305 V8 water pump

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WA1DH

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I posted here a while back about my 305 and an air pocket issue, which I thought I had solved. The truck runs cold when it's cold out, and overheats when it's warmer (in the 40s or up). I've tried 2 thermostats and am about to just remove the thermostat and run it without one to see if it still overheats.

I did some research and read that these trucks with serpentine belts use reverse rotation water pumps. Back when I got it I installed an AC Delco P/N 252-719 water pump. I just learned about reverse rotation pumps and am wondering if this pump is standard rotation. I had to install my old pulley on it so no idea if it's meant to run the other way.

When I ran the part number on AC Delco's website, it listed that pump and P/N 251-725. Neither lists rotation. I called AC Delco and customer service suggested I call a garage (thanks).

Does anyone know if these pumps are different rotation styles?

Thanks.
 
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Serpentine belt driven pumps rotate the opposite direction of V-belt driven pumps, so that's a critical piece of data to know when you order pumps. I'm not sure if there is an easy way to identify what you have, but a wrong-rotation pump will definately have the symptoms you describe.
Have you got a V-belt or a serpentine belt?
 
I believe the 252-718 is their cheaper aftermarket part. The 251-725 is the GM OEM pump. Althought they fit the same applications they are not the same part.

I used the 252-718 once in my 350 and it only lasted about a year. I replaced it with a Gates that has been in 2.5 years now with no problem although I have seen it speculated the 252-718 is a rebadged Gates pump.

Not sure about the rotation though.
 
I have a serpentine belt, so it needs a reverse rotation pump. When I got the new pump (P/N 252-719) Rockauto didn't list the rotation direction. It had the cover on the impeller already with the gasket in place, so I never opened it to look at it. I'm also assuming that they gave me the right pump, because the fact that it fit doesn't mean it wasn't the wrong rotation.

Am I correct in assuming that if I put a reverse rotation beside a regular rotation pump, the impeller blade direction would be reversed? I think the only way I'm going to know for sure is to remove the pump, pull the cover, and compare it to the old one.
 
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My research shows that the 251 number is original equipment while the 252 number is the aftermarket replacement.

You seem to have the correct rotation pump
 
Open the radiator cap when it's cold, fire up the engine, and see if there is flow in the radiator when it gets to temp.
 
I have run into this myself in the past with the different rotations and some people being "smarter" and buying the pump that was cheaper since it looks the same (gotta love those people)and had it do this to them. Now you did say you realize that there is a difference so I'm not saying this about you and don't mean any disrespect. But I have also first hand seen a pump that was the correct part number and been the wrong rotation. My guess is the internals interchange and one in a million may be assembled wrong or simply boxed wrong since they do look the same. My guess is take it apart and check or just save yourself the headache and maybe buy another or try to exchange it if you still can.
 
You can sometimes tell by the casting shape whether a pump is forward or reverse rotation. The casting will look either like this:

\O\ or this /O/

Where the "o" is the impeller chamber and the flat bars are (approximately) the water passages leading to the cylinder heads. Viewed from the front of the engine, /O/ is reverse rotation, and \O\ is standard.

Sorry for the poor ASCII artwork but hopefully it becomes clear after a little head scratching.
 
It's possible that two different water pumps were switched by an employee or even a customer who returned them. If it was mine, and since the price isn't too high, I'd buy the correct pump and reinstall it, this time verifying that the correct pump is installed..
 
Does this truck have the heater hose return to the radiator? If so, then lower the fluid level to just below it and start the engine with the cap off. Coolant should be circulating fairly forcefully right from the start. That will tell you if your pump is working.
 
Well no such luck. I was hoping it would be as simple as the wrong water pump, but it wasn't. Impellers on both old and new pump look identical (same direction blades).

I went and tested my thought that the radiator may be clogged. I drained it, pulled it out, and ran water from a garden hose through in both directions. The water flows through just fine, as fast as the hose can supply it.

I'm at a loss with this one. I'm going to put it back together over the weekend, pull the thermostat, and make sure coolant is moving in the radiator. If so, I'm going to run it without the stat for a while and see if it overheats that way.

