How do you get the thermostat to open?
 

I've tested my brand new, 189 degrees F thermostat in hot water. It opens perfectly.

I install it in my 83 d24t, and I just can't get the thing to open. Both ends of my head will be around 210 degrees F, while the thermostat housing is only at 140 degrees F. The top portion of my radiator is 175 degrees, while the lower portion and lower hose are not even 100 degrees. I've spent over 10 hrs trying to bleed air out of the system and get the water to flow.

I'm driving around with no thermostat since I can't seem to get mine to open. I even tried drilling a couple holes in a different working thermostat and installed the thermostat orienting the holes towards the top of the bore for water/air to flow. It still won't open.

I've never experienced so much trouble with a thermostat on any vehicle. Any advice?

Where are you bleeding the air from? Sounds like something is wrong. Could be a defective water pump or you have a lot of air somewhere. I've never (knock on wood) had trouble beeding a D24/D24T.
Does it overheat or just run hot? Since the thermostat sits in the block, that side should get hot first, thus opening at the correct temperature. Also, make sure that it is installed properly. The longer end (with the spring, etc.) goes into the block.

It is running hot, but not overheating. The thermostat will only fit one way: the long portion into the block.

The blades of the water pump look great. The bearing is tight too. I don't see how it might not be pumping the water properly.

I've been bleeding the air from the top of the cold start device. In latter attempts in get everything working properly I've been bleeding air from the thermostat housing hose.

95% of my radiator and 100% of the lower radiator hose is still stone cold while the head is 210 deg F.

Silly question, have you checked your radiator? Could be poor circulation through it. Also, without a thermostat in it, does it warm up at all? Could be a bad head gasket causing air pockets, though it would build up a lot of pressure in the system nearly immediately after starting it.

That does sound like radiator blockage.
What is the cooling system history prior to this incident? What led you to change the tstat? Was this happening w/ the old tstat?

If you tested the thermostat in water and it's opening fully at the correct temp, than your problem is almost certainly something else (possibly the head gasket, or water pump from your symptoms).

What temperatures is your engine reaching, and under what conditions? How are you measuring it?

I've been around and around on the cooling systems on these engines, trying to make them run at the right temp. My '83 764 TD (same year and setup as yours) runs hot on hills, and even just around town if I hammer it from stop light to stop light I can see the gauge walk up from 9:00 to 10:00. I've put thermostats in it; I've replaced the water pump; I've checked and rechecked the headgasket (which I know is new on yours anyway), I've fooled around with cam and pump timing; I've tried restricting flow in the the hoses to the oil cooler, heater and cold start thermostat in an effort to force more flow through the rad... all of that has had no effect. I believe a plugged radiator is probably the only thing left that it could be.

The only D24T I have owned so far that really has zero cooling issues is my '85 745 that I put a different motor in over the summer. It's a good motor with a good head gasket, and I installed a good (aka non-GMB-style with big impeller) water pump in it, along with a brand new Nissens radiator and a new OEM Volvo thermostat. The thermostat was in a Volvo box but it said Wahler on it and looked identical to the Wahler t-stat I can get through IMC for the D24/T, so as far as I can tell it is exactly the same... BUT, the car with the thermostat that came out of the Volvo box never budges the gauge even if I leave my foot pressed to the floor for a solid five minutes straight climbing Rainy Pass, whereas the 764 with the t-stat out of the Wahler box quickly creeps up to the red if I do the same thing. The difference could only be either the thermostat, or the radiator, which is new in the wagon and older/suspect in the sedan. But the key point to this story is that the only car that I have been confident is really, truly trouble-free is the one where I pretty much just replaced EVERYTHING in the entire cooling system. That made it happy. I think these cars have a fairly narrow margin of cooling capacity, where it has just enough to keep it cool even in extreme conditions if everything is working perfectly, but as soon as one component starts to get compromised you can start to see cooling issues come up. They just don't seem to be tolerant of much neglect in that area.

ALSO, though, the temp gauge logic changed in the '84+ cars. The '83s had a non-compensated temperature gauge circuits, while after '84 it used a compensator. In essence, that means that the gauge on an '83 760 is telling you exactly what the sender in the motor sees, while the gauge in the '85 is giving me an approximation that may be ironing out small fluctuations in temp, etc. Essentially the compensated gauge in the '85 makes me believe that the engine gets up to temp faster and stays more stable, while the '83 makes me worry more but maybe is the one that's telling me the real truth.

Any engine will fluctuate somewhat in temp as load and ambient conditions vary, and most modern cars have some degree of damping or compensation in the temp gauge circuit to keep nervous drivers from freaking out when they observe normal temp variation on the gauge. In fact that's part of the big disaster that Ford is dealing with on the 6.0L Powerstroke engines, which have cooling system issues that can cause a sudden overheat... but they put too much damping into the gauge, so if the engine overheats fast enough, it can actually get hot enough to damage itself before the gauge even registers and alerts the driver to what's going on so he can pull off an do something about it. :rolleyes: In any case, on the '83 D24T, it should be pretty steady and the head should probably not be getting that hot, so you may have a blocked rad or air pocket or other impediment to proper flow... but just because you see a little bit of normal movement on the gauge when driving wouldn't necessarily make me think there was a major problem, as long as it never moved too high or too dramatically.

Very educating post. Thanks a lot!
Having a D24 NA with a very nervous temp gauge (a steeper hill, a bit more pedal, etc... makes it go a bit up, sometimes a few milimeters from the red part) makes me think that I need to do something about the whole cooling system...

Regards

Also, I have ran into faulty temperature senders that seem way over sensitive on them. Seemed to be mainly in the late 83-late 84 model years. After putting a gauge on each suspected motor, I have always come back to replace the sensor. May also be worth looking into.

I think most D24s could benefit from a $65 tropical fan clutch from FCP, a brand new radiator, and Zerex G-05 coolant which will keep the radiator from plugging up again. Also, a totally separate digital temperature gauge of some sort. The factory temp gauge is total crap IMO, even in the 83/84.