Coolant leak somewhere around the t-stat

I'm putting an ALH into a Toyota pickup. It's an ongoing process that's been going forever. My problem does not seem to have to do with the chassis. So, I'm posting here in this section.

Well, I'm finally to the point where I was ready to double check my cooling system plumbing.

I dug out my trusty Air Lift to put a vacuum on the system and I can't get it to obtain even 1psi of vacuum!

So, since this is a project, I figured I goofed up somewhere. I took the coolant manifold off the back of the head and checked it. It was fine and the (new) o-ring was not damaged. But, I did find where I had not tightened the thermocouple for my McNally gauge.

I did the happy dance thinking that was it... NO, it wasn't.

L.O.N.G. story short. I've tracked the leak to the area under the injection pump near either the thermostat or where the "crossover" pipe comes into the engine block right by the t-stat.

I have a new t-stat, new t-stat o-ring, new t-stat housing, and I've locked the t-stat onto the plastic "ears" in the housing. So, I think it's good.

Moving on to the crossover pipe. I have a new o-ring on it and when installing it, it feels secure.

More info on the crossover pipe: this is a cut up OEM pipe. I chose to cut it off right before it would go over the bellhousing and I welded a solid, circular steel disk there to close it up. It's not leaking from that end and that end is secured in the OEM location (the 11mm bolt at the tail end of the block).

The crossover pipe only has two places coolant hoses come into it now. One is the return from the Toyota heater core and the other is the return from the VW heat exchanger/oil cooler. Those all check out OK.

Once the crossover pipe has been removed, it checks out OK - holds vacuum.

The leak definitely seems to be under the accessory bracket. Can anyone please help me think of something that I'm missing?

Since it won't even pull 1lb of vacuum when everything is installed, this is a BIG leak. Is there some sort of plug or hose or something that I'm missing?

If you cut off the end that goes to the heater core how are you routing your heater core into the loop? From the bottom of the Reservoir?

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Hell, try putting water in it and see where it comes out

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If I can’t figure it out dry, I will wait until closer to run time to put liquid in.

I don’t really want to have raw water hanging around in there, in crevices, until who knows when.

I made a video of our coolant routing.

Check it out:
https://youtu.be/COApgsyFUqk


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That's purdy, is your bleeder leaking? Can you pressurize it and listen for hissing?

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Is the oring properly seated? I had a hard time with both of mine

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The bleeder seems tidy. As a matter of fact, we have been removing the bleeder screw all together and using that as the point where we introduce compressed, shop air.

I've used a stethoscope (without the long 'needle-type' of probe - just the open hose end) to narrow down the area of where I hear hissing. The problem is that it is leaking such a significant amount that it's hard to hear where the air might be coming OUT because it's hard to differential from the air hissing IN (from the compressor).

If it would hold just a little pressure so that the compressor/input part of this equation could be removed, that would be helpful. Oh, well...

We have an ALH on the engine stand too and I can't see where this leak could be coming from. It has to be:
a) the crossover pipe or
b) the t-stat housing

I was hoping I might just be missing something simple. Like someone might go; "you checked the freeze plug below the t-stat, right?".

I thought about either:
a) spraying soapy water around the area and looking for bubbles or
b) buying an evap smoker and smoking it. I still haven't ruled the smoker out. I called local parts houses today looking to see if anyone rented a smoker. No luck.

Thank you for your help and your compliment of it being purdy. I'm trying to take my time and do it absolutely, tee-total spot on. I'm not cutting any corners.

I may hit you up this fall, I've got a 4 runner that is asking for a diesel swap and you're not too far away...and I horse traded for a alh 5 speed car that needed an engine so I've got some parts when it's time...might be a good weekend trip one day.

The fuel mileage and drivability you get from that conversion is great but I just don't know how I feel about getting to the vacuum pump

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Try slowing the airflow down like turn the shop air regulator down to 10 psi or 20 just to rid some of the hiss from the in side

Did you put the o ring between the thermostat and the housing? There really isn't that many places for it to get out and you've covered all of them.

I do wish you were closer...I'd drive there tomorrow as long as I got my pump head seal fixed...[emoji16]

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The (new) o-ring is between the (new) t-stat housing & the (new) t-stat. The three items were inserted as one as I’ve seen online.

I’m close to ruling the t-stat/housing out because I can get a bit of vacuum if I put the Air Lift directly into the t-stat housing. That is at least showing that the housing is sealing from the outside in.

