I've got a perplexing problem with my engine keeping at the proper temperature at highway speeds.

I have a 1992 with the 351/E4OD. The truck has 185,000 miles on it, but I put in a reman'd Jasper motor at 165,000 two years ago. A new water pump and radiator were put in about the same time and the heater core was replaced maybe a year prior to that. I have a electrical temp sender in the stock sender location and relocated the stock sender to the heater core input line.

The truck has always run at about 200-205 degrees, year round. It always was a bit slow to warm up in the winter for heat, but reliable. I had the coolant flushed and changed a little while back by my local mechanic and also replaced the thermostat at that time. He left a bit of air in the system, but after a few days of around town driving I couldn't hear it in the heater core any more. But the truck would not stay at 195 over 60 mph and was very slow to get up to that temperature driving around town (took like 20 minutes). Went back in and the mechanic figured we just go a bum thermostat. I should probably note that this exact same issue happened when we put in the Jasper motor (different mechanic) and a new thermostat seemed to solve the issue.

Again, truck won't stay at 195 while running at 60mph or faster and still takes a long time to warm up. Driving at 70mph it runs at about 150-165. If I slow down for tolls, temp goes right up to 195 or so. At the end of a long trip today after several stops to check cooland levels the temp came back up to 180. Doing some searching on this board everything seems to point towards bad thermostat, heater core, or maybe the rad cap. When the truck is up to temp both hoses from the heater core are hot and seeing as how it's relatively new and was just flushed I would guess it's not the problem. Since the truck runs up to 195 ok and doesn't overheat around town it seems like the thermostat is working somewhat. Today as I sat in a roadside rest area for about 10 minutes letting the engine warm up some so I had heat and a warm winshield for driving in the freezing rain we had the temp did climb to 210. I was in a hurry so didn't see if it would keep going. The old t-stat I had prior would also climb to just a bit past 210 before backing the temp off to 195 during extended idling. I haven't replaced the rad cap yet, but the system is building pressure. Oh, one other thing. I've had to add coolant twice as the air worked itself out of the system. The first time I added about 1" to the radiator itself and refilled the overflow bottle. The second time I just had to fill the overflow bottle (it was empty both times). The level seems to be holding fine and there are no visable leaks I can see.

Do I swap in a 3rd thermostat or have I missed something?

What is the best way to bleed more air out of the system if I do have the thermostat replaced again.

 

A FRIEND OF MINE HAS OWNED A RADIATOR REPAIR SHOP FOR THIRTY YEARS AND ALWAYS USES A 7LB PRESSURE RADIATOR CAP.
I HAVE A 89 FI50 AND IT REQUIRES A 195 DEGREE THERMOSTAT AND IF WORKING PROPERLY IT KEEPS THE COOLANT IN THE ENGINE AT APPROXIMATLY 195 DEGREES.
IF YOURS IS COOLING TO A LOWER TEMP THAN THAT THEN I WOULD SUSPECT THE THERMOSTAT TO BE DEFECTIVE OR NOT THE RIGHT ONE FOR YOUR APPLICATION.
AIR CAN EASILY BE VENTED FROM A SYSTEM WHEN FILLING BY DRILLING A SMALL HOLE (1/8 in) THE FLAT AREA OF THE THERMOSTAT JUST INSIDE WHERE THE GASKET SEALS THE THERMOSTAT AGAINST THE TOP OF THE INTAKE HOLE. IT WILL NOT AFFECT THE HEATING AND COOLING OF YOUR ENGINE BUT WILL ALLOW AIR OUT WHEN FILLING.

 

It's always been a 195 degree thermostat. I'm just suspicious that I'm missing something since I can understand getting 1 bad one, but not 2 bad ones in a row.

 

First off I would suspect the gauge. They're not always acurate. Use a cooking or candie thermometer and with your cap off and engine running at normal temp take a reading from the thermometer and compare it to your gauge on the dash. If it is the same then my next guess would be your radiator cap.

 

The new Autometer gauge was put in this summer and told the right temperature prior to swapping thermostats. And it stills seems to read correctly since around town the temp rises to 195 and then holds steady. I also have the stock gauge hooked up on my heater line and it reads about the same temps as the main gauge (at least correlated to my historical readings on the NORMAL written on the dial) It's only at high speeds where I'd guess I'm getting lots of cooling through the radiator that the issue occurs and the temp drops to 150-165.

