Odd misfire 2002 3.0 Ranger, please help

[alucard0822]

I've been chasing an odd misfire for weeks, have never had this much trouble finding the source, but this one is beating me up, and wanted to see if anyone could help. I was a tech at a couple independent shops from 95 to 05, got out of the shop and took a desk job ever since, pretty much only working on my own vehicles, but still have all my tools and test equipment, and access to Mitchell, along with the Torque pro app and Bluetooth OBDII adapter, but been awhile since I turned wrenches for a living, and am getting to the point of chasing my tail.

My truck is a 2002 Ranger Edge 2wd extended cab, manual trans 3.0(U) with 235K miles, babied and bought used in 04 at 30K. I only put about 10K mi a year on it now, all trips are long enough to warm fully.

The problem is what feels like a misfire under light acceleration at low RPMs, under 2K, with steady throttle and speed it will stumble and miss for a couple seconds, then run fine. There is no misfire at idle and higher RPMs, no loss of power, no loss of fuel mileage, no CEL, no active or history codes. It only happens once the engine gets to temp, and outside temps are around freezing or below. It only happens after coasting in gear at low RPMs, the longer you coast the longer it will misfire once I start accelerating, does not seem to matter how much I accelerate as long as the RPMs stay under 2K. A long gentle downhill coast followed by light 5th gear acceleration will misfire for about 5 seconds, this is about as bad as I can get it to act. It started a month ago, got progressively worse throughout a week, but now seems pretty consistent.

I have done a fair amount of work in the last year or two to it, the problem didn't happen along with any particular job.

Clutch, master/slave cyl, rear crank seal, trans reseal

cam synch and sensor using O.E. alignment tool

front cover leaking coolant externally, resealed cover, replaced timing chain, gaskets/seals, water pump and hoses.

New belt, tensioner assm, and idler pulley

fuel filter, IAC(was sticking closed) decarb with seafoam through intake, clean throttle and MAF

coil, wires, plugs, suspected ignition miss, old plugs were worn a little, wires were old, coil was original, didn't make a difference. Tried swapping a new MAF, suspected the intake temp component was bad(problem only occurs when really cold) didn't make a difference

Checked all vaccum lines and connections, replaced a couple old hoses, ziptied possible loose connections, checked for leaks with carb cleaner, none found, smoke tested, none found.

Fuel pressure test, jumped right to 62psi, and held steady while running, measured at the rail valve.

compression test, pumped up quick to around 160 on all cylinders and held pressure dry. Vaccum gauge holds a steady 19# At 750rpm idle and 2k RPM fast idle, jumps quick to about 24# when snapping the throttle closed



No CEL, no codes, O2 sensors appear to be switching normal(they are original), TPS graphing good, only thing that the torque app is showing odd is a long term fuel trim around -5% to - 10% on occasion. At this point I'm just about out of ideas. The only thing I suspect between the symptoms and slightly off fuel trim is a leaking injector loading up fuel during engine braking, then misfiring due to excess fuel till it clears in that cylinder under acceleration. Of course if I engine brake and roll to a stop, it idles fine, and only occurs under light/low rpm accel when taking off. Without knowing which cylinder/s it is happening on, It would be a $200 guess to replace all the injectors, and I'm not that confident that is what it is.

So long story short, please help

 

[HMFM Miller]

Im having almost the same exact problem with my 03. Ive swapped the same list of part as you and I also dont have codes. Im at a wits end. Ive though about pulling the injectors and cranking it see if they leak.

 

[alucard0822]

That's the point I'm at, although if I go through the trouble pulling all the injectors, I have enough mileage I might just replace them. I'm hoping I can at least make it to spring to dig into it, or at least whatever the problem is gets worse and easier to find.

 

[HMFM Miller]

I swapped out my injectors. Not that bad of a job, just have to remove the upper intake manifold. However did not fix my problem. Still misfiring. I tried to call a shop today and explained all I did to it and they said, "well Im not sure then. Youve done everything we would." So Im lost as well. Im leaning more towards a valve/rocker arm issue now.

