water pump question from 4X4M38

RICKG · Post by **RICKG** » Thu Jan 15, 2015 8:40 am

Hi Rick,
On my M38 I have a two pulley water pump on my rebuilt engine. It has a rebuild tag on the block so I'm pretty sure it's military.

The water pump has a small (1"?) hose running from the top right of the pump housing over to the right side of the engine and it is attached to the motor where the M38 temperature sender goes. There is a splice and a fitting, and an electric sender in the fitting.

Is that a Civvy thing? I don't see anything in the M38 literature about that extra line fr0m the water pump, but some photos show a plug in the pump casting.

Thanks,
Brian

_________________
Brian
1950 M38
MC11481
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This question was PM'd to me and I don't have an answer other than its not stock. Best to share it with the board and get more informed opinions..

Brian should have the bypass hose from the WP to head as shown here but apparently has another hose from the WP plug shown in lower left of foto to the temp sender port with some sort of in-line sender installed. Sounds like the PO didn't have a stock sender and improvised..

ChuckW · Post by **ChuckW** » Thu Jan 15, 2015 11:33 am

Sounds like maybe a block water heater.

oilleaker1 · Post by **oilleaker1** » Fri Jan 16, 2015 6:20 am

I have a CJ with the "T" ( more like a "y") arrangement at the temp sender hole. If the head isn't drilled at the rear top, then it draws water at the sender hole for the heater. If the head isn't drilled at the front for the bypass M38 hose,then I guess they ran it back to the temp. sender. Never seen that done before. Most likely a owner's modification. The plastic oil pressure sender line is a civilian thing also. If his engine's head has the cast flat on the front, he can drill and tap it for a bypass and eliminate the hose from the pump to the temperature sender hole and install the small 90 degree correct hose. Wonder if his head has the hole at the rear top? The civilian L heads didn't have the front bypass hose and needed the small bypass hole drilled or provided in the thermostat.

4x4M38 · Post by **4x4M38** » Fri Jan 16, 2015 10:31 am

Hi Guys,
Thanks for the replies.

I have a remanufactured engine from Pueblo, Co., 7/55. It has an M38 head as well. I took a look this mornign and there is a pipe plug in the head at the boss pointing towards the radiator. The hose in question runs from the water pump connection to the right, then back on the right side to the connection for the temp sensor.

http://willysmjeeps.com/v2/modules/gall ... .sized.jpg

So end of the day, the provisions are there to remove the pipe plug and screw in the proper fitting and bypass hose. I guess in the meantime it's ok to run as is?

Thanks,
Brian

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Post by **wesk** » Fri Jan 16, 2015 1:04 pm

Be sure to understand the purpose of the original M38 set-up. It was a water pump bypass feature that was added to improve rate of warm up. Preheated water would leave the head and mix with cold water from the radiator speeding up the warmup. M38's did not come with heaters.

On civilian jeeps with heaters the same purpose is accomplished through the heater plumbing. The water pump draws cold water from the radiator and pushes it through the block and the heater plumbing the most of the return heated water goes straight to the radiator but the heated water that went through the heater and head is returned directly to the water to help with a more rapid warmup.

The correct way to plumb in a heater is hot water to the heater from the side of the head and then route heater return to the port in the top of the pump. If the head has a front port it then is plugged.

4x4M38 · Post by **4x4M38** » Fri Jan 16, 2015 2:54 pm

Hello Wes,
This thing does not have a heater, military or aftermarket.
After reading the above posts I have no idea why that line was
plumbed that way and the front head port plugged.

Maybe one explanation.

Bubba.

I'll make plans to obtain the parts and return it to stock.
Thanks for all of the help.
Brian

ChuckW · Post by **ChuckW** » Fri Jan 16, 2015 4:36 pm

I was thinking about a jacket water heater, not a passenger compartment heater, but obviously this is neither. A photo is worth a thousand words!
Bubba strikes atgain!

4x4M38 · Post by **4x4M38** » Fri Jan 16, 2015 6:19 pm

Chuck,
That's what I thought you meant, but correct, neither!

And they are usually somewhere down low, as warm water rises, not on top!

Take care,
Brian