Update on this project. I thought for sure that the gasket you see in the video was the cause of the leak. It seemed so obvious. The shop put new gaskets on and buttoned it all back together and --- it still has a leak somewhere in the rear of the engine. I got an endoscope and looked back there as best as we could and did not see anything obvious as we pressured it up. I could see two core plugs that seemed to be intact and not leaking. I (and the shop) are scratching our heads about where it is coming from.

Could it be:

• Coolant bypass pipe? -- that tube you see in the valley seemed to be in good shape but we didn't have a way to pressure test it. It certainly is not leaking on the part exterior to the valley. Do these leak often? Perhaps we should have changed this anyway.
• Plenum Gasket not installed correctly? -- I assume this would be hard to mess up.
• Head gasket? -- maybe but I see no other indicators -- see above post.
• Any other ideas from the braintrust?

I am trying to figure next course of action but it is getting expensive as I don't have time to do this myself. I guess my choices are to tear the top end open again or to drop the transmission in order to get a good look at the rear of the engine. Any ideas?

 

What engine do you have? Also I think it would be good to post up some pictures.

How are you testing the coolant leak? I would just get a pressure tester so you don't have to have the engine running. If it is the 3.4L then it could be as simple as a the coolant lines going to the throttle body.

 

Engine is 3.4 5VZ-FE. Best visual of the top side I have right now is in the first linked video in the original post of the valley condition.

The coolant leak was a persistent leak that is evident at the bottome/rear of the engine but can't be seen near the top side anywhere. It happens after running/sitting a while and is quite evident when using a pressure tester with the engine off. The leak definitely is not the coolant lines going to the throttle body as there is no coolant up high like that and that area has been disassembled and reasembled multiple times now in order to get to the intake plenum.

This is where I see the leak is evident under the vehicle. Coolant has been changed to pink since this picture was taken. I can't see evidence of it coming from the two core plugs at the rear of the engine that I can see with an endoscope.

 

There isn't any coolant in the bottom of the engine. Gravity will bring the coolant down. I would use a mirror on the back of the engine to see where it is wet. Start from the top and work your way down.

 

We have used both mirrors and endoscopes to view the back of the engine as best is possible without dropping the transmission. I'm certainly aware that there is no coolant at the bottom of the engine and it is coming from a bit higher but I know it isn't at the top of the engine. You can't really see the back side well. I think it might still be coming from the internal valley somewhere but am hesitant to tear into that again.

I'm just hoping to surface some alternate ideas about where the leak could be coming from as we have just about exhausted possibilities with troubleshooting so far.

 

There are a ton of coolant lines on the rear of the engine. Going to the intake / throttle body also going in and out of the heater core inside the cabin. I marked some of them in red. There are more! A simple pressure test should reveal the leak. If i were you I would take this to a qualified mechanic. Should be pretty simple for an external leak. Just remember coolant cannot come out of an area that doesn't have it. So follow the coolant lines.

@BamZipPow does coolant flow through the intake? How about if you take off the lower intake on these?

I have seen many places and people say they have a rear main seal leak, when it is just the valve cover gaskets, that is running down the back of the engine.

You said you had a coolant burning smell, well there is an exhaust pipe crossover right below these lines.

 

Just remember coolant cannot come out of an area that doesn't have it.

Careful, coolant most definitely can come out of places where it shouldn't be. My money is on a freeze plug

 

96 Wolf, thanks so much for trying to help, but we (qualified Toyota shop and I) are so far beyond the basic troubleshooting you are suggesting. I wish it were that easy.

What I know is:

• Leak is not at any of the top-level hoses. These have been pressure tested and most are new.
• Leak is not at the water pump and blowing back to the rear of the engine. Water pump has been replaced and shows no signs of leaks. Hoses in that area are good also.
• There was likely a leak at the intake plenum gasket (I think that is what you call it) as this video shows when we opened things up to see the engine valley. That gasket has been replaced. It was assumed at that point that it was the leak culprit.
• Visual inspection of the two core plugs at the rear of the engine seems to indicate no leak there. I guess we (me and the mechanic) could be wrong and I think there are only two there.
• We didn't replace the coolant bypass pipe that runs the length of the valley. It looked to be in good shape. This might have been a mistake and could be a leak source.

I am just trying to surface additional/alternate ideas or troubleshooting steps at this point before we take the whole top of the engine off again and/or drop the transmission to better see the rear.

