Intermittent Cooling Problem, '01 PF

[Chuck Tribolet]

I have an '01 PF, 158K miles, manual transmission.

Around town, everything is fine. Most weekends, I tow my boat (about 2200pounds boat, motor, trailer, dive gear, PF is rated for 3500 with MT) to Montereyand back, 50 miles one way, I usually do two round trips a weekend. Whentowing, it will sometimes push something less than a litre of coolant into thecatch tank. The warmer it is, the more likely it is to do it and the more coolantit wlil push, but it's a loose relationship. Some warm days it doesn't push any,some nice cool days it pushes more than usual. I check it at each end of thetrip, if if it did push some coolant, I let it cool off, take the radiator cap off,and push it back into the radiator. Once warmed up, the temperature gaugenever gets off dead center even on steep hills on hot days, as long as I keeppumping it back.

The problem first started happening in May 2007 at about 138K miles.I had a big time overheat in VERY slow traffic on a 72F day. At this point, I didn't knowit was pushing coolant. Fortunately I had a mondo icechest full of meltwateron board, and it was blowing 25knots and I parked facing into the wind.It was impressive how fast the boiled coolant condensed and pulled thecatch tank contents back in. We just kept adding meltwater to the overflowtank and that filled the radiator back up, and we got home.

I replaced the radiator cap (original Nissan, looked good, but replace is cheapand easy) with a Stant. That didn't help. I replaced the fan clutch, thatdidn't help. I replaced the thermostat. That helped, at least somewhat,for a while. I later replaced the radiator cap again with genuine Nissan,no change.

When I replaced the thermostat, the coolant I drained was nice transparentgreen. I even filtered it through a coffee filter and got about six little bitsof grit. Cleanest used coolant I've ever seen. Since I had the rad out,I back flushed that. Nothing came out except clean water. The inside lookedlike the inside of a Coors can.

If I don't push the coolant back, it will generally go back over the course ofa week's daily driving, but won't go back the first time it cools off (which I'dexpect if I'd boiled a little coolant to "steam").

Coolant is 50% Genuine Nissan, 50% distilled H2O.

The only thing I can think of is a tiny head gasket leak that only opensunder heavier load, but:There's no sign of coolant in the oil (I change my own oil),There's no sign of oil in the coolant.There's no coolant on the plugs (they got pulled somewhere along the line).There's no net loss of coolant.

Any thoughts?

[fueler]

maybe theres a pocket of air somewhere in the system? how about water pump, was that ever replaced? (i hope it was, by 158k miles!)

i don't quite understand what you mean by "pushing the coolant back" from the reservoir.... why dont you leave it? the radiator will suck it back in as it cools down.

[Chuck Tribolet]

The radiator does not suck it back as it cools down, which says that whateverpushed the coolant into the overflow wasn't boiled coolant. I let it cool off, take the radiator cap off, and pump air into the overflow with a bulb pump(like on a blood pressure cuff).

The water pump has never been replaced. It's not leaking, it's moving coolantwell (or it would overheat going up a hill on a hot day).