I got some fantastic help on this project from mustang67408. He was an excellent resource for both knowledge and encouragement. Many thanks to him for putting up with a barrage of PMs full of silly questions
My buddy has a 1998 Taco that was missing badly on one cylinder (actually when it was fully warmed up it missed on more than one cylinder). I did a compression check and #1 was 100 psi lower than the rest. A cylinder leakdown test revealed bad exhaust valve/s on #1, so I knew my work was cut out for me. My buddy wants to get this thing running as inexpensively as possible, so he can sell it and get a Taco with the 3.4l.
Here is a simplified version of the events that took place thereafter:
I made this:
Look like this:
So I could dissassemble this:
Before teardown I measured and recorded the valve clearances. All of the exhaust valves were too tight. I knew that only #1 had burned though because of the compression test.
Here's what the valves on #1 looked like when I got them out:
The valve on the left is obviously more burnt than the one on the right, but I am going to replace both valves. I deliberated on whether or not to have a machine shop replace the seats, but this was a budget project that I wanted to complete this weekend, so I was trying to stay away from the machine shop and related expenses/delays if at all possible.
mustang67408 advised me to try lapping then in good. I gave it a shot, and after a few minutes the seats cleaned up pretty well:
With the new valves and newly lapped seats, I installed the springs. Then I turned the head upside down and filled the combustion chamber with solvent (also mustangs idea). In the picture you can see my compression tester adapter acting as a plug for the spark plug hole:
After several minutes of that with only very minor seepage, after 10 minutes not even a drop had formed in the exhaust runner,but it was visibly damp. After cleaning the mating surfaces on the block and head, the head went on and then all of the valves needed to be adjusted. I had read the FSM and measured all valve clearances before disassembly, so it was a simple matter of measuring the existing shims and comparing those two numbers with a chart to get the sizes of shims needed for correct clearances. I had to guess what shim would be appropriate for the two new valves, and I got one of them right and one wrong. I had to make an extra trip to the dealership, but all the valves are perfect now. The shims sit in buckets on top of the valve stem/springs. If you look close, you can see the setup in this picture. The valves on the right have the shims installed, the left side have empty buckets.
The FSM calls for two SST's for the adjustment proceedure, but they are for swapping shims with the cams in place. You don't need then when the camshafts are removed from the head. In my case, it would have been nice to have them because I ended up removing the exhaust camshaft an extra time in order to reshim the one new valve.
I got the whole thing back together and it runs like a champ. Purrs at idle and has much more power than it did before
(go figure!)
Thanks again to mustang67408 for all of his support! He knows his shi'ite for sure.
I hope this simple write up helps someone in the future. I spent a total of about 12 hours over the course of 4 days to get this thing done. I would rate the job at about a 7 on a scale of 1-10 for difficulty. It definately wasn't easy,but it is very doable for someone with average mechanic skills. If you need help changing your sparkplugs, this probably isn't a job you want to take on