This is the wonderful Christmas present I received:

So I have a 2003 Ford Ranger 3.0 which started setting off the check engine light Christmas day. It fired off the following codes (hereinafter referred to as "the same codes" as they never changed):

p0174 - system too lean bank 2
p0300 - random misfire
p0316 - misfire on startup
p0302 - misfire cylinder 2
p0305 - misfire cylinder 5
p0306 - misfire cylinder 6

The first thing I did was to change the spark plugs and wires. I installed duralast wires and bosch double platinum wires (gapped at .44). No change, continued to receive the same codes.

Knowing that the ignition coil pack is prone to failure I went ahead and replaced that. No change, continued to receive the same codes.

Read on the forum about the camshaft synchronizer wearing out over time. Even though the original was not making any faulty sounds I decided to replace it as well considering the part was fairly inexpensive. Installed correctly, aligned just right. No change, continued to receive the same codes.

At this time, since I had reset the computer, I had the truck out for a test drive. Although it was running a bit rough on idle, it became noticeably worse under load with some backfiring (even worse when going up a steep hill). Upon my return the catalytic converter coming from bank 2 was glowing brighter than a wino's nose on New Years (trying to keep within the holiday season). So I removed that converter (to rule out the possibility of a clogged converter fouling the system, though the converter on bank 1 was fine), reset the computer, and ran it again with no pipe attached. No changed, continued to receive the same codes. Went ahead and installed a temporary straight pipe (just to keep it quiet while working on it).

Checked the MAF sensor - appeared clean and intact. Checked all electrical connections - everything tested normal. Also went ahead and replaced the air filter (bit dirty, but not enough to foul the system).

Checked the electrical connection to the wires and spark plugs. After being zapped a few times it was determined that all the plugs were receiving juice.

Checked the fuel pressure from the fuel rail - 66psi, within normal limits.

Checked the timing - within normal limits.

Fearing that I improperly gapped the plugs I decided to install the bosch platinum +2 plugs (no gap required). I noted that there was some fouling from the bosch double platinum plug removed from cylinder 6, though the others were fairly clean. No change, continued to receive the same codes.

Removed and cleaned each of the fuel injectors. They were all opening and closing appropriately, and all tested within normal limits using a multimeter. Re-installed the fuel injectors and tested again. No change, continued to receive the same codes.

Checked the vacuum line (no EGR in this model), and it appears to be functioning appropriately.

Removed the valve cover from bank 2. Hoping, just really hoping, to find something wrong. Nothing. No bent rods, did not appear (at least from the top) that the valves were an issue (knowing that improperly seated valves is another issue with the Ranger).

Completed a compression test with the following (approximate, as I do not have the exact numbers off hand) results:

Cylinder 1 - 150
Cylinder 2 - 120
Cylinder 3 - 150
Cylinder 4 - 155
Cylinder 5 - 120
Cylinder 6 - 170

Although it was interesting to see that the highest and lowest results were coming from the problem cylinders, they were still within a limit that should not be creating a misfire situation (unfortunately, they will be something that I will have to address later though - once the misfire mess has been resolved).

Having ruled out the air intake system, the ignition system (spark), mechanical failure (the inspection with the valve cover removed) - we decided to swap parts around just to double check the parts themselves. Now things got a bit interesting. First off we removed each spark plug wire from the spark plug with the engine running. No engine change when removing plug wires from cylinders 2, 5, or 6; but noticeable change when removing from cylinders 1, 3, and 4 (lower rpms, some backfiring). Swapped the spark plug from cylinder 1 (known to work) with cylinder 2 - no change. Swapped the fuel injector from cylinder 6 with cylinder 4 - no change, which means that the injector in cylinder 6 was functioning properly. With the engine running I removed each of the electrical plugs for the fuel injectors - no change when removing them from cylinders 2, 5, and 6; noticeable change when removed from cylinders 1, 3, and 4. Now when I re-connected the electrical plug to cylinder 2 there was a slight change in the rpms! I disconnected it again and noticed a drop in the rpms! Tired the same with cylinders 5 and 6, but nothing.

I checked the ground on the pcm (mounted inside the passenger upper firewall). The ground was fine. We removed the cover from the wires going into the pcm, with the engine running we applied pressure and movement to the wires with no noticeable difference. Also with the engine running we tapped on the MAF and throttle body with the handle of a screwdriver - no change.

At this time we're thinking it may be the pcm. The one I removed was 3L5A-12A650-ME. In talking to Ford this was apparently updated to 3U7Z-12A650-FGA. So I ordered the pcm from a company in New York which will have it programmed using my VIN.

We're still not sure if this will do the trick. So while we're waiting for the computer to arrive I'm wondering if anybody has any other thoughts as to what could be causing this problem. We just picked-up an oscilloscope, and plan on checking to see if there's any interference going to the pcm. Apart from the computer and oscilloscope for further testing we are all out of ideas. Any thoughts and suggestions would be greatly appreciated! Thanks!

 

First, the lean code is from the misfires on bank 2, so it is the result of the problem not the cause, in this case.
O2 sensors detect Oxygen, not fuel, so they cause a Lean code if there is too much oxygen, not too little fuel.
When a cylinder misfires none of the oxygen is burned so.........too much oxygen in exhaust.
This is confirmed by your observation of the Cat running hot, it was burning up the raw fuel that came out of the misfiring cylinders.
So this meant you had plenty of fuel, there is no "lean" air/fuel mix, there is misfires.

This means misfires are a spark or compression issue.
You have replaced pretty much the whole spark system without changing the symptoms.
I would double check the coil firing order but outside of that your issue comes down to compression.