 

There is a possibility that it could be from the heater core switch valve. You should be able to see if there is any residue on the valve located on the firewall.

 

That has been checked as well. New as of about 6 months ago.

 

There is an outside chance that some part of the head coolant passage may have a micro crack in it. Rare but it happens.

 

I worry about stuff like this. Mechanic is pulling the top side of engine off again to see if we can actually see the source. Part of the challenge is you can't pressurize it after you get the intake plenum off to see that valley. Perhaps there is a way we can isolate certain parts of the system to see and pressure check.

 

From this thread...there are only two and they are behind the flex plate.

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To verify, you should be able to pull the bell housing inspection plate to see if there is any coolant in there or not.

 

From this thread...there are only two and they are behind the flex plate.

Thanks for getting me to look at that 4Runner thread again. I found that previously and only recalled that it said two plugs on the rear of the engine. I had my camera back there and saw what I thought was a plug on each side (somewhat obscured by grime but no fluid) and I assumed they were OK. From your pictures and a more careful review of that thread, it seems it would have been impossible to see the plugs without some disassembly. It does seem that the fluid was emanating from the small hole at the bottom of the bell housing which your second picture shows. Being it was the lowest point, I didn't want to assume it came from there.

I'll call my shop and/or stop by there today to discuss. It looks like a transmission removal might be required to access that. Costs keep racking up.

 

So surfing around on the web for more photos, this example is what I assumed was one of the freeze plugs on the rear when I looked with an endoscope. Clearly, it is not so I have to get under that flex plate.

 

But think big picture. Coolant leaking from those freeze plugs wouldn't explain the coolant in the valley. Is it possible the bypass tube has a pinhole leak in it? It would be pretty easy to remove and pressure test it with air and soapy water.

 

But think big picture. Coolant leaking from those freeze plugs wouldn't explain the coolant in the valley.

Good point and I guess the intake manifold gasket repair might not be good or that bypass pipe might need refreshing which we should have done while it was apart. Clearly, that intake manifold gasket needed replacing and I am convinced it was filling the valley. I want to assume that it is fixed now and I have two leaks so I was moving on to the next suspect but perhaps prematurely.

Now, I'm wondering if I/we can get that inspection plate off and snake a camera in there to peek at the real freeze plugs. I am thinking not as I can't find someone on the web that has done that.

It seems I might have to open the entire top again or drop the transmission still not knowing which it is. This is extremely frustrating.

 

I say use air to inject and pressurize the cooling system. Not a lot but enough that you might hear the air hissing out at the leak point.

 

I say use air to inject and pressurize the cooling system. Not a lot but enough that you might hear the air hissing out at the leak point.

That is a good idea. I'll either find some time myself in the future to evaluate or take it to another shop I trust.

The shop where it is now quoted me 18 hours of labor to drop the transmission and see if the freeze plugs are leaking. I don't think it is that much but I think they are tired of trying to find this leak and want to quit. They did fix the intake manifold gaskets which I know were bad but I am somewhat disappointed in their diagnostic skills. Regardless, I am picking it up today with it still having that slight leak.

 

I say any coolant in the valley has to be draining out the back of the block and down onto the transmission bell housing where yer seeing the coolant. If you feel comfortable with it, you should be able to test that theory by adding some water into the valley and see where it drains out of.

 

Got reminded to update another thread and I thought I would update this one as well. I ended up picking up my T from the shop that had it about a month ago. They had replaced the intake manifold gaskets which looked terrible and which we assumed was the source of the coolant leak. Reassembly and subsequent pressure testing showed that I still had a leak. I assumed at that point that I needed to open the top side up again to get change that coolant bypass pipe or drop the transmission to access those freeze plugs as those are the two most likely culprits left. I was not ready for either of those options at that point due to cost and uncertainty about which option would discover the problem.

I decided to drive the truck a bit and see how bad the leak was. I have now put 500 miles or so on the truck on a couple of trips to the coast. The leaking is substantially reduced from prior state but still evident. I assume I had two leaks and the new gaskets fixed one of them unless that job was done poorly and they are still leaking. I have not had to fill the coolant yet but I do see the drips in the same place as before. Truck runs at 192 to 194 degrees at 70 mph with ambient temperature in the 70's. It even holds at that range off road in super soft sand and I did about 50 miles total in those conditions over two trips.

It bugs me to have any fluid leak and I'll get back to this one when work allows or another project that shares disassembly steps helps me rationalize it. I'll update again when I finally find the solution.