Have 1994 4runner v6 auto with 323k and a potential head gasket problem, but shop tests say otherwise.

This all begins last April when my 4runner approached a stop light and died. Next thing I notice is the temp gauge is past red with no prior warning. Got it started long enough to get it out of intersection, parked it safely, called 2 repair shops that I've had experience with to find an opening and then a tow service. With 323k, I was concerned about other damage before beginning repairs, but the water pump housing was destroyed and losing water faster than they could pour it in. At 280k, I had the timing belt renewed as scheduled and the original Toyota water pump was replaced with a Gates water pump and other parts. Totally came apart after 40k. Thus a leak down test couldn't be performed until after $1300 plus of parts and labor, all genuine Toyota, not risking Gates again. It passed the leak down test.

I've been adding coolant regularly since. Found a bypass hose that leaked and needed replaced and had it tested afterward by another shop using a coolant exhaust gas tester and they concluded no head gasket problem, but noted pin hole leaks in radiator. Purchased and installed new radiator and hoses. The place I park at work and in my garage have absolutely no signs of coolant on them, but coolant loss is increasing. I can only conclude a head or head gasket problem. Spark plug #6 is wet and I have a miss when idling.

How can it pass two different tests for head gasket failure? Am I missing some other causes?

Whats the oil look like? When its running whats the exhasut look like, steam?

Oil looks normal and not increasing. No unusual color when I wipe the dipstick and no "milkshake". Occasional startup steam when temperature in low 30s, but noted this morning with low in 50s there was no steam at all from startup onward. Definite smell of antifreeze from exhaust. My thread may be mislabeled as "head gasket", when I think the problem is probably a crack in the head. Moot point anyway since to repair the head, the gasket gets replaced. A leak down test requires the valves to be closed. It passed the test so there are no cracks in the combustion chamber. Second, it passed the emissions gas test which means no exhaust is getting into the antifreeze which has no oil or bubbles in it and looks like it just came out of the container. Hence, coolant is likely drawn in on the intake stroke when the intake valve is open. From inspection of the spark plugs, the problem appears to be entirely on cylinder #6. Spark plug is dramatically different from others.

When I rebuilt my 3.0 the heads required welding as they corroded to the point where the head gasket could not seal properly. Did you inspect the plugs when you removed for the leak down test? Any deposits?

You could see the corrosion, it looks much worse once cleaned.

Attached Images

 

Leak down test was performed by the shop that replaced the water pump, timing belt and all associated hardware. Mechanic said no problems which I took with relief. Checked the plugs myself a few weeks later because engine performance was rough and coolant disappearing. Shop did not replace the plugs. Noted in first post that I found a bypass hose leak to fix and pinhole leaks in radiator which has been replaced. I'll post a pic of #6 plug later. It was heavily corroded and even had red deposits. I only use Asian formula in the vehicle. All other plugs were gray and relatively clean. The coolant gas test was performed by another shop about a month ago after radiator was replaced, but coolant loss was still occurring.

Sounds very similar to my girlfriends 91 with a 3vze. Losing coolant, steamy exhaust, and slightly foamy oil cap (though oil on dipstick is fine and I've been told that's "normal"?). However when she had a mechanic take a look at it, it passed the compression test, and chemical test to check for exhaust in coolant.

Is checking the spark plugs another good indicator of what could be up? I've never been a car guy but want to be more independent with our vehicle, especially as I now have a workshop to tinker in