2.5L ('98-'01) 2.5 P0303 I can’t seem to shake!

[DCarroll597]

Alright hopefully someone in here might be able to point me in the right direction.

2000 Ranger 2.5L
190k
Chasing a cylinder 3 misfire. Only throws P0303.

Replaced all these parts in the past 4/5 weeks;
new plugs all around
New plug wires
Two new motor craft coil packs
New air filter
New MAF
New IACV
New TPS
New DPFE sensor
New cam position sensor
New 02 sensors (upstream and downstream)
New gaskets at throttle body, intake manifold, exhaust manifold, valve cover, egr.
New timing belt and tensioner.
New thermostat, housing, and sensors.
New fuel filter
New injector at cylinder 3

compression at all cylinders above 165 psi
Passed leak down test with flying colors (to the point where I’m almost certain this is not the factory head. Put 70psi in the cylinder (multiple) and they didn’t leak AT ALL, could that be indicative of an issue? I know 8-10% leakage is typical)
Valve train looks damn clean (no broken valve springs, sludge, or anything like that)
Put a borescope in cylinder through exhaust ports and can’t seem to see any valves stem seals leaking or valves burnt up.
Vacuum gauge reads 16lbs/vacuum.
Fuel pressure 65psi at the shrader valve on the fuel rail.
Exhaust back pressure test ruled out bad cat.

I feel like I’m so close to fixing this, but I have no clue where else to go with parts or tests. Mind you, I’m about the worst mechanic you’ll ever meet. I’ve heard of the PCM failing on these, but I feel like it would cause other issues too? Not just the consistent P0303. The truck runs pretty rough when cold, then better once it’s warmed up. Will still on the occasion stumble and sometimes die when coming off the pedal into a stop sign/light.
Been trying to monitor live data while driving and idling, but honestly don’t really know what I’m looking at vs what I should be looking for. Could it be something with the egr control purge valves? I’m sensing it’s vacuum/air related because at one point it threw me a P0171 too lean code (only popped up once, cleared and hasn’t come back, but the P0303 did.) Could it be EGR related? I’m banging my head into the wall at this point hahaha.

can anyone maybe point me in a direction that might help me out? PLEASE AND THANK YOU

 

[franklin2]

Can you really feel the engine has a miss-fire? It should be dramatic on a 4 cylinder. Or is this the computer playing games?

 

[DCarroll597]

Can definitely feel the misfire, I just pulled both spark plugs out of cylinder 3 and looked pretty lean. I’m kind of leaning towards injector now. It is worth noting that it’s a brand new duralast injector, I have heard of these being shit right out of the box. Going to switch it with injector 2 and see if the misfire moves. Wish me luck I don’t have to keep yanking this intake manifold off hahahaha

 

[DCarroll597]

Also have not exactly ruled out a bad PCM, not even sure how to test it

 

[franklin2]

I asked google AI if your engine was batch fired injection, and it said yes. If we go with that, then the PCM is ok, since it fires all the injectors or at least two of them at the same time.

 

[DCarroll597]

Alright so just got back from a test drive, swapped fuel injectors and still got the same pending P0303. Any ideas? Could a bad IAT sensor cause this? I noticed my idle air temp on the scanner said 89 degrees then slowly rose to about 100 degrees. (Mind you the outside air temperature is mid-low 60s). Could this cause a slight misfire at cylinder 3 if the sensor is bad? I noticed longer think my issue is fuel related, but more than likely air or a sensor at this point. I will say, truck runs damn good afternoon about 20ish minutes of driving, but still can get it to stumble after a rev, but then will find its idle no problem after a few stumbles. I can almost taste the fix I’m in need of. Aaaaggghhhhhhh

 

[franklin2]

Is there a PCV line near #3 cylinder or intake runner? Or any other type of port near there that could introduce air into that cylinder? You said you have had the intake off many times. The gasket surface is perfect around #3? Check it with a straight edge?

 

[DCarroll597]

PCV is right next to cylinder 2, so doubt that would cause it. Pretty sure the gasket for intake manifold is fine. I do have another new one in the garage, so probably wouldn’t hurt to pull the manifold again and maybe hit a sanding stone with it? I’m kinda lost at this point.

 

[DCarroll597]

Alright, even though I'm getting no replies I figure I'll still update should anyone else have the same issues.

Replaced both upper and lower intake manifold gaskets and truck ran much much better. Took on a good test drive and checked codes and had a pending P1405, DPFE upstream hose clogged or unplugged. Sure enough, my stupid self didn't use high temp lines on the new DPFE sensor, so the upstream line turned into glass haha. Replaced with Dorman high temperature lines, code went away and hasn't come back. However, I am still getting the ever so slightest stumble when cold, and sometimes when coming out of gear towards a stop light/sign. It's seriously slight to the point where it'll stumble for maybe 3-8 seconds and then find it's idle no problem. Could this be a bad IACV? The one I have is a brand new Standard IACV, but we all know how new doesn't necessarily mean good. Should I seek out a motorcraft IACV? I've also heard of people getting around 18" of vacuum, and mine measures around 16", could this be indicative of a super small vacuum leak? I have yet to smoke test it, cause I don't have means to. But, DIY smoke machines sound like a fun little endeavor so I might head down that rabbit hole here shortly. I feel like I'm soooooo close to making this truck run tits again. Anyone out there have similar symptoms they've found a remedy for? PLEASE AND THANK YOU! And shoutout @franklin2 for throwing out suggestions and giving me other avenues to pursue! Anyone else please feel free to chime in and tell me I'm a retard. Thanks gentlemen! Love and respect.

 

[SenorNoob]

Honestly could be either one. Did you just clear codes or leave the battery unhooked long enough clear the adaptive memory. It'll get ya on things like that. Especially if it's trying to used remembered values from a leak or the old IAC. Also when I do clear the adaptive memory on mine it take a week of driving (only about 100 miles with a dozen or so starts) to get it to run right again.

 

[DCarroll597]

I did both. Cleared codes with scanner, and reset PCM by unhooking negative and jumping it to positive for around 10 mins. I drove it again today and symptoms continued, no worse than before, basically the same. I'll keep driving it and see what comes of it, and update this thread accordingly. I'm really leaning towards a Motorcraft IACV, because at one point I unplugged the current one and had no change in idle (and from my understanding when unplugged, it should drop idle and potentially stall out) So something is telling me that the Standard Motor IACV I currently have isn't working properly. Anyone else ever run into this? Thanks for the reply, @SenorNoob, much appreciated!

 

[SenorNoob]

If you unplug the IAC and the idle doesn't drop, the engine is getting air from somewhere. Could be a vacuum leak, stuck IAC, stuck throttle plate, or somebody has turned the idle screw in too far...

 

[franklin2]

If you unplug the IAC and the idle doesn't drop, the engine is getting are from somewhere. Could be a vacuum leak, stuck IAC, stuck throttle plate, or somebody has turned the idle screw in too far...

Yes, getting air from somewhere.