and use it to control the fans on my cars.

Pop N Wood

iTrader: (1)

Disconnect the sending unit from the head and put it in a pot of hot water along with a kitchen thermometer.

Should tell you if the unit is good or bad.

I don't know that I agree with the advice to replace the sender and gauge in pairs. I have never had that problem.

GNCLONE

you can also run it with the radiator cap off and stick a kitchen thermometer in there to get a true temp

SSilver

Checked the wiring and changed the connectors and it is still reading 250* (max temp on the gauge).

Haven't gotten a chance to pull the sending unit yet but it looks like it may be the culprit?