If it is indeed the crossover pipe, I have no idea what could be the problem. As already stated, that has a new o-ring there too & felt good going together.

I got to this point because I was just about to send my upper, aluminum coolant ‘tree’ to Swain to get coated (like the exhaust, turbo hot side, & intake manifold) but decided I’d double check my work before sending it out.

If you were closer, I’d sure let you take a look

I’m attaching an overview of that side of the engine bay:



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How certain are you that the heat switch valve isn't leaking?

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Unless it's the hard pipe that goes in beside the tstat housing

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WildChild80 said:

Unless it's the hard pipe that goes in beside the tstat housing

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Click to expand...

This was going to be my contribution as well, as I had the mystery leak from around that area and finally pinpointed it to the double-lipped seal on the end of the hard pipe that goes in behind the water pump. It was destroyed:










That interface can get rusty too.

It's part 3 in the diagram:



And I agree with the vac pump back there too, hopefully there's more room to get that thing out than what meets the eye in those photos. I've had more leaks from the interfacing seal than anywhere else on the car. I just put a new vac pump in there too due to mine being worn out. It's already tricky to get out in it's orientation in the mkiv platform

Did you figure it out yet?

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KrashDH said:

This was going to be my contribution as well, as I had the mystery leak from around that area and finally pinpointed it to the double-lipped seal on the end of the hard pipe that goes in behind the water pump. It was destroyed:

That interface can get rusty too.

It's part 3 in the diagram:

And I agree with the vac pump back there too, hopefully there's more room to get that thing out than what meets the eye in those photos. I've had more leaks from the interfacing seal than anywhere else on the car. I just put a new vac pump in there too due to mine being worn out. It's already tricky to get out in it's orientation in the mkiv platform

Click to expand...

I assume you have a 5 speed, those coolant glow plugs are a little bit of a pain to get around

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WildChild80 said:

Unless it's the hard pipe that goes in beside the tstat housing

Click to expand...

That's what I've been referring to this whole time as "crossover pipe". It has a new double-lipped seal (using Krash's description).

Where the crossover pipe goes into the block that area has been cleaned and this is a rebuilt (read cleaned, magnafluxed block) engine.

I haven't tracked it down yet but, I did buy an evap smoker today. It should be here Thursday (2 days). I will smoke it and go from there.

I'll post again if I can get it ciphered.

Thanks again for everyone's time and replies.

Two things that I haven't read from you is that you screw and seal your vacuum, or pressure hose, and the cooling hose out of the thermostat is new properly tight with no cracks, etc

I'm wondering if your issue is one of those.

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frambach said:

.............. I will smoke it and go from there.................

Click to expand...

Words to live by.....

Well, I smoked it.

Found smoke pouring out of where the oil pressure switch screws into the oil filter housing (I had that port empty because I will be installing my McNally gauge there to monitor temp and pressure).

So, just to move forward, I put the OEM switch in there and tightened it up. Still no vacuum!?

Smoked it again and we've got smoke in the area of the t-stat. Using a mirror and bright LED light, I STARED at the place where the crossover pipe goes into the block. From what I can tell, it doesn't look like any smoke is coming from there. While staring at it, I see smoke slide into the picture - not develop from there.

At this time, I'm calling that good. I can also say that there is no smoke coming from the rear of the cylinder head. That means the coolant temp sensor and my fabricated 'elbow' back there is good. That's a relief!

Now, looking at the thermostat, it definitely seems to be coming from there. I've removed the new, aftermarket housing and double checked everything. It all looks good. However, that has not stopped it from leaking!

I tried removing the t-stat and reassembled. Still leaks. I installed an old, OEM t-stat housing that was NOT leaking when installed on the car that donated it. Project truck still leaks! I'm at a loss. I cannot seem to remedy this friggin' leak at all!

Summing up; our cooling system is leaking into our oiling system (evident by smoke coming out of oil pressure port) and I can't, for the life of me, get the t-stat housing to stop leaking.

Sounds like your oil cooler is compromised

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WildChild80 said:

Sounds like your oil cooler is compromised

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Click to expand...

The oil cooler is an air exchange, not water.

Water to oil would be more likely to happen at the manifolds and the head.

Or maybe the gasket to the oil filter housing, maybe? Haven't had that apart as of yet.

Thermostat housing o-ring or wonky housing causing an issue there. You said you checked your connector, and, that was my first thought, other than the hose.
Unless, that's where the head gasket is presenting an issue?

