3.4L Cylinder 4 misfire

I have a 97 4runner with the 3.4l in the shop at work with a cylinder 4 misfire. I'm assuming it's the same as the Tacoma, which is why I posted it here. I've swapped spark plugs, coils, and plug wires and it still misfires on the same cylinder. Compression checks out at around 150psi. Did some research and found that the injectors seem to be a common problem. So, we ordered a new injector and I put it in, only to find out it still misses. I took the intake back off and we checked all the injector connectors with a noid light, and it lights up. And, we could see all the injectors spraying gas when looking down into the intake towards the valves.

Odd thing is it seems to randomly run smooth for like a second or less before going back to a constant miss. I work at a Ford dealership, and the scanner used for other brands is absolutely no help on this particular vehicle. Oh, and it has 331,000 miles on it.

Any ideas?

 

Run a leak down test on number 4. No over heating problems?

 

It still could have a partially plugged injector. Try swapping it into another cylinder that's easy to change. See if the misses moves to that cylinder.

 

The guy didn't say anything about overheating, so it's not likely. And it has a new injector, so that part should be good now.

 

Leakdown test

 

not being an ass, just experience talking here "new" stands for never ever worked.

everything below is assuming that you have a sound motor, no cracks in the head etc.

It sounds to me like an issue with a valve spring or cam lobe or valve clearance in that cylinder. This requires a visual check.

In my opinion, a compression test is a dynamic go - no go test... either the compression is good or it is not and the reading is dependant on how fast the starter cranks the motor over.

A leakdown test will tell you the static mechanical sealing ability of the assembly and where the problem lies... you have an at speed dynamic problem, a broken valve spring will cause an intermittant miss at idle and a constant miss off idle until all of the stars align just right and the valves are closed when they are supposed to be, then she runs normal for a second.