Exhaust gas in coolant

[CO MJ]

When I bought my Comanche it was maxing out the temp gauge after a short drive. After checking sensors and compression, I tested the coolant and found exhaust gases, so I pulled the head, had it magnafluxed, decked, and put in a new gasket. That did not fix the exhaust gas problem, but I am not running at normal operating temps. I just made the assumption that this means there's a crack in the block between one of the cylinders and the coolant passages.

It's a year later of driving it most days, still gases in the coolant, still no overheating, but it did start misfiring under load once the engine is warm, as well as the TCU getting stuck in fourth. I also noticed the air bubbling out around the thermostat housing seal, presumably because of the pressure created by the exhaust leak.

 

I will be going through all the sensors, coil, plugs, etc to attempt to diagnose the misfire and trans issue, but my concern is that the misfire is caused by the aforementioned crack in the block having gotten worse over the last year.

I think compression or leak down test is probably my best option to check for that crack? and if it turns out to be the issue, I'm not sure what to do. I'm still not having any issues other than the occasional misfire going through a slow sharp turn or up a hill, and blipping the throttle tends to make it stop. There doesn't seem to be any contamination in the oil, coolant loss, or any smoke or smells coming out the other end of the exhaust so I'm fairly sure the gas is going into to coolant exclusively. This block is also nearly at 200k, the gas infused coolant has been rusting out the block for god knows how long, and I'm a bit hesitant to drop tons of money into it vs purchasing a brand new crate engine (I have a renix year so putting something a bit newer into it is honestly a bit of an exciting idea anyways). 

With the reputation of stop leak type products I've been hesistant to try anything, but is that worth a shot?

Any other thoughts, comments, suggestions? I will update the post with extra information about sensors etc as I test.

[pizzaman09]

I have a very experimental project car, it's a BMW inline 6 with a a homemade engine head.  Over the project I've dealt with many exhaust leaks into the coolant.  I tired a few stop leak products.  The one product I tried , which was some ancient bottle of something made by Prestone worked great.  I also tried k seal and wasn't pleased with the amount of what appeared to be copper flakes it had in it which just made a mess of the whole cooling system.  

 

At this point it sounds like you don't have much to loose, I'd try what ever product seems like the advertising works best on you and give it a shot.  You'll have to flush your cooling system once the engine is replaced anyway.

[Huntr]

Did you ever get around to a compression test? I'd check compression numbers between all cylinders. When you pulled the head did you have the block checked for cracks or just the head? 

[AZJeff]

If you have tested the coolant and have confirmation that there are exhaust gas products present, the first thing I would do is to run a compression test or leak down test before taking any further action.

 

That test will tell you a bunch about how good the overall block is, particularly if the cylinder/ring/piston condition is worth investing more in the engine, or if it's time to swap engines.