Good luck with your search.

Do you have your timing cover on? Is there any path from the water pump to that area presently?

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You mean the oil cooler on the bottom of the oil perch that has coolant running through it...that's coolant to oil...

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Alh cars DONT have air coolers for the oil...

The oil filter housing doesn't have coolant flowing through it.

Please explain how the manifolds could have coolant passing into the oil via either manifold.

Remote possibly the water jacket and oil drain at the head could be compromised but IMHO highly unlikely...

Start isolating things and see if you can find one thing at a time. Hook air straight up to the oil cooler and see if it'll hold pressure either positive or vacuum. Then if it does try to isolate the block as much as possible. Also try the old housing with the new thermostat and new seal.



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eddieleephd said:

The oil cooler is an air exchange, not water.

Water to oil would be more likely to happen at the manifolds and the head.

Or maybe the gasket to the oil filter housing, maybe? Haven't had that apart as of yet.

Thermostat housing o-ring or wonky housing causing an issue there. You said you checked your connector, and, that was my first thought, other than the hose.
Unless, that's where the head gasket is presenting an issue?

Good luck with your search.

Do you have your timing cover on? Is there any path from the water pump to that area presently?

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Click to expand...

eddie, I appreciate you commenting but, I'm not tracking on a couple of your thought processes. The oil and coolant could definitely mix if the heat exchanger were bad. The gasket to the oil filter housing only interfaces with oil. So, that couldn't be it. But (check out below) - you were onto something:

To anyone else following this thread, we can pack it in folks. It appears to be the headgasket.

I took the heat exchanger (often called oil cooler) off and bench tested it - all by itself. It tested fine.

That left me scratching my head again but, I think in the back of my mind I was starting to get skeptical of the HG.

I took the oil pressure sender back out of the oil filter housing and sure enough, smoke. So, my testing the other day wasn't a fluke.

I put the oil pressure sender back in place and really, I mean REALLY, watched the t-stat housing.

There it (the smoke) was oozing out from the area around the top of the t-stat.

As I walked around the front of the truck (the timing belt end of the TDI), I saw smoke creeping out of the timing cover... HOLD THE PHONE!

Then, while watching the t-stat area again, I happened to look back and saw a solid stream of smoke from ABOVE the crossover pipe - from the headgasket!

Since the HG is nearly directly behind the t-stat housing, it was throwing me for a loop. At this time, I am convinced it's the HG and I will be contacting my engine builder.

Thanks again everyone.

My bad on the oil cooler, two coolant lines running right into it, duh!

Head gasket I called was right, could have been the oil cooler though.

So, I appologize for my incorrect statement earlier about the oil cooler, just seemed really unlikely.


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Didn't seem likely on the oil cooler either but nothing made sense on where the crossover was, there's like 2 places it could cross, and unfortunately it was the HG...

A new head gasket on a freshly built engine was my last suspicion...still wanting to know why and where the new gasket didn't seal

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Yeah I wish you weren't dealing with a bad gasket on a freshly rebuilt engine.

I honestly didn't want to be right about that!

Was hoping for a water pump honestly.

Seems like you may have to reassess the gasket thickness, could be too thin and the cylinder held it up.

You know the only way to tell is take it apart.
I'm wondering if there's a dye you can use with the smoker that would tell the truth.

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eddieleephd said:

Yeah I wish you weren't dealing with a bad gasket on a freshly rebuilt engine.

I honestly didn't want to be right about that!

Was hoping for a water pump honestly.

Seems like you may have to reassess the gasket thickness, could be too thin and the cylinder held it up.

You know the only way to tell is take it apart.
I'm wondering if there's a dye you can use with the smoker that would tell the truth.

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Click to expand...

The thickness of head gasket is dependent on piston height not sealing ability.

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Yes I understand, however, it is dependent upon the amount removed from the head and the block.

If it is too thin the cylinder can hold the head up at TDC and cause this very issue.

Sealing ability is exactly what depends on thickness being correct.

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eddieleephd said:

Yes I understand, however, it is dependent upon the amount removed from the head and the block.

If it is too thin the cylinder can hold the head up at TDC and cause this very issue.

Sealing ability is exactly what depends on thickness being correct.

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Click to expand...

FALSE!

you measure piston height from the deck of the block, if the HG we're too thin the valves hit the Pistons...where do you get these ideas?

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