ALH coolant flow question

Sorry to ask such a seemingly easy question to answer, but... Which way does the coolant flow on an ALH? Thermostat routed to the lower radiator hose or upper? Thanks in advance. Every diagram I clicked on was a bad link.

Tstat is on the lower hose. Coolant flows in there and out through the head to the top radiator hose...Hope this helps

Thanks so much! I was pretty sure untill I saw a few conversions done backwards!

archemitis said:

Thanks so much! I was pretty sure untill I saw a few conversions done backwards!

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yeah, and those guys will find out soon enough that when the coolant gets just a little bit low the water pump stops pumping

It is still odd the way VW plumbed it
Normally the tstat is on the hot hose on top(relatively speaking)of the engine
Took me awhile to get my head around this, as there is a passage in the block where the coolant(hot) passes over the tstat

Lone Ranger said:

It is still odd the way VW plumbed it
Normally the tstat is on the hot hose on top(relatively speaking)of the engine
Took me awhile to get my head around this, as there is a passage in the block where the coolant(hot) passes over the tstat

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Having the thermostat at the lower hose is quite common in all modern-ish engines from all manufacturers. When the thermostat opens in this arrangement, the relatively low temperature coolant from low part of the radiator comes into contact with it immediately, causing it to shut sooner or at least not open as far as it otherwise would if it were at the upper hose. The effect is less drastic swings in engine temperature during operation. Emissions

emissions, and far more stable operating temperatures. That gives you more consistent performance and makes it easier to tune

Yep, I wasnt knocking it. It performs flawlessly.
Just for a small block ford guy this arrangment is kinda backwards

In winter with no coolant glow plugs and egr put to the lowest possible it struggles to stay in the 80s. As soon as it idles its 55-60 in about 3 minutes. I've thought about a temp jerry rig to move it to the top hoses and would if I knew it would make it a little warmer haha

Get yourself a 92C thermostat and call it a day.. Napa part number ATM-143992K2 for a Calorstat brand, or ATM-TX1392D for Behr brand.

I have the Calorstat one in mine and it will run 197-204F on my Torque app on the highway even when it's -15F outside..

Lone Ranger said:

Yep, I wasnt knocking it. It performs flawlessly.
Just for a small block ford guy this arrangment is kinda backwards

In winter with no coolant glow plugs and egr put to the lowest possible it struggles to stay in the 80s. As soon as it idles its 55-60 in about 3 minutes. I've thought about a temp jerry rig to move it to the top hoses and would if I knew it would make it a little warmer haha

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Veeeery common for these thermostats to stick partly open or start opening early as they age... The location is fine, you may have a weak t-stat.

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The return coolant from the Oil Cooler, Expansion Tank, EGR Cooler, and Heater Loop all end up at the Water Pump housing where the T-stat is located. That source of "hot" coolant is what causes the T-stat to open and allow flow from the radiator bottom hose. That design or very similar, goes all the way back to the 1974 VW Rabbit engines.

AndyBees said:

The return coolant from the Oil Cooler, Expansion Tank, EGR Cooler, and Heater Loop all end up at the Water Pump housing where the T-stat is located. That source of "hot" coolant is what causes the T-stat to open and allow flow from the radiator bottom hose. That design or very similar, goes all the way back to the 1974 VW Rabbit engines.

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good point

What happens when there is no thermostat installed.
I just found that my AHU did not have one

Diesel Sammy, with no T-stat, the engine could actually run hot as the coolant would flow through the Rad too fast (especially in summer). During the winter months, the Engine would not warm up to normal operating temp.

FWIW, I've seen Threads/Posts about "trapped" air in engines, especially Subie engines in the Vanagons. My experience, the VW engine coolant system purges air extremely well. I've never had an issue purging air out of the coolant system in the ALH in my Vanagon ................ long rad hoses and heater hoses all the way to the front (engine in rear)...... never an issue!

The Water Pump is extremely efficient moving the coolant. I have the expansion tank tapped for a fitting, tube, etc., to a pressure gauge up front in my Vanagon. The gauge registers from 0-15 PSI. The coolant system very seldom goes above 7 to 9 psi. ........... As for efficiency, when I rev the engine or as I go through the gears, RPMs climbing, I can see the pressure drop because the water pump is pulling coolant out of the expansion tank (as well as the other returns)... very interesting !

AndyBees said:

FWIW, I've seen Threads/Posts about "trapped" air in engines, especially Subie engines in the Vanagons. My experience, the VW engine coolant system purges air extremely well. I've never had an issue purging air out of the coolant system in the ALH in my Vanagon ................ long rad hoses and heater hoses all the way to the front (engine in rear)...... never an issue!

