Hello, I'm noticing a couple of symptoms and was thinking maybe my water pump is on it's way out. 2000 F150 5.4L with 126k. Seems to be running fine as usual except for ocasional and very slight shuddering at or near idle. Also, this morning noticed some antifreeze on the ground from when I had driven off earlier. I looked at the fan assumbly and water pump for obvious leaks or wiggleing but found the fluid had come from the overflow reservoir. Watching the water temp guage all day and it has been at normal. Oil looks clean. I do have what's been described as the pistom slap noise upon start-up. Didn't detect any play at the fan but it's under tension of the belt. The fluid from the overflow has me puzzled. Any thoughts?

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Replace your radiator cap.

 

I'd like it to be that easy and am willing to give it a try. I'm going to watch it for any new developments. It's the fact that the shuddering started recently too. I could chalk that up to the state of tune but it started rather quickly.

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When the pump goes out, the fluid will be coming out of a weephole on the pump. Happened on my Grand Cherokee last year.

 

Just to be perfectly clear, you said the coolant was coming from the puke tank. I presumed that you meant that the tank itself had become full, and was spilling over onto the ground. If that's not what you meant, please explain.

Your cooling system is really very simple. It's a closed system, which means that air can't get in, and fluid can't get out, under normal circumstances. The fluid in your cooling system expands with heat, and contracts when cold. When sufficient pressure has built up, it overpowers the spring that seals the radiator cap, raising it just enough to allow the expanded fluid to escape through a tiny hole, into a tube, and into your puke tank. When the system cools down and contracts, the fluid is sucked back into the radiator. In theory, you should never have to add any fluid to your cooling system, other than flushing the system every other year. If no one has added any fluid to your cooling system, yet the puke tank is filling up enough to spill over onto the ground, either the pressure in the cooling system has increased beyond where it should be, or the radiator cap is no longer strong enough to remain sealed up to the designated pressure level it was designed for.

There are two main ways the pressure in the cooling system can exceed what it was designed for; either it's overheating (low on fluid, bad t-stat, bad pump, etc.), or you have a blown head gasket that is allowing cylinder compression to force air into the cooling system, raising the pressure. Since you said the temperature never got hot, that rules out overheating.

Which leaves you with two likely possibilities; a blown head gasket, or a bad radiator cap. You said the oil looked good, which suggests it's not a head gasket. When a head gasket fails to the point of allowing compressed air into the cooling system, at some point it will flow the other way, and coolant will be present in the oil.

That leaves the radiator cap.

Not being able to see your vehicle in person, and being unfamiliar with it's service history, I'm afraid it's nothing more than an educated guess, but it's a good place to start.

Good luck.

 

Thanks again for your replys and the explanation. I changed out the cap last night and watched more carefully what was occuring. I think it's just a thermostat gone bad and sticking just when starting from cold. Watching the guage from start up It would register in the middle even though the blower was blowing cold air. After a couple of minutes it would climb quickly to high at which point the thermostat would open, I'd start feeeling heat and all would resume to normal. What I think is happening is when the stat pops open it surges into the overflow for a moment and causes the fluid to excape through the cap. This same thing occured last night and this morning. Is it as easy to change out the stat as I remember on older motors? What temp is recommended?

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You can try replacing the t-stat, but that shouldn't cause your problem. T-stats open gradually, which prevents an overpowering surge. Still, stranger things have happened, and a t-stat is an inexpensive item to replace, so why not?

T-stat replacement is very straightforward, but you will likely need to remove the power steering support bracket. Make sure you use a new o-ring, and that the little nipple valve on the side of the t-stat is towards the drivers side of the the truck when you insert the t-stat into the intake (it will make sense when you have it in hand). Your replacement t-stat should be 192 degrees, +/- 5 degrees. I highly recommend any brand other than Stant. Their t-stats suck raw eggs, on a good day. RobertShaw t-stats are my preferred brand, if you can find them. Very accurate.