I would say maybe it's just the temp sensor, but if I touch the top hose when it's reading ~240-250 it feels like the coolant is boiling inside. When this is happening, the lower hose is cold to the touch.
 
I drilled the thermostat when I put it in. The serpentine belt was routed according to the under hood diagram, with the back of the belt running over the water pump pulley for counter-clockwise rotation.

While reinstalling the radiator I noticed a plastic cap on one of the end tanks (I think it's for a transmission cooler? It's a 5-spd) was loose. I tightened it. Perhaps I was loosing pressure here? My coolant level never went down and the overflow tank would go up and down like normal.

Will be putting it back together tomorrow/Monday and see what happens.
 
Originally Posted By: WA1DH
Well no such luck. I was hoping it would be as simple as the wrong water pump, but it wasn't. Impellers on both old and new pump look identical (same direction blades).

I went and tested my thought that the radiator may be clogged. I drained it, pulled it out, and ran water from a garden hose through in both directions. The water flows through just fine, as fast as the hose can supply it.

I'm at a loss with this one. I'm going to put it back together over the weekend, pull the thermostat, and make sure coolant is moving in the radiator. If so, I'm going to run it without the stat for a while and see if it overheats that way.

I would say maybe it's just the temp sensor, but if I touch the top hose when it's reading ~240-250 it feels like the coolant is boiling inside. When this is happening, the lower hose is cold to the touch.



That tells me your not getting any coolant circulation for SOME reason. The radiator is cooling whatever does pass thru it, but not enough ifs flowing.

Take that T-stat out and see if it opens in a pot of water heated on a stove... it should open BEFORE the water boils (use a cooking thermometer to get an exact temp of when it opens if you want, but the "before boiling" test is pretty good). I've seen them dead in the box- ALWAYS boil-test a T-stat before using it.
 
One other thing... did you check to see if the water pump impeller is slipping on the shaft? I've heard of that before, especially with plastic impellers that are pinned to the shaft. The pin can shear, or just be left out during manufacturing sometimes.
 
I boiled the thermostat before installing it. Seemed to open around where it should (I used an IR thermometer on the water).

The impeller is metal. It appeared to turn as it should when I moved it by hand. No slipping that I could notice.

It's not back together yet since it's snowing and freezing once again. Will update when I get it running again.
 
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Update:

I got everything back together. Filled the radiator and reservoir. Started it up and idled for about 10-15 min until the coolant level in the radiator dropped and then started surging and falling every few seconds (I believe this means the thermostat just opened). Top it off before it surges and spills over and cap it quickly. Let it idle for another 10 min or so and then I shut it off. Dash gauge reading 150F or so. Loosened the radiator cap and it was under pressure - shot coolant out as I opened it.

Start it back up and go for a test run. Drove around slowly with steady RPM. As I get around 2k RPM I get a loud gurgle in the heater core. I held the RPM there for a while and it seemed to quiet down - perhaps I was pushing air out? Didn't beat on it too much, drove about 3-4 miles and went back home. Dash gauge reading ~190ish. IR thermometer had the thermostat housing at 185-190 and the lower hose around 85.

Cooled down a few hours, checked it, and hasn't lost any coolant. Took it to work and back (~10 mi each way) and drove it hard after it warmed up, holding RPM around 3-3500 (4k redline). The gurgle in the heater core is still there and pretty loud around 3k RPM. I noticed when I stopped at a light after revving it I could hear a bubbling/popcorn sound from the heater core. Dash gauge reading ~160. I get home, stop, dash gauge now around 190. I hear the bubbling noise from the core, and check the upper hose. Every few seconds I feel a 'thump' in the hose, like an air pocket or something just moved. I shut it off, felt the hose again, and now it felt like the coolant was boiling. This lasted maybe 30 seconds or so after I shut it off. Reservoir isn't boiling and had risen to the hot position.

No coolant leaks that I can notice, and the dash gauge never went over 190-200 range. The gurgling, thumping, and boiling are concerning me though. Any thoughts?
 
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