The Water Pump is extremely efficient moving the coolant. I have the expansion tank tapped for a fitting, tube, etc., to a pressure gauge up front in my Vanagon. The gauge registers from 0-15 PSI. The coolant system very seldom goes above 7 to 9 psi. ........... As for efficiency, when I rev the engine or as I go through the gears, RPMs climbing, I can see the pressure drop because the water pump is pulling coolant out of the expansion tank (as well as the other returns)... very interesting !

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So just to make sure I'm doing this correctly...

Plastic flange that holds the thermostat - lower rad hose
Metal pipe from the similar location that goes around to the oil cooler - heater core return?
Plastic coolant doohickey on the driver side block - upper rad hose
I have the fan switch sensor in a fitting in the lower hose T fitting from the Passat and it's split to get the reservoir/expansion tank fluid too.
Radiator overflow/little line goes to the expansion bottle/reservoir
Block plastic driver side doohickey to egr, egr to heater core in, and then I have the heater core out to the rear heater core in to simplify things and take out some of the vacuum actuated valves in the Eurovan.

Does that sound correct before I fill it up with coolant? I drained a 5 gallon bucket of the stuff before pulling the old motor and it still had stuff in the passages and hoses that have spilled everywhere.

IMPORTANT > The Bentley Manual has an error relating to the flow of coolant to the Expansion Tank. Their drawing says that pipe goes to the top of the Rad (that is dead wrong) and shows the flow in the wrong direction.

So, here is how I understand that the coolant flows out of the engine and returns.

Simple things first:

Coolant exits the head on the transmission end of the engine via a Coolant Flange with three exit nipples. 1. Bottom nipple (smallest) goes to the Oil Cooler, 2. Straight away nipple (medium size) goes to the EGR cooler and, 3. the largest nipple goes to the big hose to the Top of the Radiator. (Temp Sensor is there too) Also, below the Coolant Flange is a small Stainless Steel nipple coming out of the head with a small hose on it (flow is to the Expansion Tank with other Ts coming in along the way).

Big hoses (top and bottom of the Rad)

1. Hot coolant always goes IN the top hose to the Radiator.
2. Cooled coolant returns via the bottom Rad hose. It goes to-thru the T-stat located on the side of the Water Pump housing.

Beginning at the Water Pump, Coolant flow is from the water pump housing (cast in the block) through the engine block into the head and out of the head via the Coolant Flange, transmission side of the engine (as described above).

Now, looking at the many small hoses (two sizes) going and coming in every direction is the next step in learning the flow direction.

1. Find the metal pipe that enters the Water Pump housing. It will be parallel with the block just below the head. It enters the WP housing below and under the end of the Injection Pump (use a bright light to see better). It can be seen behind the metal dip stick tube and the oil filter housing.

2. Follow that metal tube away from the WP housing. Going away is going up stream (opposite of coolant flow diretion).

(As you find a branch or fork, follow it. It will take you to the device the coolant is coming from.)

3. The first branch goes over to the bottom of the Expansion Tank.

4. The second branch goes to the Oil Cooler located on the bottom of the oil filter housing.

5. The final branch or end of the metal pipe, is for the hose returning coolant from the heater core.

As described above, Coolant exiting the Coolant Flange to the EGR cooler, goes to the heater core. Also, there is a small hose coming off the EGR cooler that Ts into the smaller hose that comes out of the end of the head (SS nipple as described above) near the Coolant Flange. This circuit (smallest of the hoses) goes to the top of the Expansion tank. There is one other small hose that Ts off the largest hose (plastic connector) that ties to this circuit.

The smaller hoses provide continuous circulation with no restriction. That circulation is important as it maintains flow and purges air from the system.

So, in summary, all continuous circulation of coolant, returns to the Water Pump housing and flows over/around the T-stat. When the coolant temperature rises to the pre-determined opening point of the T-stat, it will begin to open slightly allowing cooler coolant to enter from the Radiator. The T-stat never pops completely open ........ it is a slow procedure, opening or closing.

There you have it!

Hope this is clearer than the pink stuff flowing through the system.

thanks for the info. im doing a mtdi alh swap into mk2 golf with no egr system.

so i can use some alh hoses, some custom runs and might as well use chasis coolant bottle.

so without egr parts do i blindly follow alh routing and take a run from the side port rear to heater core, but also have a small run coming from this route that t's into the heads barb, before making it over to top of reservoir?

guessing i can't just make a straight run on each of those?

I don't have the EGR heater on my ALH in the Vanagon either.

If I remember correctly, I have the coolant from the SS Barb off the head directly to the Expansion tank with a T in between from the big hose off the coolant flange on the end of the head. I really believe just a straight shot from the Barb to the Expansion tank is all that's necessary.

