overheating issue

[scout man]

I am having a problem with the scout 345 v8. It keeps overheating at an idle. Started a few months ago, and seems to be becoming more of a problem. I can cruise up a mountain pass at almost redline with no problem, but if it sits and idles for a few minutes the coolant will boil out of the reserve reservoir. I even have an auxilliary electric fan in front of the radiator, and even with that on it will over heat at idle. I noticed the last time it happened, that the upper radiator hose was bulging really bad, and about to explode. Also frightening, I noticed last time that the fuel in my inline filter, which is at the front of the block, appeared to be boiling. I am not hugely knowledgable when if comes to engine stuff. SO what am I probably looking at here, water pump? clog somewhere? thanks!

[Rob]

Rmart74 previously posted:

[Robert B]

the weird part is if ur moving its fine??? maybe the water pump is working then for somereason but not at idle ?? or it is pumping badly and at redline the speed it is turning is overcoming the bad pumping ?? ahh i know its kinda waste of gas but can you sit still and run it at say 2 or 3 thousand rpm for a few mins and its fine ??? but let it drop to idle and see if it will then overheat ......

[Cr33p3r]

it could also be the t-stat has a weak spring and while moving it has enough rotational force (pumping coolant) to hold the stat open and while stopped it does not and lets the stat close causing it to overheat.

[Chris]

The primary function of the fan is cooling at idle so it's probably the t-stat. A clog wouldn't be selective and happen only at idle. Lots of hot rods run w/o a fan just fine because the driver shuts of the motor at idle. Keep it simple and start with the t-stat Steve.

[Hypoid]

Does it still have the fan shroud?

[Patrolman]

My vote has the water pump. At least that is my first thought. For some reason it likely isn't moving the coolant at idle, but able to spin enough at higher RPM. It is possible the impeller is damaged or corroded. Either that, or there is a clog keeping the coolant from moving quickly enough, but I would lean toward the water pump. Good news is that it is easy to get to on these engines. This article makes several other good points as well...

http://www.ehow.com/about_6317755_ca...t-idling_.html

[Hypoid]

I still think it's air flow, but, how old is the radiator cap?

[scout man]

It does not have a fan shroud on it, but hasnt since I bought it, and this is a new issue.

The aux fan is on the outside, but is definitely blowing air through the radiator. The mechanical fan is still active as well.

I have heard some thioughts here that were along the lines of my thinking. I was thinking the pump is not pumping, but when I am moving it doesnt matter because it is still getting cool enough... or the pump is picking up a little bit at speeds. Ill have to dig into this and get it fixed before moab. thanks.

[Hypoid]

What I know at this point is that the cooling system is not shedding enough heat at idle. 4finger has a valid point about the ambient temps being cooler over the past few months. Ever notice the cars on the side of the highway when the average temps change by 20 or 30 degrees? Some problems can be masked by weather trends.

If the water pump was losing it's capacity to move fluid, the engine would overheat under load. That's not happening.

If the thermostat were stuck closed, the engine would overheat quickly, under most any operating condition. If the thermostat were stuck open, especially in that big iron block, the engine would never reach operating temperature. A small aluminimum engine would be more likely to overheat with the T-stat stuck open.

What you have told me at this point, is that the cooling system has a diminished threshold for shedding heat. That narrows the problem down to adequate airflow, or reduced surface area inside the radiator. I'm leaning towards Rollies suggestion, but recommend removing the electric fan from the vehicle to see what happens. I've read more than a few threads started by someone who thought an electric fan would cure any percieved problem, only to be disappointed. If the engine still overheats at idle, while the engine is running, touch the front of the radiator to see if there are any cool spots. An area that is cooler while the engine is running is likely to not have any coolant flowing through it.

Did you ever look at the underside of the radiator cap?

Does this beast have a clutch fan?

Any problems with driveability, does it act sluggish?

[scout man]

Hypoid previously posted:

[Patrolman]

I don't believe any of the Scouts have a clutch fan. I would have to look to confirm. Just FYI, a clutch fan is built into a mechanical fan. You can then spin the fan by hand with the engine off. One that does not have a clutch fan wouldn't spin by hand with the engine off. It is basically a direct drive then and has to spin the belts in order to move.

The underside of the radiator cap should be in good shape. No cracking of either rubber gasket, etc. I had one with a really good split in the lower gasket. The lower gasket keeps the pressure to keep the radiator from boiling over at normal operating temps, since it is under a certain PSI. If it is cracked or worn, it can't hold that PSI. The upper gasket allows the radiator to draw coolant back in from the overflow tank rather than the outside air. Also forces the coolant into the overflow when it warms up.

My understanding is that the cap wouldn't be the problem. There isn't coolant gushing all over the place and boiling out. Also, the coolant level seems to stay at the proper level when checked cold. Sounds like the only problem is it runs warm and the hose starts to bulge...

[scout man]

Patrolman previously posted:

[Patrolman]

In that case, I would just replace the cap. It can visually look good, but still be bad. They can be tested for PSI, but frankly, for a few bucks, a new one might be in order.

[scout man]

thanks, Ill do that and see where it gets me.