OK guys, I need to pick your collective brains.
1985 Coupe L98.
I just last Friday completed reassembly of the TPI, repairing the front and rear manifold oil gushers! Got it all together and started, and no leaks . While resetting the IAC, timing and TPS, the temperature had climbed up to around 275. I had a quick look in the radiator, and the water wasn't circulating, so I shut it off. After a little looking I decided that the thermostat had to be sticking. I was a little pizzed, because I had recently replaced it along with hoses when I replaced my radiator. Anyway, $10 at the parts house and I was up and running again. This time I bought one that is supposed to stick in the open position if it malfunctions.
I drive it home Friday afternoon, temps are normal (190-215). All is running fine except the check engine light is on. I couldn't hook up the coolant temperature sensor on the front of the manifold because the plug had finally, after being squished too many times, given up the ghost. Saturday morning I head to the local Pull-A-Part and locate the plug I need. I replace it as well as adding coolant to the radiator, and while driving around, everything is fine. Sunday, I make a couple of short runs, and everything is fine. Monday I don't drive it at all.
Today, however I head on some errands, and go to the shop. City driving and I don't get over 50mph. The temp never gets over 160. I get a little concerned and check the oil temp, and it never gets over 180! The outside temps have been in the 40s all day today, and it was raining( It's my DD, so don't give me any crap!). Anyway, I had to head back to the shop about an hour ago. This time I take the interstate. The highest the temps reached was 150, and that was sitting at the stop light after exiting. On the interstate, the temps were 135-140. Also, I hit the switch to put it in overdrive, and nothing.
I don't know what to think. Could I have the coolant temp sensor wires crossed, and would that make any difference? I felt of the radiator hose, and it feel normal.
Give me some guidance here guys. I want my girl running hot the way she is supposed to!

 

if you could figure out what you have and bottle it, I'll take some.

 

I thought about that, but why is the oil temp low too?

 

Mine does the same thing...runs stone cold while moving. I'm guessing my thermo is stuck open but haven't checked it out yet.

 

Sounds like it did what it was supposed to ....stick open when it malfunctions back to the store........

 

Yeah, but 140 degrees? That seems a little cold doesn't it?

 

Even if your thermostat was stuck open, I don't think you'd see temperatures that low. I would suspect your temp. sensor is giving you a false reading.

Why not pull the thermostat and check it. It sure can't hurt. Put it in a pot of water and then boil it and see if it opens at the right temp. And then you'll see if it sticks in the open position. Stick a temperature gauge in the water that can read temperatures that high when doing this or use one of those laser temperature readers. This way, you'll see exactly when your thermostat is opening.

 

I was afraid I was gonna have to pull that thing out again

One other thing that may or may not be related. My security warning lamp and that incessant ding dong the witch is dead door alarm for some reason was not working today. The last time I was at the shop, I pulled the positive cable to reset the ECM after it would not shift into OD, and it(the door dinger) started working. When I got on the highway afterwards and hit the OD switch, the OD kicked in and then kicked right back out. Any ideas?
One reason I am leaning away from bad sensors and thermostat is that the water temp and oil temp are staying pretty much parallel with each other. And the temp difference is within 25 degrees like it should be. The readings are just too low.
I hope to God my ECM isn't going away!

 

I agree as my oil temp is low as well as coolant...and the oil temp sensor was just replaced. Mine will run in the high 140's-high 150's while rolling down the highway. If I'm creeping around town it will go up to about 175 tops. Only if I sit at idle for several minutes will the temp slowly creep up past 200. At least its not running too hot all the time.
Just pull the thermostat and check it as described above to remove any doubt about it. Thats what I'm going to do.

 

Well, today I run some errands and head to the shop again. During the errands, the temps slowly rise, but never get over 160. I drive it hard, soft, doesn't matter. It seems the harder I drove it, the cooler it got!
I get to the shop and leave it idling. While talking to my brother, I notice the temps start climbing. It took a bit, but it did get up to 228 and the fan kicked on and took it down to 190 when the fan kicked off. I continue to let it idle for about 45 minutes, and the temps stayed up and the fan cycling on and off.
I get in and head to a friends ahop. Get on the interstate, and within 2 miles the temps are down to 180. Before I exited the highway, the temp was down to 170. I let the car idle and talk with my buddy. Temps climb back up to 228 same as before.
So, it seems I am going to have to change out the thermostat again. BTW, I don't think I mentioned it before, but this last thermostat is a 180.

 

try a little foreplay before driving