And, I have my heater hose as you explained... works great in my Vanagon.......

Keep it simple to keep it circulating ....

AndyBees said:

IMPORTANT > The Bentley Manual has an error relating to the flow of coolant to the Expansion Tank. Their drawing says that pipe goes to the top of the Rad (that is dead wrong) and shows the flow in the wrong direction.
So, here is how I understand that the coolant flows out of the engine and returns.
Simple things first:
Coolant exits the head on the transmission end of the engine via a Coolant Flange with three exit nipples. 1. Bottom nipple (smallest) goes to the Oil Cooler, 2. Straight away nipple (medium size) goes to the EGR cooler and, 3. the largest nipple goes to the big hose to the Top of the Radiator. (Temp Sensor is there too) Also, below the Coolant Flange is a small Stainless Steel nipple coming out of the head with a small hose on it (flow is to the Expansion Tank with other Ts coming in along the way).
Big hoses (top and bottom of the Rad)
1. Hot coolant always goes IN the top hose to the Radiator.
2. Cooled coolant returns via the bottom Rad hose. It goes to-thru the T-stat located on the side of the Water Pump housing.
Beginning at the Water Pump, Coolant flow is from the water pump housing (cast in the block) through the engine block into the head and out of the head via the Coolant Flange, transmission side of the engine (as described above).
Now, looking at the many small hoses (two sizes) going and coming in every direction is the next step in learning the flow direction.
1. Find the metal pipe that enters the Water Pump housing. It will be parallel with the block just below the head. It enters the WP housing below and under the end of the Injection Pump (use a bright light to see better). It can be seen behind the metal dip stick tube and the oil filter housing.
2. Follow that metal tube away from the WP housing. Going away is going up stream (opposite of coolant flow diretion).
(As you find a branch or fork, follow it. It will take you to the device the coolant is coming from.)
3. The first branch goes over to the bottom of the Expansion Tank.
4. The second branch goes to the Oil Cooler located on the bottom of the oil filter housing.
5. The final branch or end of the metal pipe, is for the hose returning coolant from the heater core.
As described above, Coolant exiting the Coolant Flange to the EGR cooler, goes to the heater core. Also, there is a small hose coming off the EGR cooler that Ts into the smaller hose that comes out of the end of the head (SS nipple as described above) near the Coolant Flange. This circuit (smallest of the hoses) goes to the top of the Expansion tank. There is one other small hose that Ts off the largest hose (plastic connector) that ties to this circuit.
The smaller hoses provide continuous circulation with no restriction. That circulation is important as it maintains flow and purges air from the system.
So, in summary, all continuous circulation of coolant, returns to the Water Pump housing and flows over/around the T-stat. When the coolant temperature rises to the pre-determined opening point of the T-stat, it will begin to open slightly allowing cooler coolant to enter from the Radiator. The T-stat never pops completely open ........ it is a slow procedure, opening or closing.
There you have it!
Hope this is clearer than the pink stuff flowing through the system.

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Andy, this is excellent, thank you!! Since I'm using the BHW from a Passat in the longitudinal orientation, I think a few things may be slightly different. based on the diagrams I've been using from OEMEPC.com and Jim Ellis VW to look at the exploded parts diagrams, things are a little differently. I've been using the 2004 BHW passat TDI and the 2002 Eurovan 2.8L VR6 diagrams.

Here's an exploded view of the BHW hoses.



Based on the way the BHW is hooked up, the fan coolant multiswitch is in the lower hose, which makes sense to me, as this will measure the temperature of the fluid coming out of the radiator and kick on the fans depending on the temp to cool it down. The other thing is the long metal pipe that gets input from the heater core, coolant reservoir and oil cooler has the small line that goes to the overflow, it comes from the top of the radiator on the Eurovan as referenced here. Otherwise it's great to know how the TDI coolant flows, thanks for the great info!

damac said:

thanks for the info. im doing a mtdi alh swap into mk2 golf with no egr system.

so i can use some alh hoses, some custom runs and might as well use chasis coolant bottle.

so without egr parts do i blindly follow alh routing and take a run from the side port rear to heater core, but also have a small run coming from this route that t's into the heads barb, before making it over to top of reservoir?

guessing i can't just make a straight run on each of those?

Click to expand...

the small hose on the corner of the head can go straight to the small nipple on the expansion tank, along with any other air bleed lines (sometimes there's one in the upper rad hose and/or heater core connection). The fitting that comes straight out of the cylinder head on the glow plug housing is the outlet to the heater core and the one coming from that steel line on the block is the return. The lower hose on the expansion tank goes to the pipe that points downwards near the dipstick tube..