In a previous post I mentioned I just purchased an 03 4.0 v6 a few days ago. The coolant in the reservoir was low, and upon checking at home with a combustion tester, realized that the coolant was low in the radiator (could not see any upon removing the cap). It passed the combustion test (used one of those fluid testers). Has no codes, normal idle on cold start. No film under oil cap, tranny fluid is clean. Haven't investigated for radiator/water pump leaks, but haven't seen anything noticeable. Decided to pull #6 and #4 spark plugs and put a borescope inside to check for coolant in the cylinders.

I am not familiar with HG leaks, or what a healthy cylinder looks like. I did, however, get a few pictures of the spark plug wells, and inside the cylinders. The wells were dirty at the bottom, and there was this greenish liquid about midway up the outside of both of the coil packs, it didn't smell like coolant, just smelled like combustion. #4 spark plug well looked dirtier than #6. The heck is all that gunk? It doesn't look like there is any liquid inside the cylinder, but the tops of the heads look a bit corroded/rusty? Like I said, I have no experience.

Here they are.

The spark plug hole photos wouldn't have any indication of head gasket failure, what I see in those photos is just a bit of a small possible oil leak from where the tube seals inside the valve covers.

The other photos of the top of the pistons dosen't seem show any coolant on the pistons, which is what you would be looking for as one possible indication of the V6 head gaskets failing.

To follow up I would test the coolant for any exhaust gases contamination, which is a tell tale sign of head gasket failure for the V6, if you find no contamination in the coolant I would say the gasket are still holding up at this point.

Most common issue is misfire upon cold starts because coolant pools in the combustion chamber over night. Might want to pull all the spark plugs out, get a cooling system pressure tester and apply about 15-18psi to the cooling system and let it sit like that over night. In the morning bore scope the cylinders and see if any of them have coolant pooling in them

All the photos of head gasket failures that I've seen had nice clean carbon-free pistons because water cleans out the carbon. I think you should flush the system and put the proper Toyota pink coolant in there and watch for leaks.

Agreed on the above - head gasket failures that allow coolant into a cylinder will create a steam blast that will clean pistons to an almost new looking condition. If you find a cylinder that is super clean - there is your leak. I used to have to shut the lights off in the shop and use mirrors to see down into the cylinders to verify suspected head gasket leaks. Now, a simple and cheap USB camera will give a decent picture of the guts inside with no more than simply removing the spark plugs and dropping these tiny devices down inside for a look. Wish we had half this technology back in those days - but then again, vehicles were also a whole simpler.

Sarge

If you're missing coolant, have you checked the radiator end caps for leakage?

Might even notice that the inside lower corners of the bash plate are moist from the lower end cap corners leaking.

Both my 4runners developed radiator leaks around 105K miles.

Agreed. If you're losing coolant and think it's a headgasket leak, then the engine should be misfiring at startup. The pressure tester would be a good way to diagnose the problem.

Ues pressure test is the only way to do it. Needs to be done overnight and up to 24 hours to be sure. That said, may be your water pump, a hose or the radiator causing low coolant.

@thewaydown . Look very closely at the drivers side lower edge of the radiator, near the petcock. Mine had a very slow leak there. You can see the drips. I posted about it here recently.

That makes sense. Maybe I should see about replacing the valve cover gasket.

I actually tested the coolant with a combustion gas tester and it passed. Gave me a little peace of mind, but i'm still a little uneasy, haha.

very helpful. I'll be checking this asap.

I know this is a great way to test, but i've been weary about doing it. Would applying pressure to the system and leaving it over night cause/worsen any problems that didn't already exist?

I also recommend the pressure test overnight, it’s a common practice at the toyota dealer as well. Keep it at the radiator cap specification and you’ll be fine.

Advance Auto usually has a very nice cooling system pressure tester kit in their loaner tool program. Ensure your cooling system is completely full before testing...don't want air as it is compressible. As stated above, pump it up to the cap's rated pressure and watch it for a few minutes to verify pressure doesn't drop off. If it does drop off, reapply pressure and start inspecting for fresh coolant leaks including the combustion chambers, radiator end tanks, and hose connections. If you're losing pressure but can't find the source, UV dye and a black light can help.

I'm going to be doing this tonight. Just want to clarify that I should take the spark plugs out, and leave them out while the pressure tester is loaded over night?