Temperature gauge jumping

Denisefwd93 · Aug 8, 2018

Near 100 deg days, I'm guessing it's the coolant sensor/sender? gauge is going all the way down to cold and slightly above normal and occasionally moving up and down it's been fine for weeks and weeks, new fan clutch been through 3 different thermostats I'm now on a 192 thermostat, coolant water is right where it should be, Reservoir is right where it should be.

RonD · Aug 8, 2018

By pass heater core temporarily, use one heater hose to loop to other connection on the engine or use a coupler to connect the two hoses together.

1994 and older use the heater core as the coolant bypass so it needs consistent flow thru the core or temp gauge will go up and down almost randomly
As core gets older and partially clogged up flow will go up and down, which shows on the temp gauge.

Bypassing the core will tell you if thats the problem, and you don't need heater on a HOT day in any case, lol.

Denisefwd93 · Aug 8, 2018

Did that per your previous advice Ron, about a month-and-a-half ago, it's been fine up until yesterday.

RonD · Aug 8, 2018

Copper elbow is getting plugged up..................lol, joking

Then yes, next stop would be ECT SENDER, the one wire temp sender

You could also check radiator surface after engine is shut down warm.
Pull shroud back and run your hand over the fins, should be warmer/hot at the top and get cooler toward bottom, this is a side flow radiator so toward upper rad hose connection it will be warmer and then cooler as you move towards lower rad hose connection.
You are feeling for very cool spots, those are blocked tubes, so they get the cooling air flow but not the heat of the hot coolant inside.

Partially blocked tubes lessen radiators ability to cool the engine, which is often not noticed until outside air temp gets hotter so less cooling at the fins and you get the higher running temps, but I would expect it to be consistent, based on engine load/heat, not fluctuating, randomly

Denisefwd93 · Aug 8, 2018

Will do! but this radiator is not the original one because it has transmission cooler connections. I'll check with my infrared also but it doesn't seem to get anywhere near and overheating situation. Also, getting ping even on easy acceleration.

RonD said:
Copper elbow is getting plugged up..................lol, joking

Then yes, next stop would be ECT SENDER, the one wire temp sender

You could also check radiator surface after engine is shut down warm.
Pull shroud back and run your hand over the fins, should be warmer/hot at the top and get cooler toward bottom, this is a side flow radiator so toward upper rad hose connection it will be warmer and then cooler as you move towards lower rad hose connection.
You are feeling for very cool spots, those are blocked tubes, so they get the cooling air flow but not the heat of the hot coolant inside.

Partially blocked tubes lessen radiators ability to cool the engine, which is often not noticed until outside air temp gets hotter so less cooling at the fins and you get the higher running temps, but I would expect it to be consistent, based on engine load/heat, not fluctuating, randomly

RonD · Aug 8, 2018

Pinging means at least one cylinder is getting too hot so gauge is reading a real temp or over temp situation, I wouldn't change sender in that case.

Could be dirty injector, they can cause a lean mix which burns HOT and pings easily.
But 1993 is probably running Batch fire injection(3 injectors open every 1 RPM), so would be hard to track that down.
Could check spark plugs on each bank, one may show lighter color than the other two, its injector would be suspect

I would unplug IAC Valve after engine is warmed up, just a quick test to see if there is a small vacuum leak, RPMs should drop down to 500 or engine may stall, thats what you want

Could be lower intake leak is causing Lean burn on one cylinder

Denisefwd93 · Aug 8, 2018

Pretty sure the 3rd "new" IAC is no good. Cleaned the maf not too long ago. The new brake booster is hard to push so that's coming out for another. This just goes on and on LOL Do the older OBD1 4.0's only have the one temp sensor Ron?

RonD · Aug 8, 2018

No, they have 2, ECT Sensor for the computer, always 2 wire, runs on 5v from the computer

And ECT Sender, it runs on 12v from the dash gauge, actually a Ground for the dash temp gauge, 1 wire version uses its threads to get a good ground on engine, later versions had 2 wires, 2nd wire was just a Ground connected in engine bay ground point via wiring harness

Not interchangeable, they read temps differently

ECT sensor usually has a cone shaped tip that goes into the coolant flow

Sender has cylinder shaped tip

Fuel injection can't use a Choke Plate, no Jets to suck extra fuel from which is needed to run a cold engine, so computer has to know accurate engine temp to set Choke Mode(rich mix, high idle, advanced spark) so cold engine can idle and not stall.
The senders are OK for the gauge but not as accurate as needed.
In later years(around 2004) just the ECT Sensor was used and computer ran the dash temp gauge, no temp sender on engine

Denisefwd93 · Aug 8, 2018

I kind of knew that, but it doesn't come up on internet search only 1wire sensor comes up. I know you told me wear but I already forgot where that one is located on the engine guess I better get a new battery in my multimeter lol

RonD · Aug 8, 2018

On the 4.0l OHV both are located just above thermostat housing on lower intake

Sender is usually just off on passenger side, sensor drivers side, but very close together

Denisefwd93 · Aug 11, 2018

Before I start changing sensors: I want to reexamine grounds, ground wires and connections the, original ground was disconnected and they ran a new ground wire from the battery to somewhere down below lol.

pillow there are also some ground wires connected to the battery terminal to the sheet metal near the battery.

I found a ground strap from the engine to the firewall that I reconnected.

Ron, do I need to improve on any of this? thank you in advance

RonD · Aug 11, 2018

Battery Negative to engine is the High AMP cable, for starter motor 75amp, and alternator 40 to 100AMP

Then rad support and inner fender needs a ground, for headlights and relays

Cab firewall needs a ground, for cab electrics, yes this is usually done with ground strap from engine's head to firewall, and often left off after engine work

Frame needs a Ground, usually done with ground strap from engine block to frame near a motor mount, this is for tail/trailer lights, but also for fuel pump on some models.

All body parts and frames are painted BEFORE assembly at the factory, and most of these parts are bolted together using rubber isolater washers so not great Grounding between these parts, even if they are bolted together metal to metal it is still PAINTED metal to PAINTED metal so not a good Ground.

If you get a longer wire and hook it to Battery Negative and your OHM meter, then check Ground bolts in the cab for 0 OHMS and grounds on frame and rad support for the same

Denisefwd93 · Aug 11, 2018

I did check with ohmmeter from the battery to the old cable, I did find a very slight resistance, just assumed that is why they ran a new one which have to be much heavier and red LOL.

What also has me wondering, about 2 weeks ago on a rainy night, all the 4wd lights came on and stayed on till I got home, and when started the truck again, the 4wd lights did not light up again, this was all a surprise because I assumed the 4WD was disconnected or inoperable. Even the low range light came on. The T- case of course was not engaged.

Temperature gauge jumping

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