 

[alucard0822]

Seems to be misfiring more now, doesn't need to coast in gear as much to misfire either, and does it when taking off from a stop. The bad thing about eliminating all the easy stuff is the expensive and tough jobs it leaves. Looking less and less like a simple ign/fuel issue, and more like a valvetrain issue for me too.

 

[RonD]

There is a Ford TSB about the 3.0l heads in the early 2000's.
Valve seats were coming loose causing misfires, that could come and go but would get steadily worse.

 

[alucard0822]

Looked into it, mine is an early 02, so doesn't fall under the TSB. I did a compression and vaccum test recently, and both were normal, so even if it did fall into that year, doesn't have the symptoms of it.

I did run a fuel volume/pressure test last night(both in spec), replaced the fuel filter while I had it off, and re-ran a vaccum test and leak check, all stemming from an occasional blip in short term fuel trims while driving, they were heading up near +12 on the datalogger, a little elevated, but not enough to cause a misfire IMO. I got a new OBDII Bluetooth adapter, and got a new app capable of mode$06(have been using torque pro), plan to check again after work, so hopefully it ends up being helpful. I'm really hoping it is something simple and embarrassing I overlooked instead of something expensive at the bottom of the "causes of misfire" list. The main issue that has me worried is that it runs fine till it reaches operating temp, often a sign of worn rings and low compression, but also could be heat affecting something not as expensive, will try re-doing a compression test with the engine hot as I can get it.

ISSUE:
Some 2004-2006 Rangers 3.0L-2V "Vulcan" vehicles may exhibit:

Malfunction indicator lamp (MIL) on with diagnostic trouble codes (DTCs) P0300-P0306 and P0316
Rough running at various RPM
Rough idle
Misfire
ACTION:
This may be due to an engine exhaust valve seat recession causing a loss of compression. Refer to the following Service Procedure to diagnose exhaust valve seat recession.

SERVICE PROCEDURE
Perform a power balance test to clarify if a cylinder is misfiring. Identify and keep record of any cylinder that has a concern.
Perform a manual compression test with the spark plugs removed to locate any cylinder that may have low compression. Refer to the appropriate Workshop Manual, Engine System, General Information, 303-00.
If no low compression is found per shop manual instructions, proceed with normal misfire diagnostics per the Powertrain Controls/Emissions Diagnosis (PC/ED) manual for the appropriate model year.
If low compression is found and it is a cylinder that had a misfire on power balance, perform a leakdown test to determine the source of leakage that may be causing the low compression. Refer to the appropriate Workshop Manual, Engine System, General Information, 303-00.
If low compression is found in a cylinder that did not have a misfire detected then proceed with normal base engine diagnostics per the Workshop Manual, Engine System, General Information, 303-00.
If the exhaust valve is identified as the source, replace both cylinder heads with the newly released service cylinder heads. Refer to the appropriate Workshop Manual, Section 301-01A.

 

[alucard0822]

WOOT!!!! got it.

The mode 6 capable OBDDIAG app I downloaded for 99cents paid for itself 100 times over. Torque is a lot easier to work with, and prettier to look at, but doesn't have a mode 6 raw misfire display. Takes 13000 misfires in a cycle to set a code, had about 10000 on 1 and 5 and about 100 on 3, just under the number needed to set a code, but plenty to feel at low RPM. Swapped plugs and wires on 1 and 2, no difference, got a bit worried, figured I would backprobe the coil and check voltage/frequency, coil is fairly new, but I suspected an ECU or wiring issue, and found it, moisture in the connector, corrosion on the 1/5 coil pin(3 coils each runs 2 cylinders) to the ECU. Just enough resistance to mess with it apparently, cleaned it, put dielectric grease on it, and there it was, misfire cured, turned out #3 had a small burn spot on the plug wire where it was laying next to the #1 wire, ran plastic tubing around it to isolate them, test drove it, and now the misfire count on all cylinders is 0. Luckily enough it was a free fix, but damn was it tough to find.

To top it all off, my parking brake cable snapped when I parked it after the successful test drive, simple replacement, but between the tight rusted cable ends, and laying on the frozen ground pulling hard as I could to get it in the rear coupler, it soured the earlier victory a bit, but good news, my Ranger is back in business.

 

[RonD]

Thanks for the update

Good work finding that issue