The 6.5L TD in my Breakway has developed a strange cooling issue (see below).

The system is a bit weird. The water pump suction and discharge through the thermostats are routed directly to the radiator (crossflow, side tanks). On top of the radiator is a plastic tank (based on its markings, it was originally designed for a system without an overflow tank - but the neck has an outlet and the system has an overflow tank).

This tank has the low coolant sender. There's a 1" hose from the water pump (discharge side) to this tank (which appears to be the way the fill is accomplished) and a 1/4" hose from this tank to the radiator core (top of the suction side).

about 2K miles ago, it would fill the overflow tank, but not suck the coolant back in as the engine cooled. I replaced the cap; the third one seemed to be the charm.

So after running a few hours on the road, the coolant is pushed to the overflow tank, and the top tank has only a little coolant in it - running temps are normal.

Because the 1" discharge hose from the pump is now forcing air (which compresses), the flow through the thermostats is reduced (based on engine temps rising when exiting the Interstate - no overheating); revving the engine a bit brings temps back to normal.

My first thought was that there's a head gasket leak, but the engine runs smoothly through its range, even on a long, steep grade pull, and the system retains pressure. There's also no coolant loss - it ends up in the overflow tank, and seems to like it there.

I'm beginning to suspect that the top tank has distorted, and the cap isn't sealing properly, but I'm interested in any other opinions...

I've got some experience with my 93 k3500 truck with 6.5 Turbo. I bought the truck used with less than 50k miles on it about 9 years ago. I tow a 5th wheel camper that weights around 7k, the truck is 7k so, probably about the same weight as most Class A's. I had always had a very weird over heating condition. The Truck would run fine all day long on the hottest of days as long as I was not on a hard pull. Virginia to Florida in the middle of July was no problem. BUT, when I hit a grade I could watch the engine temp go up and hear the thermostatic fan turn on, which is good. Then the stinking fan would turn off! I then watch the engine temp sky rocket up along with transmission and oil temperatures. Fortunately on the east coast I have only a few places that I traveled that caused this. One is 211 east from New Market to Luray VA. When I traveled out west I hit some stuff in Wyoming that caused it and God forbid that interstate that crosses Colorodo, I had to pull over several times.
Each time, I could pull over and let the truck idle and the engine would cool off. All the coolant was in the resovoir, and it would migrate back into the engine in no time. I would loose some, but not that much.
A couple years ago I was fooling with the timing (for no good reason) and I had a hard time starting the next cold morning. I over juiced it and blew a head gasket. What I found when I pulled the heads off were very small cracks between valves. I believe that these are actually normal, but in my case I am convinced that they made it to the water jacket. I have a kit to sleeve the water jacket and I did that, but chickened out and bought new heads when I put it back together a couple months ago. I have not towed with it yet, but I am convinced that I took care of the problem. The water jacket repair can be done with the engine in( I believe) I have the sleeves, drill bit and instructions to do many heads incase you want to try it, but I hope that is not your problem.

Sorry for such a long post...
I follow with a picture of the crack

Doug in VA.

quote:

Originally posted by Rusty:
... but I'm interested in any other opinions...

Did you drill a small vent hole in the T-Stat? If you think your system is getting airbound then this should help. They also make radiator fill necks that attach to hoses - you could T into the system to check your theory on a distorted tank.

Bill N.Y.

Thanks, Bill N Y - I don't think airbound is the issue; the thermostat manifold has a bleeder valve on it, and it was run clear.

Doug, oddly enough, long steep hills aren't a problem. The only manifestation is the insistance of the coolant to homestead the overflow tank. Long pulls on grades barely crack the thermostats (there are two, since I installed the high-capacity cooling system - but this issue arose before that mod).

What I find intriguing is that the coolant is blown out of the top tank, and declines to return from the overflow tank, leaving an air space. This to me indicates a bad seal between the top gasket of the cap, allowing air to enter the system rather than sucking coolant back into the radiator...

So far, excluding some miniscule crack in the head or head gasket, there's no sense to this - on runs of 25-60 miles, there's no symptoms - only on extended (200+ miles) runs does it appear - and it's not been disabling.

Bill N Y's suggestions are also excellent - except for the fact that shorter (less than 100 mile runs) show no symptoms, and that the system shows no air intrusion.

"Other than that, Mrs. Lincoln, how was the play?"

quote:

Originally posted by Rusty:
I don't think airbound is the issue; the thermostat manifold has a bleeder valve on it, and it was run clear.

Does this have an automatic air bleeder on it? If it is an your cavitating your air would be bleeding out. Your lack of coolant psi wouldn't turn into a suction and draw out the overflow coolant from the surge tank.

If you drill a small hole in the T-stats then any air pockets would go higher and out to the top of the radiator.

Are there any low spots from the t-stat housing to the radiator?

Am I missing something in my minds eye on your setup? Like you, I don't understand why (unless it is distorted) the coolant doesn't get drawn back out of the surge/overflow tank.

Bill N.Y.

quote:

Originally posted by Rusty:
What I find intriguing is that the coolant is blown out of the top tank, and declines to return from the overflow tank, leaving an air space. This to me indicates a bad seal between the top gasket of the cap, allowing air to enter the system rather than sucking coolant back into the radiator...

You can have your cap tested, but that will not test the interface between the cap and tank neck. I cured a problem like that once by cutting a ring of brass shim stock and placing it under the rubber neck seal.