The 120 psi compression along side 150+ psi means bad valves or rings on #2 and #5, 3.0l engine in the early 2000's had known issues with recessed valve seats, so that would be the best guess, could be valve train issue, but valve seats would be my guess.
You could retest and add a tablespoon of oil to #2, compression will go up but if it doesn't come up to 150 or higher then it is a valve issue.

#6 misfire could be caused by the Lean code.
O2 sensor tells computer oxygen content, if oxygen level is too high computer opens injectors on that bank longer because high oxygen means not enough oxygen is being burned so more fuel is needed, when computer reaches its pre-set limit for injector open time(pulse width) it sets the Lean code to notify driver.
As the air/fuel mix gets richer it can cause other cylinders on that bank to misfire.


Not sure where you read 120 is within spec, when you are seeing 150+ on other cylinders.
Compression test is not about the actual number, this is why a compression test must be done on all cylinders at the same time with the same gauge or it is of no value.


Computer would be a long shot with the compression issue.


This is the TSB for the '04-'06 3.0l, but I believe it was expanded to earlier 3.0ls back to '01
TSB 05-26-3

FORD: 2004-2006 Ranger

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
1. Perform a power balance rest to clarify if a cylinder is misfiring. Identify and keep record of any cylinder that has a concern.

2. 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.

3. 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.

4. 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.

5. 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, 303-00.6.

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.

PART NUMBER / PART NAME
6U7Z-6049-A / Cylinder Head Assembly
2F1Z-6079-BB / Upper Gasket Kit
F8 DZ-6065-AA / Cylinder Head Bolts
F7DZ-9448-BB / Engine Exhaust Gasket
4F1Z-658-AA / Valve Cover Gasket
F6DZ-9439-C / Side Gasket
F2DZ-9A425-AA / China Gasket (Front)
F3DZ-9A424-BA / China Gasket (Rear)
4L5Z-9H486-AA / Gasket-Upper Intake Manifold

 

Thanks Ron! That was our thought as well - one of the valves being recessed. When I pulled off the valve cover gasket I was really hoping to see the valve stems from the misfiring cylinders (#5 & #6 - didn't pull the cover off bank 1) being higher than the properly functioning cylinder (#4). Granted this was just a visual, I was only going to pull the head if it looked like there was a problem (and by that point I was really hoping it would have been a problem as I was running out of ideas).

As for the compression based on the calculations all cylinders should have been above 144 psi. We temporarily ruled out the compression for the time being (thinking it was another problem all together and not one contributing to the misfiring), but that again was based on the visual inspection of the valve stems/rods/rockers (all of which appeared intact and undamaged). If the computer doesn't work I think I'm going to go back in your direction and do a wet test on the cylinders, or just jump right into the leak down test.

What are your thoughts as to the #6 having the highest compression at 170 psi (in comparison to the properly functioning cylinders averaging 150-155 psi)? Do you think it's just a coincidence or a symptom? Thanks!

 

3.0l has 9.3:1 compression ratio, rule of thumb for stock gas engine is to use a multiple of 18, so 9.3 x 18 = 167.4psi
Some use 18.5.

My 4.0l has 9.0:1 and tests out at between 160-170 per cylinder, 9 x 18 = 162 so fairly accurate multiplier, I am at sea level.
9 x 18.5 = 166.5 so maybe better multiplier for my engine and in general.

Using the math formula for compression ratio to actual PSI has never really worked for me and the actual results in the real world, and that's been for over 20 years of wrenching, lol.
So I like the 18, seems to work in the real world.


OK, now for the "misfire" codes.
The computer gets the engine RPM from the CKP(crank position) sensor.
It has a #1 TDC mark and then "marks" equally spaced around this "tone" ring.
Computer uses these other marks to determine power added or power lost on the rotation of the crank shaft.
If a cylinder misfires completely there is power lost and spin speed on crank changes.
We all know that higher compression engines produce more power, because of the higher compression, so it is also true that lower compression equals lower power.
A cylinder with lower compression than the others will produce less power, which can also show up as a misfire code.
So a misfire code can be there even if cylinder is firing most of the time, but under load the lower compression cylinder is not adding the expected amount of power to the crank spin, this is a misfire as far as the computer is concerned.


170 in #6 could be for several reasons, could be it has a leaky valve guide seal so better seal on the rings during the test, could be fuel from injectors, if they weren't turn off, sealing the rings better.
Could be that's the only cylinder with correct compression, lol.

I doubt you would see any difference in valve stem height if a cylinder still had 120psi compression, the valve isn't "tulipping" the seat is down on one edge so valve height wouldn't change.
Recessed exhaust valve seat will come and go at first then just come and stay, but as exhaust valve doesn't seal completely hot gases escaping will start to wear a larger gap.

Leak down won't tell you any more than a spoon of oil, BUT, it can be helpful to pump air into a cylinder and listen at intake and exhaust to hear air escaping

 

Thanks again Ron! I'm going to try the oil test tomorrow morning or friday after work. Now I'm a bit concerned because you may actually be right - that the #6 with 170 is the only cylinder at proper compression!

After reading the TSB you posted - is it possible to have the heads currently on the truck machined with new seats, or would it be cheaper to purchase new reconditioned heads from a place like ebay?

 

I doubt redoing the seats would prevent it from happening again.
You could call a machine shop, or a few, and ask first if it would last and then how much to do it.
It could have been a batch of bad seats made it into these heads in the early 2000's, Ford is never forth coming about why, and they can be downright **** about admitting fault...................much like the rest of us, lol.

A popular machine shop should have repaired or replaced many of these by now so they would have a better idea of the best path.
Also ask them replacement cost, machine shops can often get good deals on heads or can get used heads and rebuild them for less that you would pay for a set of heads off ebay when shipping is added, heads weight ALOT, and you don't know for sure what you will get.