I normally post in the van form but wanted to get you guys opinion.
I have a 88 E150 with a 302 EFI and AOD transmission.
The van has 104k miles on but as best we an tell it sat for at least 10 to 12 years.
Have been fixing little things here and there and it is doing pretty good now except for the engine temp seems to vary quite a bit and very rapidly.
Replaced fan clutch, 195f thermostat, idler pulley, serpentine belt, had the radiator vatted and rodded out. The stock temp gauge was showing hotter then should be and swinging from between the A and L on NORMAL all the way to the next to the last line before hot. SO we replaced it with a SunPro mechanical gauge. Also did a combustion leak test and there are no exhaust gasses getting into the radiator. The oil looks great and there is no white smoke out the exhaust. The engine seems to be running great.

The problem is that the mechanical gauge swings back and forth between 190f to 202 during normal driving. This seems simular to what the dash gauge was doing, bouncing up and down.

Around town in stop and go traffic with many lights the temp stays around 192 to 194f. I do not use overdrive in town. On the interstate once in OD, the temp quickly goes to 200 to 202F at any highway speed. If I pull it down in drive where it raises the RPM’s, the temp quickly drops back down to around 192 or 3F.
The system was flushed and burped then driven about 100 miles before these videos were made.
If I just let it sit and idle from 200F, it will slowly drop down to about 192f.

Here is a video of the temp gauge at around 200F in OD and running about 65mph on the freeway, I was just leveling off after pulling a not so steep but long grade in the truck lane. I took it out of OD and put it in D and you will see the temp drop rather quickly. I put it back in OD just before getting off the Exit ramp and decelerating, you can see it raise again..
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In this video I had pulled into a parking lot immediately after getting off the interstate, turned around and pulled back out on the the main road. You can see how fast the temp drops almost 10 degrees.
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Thanks for the advice... Before I had the radiator cleaned it would raise in temp at a stop..

 

Looks to be air pockets in the coolant passages or head gasket. I had the same exact issue with a chevy 1500, no steam smoke or anything. Changing the thermostat did nada. I rebuilt the engine and all is well. I did find that the cooling passages were all gunked up. Had it completely washed at the machine shop.

 

I checked for combustion gasses in the coolant system and it showed up negative. I drained a little antifreeze out so it wouldn't be sucked up into the tester then started the test while the engine was cold. As you can see at 190F the fluid was still blue meaning no exhaust gasses in the radiator.
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I took the tester to the tail pipe to check that the fluid was good and as you can see, it changed colors once mixed with exhaust gas.
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Yes so the head gasket is ok, there still remains the air pocket issue. For temperature to change like that it would have to be air in coolant passages or partially clogged passages. The only way to ensure that passages get cleaned out is to rebuild the engine. I don't see any other way.

 

Just a complete guess but I would be interested in the transmission temperatures, The transmission is capable of overheating the engine and since the cooler is in the radiator after the thermostat it can cause some odd reactions for the gauge.
If it was the Engine in my thinking it wouldn't matter what gear the transmission was in as long as the RPMs were the same... Something to check.

 

If it was air pockets, they should have worked themselves out by now I would think. I have driven over 100 miles since the last time I had the system opened..
Thanks for the suggestions..
Interesting about the transmission temperatures,,, I will find out how to hook up a temp gauge and check that too..

Maybe I'll try running some of the flush where you drive it around for a few days then flush it out.. Maybe just by chance it will do something other then plug the radiator..

 

I'm gonna suggest replacing the water pump, the fact that temps drop when you raise engine rpms suggest to me that the pump isn't moving fluid the way it should, and if the motor had a bad head gasket you typically see temps rise with more rpm.

 

I wa afraid it might come down to changing the pump. I believe it is the original so it has been on there 26 years. I broke a bolt removing the T-stat housing. The steel bolt was corroded inside the aluminum housing. With one bolt still intact in the housing, I snapped the ear off of the t-stat housing. It ear broke right at the bolt and I was able to take a hammer and long screwdriver and tap the piece out. Once that was done the bolt came right out. I can only imagine the corrosion where the bolts goes 1" and 1/2 through the water pump housing!

 

At 200F if you let it sit and idle. the temp will very slowly drop down to about 192 or 3F, but it takes 10 or more minutes.

 

Have you checked for diagnostic codes? I just remembered something from when I was a kid, Dad and I replaced the intake manifold gasket on our 88 E150 5.8 C6 and the factory temp gauge was all over the place when driving and pulled a code for the computers temp sensor (The temp gauge has a separate sensor) and that lead us to finding the plug wasn't completely seated into the sensor and not giving a reading to the computer. Surprisingly the van ran very normally and really the only thing that changed was the coolant temperature?

I was 15 and just got my learners permit and didn't know much about anything vehicle wise at the time so I'm not 100% sure this is correct but it's how I remember it working out and was a good lesson for diagnosing before throwing parts at a problem.

 

I haven't tried pulling codes yet. IT has a CEL light that comes on when turning the key on but then goes off once the engine is started. Yes I know about the two temp sensors. When I was changing the alternator I had to take the connector off of the temp sensor. I will double check it. The temp sending unit, I replaced it first and them dash gauge was reading higher so I installed a mechanical gauge. I think that gauge is probably 5 degrees off as it reads 190 when the stat opens but the stat is a 195.