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2000 3.0l misfire under load

99supercab

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Hello everyone I just bought another 3.0ranger and under load it has a constant misfire. I just put new plugs and coil pack on it today and it does the samething.Any help would be greatly appreciated thanks in advanced.
 


RonD

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Are there any codes?

Is it a single cylinder misfire, steady miss, or is it a random(multi-cylinder) misfire?

What did the old spark plugs look like, did one show the misfiring?

When engine is under load a lean cylinder can misfire, so dirty injector, this can also cause pinging/knocking noise.
Fuel demand also goes up under load so low fuel pressure would start with misfires and get worse as RPMs and demand increased.

EGR valve opens only when engine is under load, this helps cool the cylinders but can cause missing if it opens too much for the RPM/load, which could be a DPFE sensor issue or one of its hoses, this would usually cause multi-cylinder misfires.

I think your 2000 year falls in that range of the heads that had bad exhaust valve seats, they could recess and cause misfires, and it was a floating seat issue so could come and go.

The 3 basics apply
Spark, at the right time
Fuel, in the right mix
Compression, above 150psi

Lack of one causes misfire
 
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99supercab

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I disconnected the DPFE and the vacuum line to the egr valve and it did the same thing. It has two pending codes p0171 and p0174 lean on both banks.the plugs looked good no fouling or discoloration.
 

RonD

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Lean codes could be from the misfires, as air is just pumped thru the cylinder if there is no burning, O2 sensors measure Oxygen in the exhaust not fuel, so unburn fuel goes to Cat to be burned off.
Probably multi-cylinder misfires if both banks show lean.
And the lean mix will also cause the misfires, so it could be a cause or an effect, lol.

The lean codes mean the computer has tried to richen the fuel mix but O2s are still showing lean, so the code is set.

The computer sets a 0 fuel trim based on MAF(mass air flow) sensor, the MAF data reports the amount of air coming into the engine, the computer uses that data to set the 14:1 air:fuel mix(0 fuel trim), the O2 sensor then reports if that mix has too much or too little oxygen left after it is burned.

So it all starts with the MAF sensor data, that sets the 0 trim marker, a dirty MAF sensor will under report the amount of air so engine is running lean even though computer thinks it isn't, it adds more fuel based on O2 sensor data but it has a pre-set limit, when that is reached it will set the lean code.

I would clean MAF sensor and inspect it's air tube to the intake, also PCV hoses and larger vacuum lines.
Any air entering the engine without passing thru the MAF sensor means computer is using the wrong 0 trim for fuel mix.

Lower fuel pressure has a similar effect, the computer can not monitor fuel pressure, the 0 trim air/fuel mix is based on computer opening a fuel injector for XX milliseconds based on 65psi fuel pressure, if fuel pressure is low then less fuel comes out, O2 reports lean computer opens injectors longer until it reaches its limit and sets the code.


And just as a heads up, the Ford DPFE sensors are only good for about 70k miles, non-Ford replacements are cheaper and tend to last longer.
The wrong EGR flow will cause misfires under load.
 
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99supercab

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Fuel pressure KOEO is 46psi and I just cleaned the MAF the one resistor was completely black. I also unhooked the battery to reset the PCM. Short term fuel trims are running around 20%. I'm going to go see if I can see if there are any cracked or broken vacuum lines. The DPFE was replaced a year ago from the receipts left in the truck.
 

RonD

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2000 3.0l has returnless fuel system so needs 65psi fuel pressure, I would check it with engine idling and then at 2,000 RPMs for 1 minute.
The fuel pump on the returnless is the only fuel pressure regulator there is, so low pressure usually means fuel pump problem, although for $10 changing the fuel filter wouldn't hurt.

1997 and earlier used return line and FPR(fuel pressure regulator) on the fuel rail, these ran at about 40psi.

Yes, high fuel trim(+20) would mean computer is trying to compensate for an O2 lean reading.

As said 0 trim is based on MAF sensor data, computer then switches from -5 to +5 by adjusting the open time on injectors, and watches the O2 sensors response.
Short term trims(STFT) should change rapidly, that is the computer trying to get the best MPG.
At idle +5 to +10 is not unusual, idle should be rich.
+20 means computer has to set a lean code, that is about the limit, although I have seen +25 on live scanner.

-20 would mean rich exhaust and code
 
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99supercab

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It has 60psi I'm going to get a fuel filter tomorrow at work and see if that helps any.thank you for the help.
 

RonD

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?? what was the 46psi??

If you have 60psi at 2,000 RPM after 1 minute you should be fine.

Look at the air side of the equation.
 

99supercab

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It had 46psi when you primed the system before starting it.
 

RonD

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That is probably OK, but I would expect no more than a 10psi drop after vehicle sits overnight.
And if key was turned on once before testing it should have been back to 65psi for starting.

Fuel pump has a check valve(back flow preventer), fuel can flow out of tank but can't come back when pump is off, these valves can leak or fail, reducing fuel pressure when running and loss of pressure when vehicle sits.
 

99supercab

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I left the gauge on it I'll look at it in the morning and see how far it dropped
 

RonD

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I wouldn't leave the gauge hooked up that long, you wouldn't know if it was the check valve or the gauge that was leaking, lol.
 

99supercab

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Ok I'll pull it off when I go back out.
 

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