75 Chevy 400 SB cooling issue

[Dan 75]

First post so hi everyone!
So here's my dilemma.
Just got a 75 c10 and I'm having a hell of a time figuring out a cooling issue.
Already flushed and replaced 180 thermostat, radiator cap, hose to overflow. It will build pressure, run at 160-190 at idle/ low rpm soon as I drive it and get it up the temp up to 200 it looses firmness on hoses and will suck the top hose shut and allow the truck temp to spike to 220-230 let off the gas the temp drops back to normal. Once it gets hot hoses collapse at 2000+rpm and won't build pressure. I'm at a loss if a bad water pump can cause this or bad hoses? It has a Weiand pump old school copper radiator. When I flushed the system it flowed fast both ways, but there was a lot of rust and crud in the system. Can a water pump intermittently pump fluid or could the pump just be in the beginning stages failure? Or is it as simple as the hoses have gone bad and don't collapse until they heat up?

Any help would be great,
Thanks

[cadillac_al]

I think when the top hose collapses it means your system is building vacuum due to a clogged radiator. If you don't have a stack of radiators lying around like me then just buy another one. Maybe a radiator shop could do something with it but new ones are only $100 or so. I run a 400 with a 195 thermostat and temps are rock steady.

[hatzie]

I'd replace the hoses but I agree that a collapsing top hose usually means the radiator is restricted by crud.

[Dan 75]

Thanks for the replys guys, I'm going to just replace everything since the truck apparently sat for a long time before I got it. And I've already had to replace a bunch of stuff that went bad from sitting. It's a shame since who ever built the truck originally put a lot of nice parts and money into it. Wish I knew more about the truck. Feel like I'm chasing my tail on a lot of the stuff I've been dealing with.
Thanks again
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[hatzie]

If it's pulling enough pressure to collapse the top hose I doubt there's an issue with the water pump.

[Dan 75]

Ok so new update, start up cold give it gas about 1500-2000 rpm builds pressure top and bottom hoses. Temp keeps climbing up to dangerous levels, back off throttle lose pressure and temperature drops. From this point on system goes from pressure to vacuum and will stay cool under idle but overheat under throttle due to hoses collapsing and no flow. Not loseing coolant none in oil. I'm at a loss for what is going on. HELP!!!

[GM-power]

Have you put a new cap on the radiator?

[Dan 75]

Yes, the truck had a 16lb and I replaced it with a 13lb thinking maybe it was too high of a pressure. With the the cap open and a funnel in the radiator it will go up and down and under a load 2-3k rpm it doesn't seem to move much and as soon as you let off the throttle the coolant pukes out. But with the cap on it doesn't lose coolant. That being said I haven't been able to drive it very far due to the temp surges I don't want to smoke the motor. Like I said any rpms over 2k and it starts sucking the hoses closed and over heating. Lift and it cools right off. Seems like a flow problem but I can't figure out what is causing it. Already did fluid, thermostat, cap, and coolant return hose. So it's either the heads, pump, radiator, clogged coolant passages or a combination of all. It's driving me crazy.

[cadillac_al]

The more I think about it, if the radiator was clogged it shouldn't affect the top hose. I think your restriction is in the thermostat or the engine some how (no idea how). I would try running it without the thermostat. You may have gotten a dud thermostat that isn't opening enough.

[Dead Parrot]

Does the heater hose return go to the radiator or the pump? If radiator, is the flow good at higher RPMs? If it goes to the pump, might be worth moving it to the radiator for testing purposes as a way to observe your pump output. Easier then replacing the water pump.

If you haven't already, replace all the hoses.

[Dan 75]

No heater it's been removed along with the wheel wells for the wheels to clear. Here's a pic of the pump. You guys think it could be caused by cavitation, or do you think it's a blocked cooling passage or radiator tubes. Either way I'm replacing the pump.

[crewcabshorty]

Has the truck had its cylinder heads changed recently ? The 400 has steam holes in the heads. Or head gaskets that dont have the steam holes in them .

[Dan 75]

Unfortunately I don't know much about it just what I was told from the guy I bought it from that told me it had been sitting for years when he got it and supposedly it has a built motor. Forged pistons crank aggressive cam ect. It's got a msd electronic ignition pro billet msd distributior mechanical advance Holley 750 double pump with vacuum secondary victor jr manifold headers with 3" to flow masters. That's the stuff I can see. The engine sounds healthy and hauls ass the little bit I've driven it. There's just been a lot of things that have gone bad from sitting as well as things that were not done right that I've fixed. When I got it it would barely run due to the floats being stuck and flooding the crap out of it. I did do a block test today and it doesn't have a blown head gasket. Also drained one side of the block tonight and it had a lot of crude that came out. The hole was completely plugged and I had to jam a screwdriver to unblock it. I haven't been able to pull the other plug yet it's too tight and because of the header I can't get any leverage. Anyway I'm almost positive with all the crap in the cooling system that there's a block somewhere either in the block manifold radiator or all of the above.

[Bluestep]

Originally Posted by Dan 75

No heater it's been removed along with the wheel wells for the wheels to clear. Here's a pic of the pump. You guys think it could be caused by cavitation, or do you think it's a blocked cooling passage or radiator tubes. Either way I'm replacing the pump.

In your photo it looks like the bypass port is plugged up. If it is make sure that the corresponding hole in the block is clear before installing a new pump. It sounds like it may be getting an airlock. The bypass would be a factor in that.

[Dead Parrot]

If there is no heater, the PO may have plugged the output to the heater from the intake. I think GM intended that there be some water flow through the heater core, even with the thermostat closed. Keeps water moving through the block and avoids hot spots with a closed thermostat. Probably more important on a SBC 400 since the cylinder walls touch.

If it doesn't have one, might be as simple a fix as running a heater hose from the output on the intake to the input on the water pump to restore the pre t-stat open circulation.

[Dan 75]

Originally Posted by Bluestep

In your photo it looks like the bypass port is plugged up. If it is make sure that the corresponding hole in the block is clear before installing a new pump. It sounds like it may be getting an airlock. The bypass would be a factor in that.

Yes the pump that came off doesn't have a hole it looks like it's manufactured that way. Also the bypass hole on the block is plugged with silicone. What would be the reason to eliminate the bypass? I haven't done any research on it and just noticed the red rtv in the block today when I was flushing the block.

[hatzie]

This may sound like an odd thing to look at...

Check the hose to the overflow bottle and the overflow bottle vent for mud dauber nests or other blockages.
The radiator cap should open when there's a vacuum in the system but it only works if the vents are clear.

BTW that water pump looks fine mechanically but I'd change it now that you have it removed. The seals may fail fairly quickly from sitting.

[hatzie]

If you have blockages in the engine itself you can spot check with a laser thermometer.

At $200-$300 lower priced infrared cameras are still out of most casual hobbyists price range.

[tucsonjwt]

If you plan on keeping the truck for a while, I would replace the radiator with a cheap Spectra plastic/aluminum one and also replace the water pump with a Delco pump. That should cost about $250 if you shop around. My experience with radiators is that they can hold a pressure test but be so clogged that they don't allow enough coolant flow to keep the engine cool.

Also, check to see that the fan is actually working. If you need one of those, I would go with Delco again - about $115.