The Stovebolt Site Forums The Shop Area Engine Shop water pump bypass ?

I have what looks like it might be a water pump bypass on my gmc. Didn't notice/think much about it til today when I pulled it out of storage and was checkin it over. It connects from the lower part of the thermostat housing to the water pump. My truck likes to run warm (190) on the interstate and I am wondering if this bypass is really necessary since water is going around in circles instead of through the radiator. Before I blindly rip it off I have to ask: Why did gm put it there in the first place?
jason

no leave it be.

it allows the water to keep moving thru the block when the thermostate closes.

if the water flow stops moving thru the block it will cause steam pockets to form on the the head and cylinders.

and the water pump will take more horse power to drive while the thermostate is closed

I did some checking of my parts manual and there is supposed to be a "restrictor plug" in the bypass tee when a heater is installed. Has anyone ever heard of it? Even in 50 F weather this engine (302) likes to run warm but only above 55 mph. Putting around in 100F doesn't faze it. I've got a 4 core radiator and a 5 blade fan. I've adjusted the timing both ways, jetted richer, then leaner all to no avail. The water pump is original, does the impellor rust out? After turning the engine off, the temp climbs to 220F+. What is normal??? :confused: :mad:
:confused: jason

Jason, as you well know, you will either get an answer from someone who knows for sure, or an answer from someone who is guessing or an answer from someone who can figure it out on the fly. You strike me as someone who can figure it out on your own given enough information. I would start by flushing the block, I hooked a garden hose to the upper coolant hose connector and took the water pump off and flushed a ton of rust / crap / sandy looking stuff out of my 57' 235 in my 49. This solved the running hot problems for me. My impeller on my pump was somewhat rusted out, but still had the basic shape and reasonable clearances.

If by putting a restrictor in your bypass, you elevate pressure in the block, you will prevent steam pockets that allow areas of the block to get hotter and hold heat.

Hope this helps.

Jason, what temperature thermostat are you using?
I am using a 160* in my 248, because the PO didn't have any themostat installed! I have flushed my cooling system several times and always get lots of rust and scale. No heating problems so far, it runs about 170* down the highway on an 80* day. I used the old "hot rodder" trick of drilling a 1/8" hole in the flat area of the thermostat. This allows air to escape to the radiator and prevents "steam pockets" from forming. It also makes the 'stat act like it's a slightly lower temp model. I just didn't want pressure to build up in my cooling system, since it was operated as an "open" system for so long before I got it.......Sure don't want to have to drop the flywheel because I've blown out one of the expansion plugs in the block .
Just a suggestion, that's all. I'm no expert, but that's what worked for me

Couple things .....You have no heater or it's outta the loop?

Ken's correct about flushing everything and the need for a restrictor

If you have time and want to help this pull the freeze plugs and look at the rust....If it's bad
You need to get at the rear one too. I machined the bell housing to get to it easy.Use wire or whatever to get the junk out.

But here's some guesses....Half the rads blocked.Get the motor warm and feel for cooler spots on the rad....better yet use a temp gun to check it
second ...check for a leaking head gasket It's possable to force hot combustion gases into the cooling system underload and run cool at idle You can get a kit that sniffs at the filler for exhaust gases and turns color!!
Third run a water line from the rear of the head to the front where you have this line....
What this does is 1..Allow water to syphon after you shut the engine off instead of blowin out the overflow and 2.. You get ALL the motor filled when you fill the system

Is your truck a pressurized system?

Oh one more thing.....the 39 runs @140 -160 most of the time
I ustta think that 180-190 was where HP lived ....the GMC LIKES lower temp !!!

The problem could be caused by a number of small factors.

I've drained the radiator (not block) several times through the course of its life and didn't see any evidence of sediment, so I didn't expect rust or scale in the engine. I was probably wrong. The radiator was serviced a few years ago and the guy didn't think that there was a flow problem. He didn't do a flow test, however. The bottom tank always seems very hot like it isn't cooling.

I am using the chevy water outlet neck with the small hose and small water neck on the radiator. The gmc used a larger diameter hose but is it really necessary? I do have a heater but it has a valve which can completely shut off the water so I guess that a bypass may still be necessary?

I have a line from the back of the head through the manifold (heated) and up to the lower part of the thermostat housing. I am using a 7lb cap but without an overflow tank so the radiator has some air in it all the time.

I tried a high flow thermostat and it ran hotter on the hiway and then wouldn't warm up around town. I know this contradicts all the screaming that I did earlier about needing high coolant flow :rolleyes: . I did a lot of reading on the topic and think that there is more to it than I realized. Sorry if I offended anyone.

I guess I know where to begin,
thanks for the suggestions.
jason

I forgot to mention that I do have an overflow tank, the built-in pressure valve is missing and I am using a 0 pressure cap.........overflow tank is connected to the "dump" tube on the radiator .

Jayson

I've cleaner and recored Rads only to have em fill as soon at the truck gets hot!!
Try this unscrew the tap at the back and see what comes out.You might get a trickler or nothing at all

I get nothing. Soon as the weather warms up...
thanks,
jason

Jayson.....nothing means the block is fulla crud!!!