I just purchased a 1988 E150 conversion van with a 302. I have around 200 miles on it so far with the OD reading 115,000 miles. Recently, with in the last 2000 miles, the water pump and thermostat has been replaced. I noticed on a couple of occasions the temp gauge spiking up to the hotter than normal range. It usual lasts for a block or so
than goes back down to normal. I replaced the thermostat with another new one, not a fun job... the water pump is in the way. I also replaced the radiator cap. Anyway, about 40 miles after replacing the thermostat, it happened again. Watched the guage go from the "R" in normal to above the normal range. I was 1 block from home, so I parked it in front of my house and when backing in to the spot, the temp went back down to the "R."
My next thought is that I may have a malfunctioning temp. sensor or possibly the fan clutch is on the outs. Any suggestions would be greatly appreciated. Not sure how to diagnose this problem. It is usually the thermostat sticking shut. But two new ones in a couple of thousand miles with the exact same temp. spiking has got me stumped.
Sounds similar to my problem on my 4.9L. The temp would jump up to like the "A" or "L" then just go back down to "N", I replaced everything! Waterpump, 4 t-stats, Fan Clutch, Belt, Hoses, Radiator, and sending unit. Finaly I broke down and took it to the dealer and they said it's a clogged cat. You should try replacing the sending unit first...on my 4.9L there is 2, one for the gauge and one for the computer....the one for the computer is what I replaced because I know the one for the gauge was correct because the truck ran like crap when it got hot.
On some vehicles especially with older hoses, sometimes the rubber gets soft. At higher engine speeds(Cruise, normal street etc) sometimes the water pump will create a vacuum on one of the hoses. If the hose is soft enough, it will collapse and cause over heating.
To check for this warm up the truck so that you are sure the thermostat is open. Probably best bet is to let it do what it has been, ie warmup and then spike on the temperature. Once warmed up. park it and leave it running. Pop the hood and carefully look and squeeze the big radiator hoses. Be sure to watch out for fans and running belts etc.(I know this is dumb but some people would touch them). While watching the big hoses, rev the engine up to freeway speed and hold it. If you have a soft hose problem you will probably see the hose collapse. The fix is a new hose and or those spring thingies that go inside some radiator hoses.
I drove the van to the parts store tonight and picked up a new fan clutch. Themp. spiked a couple of times while driving. When it started to get hot, I turned on the heater and it went right back to the normal range. Cannot work on it to night, but I have some time on friday to do the job. I will let you all know how it turns out. Thanks for the help so far. Hopefully I won't have to go to plan B.