Im battling a running hot issue only at highway speeds (has climbed to about 230 degrees)with my 1991 dakota sport 318. I have done a taurus electric fan and have a 180 t-stat. When im going over 60mph my temps will gradually raise until i slow down to below 50mph or so. It doesnt matter if my fan is on high low or off the temps act the same. Im judging my gauges to be accurate as the stock gauge and aftermarket gauge are pretty synced together (of course the stock doesnt have actual numbers but i know roughly where 180 and 195 degrees are on it) and my fans temp switch is turning my fan on at the right times as well (based on the aftermarket temp switch). Radiator fins are clean as well.

Recently i did the timing cover, waterpump, thermostat, rad cap, and the little bypass hose. Ive tried flushing the system and burping it more. It gets good heat. The radiator was changed a year ago, but the truck sat for 8 months of that period since. Any other ideas of where to Start fixing this, i forsee it getting worse as the weather gets warmer...

 

Id be looking into a defective thermostat perhaps(only opens part way??). Also, is you're fan sucking or blowing? haha never know. Man that is a real head scratcher.

 

when it is at 230 is the AC running? If so, does the engine cool down some if you turn it off? If it does, then the AC could be overcharged.
Even if the above does not apply. Is the condenser clean? Remember the air has to pass through it first to get to the radiator.
I don't know what temp. these trucks run at. I have a corvette and its not uncommon at all to see them run 20 degrees above the t-stat opening temp. especially on a hot day with ac running they can go even more.

 

I suppose you know you can test the thermostat by placing in a cup of boiling water to see how far it opens or if it does at all. It sure sounds like air in the system but your heat is hot so that probably isn't it. Maybe the new cap isn't sealing? Do you still have the cowling on? The cowling helps it cool. If the cooling system is set up right then something is running hotter than it is designed for but I expect you already know that too.

 

It has occurred to me today to ask why the radiator was replaced and if the motor was flushed when you replaced it. It may be that some of the internal coolant ports are plugged or narrowed by build up and that the motor could do with a flushing.

 

the thermostat has been removed (after changed 3 times with 3 new thermostats and not affecting anything) The temps still climb just takes a little longer to do so. The fan is also set up as a puller (or sucker in your terms LOL)

My truck has had the ac removed and the condenser is removed as well (everything ac related except the hvac box under the dash has been removed).

The radiator was changed due to a leak and just flat out being old. figured when i did the electric fan swap id start with a fresh radiator and as well. The cooling system was flushed before and after the radiator replacement. as well at 3 times now trying to solve this issue.

When i first did the electric fan swap the truck ran constantly at 180 on the highway of course in town it would heat up to the 195 mark, kick on the fan and cool back off to the temp the fan shuts off at. but im still not having issues in town, stop and go traffic is fine, anything under 55-60 it acts fine. even on warmer days. but kick it up to 65-70mph and thats when it climbs. Im really at a loss here....

 

Hi! You said in your post you wired in an after market temperature gauge. Did you wire the after market Temperature gage straight to the sensor or are you piggy backing off of the existing dashboard's circuitry? Honestly, I've had four 80's and 90's chrysler products. And a few more sprinkled throughout my family. I've always eye'd the dashboards in them with some measure of suspect. For example I've noticed in my truck (Which I purchased recently) That when I have the A/C on the oil pressure gauge will start to creep up. Also from time to time my speedometer tick's in time with my left turn signal. I'd suggest seeing if it's actually running warmer than it's supposed to or if there is static bleeding into the dash cluster at increased RPMs. Perhaps the next time it starts saying it's hot pull over and check under the hood. Maybe see if you can borrow one of those stylish temp lasers or something?

 

Sorry man the only other thing I can think of is the fan shroud. It is rather important to the airflow through the radiator as well as directing it over the motor.

 

I thought fan shroud as well but thought of one other odd ball thing to look at...transmission cooling and or temp. Line of thinking, faster speeds = higher rpm due to overdrive. Maybe trany is getting hot. Could also be just higher engine rpms causing it to heat because of cold spots on the radiator. Speaking of...have you felt your radiator for cold spots?

You could try overdrive off or lower gear at lower speeds (try for same target rpm that occurs at the 65-75mph overheat zone) and see if the temp goes up. At least then you would know it's ether trany heat or engine heat at a constant higher rpm causing it and not just an indication problem. Also try to turn the dash heat up (in that way your heater core is used for additional / supumental cooling) see if the engine temp goes down.

 

My aftermarket temp gauge is wired with its own temp sender. I have checked for cold spots on the radiator and its all evenly hot when this happens. I havent used a fancy temp gun or anything on it but i know the truck is warmer than normal when i pull over after this happening.

Last time i was out wheeling with it it was alot of low speed 1st gear high rpm and my electric fan did perfect keeping it cool. (and this was after it warmed up on the highway driving to the wheeling location)

The transmission is running at around 135-145 degrees while down the highway. It is not plumbed in through the radiators cooler for summer. I have it ran through its own cooler as well with its own fan.

running the heater does not affect the temp im running at either. Something someone mentioned elsewhere, what about possibly a small head gasket leak? where the head gasket blows just enough that it lets exhaust gasses into the cooling system essentially airlocking it at higher rpm but not causing a noticable loss of coolant? Never seen it before but thats what someone else mentioned....