That does make sense. Can you get your borescope down to those passages?

there's a use for a bore scope I never would have thought of.

 

The thermostat housing and water pump will have to come off again to look. I don’t think I’ll be able to see all of them, but I’m hoping to be able to see enough to determine if that is the issue. I might also be able to do a general inspection of the block passages to look for a source of contamination - I’ve accidentally left paper shop towels in engines before during repairs. Never in the cooling system, but it wasn’t fun trying to get them unwrapped from the intake valves.

 

So after looking at some pics of the head gasket, I’m thinking I may have the right idea. The head has coolant passages on each side of the cylinders and the bolt holes, with 2 primary passages at the back of the head; but the gasket only has the primary passages at the rear of the head and select passages around the cylinders.

The pics below are of the gasket and head with the passages marked with red. The gasket is a fel-pro but illustrations of the original show a similar design. There’s a clear difference in the locations of the passages, so even a partial blockage at the primary passages would impact the coolant flow.





Not sure I’ll get my camera all the way to the back of the head/block, but maybe I can get a good enough look to see if there’s restrictions.

 

Has the head been off the motor? Wonder if the gasket was installed wrong?

 

Not that I know of, but I don’t have a complete service history on it either.

It’s probably going to be next week but I’m definitely going to get as good a look as possible at those rear passages - I’ll pull one of the block plugs if I have to.

 

Is there a plug on the front/rear of the head??

 

There is a plug on the rear of the head, but the engine/head is tucked up against the firewall. I could probably get it out without much trouble but correctly installing a new one might be more trouble than it’s worth if it doesn’t lead to a solution.

The block plugs are all positioned on the manifold side of the engine - on a hoist, working room is good enough to remove and replace one without too much trouble.

 

Give it a shot, see what you find. Let us know.

 

And 2 more possibilities have been ruled out, with no headaches spared.

My boss informed me that the customer requested an OEM heater core be installed. I’m not sure if that decision was the result of a discussion they had or if the customer up and insisted on it, but I wasn’t quite ready to go that far.

I figured if they were willing to pay for a heater core install, they probably wouldn’t mind if I tried a new belt and tensioner first, just to see if it somehow fixed it. As with reversing the heater core hoses, the heat marginally improved, and even though it didn’t totally solve the issue, I’m glad I tried anyway...





I’ve never seen the alignment tabs break off like this. I didn’t have to beat on the tensioner at all to remove it - just a gentle prying is all it took, which isn’t surprising as I’ve seen several of them seize to the mounting base.

The red mark in the picture below is where the indicator mark was with the belt on.



This tensioner was definitely shot - out of curiosity, I installed the old belt with the new tensioner just to see where it rested and it was at about the halfway mark - still closer to the replacement side but not as bad as the old tensioner would make it seem.

As for inspection of the rear block to head passages, it was a bust. Not only were they not plugged, but the internal walls of the head and block that I could see were surprisingly clean for the mileage.

Unfortunately removing the rearmost block plug wasn’t good enough for the inspection. My bore scope camera is about 1/4” in diameter, but that was still too big to slip between the #6 cylinder and block walls. I then tried removing the thermostat housing, and I was able to get my camera all the way to the back of the head, but unable to rotate it in such a way as to see the passages. I finally grit my teeth and removed the plug in the back of the head, only to find no indication of restriction.

It’s gonna be fun trying to install a new plug back there... but I’ll have time to think on it as I’m [probably unnecessarily] installing a heater core.

 

Wonder if the tensioner had rotated some, and wasn't applying proper pressure on the belt?

Have a second look at the blend door while you are back in the heater box.