How does the cap help the temp? I would think that by pressurizing the system you increase the boiling point, which allows you to run hotter without increasing the chances of boiling. But does the pressure have other effects as well that might influence my problem? I'll still get a new cap, as they aren't expensive and no harm done to replace it, but was curious as to how this all works.

 

If you have a properly operating 195 degree t-stat, then the truck should not dip below 195 once it reaches operating temp. Almost assuredly, your T-stat is bad. I would replace it, then you should not have any more problems.

There is no reason to suspect the heater core if the engine itself is not reaching operating temperature in the first place.

As far as radiator caps and pressure, while pressure makes a difference, it is usually the opposite - not having enough and boiling over. I've never heard of a cap causing an engine to run too cold, but it could happen I suppose. Like you said, they are cheap and easy to replace, so go for it.

But I am certain that the T-stat is your problem.

PS - if you absolutely cannot get it to run at operating temperature at freeway speeds, you can always block the grill with cardboard or with special grill inserts that they make (check Lund, etc.). The idea is to block all of that cooling air rushing through the radiator at freeway speeds.

 

Does anybody have any recommended brands of t-stats (and ones to avoid)...? I have no clue as to what my mechanic put in there, but since this will be the 3rd trip in the past few weeks I think I can be picky as to which brand we try next

I do have the radiator cover, which was useful when operating up in Canada in -40 weather, but around Boston it rarely gets below 0 and it's pretty easy to overheat the engine with it if you're not careful.

 

Robert Shaw and Stant are two brand names that come to mind. You could always get a stock one from the dealer, but I don't know who they source from. You may just be paying double for a Stant or something.

 

I've always had really good luck with a Stant Superstat. They're usually about twice to cost of a cheapo, but the've always worked for me. They're plated with a different material, so they're shiny. I like to think they actually work better, and I'm not just captivated by shiny objects. I've also had excellent luck with stock Ford thermostats. They're more substantial looking than a plain Stant, and they have a Ford script on them. I don't think they're reboxed. Sometimes the stock stuff works best - and it wasn't expensive at all.

 

overheating issues...wow i wish i had them. and i have my radiator over 3/4 coverd in cardboard..and it got to 57 today. but. i need a new themostat. runs fine during the summer, gets to right temp and all but in the winter is sucks. if it wasnt so hard to get to and i didnt have to make custom tooling( cut a 1/2 wrench in half) to take the thermo out id change it lol. o by the way i dont cover it in the summer time. when i first got the beast it had a 180 thermo in it and i couldnt get the thing hot if i sat with a torch to the radiator...not sayin that ive tried it but lol. im actualy suspecting the fan clutch more then the thermo cause it gets warmer at 50 and above then in city drivin...go figure

 

by the way i ment 50 mph and above

 

You have to use custom tooling on a 302 to get a the t-stat? That doesn't sound right... My 351 requires 3 bolts to come off of the housing right at the top front of the motor. Easy as eating pie. I can't imagine that the 302 is much different. My big problem is I'm not set up to deal with used coolant and the mess isn't worth my while to do it myself... That, and it's darn cold working on the truck in my driveway this time of the year.

 

the disty is in the way for the most part. but trust me its realy hard to fit your hand in there to turn the bolts, and ive tried swivles and crows feet, and there to big.. if i were to take the disty out it might make it a little easyer but thats not the only thing in the way

 

Yeah the worst part is the coolant that spills out and the mess that it makes. As far as swapping out the t-stat, it's about a 1/2 hour job.

I've done it a couple of times, and used common hand tools to do it. Nothing special. I've got the 351, but it's about the same, if not exactly the same, job to R&R the t-stat, right?

 

An easy test for a thermostat is as follows. Heat some water on the stove and put the thermostat in the water. Using a temperature gauge you will soon know if the thermostat is working or not. A good way to test them before you spend all of that time replacing them. Have fun, I'm saying you bought a bad T-stat. I experienced the same problem a couple of years ago. Much easier testing them first, just my 2 cents.