I need help from the mechanics here

[sparky9]

Saturday I noticed the temp gauge on my old truck ('91 Chevy 4x4) reads about half of what's normal. I'm thinking it's time for a thermostat.
Monday morning, the temp. gauge doesn't read at all.
It's not really acting like the thermostat is stuck open. It seems to be warming up pretty normal and there's plenty of heat. Coolant level is ok, too.
I checked the temp. sending unit after work today. At least I think it's the sending unit. It's located beside the thermostat housing. It reads 625 ohms (unplugged) if that's any help.
Any suggestions?

 

[sparky9]

Saturday I noticed the temp gauge on my old truck ('91 Chevy 4x4) reads about half of what's normal. I'm thinking it's time for a thermostat.
Monday morning, the temp. gauge doesn't read at all.
It's not really acting like the thermostat is stuck open. It seems to be warming up pretty normal and there's plenty of heat. Coolant level is ok, too.
I checked the temp. sending unit after work today. At least I think it's the sending unit. It's located beside the thermostat housing. It reads 625 ohms (unplugged) if that's any help.
Any suggestions?

 

[tomf52]

Been retired a while but on many engines there are two temp sending units. One is actually called engine temp sender (it feeds info to eng computer). Make sure you are testing the correct one.

 

[Spotteddog]

Digital or analog gauges on the instrument cluster? If analog, the gauge and wiring to it can be quick and dirty tested by grounding the connector at the sending unit with the ignition "on". If it goes to "H" side of the meter range when done, typically it's a sending unit.

 

[sar4937]

Almost all technicians use a scanner to begin the diagnosis. First make sure the coolant is full, remove the radiator cap to verify. Next make sure the face of the radiator and condensor is clear of debris. Clean if necessory. I would not recommend overheating the engine, that said the proper operating temp can usually be determined by a hand on the upper radiator hose above the thermostat housing. 195 degrees is generally just above the threshold of too hot to hang on to. Some of the GM's will develop head gasket problems, they will usually not overheat at idle in the driveway. But if you rubber band a light duty sandwich barg around the radiator cap opening after the engine is fully warmed up and the coolant is full, and increase the idle speed if the bag inflates like an old time Jiffy Pop Popcorn you have found a need for professional diagnosis. Watch the temp gauge for the normal range and do not let it exceed that. If you get through that without a problem, replace the thermostat, and test in the drive. There are times when the radiator, waterpump, serpentine belt tensioner, sending units can be at fault. But really most times the coolant jug is empty to the point the radiator is also low, but keep in mind that if it is there is a leak somewhere. If so heat it up full, and check thoroughly at the area above where you see the dripping. And hopefully you have a trusted mechanic to watch your back.

 

[Jeepnut]

Have a look at where the line from the block heads to the heater core. It'll be right next (under) the rotor cap/distributor...if the fitting is plastic, it might be worn and allowing pressure to bleed out, wich reduces the temperature you see. Have it replaced with a threaded fitting.

The down side to that plastic bunny-ear fitting is that the steam and moisture is directed right under the rotor...which has small screen vents to allow moisture to drain from it...not as a direct point of entry for pressurized vapour to access and corrode distributor contact points!

How's the operation otherwise?...have you had to replace rotor's/caps/wires anytime lately?

 

[sar4937]

There should be a noticeable leak at the quick-release fitting at the passenger side rear (of intake manifold) usually dripping off the starter motor, again the coolant will be low. Check both the coolant tank and the radiator. You can check tha gauge in the dash if you like, the yellow and black wire at the coolant sensor is the computer, it should turn on the check engine light, the other wire green/black? is the temp. gauge disconnecting it should drive the gauge all the way way on direction, and grounding it should make it go all the the way the other direction. If you are not achieving full temp it's probably the thermostat.

 

[m1gunner]

The temp sensor you checked is for the computer. The one you are looking for is on the driver's side cyl head in the middle (near the spark plugs).

At 104 degrees F, you should have 1200 ohms.

At 257 degrees F, you should have 53.4 ohms.

 

[shovelwrench]

Originally posted by Spotteddog:
Digital or analog gauges on the instrument cluster? If analog, the gauge and wiring to it can be quick and dirty tested by grounding the connector at the sending unit with the ignition "on". If it goes to "H" side of the meter range when done, typically it's a sending unit.

+1

And hey, put a thermostat in it, they're cheap and easy.