OK, 2000 626 2.5L started out with head gasket on cyl. 3 leaking into coolant jacket. Cyl 3 was missing pretty bad. Sent heads to machine shop. they milled heads about .003-.005 due to warp. Also replaced 2 bent valves. Reinstalled all parts. started engine. Misfire moved to #4.

Now, compression check on #2,4 & 6 show within 10% of each other at 128 lbs each. Cylinder 4 is COMPLETELY dead. You can watch it on a power balance test. Replaced spark plugs, swapped spark plug wires, swapped fuel injectors between cylinders & #4 still dead. Changed ignition coil with known good.

I ohmed wiring harness from ECM to fuel rail. All 4 wires to the front bank measured .4-.6 ohms. Checked wires for short between each other, to ground & power. No shorts.

leak down test shows no leaks into coolant, intake or exhaust. Does show 80% leakage into crank case. But #2 shows about 35% into case & #4 is the ONLY cyl. misfiring. Have triple checked timing, have 13.5v getting into & thru injector rail.

Code wise, I'm getting P0420, which is due to the cyl 4 misfire.

Stumped. Has anyone ran across similar issues?

 

Put a noid light on #4 injector connector. The PCM might be holding the injector open. That would wash the rings out and keep them from sealing. Remove the dipstick and see if the oil smells like fuel. Those are no fun timing the cams either so re-check the cams. I don't understand why it would have bent valves if it just had a head gasket problem. Was there a mark on the piston where a valve hit it? The injectors have 12 volts with the key on. The PCM grounds the injector to open it. A noid light is your best option to diagnose it if it is an injector/harness/PCM problem aside from an ohm meter if you find that the injector grounding at all times.

 

agreed with the bent valve. no, there were NO marks on any of the pistons. The timing marks on the cam to cam gears are dead nuts on for both banks & the timing BELT is new along with all idler, tensioner pulleys and tensioner itself. the timing marks for the belt on, checked them 3 times.

The cylinder is dry, not flooded. with a stethoscope I can hear all 3 front injectors clicking about the same & the oil is clean & does not smell of gas.

All this info is why I'm stumped
Oh, forgot to mention. swapped the PCM with another also. still dead on Cyl #4.

 

also the teeth on the tone ring for the crank pulley are all even & straight.

 

I would still put a noid light on that injector connector. They are maybe 5 bucks at a parts store. If number 4 cylinder is dry and not firing then something is going on with the injection circuit or you have a vacuum leak on that intake runner.

 

bump, sorry it's taken so long to get back on this 1. the noid light is flashing evenly on #4,#2 & #6

 

Is the spark coming out of the coil for #4 cylinder hot? The PCM grounds the coil to charge/discharge it. I have seen a few bad PCM's due to a shorted coil pack. Remove the #4 wire from the coil and place a 12 volt test light near it to see if it arcs to it. Without being there i can't really say. Obviously look for a vacuum leak with a flammable spray. Brake cleaner etc. Be aware that not all brake cleaner is flammable and you can't find a leak if it is not a flammable liquid.

 

You've had issues with coolant leaking into the catalytics as well as misfires sending raw fuel into catalytic converters.
Have you ever changed the cats?
I had a misfire on cyl 1. After changing,spark plugs, injector and many other things, the misfire was from poor exhaust flow. In my case it was a **** poor exhaust design, in your case it's the catalytics! My cyl #1 is beginning of bank, just like your #4?!
READ: https://www.mazdaforum.com/forum/maz...-design-28940/

 

Thanks all for the replies, Hixx, yes the spark is there & nice and bright. i have a spark tester set for a mazda so that it jumps the gap. I also have ran around the intake (upper & lower) with brake clean that I KNOW is flammable!! UseYourNoggin, the #4 cylinder is in the middle between 2 & 6, i have not actually looked IN the cats( but i will) due to the fact that 2 & 6 are on the same bank but not misfiring.

 

Hmmm, if it has fire/fuel/compression and no vacuum leak on that runner then it obviously should not miss. I did have one many many years ago that i put back together after replacing a bent rod that was similar. It turned out to be a bad intake plenum gasket that they punched wrong. Factory gasket too. I am afraid you need to tear it back down and inspect every part. Maybe the machine shop screwed up something. The fact that they claimed a bent valve is bothersome.