Hello, I am asking for solutions to save me some time and effort. My 90 runner with v6 has a dead cylinder. It is Cylinder #3 and gets about only 75 PSI. I have no idea exactly if the rings are bad or if the valves are warped or burnt. But 253,000 miles means it can’t be the best. Is there anything I can do without having to remove the head?

 

Put a few drops of oil down the spark plug hole and run the compression test again. If the rings are worn, the oil will temporarily seal them up and your compression reading will be higher, sometimes upwards of 20psi or more. If there's little to no change, it's likely in the valve train.

 

Will try, but any particular oil? Like what weight or even just marvel mystery? Also, if it is the rings, why that cylinder and what does that mean for me?

 

Just use whatever oil you run in your car.
Things like detonation (aka pre-ignition or spark knock), dirty injectors, weak or no spark can all cause 1 or more cylinders to wear unevenly. If your rings are worn, you're looking at at least a top end rebuild. That includes, but is not limited to, honed or bored over cylinders, new pistons and rings to match, new piston pins, and all the corresponding gaskets. Usually this is a good time to look at your rod bearings and replace if necessary.
If your problem is in the valve train it's a little simpler fix. Only the head needs to come off and you can then replace valves and repair the seats if needed.

 

Update: Lucky me it’s not the rings. There is also still crosshatching from what I saw on the borescope. I’m going to begin taking stuff off to see if it needs a valve adjustment, just needs to be reseated, or worst case is a burnt valve

 

Another good test would be to pressurize the cylinder at TDC with about 50 psi and listen to see where the air comes out…intake manifold, exhaust, crankcase, radiator. So you can tell if it’s leaking past rings, intake valve, exh valve, head gasket.

Looking forward to your next post.

 

Interestingly enough, I have already done that. We put a few pounds of air into the cylinder with the end of a compression tester. I opened the butterfly valve without the intake on and hear some hissing. However, this was on the compression stroke, so I’m thinking the valve was slightly open or there is not a very good seat holding. Either one is possible but I could try it again. That time my father was holding by the crank bolt. Wouldn’t it also be better to try on bottom dead center?

 

I don’t think BDC or TDC matters as long as you can be sure both valves are closed. If standard shift, put it in gear and e brake on.
sounds like it could be a bent intake valve if leaking into the intake.

 

So it is leaking air from the cylinder. And correct me if I’m wrong anywhere. The intake valves are fine. I put a vacuum gauge to one of the vacuum lines and the gauge read dead steady. And if I am wrong in saying it is the exhaust valve, then I’ll do it again on btdc to ensure no mistakes in my findings. If so then I will tear into it tomorrow regardless

 

Yes sir it sounds like you've narrowed it down correctly. Steady vacuum gauge means intake valves are sealing.
You're already on track to pull the head, and now you know exactly where to look once you get it off.
Nice job diagnosing!

 

A discovery has been made. The exhaust cam lobe was always in contact with the shim. It has a 2.8mm which keeps it open at all times. As for who put it in there I have no idea who or why. Problem is now finding who has the shims

 

Nobody put the wrong shim in there. Valve seat recession and valve stem elongation reduced that valves clearance to less than zero over time.

It's not likely that merely restoring the exhaust valves correct clearance will fix that cylinders low compression.

Can't hurt to try before you remove the head though. Hot exhaust gasses blowing by a partially open valve burn it very quickly.

A valve grind and subsequent re-adjustment of all valves are in order if you expect any continued service from this engine.

 

But why from that cylinder alone?

 

Toyotas are not such as the "Miraculous One Horse Shay" where every piece is so perfectly constructed that no part fails an instant before every other part does.

Perhaps a fuel injector that did not deliver 100%, or a vacuum leak that lean'd out that single cylinder so that it ran hotter, perhaps some slight imperfection in that valve, or its seat that left it ever so slightly more frail than its mates.

Who knows what happens in near a quarter million miles and 30 years??

Your engine should have had several valve clearance checks in its long life that might have nipped this in the bud.

Likely, it has never had any.

 

Most likely. The previous owner, my grandfather, who bought it brand new only took it in to have the little things maintained or replaced. The odometer was the only part changed by the dealer at the time. But that was 203 thousand miles ago. As far as I know, the oil pan gasket and passenger side valve cover has been replaced by the sketchy mechanics in his area. Astonishing to me how it has come this far.

 

I have a 2000 cr-v with 250k+ miles that suffers from valve seat recession on one cyl. I had run out of adjustment on that exh valve and was able to remove the rocker and slightly modify it for additional clearance. The fix won’t last forever but for now is good. As suggested, i think you should reshim yours. Go for the loose side of the tolerance. It will probably work, for a while at least.

 

Regardless, thanks for helping resolve my situation and shine some light as to how it can be fixed.

 

Agreed, but that's why people love Toyota's. Drive them 300K+ miles with nothing more than timely oil changes.