3.0 flex fuel engine missing problem.

I have a head scratcher...my 4x4 2000 ford ranger 3.0 ff vin V is missing like crazy. It has no knocking or pinging. This is the second engine I have put in this truck and both engines have displayed the same problem. I replaced the old engine due to a rod bearing spun. The new engine was low milaged. It ran smooth as silk when I first installed it, but 200 miles later, the same old problem came back. If I clear the codes, it is good for another 200 miles. The codes displayed reveal a misfire code and a camshaft position sensor code. It also had a wiring fault code for some circuit and O2 sensor codes (yeah, a mess I know). I compression checked bank 1 and 2 and it read as follows...Bank 1-90,120,90 Bank 2-120,120,120. The code scanner revealed that the misfires were occuring on the cylinders with lower compression. No suprise there. I assumed a head problem/valve prob. I didn't do a wet compression test or a cylinder leak down test, but I am pretty certain it is in the head. Now the puzzling thing...why in the world would clearing the codes make it run perfect for 200 miles? Also, I assume since this engine doesn't have variable valve timing or anything, then there is no timing issue that can cause this...other than a slipped timing chain...right? Could anything throw off valve timing enough to cause compression loss? I checked it running by the way. Anyways, just wondering why clearing the codes causes it to run perfect for a short time and if any electrical controled timing components exist to cause it to have delayed or advanced valve opening/closing. Sorry for the book long question, and thanks in advance for any responses.

Forgot to mention...

When it starts missing, I can push the gas down and it will smooth out at 3000 rpms. It just won't run good at low rpms.

Clear the codes and you need to get CPS code cleaned up, adjust the CPS to specs and see if that helps, it is low on compression on 2 cylinders, could easily be a valve seat moving in the head. Next work on the O2 code as it could be causing a lean or rich condition. I doubt if you have a timing chain issue, more of a CPS adjustment issue.

JP02XLT

cowboycowboy91 said:

why in the world would clearing the codes make it run perfect for 200 miles?

Click to expand...

You also cleared memory so the ECU is using factory defaults until the fuel trims and sensor readings learn from actual operation.

The 2003-2006 3.0l heads had exhaust valve seat issues, lowering compression, do you know the year of the newer engine?

Yes, as said, resetting the computer would cause factory fuel trims to be used.

O2 sensors see OXYGEN levels in the exhaust, not fuel
Too much oxygen is Lean
Too little oxygen is Rich

Computer adds more fuel to a bank if O2 says Lean, and reduces fuel on that bank if Rich.

What happens with a misfire is that none of the oxygen is burned in the cylinder, it is all dumped into the exhaust and O2 "sees" Lean, computer adds more fuel to that bank.

With lower compression a cylinder can usually ignite a leaner mix, but not a Richer mix.

So as the reset computer starts to relearn fuel trims the once in awhile misfires from low compression becomes worse and worse because computer is adding more fuel to that bank causing more misfires, which adds even more fuel.

The O2 codes are a result of this, computer has a limit of how much fuel it can add, computer reaches its limit and sets a code that O2 still shows Lean even after extra fuel was added.

Posting the exact codes would be helpful, because the codes that are NOT used by the computer are often as or more important than the codes it does use.


The compression numbers are low overall but that could be the gauge or the method of testing.
At sea level to 2,000ft elevation
cold engine
All spark plugs removed, to get fast cranking speed
Good battery, again for crank speed
Throttle held open, foot pushed down all the way on the gas pedal when cranking, this also shuts off fuel injectors
3.0l should run above 150psi

But your test is fine, it shows 1 and 3 are more than 10% off of the other 4, which is the point of a compression test.

No on valve timing, you could pull the valve cover on bank1 and check rockers for 1 and 3


CPS(cam position) sensor is important to fuel delivery, Ford was changing from a 3-wire CPS to a 2-wire CPS around your year.
These two sensors are not interchangeable, if computer is programmed for 3-wire sensor then it won't understand 2-wire, and visa versa, there is no "wiring" fix.
3-wire is Hall Effect
2-wire is Variable Reluctance
Totally different signals going to computer

RonD said:

The 2003-2006 3.0l heads had exhaust valve seat issues, lowering compression, do you know the year of the newer engine?

Yes, as said, resetting the computer would cause factory fuel trims to be used.

O2 sensors see OXYGEN levels in the exhaust, not fuel
Too much oxygen is Lean
Too little oxygen is Rich

Computer adds more fuel to a bank if O2 says Lean, and reduces fuel on that bank if Rich.

What happens with a misfire is that none of the oxygen is burned in the cylinder, it is all dumped into the exhaust and O2 "sees" Lean, computer adds more fuel to that bank.

With lower compression a cylinder can usually ignite a leaner mix, but not a Richer mix.

So as the reset computer starts to relearn fuel trims the once in awhile misfires from low compression becomes worse and worse because computer is adding more fuel to that bank causing more misfires, which adds even more fuel.

The O2 codes are a result of this, computer has a limit of how much fuel it can add, computer reaches its limit and sets a code that O2 still shows Lean even after extra fuel was added.

Click to expand...

Yeah, that makes sense. The engine was a 01 or newer. I seen the truck it came out of and it had 120,000 miles and had been wrecked pretty bad. The engine was clean and no leaks. The truck had the newer body style. The cps was a 2 wire, and so is mine.

Hard to say why you would have 2 engines act this way though...............

Did the other show low compression as well?

The low compression in 1 and 3 are absolutely the reason for the misfires, and nothing but a physical mechanical failure can lower compression.
So nothing transferred over off original engine or original truck wiring can cause that.

To be honest the exhaust vavle needs harden vavle seats especially flex heads might not want to drive it daily till fixed. If not fix it can cut your head or vavle like a plasma cutter not good at all.



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Would it hurt it beyond repair to keep driving it?

I have put a little over 6000 miles on it running this way. It is currently my daily driver. I understand I need to fix it, but I really need it running right now. Will it completely destroy the heads to keep driving it this way for a month or two? I will start looking for another daily driver so I can work on it.

Really depends on what's wrong with the engine causing the low compression.

If it is recessed exhaust valve seats then you should be fine, but...........
It will lower MPG and possibly cause early O2 sensor issue/failure on that bank.
Cat should be able to handle the richer exhaust, Cat might run warmer, so you may notice more noises from it after driving and then parking.

Just keep resetting computer to keep air/fuel mix leaner.