Cooling System Under Pressure Even Cold

I've got the chassis on my MK4 build about complete. It's a Blueprint 347 from Forte's Parts with the Edelbrock Pro Flo 4 EFI. At this point I only have a few hours on the engine, moving it in and out of the garage. I do have an odd situation with the cooling system. After I run the engine up to temperature and let it cool down, the cooling system is still under some pressure. Even if it sits over night and is ice cold, if I remove the pressure cap it will puke water into the recovery tank. I've never seen a car cooling system under pressure when stone cold. For what it's worth, I don't have the old style expansion tank. Any thoughts? Just seems weird to me.

Can you show us how you've plumbed your expansion/recovery tank? Also, did you install your thermostat correctly (spring toward block)? Does the upper radiator hose section between the t-filler and radiator get hot when the engine gets up to temp?

The system should pressurize and overflow as it gets up to operating temp, then fully depressurize and pull the coolant back into the system. I had the same symptom you're describing on my first build. I rebuilt my donor 302, and didn't realize the stock head bolts were a TTY one-time use bolt. I torqued them to spec, but that wasn't tight enough, and I was blowing compression into the coolant passages, past the head gasket. I ended up putting new head gaskets in, and new bolts, and that solved my problem. I doubt that's your issue with a fresh BPE engine, but just sharing the experience. I also assume they (Forte) installed your thermostat when they installed your ProFlo4 manifold, so hopefully that was done correctly. That leaves the expansion/recovery plumbing, that you likely did and not BPE, so let's start there with some pics.

Assuming that's all correct (thermostat orientation and overflow/recovery plumbing), then perhaps you still have some air in the coolant system. I'd start by taking the water temp sender out of the ProFlo4 manifold, and slowly fill through the t-filler until coolant overflows that sender fitting, or even fill via the sender fitting. You might need to 'milk' your upper and lower radiator hoses to burp the air out.

Greg

Suspect there's a spot on a head gasket that's allowing high pressure combustion gases into the cooling system. When you shut it off, the 16psi cooling system can't vent the pressure. Fixed this same thing on a crate engine last spring. Replaced the head gaskets and used ARP head bolts.

I don't have the expansion tank, just the overflow/recovery tank. It's not under pressure unless I remove the radiator cap. The setup is pretty simple, just the rubber and stainless upper radiator hose with the inline radiator cap. The comments about the head gasket are what I'm concerned about. Everything is brand new and the entire drivetrain was complete when I received it from Forte's. I'm assuming the thermostat is in correctly. It does seem to warm up and run well. No overheating, and the fan cycles on and off correctly. There's no evidence of water in the oil or water entering a cylinder, as all 8 appear to be firing fine. I'm gonna be really bummed if there's a head gasket issue on a new engine.

Replace the radiator cap?

I thought I had this very problem but ffr sent a cap that is 20lb I really think that's to much should be around 12 to15. To be sure of your head gasket you can buy a test kit on Amazon to check it for around $20 much cheaper than just guessing. I also did a pressure test of 20lb overnight with no no leak down in 24hrs. This confirmed that their was no head gasket failure. And yes make sure that there is no air in system for it expands at a much faster rate than coolant Hope this helps

My guess is either the cap is bad or the head gasket is bad. Start with the easy stuff first. Maybe get yourself a bleeding funnel kit, start the car and see how the coolant looks in the funnel all the way up to operating temps. You can also verify that you don't have any air in the system. Fingers crossed it's not the head gasket!

Maybe call Mike Forte and see what he thinks. I'm sure he'll take care of you if their is an issue.

https://www.harborfreight.com/no-spi...kit-58423.html

We need pics of your cooling system layout.

Bet its a bad cap. Even if you had a bad head gasket, the cap would bleed off the pressure once it cooled. Get a new cap.
Mike