I have 1994 F-150 5.8L XLT rear wheel drive truck. It has 215000 miles on it.

Recently it start to show temperature gauge going all the way to RED hot line when driving on Freeway.

Went to Industrial Radiator shop and got full check done. Replaced water pump, thermostat (190° F), bottom hoses (Not collapsing), fully flushed the Radiator and changed the Radiator fill cap.

Even after all this it has been overheating.

One strange thing I have noticed, when the RPM goes above 2500 RPM the engine get cool right away and temperature gauge goes to normal temperature.

So if I get off the freeway and park the truck at gas station and then just rev it up to 3000 RPM, the engine temperature drop to normal. Very strange situation here.

Please note I have also replaced the temperature gauge sending unit.

I took my Thermo temperature laser scope and measure the temperature and I can see when it is hot at RED zone, engine block is at 240° F, but when running normal and cool, the engine block is at 190° F.

I thought I may have blown head gasket, but I don't see any white smoke from tail pipe and I don't see Cafe Mocha kind oil on engine oil fill cap and dip stick.

I am very puzzle what is going on with this truck.

Any feed back to help will be greatly appreciated.

 

What speed are you driving on the freeway? One thing I didn't see mentioned was the fan(fan clutch) behind the radiator. If you are driving fast enough, you don't need help from the fan behind the radiator, but if say below 50 MPH.

 

On Freeway I drive 65-70 mph and temperature get hot.

Even on street when I am driving 35 mph, I see temperature swing to all the way to RED line.

The Industrial Radiator shop and I verified that fan and fan clutch is working fine.

The Friend of mine recommend me to take it to shop and get engine check to see there is not blown head gasket.

Like I said, I don't see any white smoke from tail pipe and I don't see Mocha kind of oil color on oil filler cap and dip stick.

I am real puzzle what is going on here.

 

I'm assuming the trucks running in these pics, right? Your oil pressure is low.

 

Yes, Truck was running.

I went back today and check the oil level, it was little low, so I added 1 qt of oil.

Then retarded truck and let it run for 5-6 minutes, I can still see low oil pressure.

Truck start to heat up. Then I Rev up the RPM to 3000 while park and temperature just drop.

See below video link.

 

What can be causing the oil pressure to stay low?

Also why temperature goes down on higher RPM above 2500 to 3000?

Like I said, I don't see any white smoke from tail pipe, I don't see Mocha looking oil on filler cap/filler neck and oil dip stick.

I love this truck and don't want to get ride of it. Please help and give me any thoughts or idea or feedback you all have.

Thanks in advance for your help and support.

 

Worn main bearings and/or rod bearings, Plugged oil filter. When was the last time you did an oil/filter change?

 

Oil was change in August 2015. I do Oil change every 6 months. Last oil change was done at Walmart. I paid extra $20 to get extended mileage oil.

What we are looking here for worn main bearing and/or rod bearing repair?

The truck worth about $2000.

I like this truck, it was running so good, fun to take it to Central California Coastal Beachs and drive/float this baby on sand and enjoy summer day. Weather still warm here in California Central Coast.

 

Don't be concerned about the low oil pressure reading. That is just an idiot gauge, it registers somewhere in the NORMAL range when you have adequate oil pressure and drops clear back to the left when you have very low or zero oil pressure.

The oil pressure gauge in my '94 use to read about where yours does and the oil pressure sending unit started leaking. I replaced the sending unit with a genuine Ford part now my oil pressure gauge reads a little past half way on NORMAL. I believe the old sending unit was also a genuine Ford sending unit, go figure.

 

Are you sure the water pump is going the right direction?

 

88n94, thanks for confirming the oil pressure concern. Like I said I added 1 QT today and now the oil level is just fine, I don't see below or over the fill line.

Jaagen, Yes, the water pump is going in right direction. I worked with the Industrial Radiator shop and we put bypass to see the flow of coolant and is flowing fine and pump is pumping in correct direction.

If you all look into video link, you can see as I rev up to 3000 RPM the temperature went down to normal, but as I idle the truck at 1000 RPM, the temperature start to creep up to RED line.

And it is true temperature, I used Thermo Temperature laser gun to measure the temperature. In normal temperature it is at 190° F, but when it hit the RED line, the temperature reached 240° F. There is 50° F temperature rise so quick. It is puzzling.

I don't want to drive the truck and blow up the engine or crack the cylinder blocks etc.

No codes, No Check engine light, No oil light or no any kind of warning light on Dashboard/instrument cluster. All look normal. But this baby is HOT.

 

What kind of oil filter did Walmart install? Just to be on the safe side, I'd install a Motorcraft oil filter. What's the worst that can happen, you're out $5?

 

Its a FRAM Core oil filter. See photo. I can remove this filter and install new Motorcraft oil filter, no problem.

One questions, if I try to swap the oil filter, do I need to drain the oil and put new oil, or just remove the old filter and quickly install the new filter and will drain some oil that I can top off. Let me know.

 

You can change it without draining the oil. It'll be kind of messy. So be sure you have an oil drain pan underneath. i used to use Fram filters until i saw the inside of one. They're cheaply made compared to other filters IMO.

 

When was the last time the belt was replaced?

If there is adequate slippage or not tight enough, then it can cause the engine to overheat and not make a noise.

Also...hold old is the Radiator cap? I change them every 4-5 years or after 30000 miles. I would NOT recommend anything other than a Motorcraft radiator cap. If the cap is bad, and loses pressure, it can cause an overheat problem. A new Motorcraft rad cap is like $10-15, and worth every penny! Should be 13lb cap too.
Anytime your replace a water pump, a new radiator cap should be installed.

I wasted $250 for a new water pump install years ago thinking it was the problem, and all it was the radiator cap.. I could have kicked myself in the *** for that!!