Please Help 4.0 ohc Missing running like crap

[Firmstrokes]

I have a 94 Ranger 5 speed manual 4x4 4.0 OHV. I have replaced the following parts.. Fuel Pump and Relay checked ground and power wires to pump. Fuel Filter. O2 sensors "all" plugs and wires. cleaned mas air flow sensor replaced tps and cleaned the Iac... This son of a gun runs perfect when you first start it. after ooh 10min or 2 hours at times it starts to stumble and act like it dropped a cylinder.. has 130k miles

 

[RonD]

Did you do any tests before replacing parts?

What was fuel pressure before and after fuel system work?
Have you checked Fuel Pressure regulators Vacuum hose for fuel?

Have you pulled out the spark plugs and looked at each one's tip to see if it is running rich or lean or has oil on it.

Next time it starts to stumble unplug the MAF sensor, this will put computer in Open Loop and it will use factory air/fuel mix not "sensor" derived air/fuel mix.
See if that helps or hurts operation

 

[ratdude747]

Any check engine lights?

 

[adsm08]

Why is "all" in quotes like you didn't actually check all of them.

Most of your tests so far have been going in the wrong direction. Aside from the plug wires none of that should affect a single cylinder.

Pull the plugs and see what those tell you.

 

[Firmstrokes]

"All" would refer to me checking and replacing ALL of the plugs and wires, and O2 sensors. Plugs looked good to me they are light brown not black or pure white on the tips. It was about 2 years ago I did wires and plugs so I threw a set at it as well.. not very pricy.. I have unplugged the fuel pressure regulator and no fuel came out of the vac. line. fuel pressure is at 35-45 lbs ,each port on the coil pack seems to be firing and firing order has been checked and double checked...whats confusing me is it doesn't give me a hard fault on a cold startup it ALWAYS runs 100% right at first.. I have unplugged the maf and stays the same .Im wondering if its the injectors or a connection to them?

 

[RonD]

Does your 4.0l OHV have a Cam Position Sensor(CPS)?
It would be in the distributor hole at top rear of block, behind lower intake.

Most 4.0l's didn't have them in '94, but if you have one going bad it can cause odd operation.

4.0ls without a CPS usually run Batch Fire fuel injection, on a V6 this means that 3 fuel injectors open at the same time, then the other 3 open the next time.
When engine is cold there would be extra fuel in the intake, Choke Mode, as engine warms up less fuel is added so if 1 injector was clogged/broken then it's cylinder could get too lean to fire all the time.
Batch fire relies on running engine air flow to circulate the fuel mixture to all cylinders.
If you were to unhook the coil for a no start, and then cranked the engine over a few times, only the cylinders with working injectors should get fuel inside the cylinder.
Then pull out the spark plugs and check the tips for fuel, if you find a dry one then that cylinders injector would be questionable.

Evanston, Wyoming is at almost 7,000ft elevation
Computer runs cold engine with a Richer mix, Choke Mode(factory air/fuel mix), as engine warms up O2 sensors take over and computer will start to use Oxygen levels in exhaust to fine tune air/fuel mixture.
It reads like when this switch to MAF, Computer, O2s only occurs is when you start to have trouble.

1994 is when Ford got rid of Barometric Pressure(BP) sensors with MAF sensor, they figured the MAF sensor can figure out the BP by the thinner air not cooling off the MAF sensor's wire as much as thicker air, the engine RPM(Crank Position Sensor) could be used to in that calculation assuming a good baseline from factory.
And then the Intake Air Temp sensor would also be used, as cooler air skews MAF air flow.

I assume the Ranger has been running fine at this elevation until just lately, so not just purchased and then you found this problem?

 

[Firmstrokes]

This is a recent problem with it missing. Other than that it runs like a top only issue I have had with it in 3 years is, i vapor locked it in Moab this year EJS. Really slow speeds on HOT rocks, i let it cool down no issues till this missing bs.. I replaced the fuel pump because i vapor locked it and was showing a high draw for the pump from the computer i don't recall the code.. also had codes for both upstream o2 sensors so what the heck there 23 years old. I don care about the $$ really the places i take this thing i need it to run 100% a 100% of the time.. spare parts are cool too!

 

[Firmstrokes]

And no it doesn't have a cam position sensor there only connectors back there are for the o-2 sensors. i have replaced the crank sensor on the frt. by the dampener also. lol
Thank you for taking time out im at a loss. i don't want to buy stock injectors because i have a m-90 eaton blower that's going on top this spring. so i would be buying another set for that..

 

[Big Jim M]

When it is up to temp and running badly pull the wires one at a time to see if a particular one gives a different result in idle than the others.
That should tell you it is either THAT injector or if you have a vac leak somewhere.

Then the test is to replace THAT injector or swap it into another hole to see if the miss changes to that hole.

Big Jim

 

[jeremysdad]

"I did wires and plugs so I threw a set at it as well.. not very pricy.."

Is a problem. Double plats aren't cheap, unless you're a 1%.

My last 'semi-tune-up' was almost a bill. And I did the labor myself. And that was shopping online (seefoam ain't cheap by the case!)

Your attitude, in general, makes me not want to help you.