2000 Ford Ranger 2.5L Rough idle, misfires at idle, stalls

2000 Ford Ranger 2.5L 4 cyl w/ A/T at 260,000 miles.

This issue has already been diagnosed but I wanted to share my experience with a head scratching problem so that others may learn from it.

My issue started about 2 years ago with some random stalling when first started and a mild rough idle. It has progressed into stalling anytime the engine has been revved (either free rev or letting off the gas after hard acceleration) and rough idle most of the time, sometimes stalling when sitting at an idle. Felt like it was missing at idle and smoothed out anytime the gas is pressed even the slightest bit.

I originally didn't have any codes but there were misfires being detected. It wasn't hitting the code setting criteria so i kept blipping the throttle to get it to a misfire state and finally got P0301, P0302, and P0300 at random times.

Went into mode $06 with misfire count % at
1.28% for 1
0.79% for 2
0% for 3 and 4



Checked for spark. This is the dual plug design. Both plugs fire at the same time on compression and exhaust, contrary to some popular beliefs. Anyways good long spark from each port on the coil packs. Wires ohmed out good. Been 100k since last tune up so went ahead and changed plugs and wires. No improvement.

Compression test showed fine: 185,180.180,175

Vacuum gauge showed good numbers.

Fuel trims were good. Both ST and LT were in the range of +/-3. A few times when coming to a stop STFT would go to 18-20 for a few seconds.

Went ahead and check fuel pressure (good), injector resistance (good), and flow tested using pulse box w/ fuel gauge (all within 1 psi at final pressure).

Finally started thinking it was an intermittent low compression issue. I have heard of recessed valves on 2 posts online, but didn't think it was that big of a topic. Most of the Ranger forums have dead end threads for the same symptoms with either the user no longer responding or they got rid of the truck. Usually after about $400 worth of parts have been thrown at it (plugs, wires, coils, crank sensor, cam sensor, TPS, MAF, IAC valve, etc).

Did a dollar bill test the tail pipe. I've heard this can give false positives on newer engines with valve overlap but this simple SOHC engine should show fine. Dollar bill would periodically suck into the exhaust.Exhaust valve sticking?

Finally broke down and got on IATN and there are a high amount of techs who saw recessed valves.

Decided to pull the head this weekend and sure enough 1 and 2 have recessed valve seats, 3 and 4 were good. #1 was sunk in 0.030" and #2 was in 0.020". Goes along with the mode $06 data.

If it wasn't for the dollar bill test I probably wouldn't know where to attack. I don't have a scope so that prevented a lot of tests for me. I'm assuming it could show up on running compression test or do a static test immediately after a stall.

Theory of the failure and root cause analysis:

Truck misfires or stalls due to loss of compression on 1 or more cylinders.
Why? Exhaust valves aren't seating all the way.
Why? Hydraulic lash adjusters have pumped up during high revs.
Why? Valve spring force is less allowing HLA to pump up higher than normal.
Why? Valve height is taller allowing for less valve spring force.
WHy? Valve seats are recessed causing taller valve height.
Why? Valve seat material has worn away from high millage.





Cylinder 1 (Bad 0.030" recession) (maybe small crack in seat as well)


Cylinder 4 (good no recession)