2000 Mazda Truck Stalls when driving, misfire after shiftin

P0301 Cylinder 1 Misfire Detected

This code appears after cold start if I give it too much gas, it dies, and then becomes hard to start. If I avoid the condition that makes it hard to start (like a plug gets fuel fouled) the code will not reappear.

P0402 Exhaust Gas Recirculation Flow Excessive Detected

This is linked to a differential pressure switch that activates the EGR valve. I replaced this switch a while back. The code has reappeared intermittently. I can avoid the driving condition that has caused the code, ie: driving up a long hill on the interstate where I have to downshift and run the motor at a rather high rpm in order to maintain the speed limit.

I will check fuel pressure, but don't think it's the problem. The motor seems to run fine at higher rpms once it gets past the stumbling after shifting.

Beyond checking the MAF connector resistance, are there any other MAF test?

Disconnected the MAF and it ran worse. Cleaned the MAF and no improvement.

I removed the EGR and fabricated/installed a block-off plate. No improvement there either.

I guess I'll do a compression test tomorrow.

Thoughts?

The numbers are in. Warm motor, t/b held open, & 4 plugs removed. Front-to-rear.

135
132
138
138

The plugs look the normal creamy tan color.

Another note, if I don't rev the motor over 2500 rpm, it doesn't seem to miss, warm or cold. Yet, while parked, if I rev the motor and hold it for a bit and bring it back to idle, it seems to be missing on one cylinder.

I also vacuum checked the fuel pressure regulatpr & evap canister, and checked the intake manifold and t/b bolts for tightness.

Any advice?

After getting to the end of being frustrated, I finally took it to a local shop to have them look at it. Among other things I told them that the compression was at 132 - 138 across the four cylinders. After they looked at it for a day, they were also baffled. They ran compression and got it in the low 140's. We were both figuring that was significantly different. After doing some more testing, they did the compression again, and a leakdown test. This time compression was low 130's. They ended up looking inside the combustion chambers with a camera, and could see that the exhaust seats were pushed up into the head a ways. Apparently what was happening, was the motor ran fine at low RPM's, and when oil pressure built up at higher RPM's, the lifters were holding the valves away from the seats, not allowing them to seal.

I thought cool. Pay them a coupla hundred bucks for the diagnosis, pull the head myself, and get it rebuilt. That wasn't hard, except when I took the head in for the rebuild, the guy found the head had cracks, two cracks each at the #1, 2 & 3 exhaust seat area. I didn't have the time to go and find another head at the junkyard, so I got a rebuilt head and paid the core charge. Like $550 total for the head with core.

So, in summary, If everything checks out o.K. And it still runs bad, check the valve seats.