OK, first the facts:

1986 F350 Crewcab Dually, Stock 460 with Edelbrock performer intake, and Road Demon 625 carb, C6 tranny, 4.10:1 10.25” rear end.

Recently replaced radiator, thermostat (180 deg), fan clutch, all hoses, new coolant, etc. All parts are heavy-duty cooling. Have not replaced water pump yet, but it’s not leaking. It’s not eating or leaking coolant, but when running the overflow (normally empty) will fill up to almost full (if I put anything in the overflow when cold, it will puke it out when hot).

OK, here’s the issue: When towing, the truck will run hot, but not consistently. I tow a 10,000lbs trailer (limit of the GCWR) and it tows well, but after about 50 miles the temp will start to creep up to an uncomfortable level. It does not ping or anything, but it does creep up to close to the “H” on the gauge (the stock gauge has “L”, then the letters “NORMAL”, the “H” at the top). The truck normally runs at “O” in “NORMAL” when not pulling anything.

One would think that this is an issue with towing, but (here’s where it gets interesting), sometimes (random) even if it’s not towing, the temp will rapidly creep up for no apparent reason…then slowly creep back down to the normal area. What’s even more interesting is that the oil pressure gauge and the fuel gauge (but not the voltage gauge) will also be reading higher than normal when it does this. Very strange.

Now, I would just put this down to a bad instrument cluster (voltage regulator), but in fact the engine really is running hotter than normal. I think the gauge thing is a different gremlin (no doubt to confuse the situation).

Doing some research I’ve thought about this and I think it may be either a cracked head or a leaking head gasket, but that’s just theory (I’d love someone to either shoot that down, or back it up). I have no proof of this (all plugs look normal).

One other theory is that the carburetor or fuel pump may be giving me issues. I have installed an air/fuel meter in the truck, and I’ve noticed that after about 50 miles of freeway driving (when towing) the A/F starts to lean out to where it goes off the “bottom” of the gauge (reading lean). A quick stab of the gas will bring it back up to normal (just slightly rich when towing, as I’m deeper in the throttle). I’m currently running a Barry Grant “Road Demon” 625 carb. I’ve jetted it up a bit, and it runs fine most of the time. I’m considering moving up to a newer Road Demon 725 carb, but doing the math that 625 carb should be enough. If I replace the carb, a new fuel pump will also be a part of the swap to eliminate any fuel starvation issues.

I really like my truck and keep it in tip-top shape, but I’m running out of ideas (and experience), so I’m hoping that some of the smart people on this forum can help me out, or at least point me in new directions that I may not be thinking of.

Thanks in advance for any information that may be helpful.

Dan

 

Ford guages are highly inaccurate! I would verify the temp with a mechanical unit. Also you may think this is bogus but put the factory high temp thermostat back in. With a 196 it will allow the coolant to have more cycle time in the radiator. Ive seen this cure many HD/Tow vehicals with the same problem your having.

 

SSCX2,

Thanks for the feedback. It did the same with the 192 thermo, so I don't think that was the issue.

I also agree with you that the gauges are innacurate, but I have verified that the engine is in fact running hot (but not overheating to the point of boiling over or anything). It will actually start pinging slightly under full throttle when it's in this state, and the heater starts belching out *really* hot air.

I will verify with a non-ford gauge so I can get an actual temperature, but that will have to happen next spring as I have put the trailer up for the winter.

Dan

 

Several thoughts:
Leaking/Defective Radiator Cap - not holding pressure and allowing hot coolant to be pushed out - too little system pressure will cause an overheat because the coolant will boil.

Corroded/Loose Water Pump Impeller - could cause variations in cooling if the impeller slips.

Faulty Thermostat - even when new, some just ain't right. Your engine will be happier with a 192.

Faulty guage - My gauges read in two distinct positions - I don't know why, but I've gotten used to reading them that way. Try it with a mechanical guage - a properly functioning cooling system should keep the temperature consistant. It shouldn't go up or down a whole lot once you're driving. Mine reads nearly the same from winter to summer, towing or not.

Good luck - sounds like a good hauler!
Heggie

 

Cheggie,

Thanks for the info. Let me address your points:

The old radiator cap was replaced with a new one, and it is not leaking. I should have stated that in my original post...sorry.

I've thought about the water pump impeller theory. I think that will be the next thing I do after some gauge work. I'm thinking of putting on a nice Edelbrock water pump, as it provides equal flow to both heads.

The thermostat was checked before it was installed. I still have the 192 handy...I'll be putting that in for winter soon. I verifed that it opened, and my digital kitchen thermometer showed it opened at the correct temperatue and the correct amount. I've been bitten by this gremlin before, so I always double-check the thermostats before installing. Oh...and it's installed in the correct way as well.

I've gotten some feedback from another forum on changing out the voltage regulator for instrument cluster. That will be the next thing I do. I will also change out the sending unit. I'm also thinking of mounting a mechanical gauge to the engine, right on the water neck. I thinkg between those tasks, I will eliminate the gauges from the problem.

Again, thanks for the input...this is all good stuff.

Dan

 

Are your hoses new? Check for soft spots in them, as they may "crush" under harder use, and technically collapse when you need them most, causing a slow in the flow of coolant. Just one more possibility to deal with here. If you suspect head gaskets, check for coolant in your oil, and oil in your coolant. Either one, or both would be a definitive indication of a bad head gasket.

 

RR4E,

All hoses are new. In fact the new lower hose has the "spring" in it to prevent collapsing (the old one did not!).

I've checked the oil and the coolant, and no contamination. I'm going to take it in to a radiator shop if the problem continues after I've tried a few more things. They can "sniff" the coolant to look for exhaust gasses in the coolant in case it's a combustion chamber leak. I think that's doubtful, but worth a shot.

Thanks again for the great suggestion!

Dan