99 5.4L heater hose routing

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FlyinMatt75

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Hey guys, first post and new owner of a 99 5.4L Expy with 230k. Need some help with some heater issues. Background, previous owner bypassed both front and rear heater cores years ago due to a leak on the rear core. They hooked the inlet and outlet from the block together as a quick fix. Well I'm gonna need heat this winter so it's time to fix this.

I verified flow through the front core in both directions and have a new rear core ready to install. I'm ordering the fittings to make my own hoses (quick connects, tees, spring clamps, hose, etc). So a couple of questions.

1. What is the flow direction from the block? Which tube is the inlet and which is the outlet. Can't find any info on this. Found the 1st pic below on the forum but my intake manifold does not have the vertical pipe on the front left side. Second pic shows my situation with the current bypass. One pipe comes up and make a 90 towards the passenger side and the other is laid over at 45 degrees up against the fire wall. This is the source of my confusion.

2. Does flow direction through the cores matter? If so, what is the connection pattern? Pics of your lines would be helpful.

3. Since I have good flow through the front core and will be putting in a new rear core, would it be best to do a flush before I hook up heater core to the system?

4. My coolant in the overflow tank looks like muddy water. Think I might need a coolant filter? Recommendations?

Appreciate any help, especially pics!
Thanks!!izdwgb_18a35721f9cb02b83ef8743ccbc86e721eb0f153.jpg20220926_174551.jpg
 
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rjrodney

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Is it leaking under the intake? That’s a super common fail point and I see the original metal pipe there. The metal pipe under the intake is the return.

Follow the arrows and labels on your diagram. 1 is coming out of the engine. I don’t have a rear core so my pic would be a bit more simple but that’s the diagram for both. And it’s showing the flow direction for both. If it runs backward through the core they never stop gurgling an air lock.

I would start with drain and fill a few times personally and see how things look.
 
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FlyinMatt75

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Is it leaking under the intake? That’s a super common fail point and I see the original metal pipe there. The metal pipe under the intake is the return.

Follow the arrows and labels on your diagram. 1 is coming out of the engine. I don’t have a rear core so my pic would be a bit more simple but that’s the diagram for both. And it’s showing the flow direction for both. If it runs backward through the core they never stop gurgling an air lock.

I would start with drain and fill a few times personally and see how things look.
I haven't noticed a leak under the manifold but I'll double check. This truck lived its whole life going up and down mtns in SoCal so everything is caked in dirt and grime which makes seeing anything deep in the engine difficult.

You mentioned that my truck has the original metal pipe still. Is that number 1 in the first pic below? If I replace my manifold in the future will it have the pipe in the front on the crossover piece?

So in the 1st pic below is 1 the output from the heads and 2 the inlet to the pump? The 2nd pic is from a video I found and it looks like #2 is the return to the pump...which would be the same as #2 from the 1st pic (my engine)...correct?

Appreciate your help...it's confusing having a different setup than all the videos I see.
 

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rjrodney

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Under the manifold is the return. #2 in 2nd pic.

Also the metal part in back of your engine I’m pretty sure is the return. It’s hard to see under the original manifold, maybe move the alternator a little.

Shine a light under the intake to look for caked coolant. I bet your loop back pipe is blocked intentionally somehow and not leaking now but it was.

The intake is known to crack and leak and the metal return pipe leaks at the junction behind the water pump in that valley. Both cause tune up problems with the coil packs eventually.

The Doorman intake has more room to route the return heater hose around in front and down behind the water pump especially with another 90 bend like you see in your loop back. Then you can use hose clamps to put it together instead of goofy o rings and press fit metal pipes.

I can post a pic of my setup in the day time but it looks pretty simple compared to the dual core rig.
 

Alexander Tan

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See if this helps for the coolant routing. The problem I have is on part item no. 9. The heater coolant outlet tube that goes to the engine block needs to have on O ring. I can't find manufacturer recommend size and it's causing coolant to leak. Anyone who knows the correct tube O ring size? The service manual does not show it.
 

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