Just bought a 2003 Explorer XLS. The engine was running roughly so I took it to the local garage. They say the #5 cylinder is a little down on compression but we can hear a popping out of the exhaust. What should we be looking for, are these things known for head or valve problems? FYI, all of the cylinders have 155 pressure, the #5 has 125. Thanks!

 

I had a bad cylinder head on my '95 Aerostar 4.0 with the pushrod motor. I forget the exact numbers for compression, but it was within the limits in the Ford shop manual. The numbers for the compression test should be within 25% from the highest to the lowest. Mine was 18-20%. The engine idled a bit rough, but seemed fine once you were moving. If you listened at the exhaust pipe, you could hear more of a pronounced "puff" than a "pop" at idle. Before I took the heads off, I did a leakdown check to confirm the cause of the low compression.

When I took the heads off at about 80k miles, there were two valves that were badly recessed on the driver side head. It looked much worse than the compression numbers would indicate. Also, I found that the head on the passenger side had a casting date a year or two later than my Aerostar, so that head had been replaced within the 45k miles before I bought it. The machinist I used said Ford produced some crappy heads for the 4.0 in Spain during the mid-90s. I was the unlucky recipient. I doubt that the problem extended beyond the mid-90's. I'm guessing you have the overhead cam motor. How many miles?

 

The story today.
I took it to the Ford dealer in my area. They had it for 4 days and this is what I learned.
134038 miles. Verified miss on cyl. #5. Checked plug, plug wire, ok. Coil pack seems good. possible injector problem, need to remove intake to check. Replaced injector, did not fix. Remove valve cover found rust in engine- NEEDS NEW MOTOR.

I asked the mechanic and he said there were "traces of rust", but couldn't or wouldn't explain how this would affect a misfire in #5. The phrase "coil seems good" sounds fishy.

Advice?