Long story short, I figured I needed to change my waterpump as I didn't think the old one was moving water very effectively, as I've always had cooling issues, even with a new radiator, and now the mercedes fan.

I just got the old one out, and as you can see in the pictures below, I think it may be the better choice. There is less clearance between the vanes and the front of the pump, and less clearance to the timing cover than the new pump. Both pumps have 8 vanes.

My pulley measures ~7" diameter to the outside edges. How big is the small pulley? I should probably head to the junkyard to see of I can find one. I rarely go over 3500 rpms, so I don't think high RPM cavitation will be an issue.

Sooo, what say ye moparts? New pump? Old pump? Old pump & smaller pulley?

To throw a wrench in the whole process, I plan to replace this engine with a magnum 360 hopefully this summer, so I didn't want to drop $100 on a new water pump.

Thanks for the thoughts.

First, the old pump:

And the new pump:

IMHO,use the new pump

you can see where the vanes on the old pump are rubbing on the pump housing

I bet that if before you removed it with fan and pully in place you could move the shft by hand..the bearings and seal are on the way out when it does that.

the 6 vane pump goes with a small a/c pully

the 8 vane pump goes with the larger non a/c pully

6 vane spins faster to move x amount of water at idle
8 vane spins slower to move same x amount of water at idle

small pully on a 8 vane pump can cause it to cavitate at sustained high rpm

you can use a HD thermoclutch and a 7 blade fan for max cooling on either combo,but need to be the right ratio on the pullys

any play on the impeller to shaft?

if it is tight then put the old one back.

may be you have feets problem, sucking the lower hose shut at speed?

The lower hose has a spring in it, so it shouldn't be sucking closed.

I just put the old pump in my vice, and can now feel just a tiny bit of play in the shaft. Soo, new pump it is.

I did have a 7 blade fixed fan on it, and now have the electric mercedes fan. At idle with both set ups, it will maintain itself, but wouldn't actively and quickly cool itself off.

Quote:

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small pully on a 8 vane pump can cause it to cavitate at sustained high rpm

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On the highway it usually runs around 2700 rpm. It's had a couple 4k romps, but few and far between. I wouldn't think that would be high enough to cause cavitation? This is a work truck, so it rarely sees anything a drag car would consider normal. It's also old and tired, and I don't want to blow it up

Pull your plugs. Make sure it's not a head gasket issue. That was the problem I had with the hot rod. You can usually find it by a loss of coolant.
My lower hose was being sucked flat at a bend where I couldn't feel it. If your hose has a good spring in it that should not be your problem.
There's always the thermostat and radiator to look at.

Since your pump is out pull the block drains and flush the block through the water pump holes. That will make sure you don't have a heavy layer of ickies in the block. I also had that problem but never knew it because the coolant wasn't nasty looking. The sediment simply didn't get stirred up.

got the new pump tested today. If anything, it's a little worse than the old one

At 205*, I watched two little bubbles sit on the coolant in the radiator, even after I revved it up. I presume I should see more flow than that? I think this is the crux of my issues.

I got a (slightly) smaller diameter pulley from the junkyard, so I'll put that on later.

The radiator however, IS dropping temperature pretty well. I can hardly touch the top hose while the bottom hose is nice and cool.

No way the new 8 blade pump in the 2nd pic w the smaller vanes is going to cool as good as the 8 blade larger vane pump. Is the temp you get at an extended idle at a stoplight w the changes a # you can live w? May be another issue, crud accum in block/heads/slight vac leak/timing or ?. Sounds like it's liveable till th eng ch but it'd b a feather in your cap to correctly diagnose/solve what's behind it.

It seems to like to stay around 210*F with the old and new pump and with the mercedes fan at full speed. This temp is fine with me, although its only going to get warmer out now that summer is here.

I redid the top end of the engine a couple (3 now?) summers ago, and at that time knocked the freeze plugs out and removed the gunk in the bottom of the block. Heads were hot tanked and rebuilt. New radiator about the same time.

I agree I'd like to figure it out, but at this point...

And I just discovered the smaller pulley won't fit. If I didn't have to remove the fan to get it on, it would run in to the crank pulley. Ohwell


In a properly working set up, should I be able to see lots of liquid moving in the top tank of the radiator?

yeah it should move.
take out the thermostate and watch. if it still doesn't move, you either have a plugged bottom of the rad, or some other blockage.

based on your post in the fan thread, I reiterate you have a blockage or restriction.
reving the motor puts a slug of cooled water from the rad into your motor, "dropping" temps that you can see.

it may not be the thermostate since you changed it, but something is restricting flow.

a/c pulleys require the full set of pulleys. you can't mix and match. Sorry wasn't thinking, or would have mentioned that before.

not only are diameters different, the off set will be different too.

You should see lots of movement inside the radiator once the t-stat opens. If that's not happening the these things:

1) Remove the hose from the thermostat housing and look inside. Is the thermostat pointy side up? If it's upside down (spring end up) it could restrict the water flow.

2) Pop off the upper hose and start the engine. Make sure the radiator has water in it higher than the water pump. Does water come out once the engine reaches temperature? If not, there's some sort of restriction in the engine's cooling passages such as the thermostat not opening.

3) Pull the spark plugs. Are any of them cleaner than the others? If so, you probably have a head gasket that is allowing exhaust to pass into the cooling system. You usually see bubbles in the coolant when that happens.

Let us know what you find with these things and we can take it from there.

There was enough light left so I went ahead and pulled the thermostat just now. WOW, so that's what it looks like when the water is moving

Project for tonight: boil both thermostats, and watch them not open


I pulled the front 4 plugs a couple nights ago. Actually looked pretty good; just a tinge of brown on them. The back couple are a PITA to get to

i run 160's in mine, and i drill a small 1/8" hole into the stat, so it wont develop an "airlock" in it. i also block off the bypass from the inside of the stat housing, filling up the nipple with plastic steel, and smoothing it into the top of the manifold at the nipple. this directs ALL the water moving through the pump, into the radiator. the IR gun shows, it works. sounds like you have a bad stat, or two. you can run without it, if needed. or, make a solid gasket out of cork, and pop a hole in the center, the size of a quarter. it will restrict the water a bit, and warm it up a bit faster. i like the 160, as once the engine warms up, it pretty much just stays open, no cycling open and shut.

I boiled both thermostats and they technically open. It opened maybe a 1/16", and once back in the air the promptly shut again.

IMO, 160 is too cold. I'd like to have heat in the winter

I wonder if just restricting the bypass would gain any benefit...

There's the issue.

Drop a GOOD t-stat in the truck and hit the road. I usually run a 180 or 190 all year.