Misfire?

Hey guys. I have a 95 4Runner with 160k original miles and about 35k after the motor was rebuilt. About a month ago I devoloped only what I can describe as a pretty bad misfire in the engine. At idle it’s jerky and sounds like it’s missing. The three injectors that you can easily reach all read good. New spark plugs, spark plug wires and distributor cap DIDNT fix the problem. Any suggestions before I dump hundreds of dollars into taking off the manifold and having it diagnosed?

 

Is this happening all the time regardless of the engine being cold or warmed up? Is the timing correct?

I would suspect perhaps the other three injectors or maybe the TPS is going south. The VFAM might also be acting up or even the knock sensor but after its warmed up this would be getting consulted by the ECM. Is the new distributor cap free of any cracks? Are the plug wires from Denso if not what is the resistance of the longest and shortest wires. Is the distributor housing original, if so perhaps the signal coils in the distributor are getting tired and nearing end of life so check the resistances when cold and hot.

At 35K this would have to be a broken ring or a valve that has not got enough clearance in lash or perhaps the valve spring is collapsing. A compression test will tell. There are two types of tests you want to do. The first is the base compression pressure that is done with the throttle plate at WOT and the engine is spun with all plugs removed from the heads. Then the leak down test is secondary to see how well the piston rings and/or valves are sealing. If you get the test unit that threads into the spark plug hole like the spark plug does then you can do both types.

The pressure should be north of 100 PSI and the cylinder to cylinder variation should be less than 15-20 PSI. The more the same they are the better from cylinder to cylinder. The FSM has values for the expected range. The leak down rate is what matters on the second part of this test. If you have one cylinder that is losing pressure quickly then you need to understand if its the rings or the valves. So pour in some motor oil like a table spoons worth into the cylinder and that should seal up the rings. Then run the leak down again and see what happens. If it is slower, then it is the rings. If it is no different then it is a valve and seat that is not sealing or perhaps a cracked head.