Hi folks..

I am going over to my father's tomorrow to help with the pickup there (see my signature for details). He seems to have it starting consistently now. However, the coolant system is inconsistently reading at the top end of the NORMAL range. Is there to tell if it really is overheating or if I have a broken sending unit or gauge (see below for the details of a resistant test on the sending unit)? He nor I have ever noticed any other signs of overheating, including any sounds, smells, running issues, etc. We never let it get above the normal range under any circumstance.

So, I need your suggestions before I go over there. They will not have internet access.

It is a 5.8L and a 1990 (as the signature shows). The gauge seems to show it getting hot (only when driven above 45mph) but once the vehicle is idled or even shut down, within 5 minutes the gauge shows on the R in Normal. It does take a little time to begin to "overheat" when cold, about 10 minutes of slow driving.

I am going to purchase a new sending unit and we will go through those motions. He checked the resistance of the sending gauge and it matches the Chilton list for -4 degrees, not the genuine 50-55 degrees that the ambient air temperature was at their house this morning. So, that seems very suspect to me.

I have had the pickup for 10 years with a hammered down temperature needle, always pegging at the same spot once up to operating temperature.

A little background... He says that he found it had collapsed the upper radiator hose the other day and the expansion tank was full. He replaced the radiator cap and that behavior MAY have stopped. No recognizable coolant loss and the engine oil looks good.

I have a multi-meter with a temperature sensor built in. I will take it with me. Is there any way to check engine surface areas for a reasonably safe assumption of true operating temperature? I do not want to overheat the engine and need to figure something out in case the gauge is really the problem and not the sending unit.

Yes, I changed the thermostat this summer, primarily because it seemed to be running hot to me, the gauge had begun showing in the L in NORMAL.

Anyhow, would appreciate all of the wisdom you can share before I run over there.

Thanks again folks...

Jason
Bandon, Oregon

 

I don't know about a multimeter temp probe, but I use a meat thermometer 0-250ish pushed in between the fins and tubes (carefully) to set the electric fan control on my nova and other such vehicles. That may be a good spot.

 

I would check the t-stat just to rule that out. I know you stated that it was replaced this summer but they do fail, often. If it is stuck in the closed position, this will result in the over heating. Worth a look. Also, check out your fan temp. switch and make sure your fan is kicking on when eng. is warmed up. http://www.google.com/url?url=http:/...5i223pWtS4_d5g

 

Okay guys... Thanks for that so far. I will keep it in mind for the temperature probe placement.

One question about the fan temperature switch... Since I am not right on it, I cannot be 100% sure, but it is a 1990, does it have that fan temp. switch? I do not believe it is an electric fan, though I have not really looked super close at it. Seems to be a large steel unit, perhaps with a clutch? Just double checking that a large metal fan would have a switch such as that. I am accustomed to electric powered fans being plastic blades.

THANKS TO ALL!

 

By the way... I did pick up that coolant temp sending unit and it is reading correct resistance according to the Chilton's manual based upon ambient air temperature. A far cry from the one currently in the pickup which is reading that the temperature is -4 degrees when it is cold. So, I think that might be a big plus.

Jason
Bandon, Oregon

 

That's a good sign. I'd replace that sensor first before testing/replacing any more parts.

 

Your truck should have a mechanical clutch fan.

If the t-stats was replaced with anything other than Motorcraft, consider it junk. I know that sounds blunt, but those Stant or otherwise cheap t-stats fail and fail often.

Clean the threads of the hole where the temperature sender threads in. To much corrosion creates HIGHER resistance, this shows higher temp.

 

Excellent info... I'll bring a brass brush with me. On second hand, maybe a new battery terminal brush, might be better in the hole? I'll see if I can get a Motorcraft t-stat from the local dealer.

Also, I noticed you are in Mexico, Missouri... Are you familiar with these folks:

Visiting the Zenith Aircraft Company kit production facilities in Mexico, Missouri, USA (MYJ)

Take good care out there, thanks again folks...

Jason
Bandon, Oregon

 

An infra-red thermometer that you can aim at various engine parts (thermostat housing, front & rear of heads, top & bottom radiator hoses, etc) will show you what is happening, & where.

 

A battery terminal brush is what I have used. In fact, I chased and chased a "supposed" overheating condition in my '92, 302 for 6 years. All it ended up being was rusted threads in the block!! After years of chasing the problem, it only took me an hour to fix it with my battery terminal brush.

The only difference between the 300 and the 351 is the fact that the sender is in the cast iron block on the 300, so it developed rust and on the 302, 351 its in the aluminum intake.

I always boil and test any t-stat I install and use a temp probe to check.

And yes, I'm familiar with the Zenith Aircraft.

 

Hello folks...

I just got home from being with dad and mom and working on the pickup. After all was said and done, it is actually the gauge! Yes, seriously. All of this effort and thought, and about nothing more than the instrument cluster gauge failing and the sensor getting replaced.

My goodness, what a relief!

We changed out the coolant sensor, and still got strange readings. Well, after all is said and done, after testing external locations (as suggested above) and taking direct resistance readings with the multi-meter on the new sensor, along with plenty of test miles, the truck is not overheating, in fact, the radiator cap is cool to the touch, the readings are all about 195 degrees, tops, and we are amazed.

So, there you have it... I'll post another thread related to this shortly. THANK YOU ALL AGAIN! Mom and Dad can get moving again!

Regards to all,

Jason
Bandon, Oregon

 

Glad to hear you got it figured out and it was inexpensive.

 

Good to know. Perhaps down the road, installing an aftermarket gauge could be done. I'm planning on it eventually. I already did an aftermarket oil pressure gauge because the indash one was all wonky even after installing a new oil pressure sending unit.