Bill N Y, the thermostat on this is actually on a manifold, and is in a high point in the system, and the bleeder valve is at the top of the housing. The 6.5L TD is relatively easy to bleed the way this system is set up. Air in the water pump chamber would exit the top-mounted fitting through the hose to the top tank (and I can see that). So air can escape three ways:

1. The bleeder valve (actually two, as I left the older one at the top of the block in also).
2. Through the 1" hose/fitting at the top of the water pump chamber.
3. Through a 1/4" bleed hose connected from the top of the suction side tank to the top tank.

I've bled this system many, many times (first flush, rinse, refill, again when the second A/C compressor failed on the road and some of the coolant boiled off, and again when I changed the thermostat shortly after, figuring the wax pellet might have been stressed, then another flush-rinse-refill after instaling the high capacity parts). This issue started before I did the cooling system mods.

bill h, I agree, and I used some aircraft-grade sealant on the top cap gasket to ensure a seal and to use it as a tattler on the contact the gasket was making. The top gasket is sealing properly, without the sealant. BTW, I have a cooling system and cap pressure tester - the caps (all three) are OK. I harvested a gasket from an old cap to use as a shim if the seal turned out to be the problem.

My latest WAG is that there's a very small leak in the top tank - I noticed that the filler neck is at a slight angle, which isn't normal - so the tank is deformed somewhat.

I also noted yesterday right after return from TN that the pressure equalized quite quickly with the newest cap, which has a gravity-assisted retrieval valve (the other two are spring-loaded), and pressure/suction held for hours with the spring loaded caps.

I had first suspected a leaky head gasket as mentioned by Goldwing Rider, but there are no supporting symptoms - on a 30 mile trip from the star party to the RV park, all appeared to be normal - the overflow tank had gained no coolant...and the engine runs very smoothly.

Because the problem isn't apparent on shorter runs (<40 miles), I picture this scenario:

There's a small air leak in the top tank. Each time the coolant expands (like on a long grade pull), the extra goes into the overflow tank. When it contracts slightly, instead of retrieving coolant from the overflow tank it sucks air into the top tank. This cycle, repeated many, many times nearly empties the top tank. My suspicion is that the leak is somewhere in the filler neck.

Anyway, today I called Spartan and they provided me with a Ford part number (GM drivetrain, Chrysler HVAC, Cadillac headlights - I'm not surprised!), which I'll check out tomorrow at the dealer. I'm also going to pull the top tank and inspect it visually.

quote:

Originally posted by Rusty:
My latest WAG is that there's a very small leak in the top tank - I noticed that the filler neck is at a slight angle, which isn't normal - so the tank is deformed somewhat.

Not WA at all. Have you done an air pressure and soap bubble test before removing the tank? This should be done at different temperatures due to the possibility of strange stuff going on....i.e. the miles driven when it occurs.

I have taped a Kleenex over a suspected leak area and let the color of the antifreeze tell me if there was a leak at any time.

Thanks for the ideas, bill h - I haven't done a leakage test as I have to unbolt the top tank from the chassis to fit the tester in there. But I'll do that tomorrow and play Bubble Man before I disconnect the hoses.

The system doesn't leak coolant and does hold pressure, but it's possible that this recent quick loss of vacuum was due to the leak enlarging rather than the different retrieval valve...

quote:

Originally posted by Rusty:
- I haven't done a leakage test as I have to unbolt the top tank from the chassis to fit the tester in there.

Sometimes it is handier to feed in regulated air at another point by way of a cobbled-up fitting. Or just pressurize into one of the block drain plugs, as long as they are not plugged, themselves. Or a heater hose.

I agree , but I have a policy of doing no leakage tests except with a manual pump, even though I have double regulators on the air compressor - at 15-16 psi, they're at the bottom of their range.

But I can't even get the leakage tester onto the fill neck without removing the top tank, and I have no other gages as accurate in the range I need.

quote:

Originally posted by Rusty:
I agree , but I have a policy of doing no leakage tests except with a manual pump, even though I have double regulators on the air compressor - at 15-16 psi, they're at the bottom of their range.

Yeah, you have a point about the regs being at the bottom of their range. One of my regs is for sprayers and airbrushes, so it has a more favorable range.

The cooling system pressure cap will limit the pressure to its rating, anyway. They are built to be pressure relief valves. With a suitable restriction in the line, it would always be able to dump any excess pressure.

I agree - again - I have an airbrush regulator (far more precise than the compressor regulators), but it's permanently piped upstairs in my spray booth...

Dislocating the top tank is no big deal - it's secured with four bolts, easy to reach, and it's coming out of there no matter what.

This won't be my first experience where a plastic tank passes one test (pressure with liquid) and fails another (vacuum with gas).

Unfortunately, my vacuum pump is dead (motor's dead)...but in its day it could pull 29.995 in Hg (labratory model - used to use it to service A/Cs before the EPA and Biddy Committee got into the act - took a day to pump down a system, but that sucker was evacuated when that pump finished).

quote:

Originally posted by Rusty:


This won't be my first experience where a plastic tank passes one test (pressure with liquid) and fails another (vacuum with gas).

I wonder if it might be a good idea to seal or reweld the seams on the tank just as a precaution. Do you have a plastic welder?

I don't have a plastic welder, but there's a very good RV dealer only a few miles from me that does; welding the tank was Plan B. I may have it welded if I can find the leak, and keep it as a backup.

The item is still a current Ford part, and if it's the right one (the part number from Spartan was obsolete, but the parts guys found the new number), I'm going to order a second as a backup in case this part becomes obsolete in the future.