cylinder #3 & #6 misfire - Ranger-Forums - The Ultimate Ford Ranger Resource

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Old 01-10-2015
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cylinder #3 & #6 misfire

I have a 98 ranger 3.0 4x4 that is misfiring. Took and had it put on a computer at the local mechanic and it's showing misfire on cylinder 3 and 6 as well as lean on both o2 sensors. I have changed the coil, wires, plugs, injectors, and the computer and still having the issue. The mechanic claimed the injectors were the problem and it wasn't. Does anyone know where else to go with this?
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Old 01-10-2015
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It is showing misfire on both Rear cylinders, of a V6, I hate coincidences.
My first guess would be vacuum leak on rear of upper intake or lower intake.

Second guess would be Cam position sensor(CPS) 3 and 6 are next to each other in the firing order
Firing Order 1-4-2-5-3-6
CPS runs the injectors, if it has a bushing going out 1/4, 2/5, 3/6 could be effected as a pair, as could 4/2, 5/3, 6/1.
CPS on 3.0l lasts for 80 to 100k miles

What was the Fuel Pressure?
'98 should be 65psi

With the Lean codes above would be my best guesses.

Lean code on both banks could also be MAF(mass air flow) sensor issue.
The way a computer decides how much fuel to add and when to set a Lean code starts at the MAF sensor.

At 1,500 RPMs, XX air is passing thru the MAF sensor, computer sets 0 Fuel trim based on XX air, 0 is 14:1 air fuel mix
Computer opens fuel injectors YY length of time(the pulse width) for XX amount of air
O2 sensor shows Lean(too much oxygen in exhaust)
Computer adds more fuel, say +5, so injectors open longer, YY+5
O2 still shows Lean
Computer opens injectors YY +10
O2 still shows Lean, ect.............

When computer gets to +20 it will notify Driver by turning on the CEL(check engine light) and set the Lean code
XX amount of air is using too much fuel so something isn't right.
If there is an air leak/vacuum leak then all the air coming into the engine isn't going thru the MAF sensor, so MAF data is wrong, so 0 starting point is wrong.
If MAF is dirty it can under report the amount of air, so 0 is wrong.
If fuel pressure is low then injector open time, YY+20, is wrong, not enough fuel is spraying out an injector at 30psi when computer is expecting 65psi

Last edited by RonD; 01-10-2015 at 08:52 PM.
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