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GMPP 350 HO, unmentionable pressure under thermo

Cornfield creations

Cornfield creations

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Jasper, Indiana
I have had this problem since I put this motor in and it decided to show its ugly face....again. I am at a loss and so is the dealership. All this motor is is a 1-piece rear main block with vortec heads and a bigger cam. I have a GMPP intake that accepts the quadrajet.

Temp will suddenly spike really high as long as you are touching the gas. Once you let off the thermostat will open. I had a bypass hose from the intake to the water pump and it seemed to fix it..well I guess it didn't.

So far this untold pressure has ruined 3 water pumps, my radiator and also my heater core, which all was new. I put a new water pump on today because it started leaking again..

My guess is that the engine is building up pressure underneath the thermostat and holding it closed.

Do these require a reverse rotation water pump or something, or is my motor messed up? I am about to pull my hair out. PLEASE HELP!
 
Dealer should be able to tell you if it requires a RR water pump.

If it doesn't, what happens if you remove the t-stat? Have you replaced IT with another Delco?
 
what year is the engine? the LT series engines are the only ones that are reverse cooled. what pressure is your radiator cap?
 
I have put I believe 4 AC delco thermostats, all were 195* My radiator cap is a think a 16 pound. The engine is a GMPP 350 ho crate engine. The gauge checked out as correct with another mechanical gauge. Do the vortec heads have a coolant crossover? I was told they don't and that would be a cause for pressure to build up under the thermostat. I ran a 180* high flow mr gasket thermostat and it worked, but the engine ran soo cold it had carbon build up and fouled out plugs.

I am just about to get a coolant bypass adapter to run below the thermostat and run the hose to the pass. side of the radiator. I don't see why I would need to run all these extra hoses or why I am having all these problems. THe dealership changed out the driver side head gasket under warranty thinking that was the problem that maybe it was installed up-side down but didn't help any. I am at a total loss as to what makes my 350 crate so much different than everyone elses.
 
It just seems to take out the weakest point in the coolant system. In the morning when its cold and the thermostat never opens you can hear the pressure thumping in the heater core, I guess releasing through the heater core. My heater hoses are hooked up to the intake and the water pump.

I'm gonna call GMPP and see what they say. Maybe my intake has something to do with it.
 
I'm running vortecs with no bypass, but I BELIEVE the actual vortec setups did use a bypass. Scott will say I'm sure.

Odd that a 180* would be that cold, that is well within "safe" operating temp for a motor, especially with a carb.
 
Im sure you would have figured this out by now but does it have any symptoms of a blown head gasket?

I ask because I once had an engine that blown a head gasket and acted very much the same way you are talking about. the thermostat acted funny and sometimes wouldnt open and there was a lot of pressure build up, blew a radiator and heater core both on the way to school before we found out what it was.

Im sure you would notice other symptoms, like poor running, loss of coolant and white smoke from the tailpipe but I thought I would ask just in case.:wink1:
 
The vortec engines had a coolant crossover in the back of the head, parallel with the firewall, aftermarket (not sure about GMPP) intakes don't have this, but a bypass from the top of the waterpump to the front of the intake (below the thermostat) should take care of this gurgling. If not I would try without a thermostat to see what happens. Keep us posted.
 
Well the motor only has about 5,000 miles on it. I ran it without a thermostat and it ran fine only it never reached temp.

My fitting below the thermostat pointing forward I have my temp sensor for my windstar fans, on the pass side I have a heater hose. Could I put a "T" into that hose and connect the pass side heater hose hole to the water pump? Or would that not alleviate the pressure?
 
I know at one point I was having weird problems with pressure in mine. Ended up being the hose that runs from the radiator to the overflow bottle had gotten some trash in it, plugged up and then wouldnt let the system vent. I think it was dust & dirt in a new radiator I put in that looked like it had been sitting on a napa shelf since 1932......
 
I think the bypass would have to go into the waterpump. Everything AFTER the waterpump (to include the heater core) is pressurized the same as the block itself is. This is why very few, if any waterpumps can be used as a source for the heater core.

The heater core hose to the radiator is a "leak" in the block pressure developed by the water pump, heat generation/absorbtion of the coolant, and restriction caused by the thermostat. I assume the Vortec/big block bypass is the same way, although since the heater core is in actuality a bypass, not sure why either of those factory systems need it.

Olds motors (at least some of the RWD V8's) had a bypass in the pump, but the ones I've dealt with also had a shutoff valve on the heater core hose, so there was no fluid bypassing through the core with the heater off.
 
What kind of belt arrangement? If it's a serp belt then it needs a reverse rotation pump. V-belts are standard rotation.

Does the block have a bypass hole below the driver's lower water pump bolt hole? Some Vortec blocks do and some don't. I'll bet yours doesn't and is causing the problem.

Where do your heater hoses connect to the motor at?
 
It has the v belt setup and I have a waterpump for my year and engine size, the one originally on it. The block has the hole on the pass side of the water pump. I have the 5/8 hose on the water pump, and the smaller one on the intake. I would like to move the 5/8 hose off the water pump and back onto the radiator.

Yes it is the 350 ho, 330 hp. I got the long block and put on my own intake and other parts. I wanted to use my quadrajet again so I got the GM intake that accepts the quadrajet.
 
Wow that is wierd. Definitely try moving the heater outlet hose to the radiator and see if circulating water that way will help.

I don't think I've ever seen one that is set up like yours act like that. It acts just like there isn't a bypass.:confused:
 
I don't think you can post photos, without membership, If you could post photos, that might help some.
 
Yeah it was running alright for a long time, now since I put on a new water pump that isn't leaking out the weep hole the whole thing started up again. I had a bypass that ran from that huge 5/8 NPT flare plug on the driver side of the intake that comes out at a 45* towards the driver side fender, and had it connected to the water pump. It would never reach temp in the winter and in the summer with the a/c on it always got hot, plus it looked horrible with all those adapters. So I thought if I move my hose from the radiator to the water pump and took off that bypass hose from the driver side of the intake, it would indirectly relieve the pressure through the heater core. It sorta worked, same thing as the bypass did, got hot in the summer, and cold in the winter, thermostat would never open unless I had a piece of cardboard in front of the radiator. I replace my leaky water pump and here I am.
 
Update on my engine problems. I did a TON of searching. Turns out others have had this problem. These engines REQUIRE an external coolant bypass. I am unsure if the block I have is an L31 or not. The L31's do not have the passages in the block, and the vortec heads do not have them either. It seems my block had the passage on the passenger side of the water pump but I am unsure if it was functional or not, I didn't build it. So anyone looking to buy a vortec headed crate motor from GM make sure to run this bypass. I ended up putting on a straight thermostat housing like the older gm's and using a upper radiator hose from a 95 k2500 454. It the hose was a little wide but it worked, and running the hose from the top of the water pump to a fitting that otherwise would have been underneath the original style thermostat housing( which is why I ran the older style housing). I used a 90* fitting on that hole and faced it towards the water pump. THe 3/8 hole on the front side of the intake I used for my fan switch, and plus it would require a hose that just would not work out to connect to the water pump. Insane. All GM has to do is put a note in with the engine when you buy it to make sure to run an external bypass.

THanks for the help guys, as always the work is fun and it's nice to learn new things.
 
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