I have a 1994 Ranger 4.0 that won't stay running on all 6 cylinders. The right bank center cylinder won't run. I have done a compression test, leakdown test, the spark is strong. I replaced the spark plug and wire, the coil pack, ignition module, tried a good borrowed computer and have new fuel injector in it. Even switched injectors with a new Ford unit and made no difference.

Here is the strange part. If you pull all the plug wires off the left bank the bad cylinder will fire. Then you can hook them back up and it runs perfect for about 20 seconds then the right center cyl cuts out. After we switched out the coil it ran great for about 20 seconds then quit on the same cylinder. If you unplug the coil then plug it back in it runs great for about 20 secs then quits on that cylinder. I grounded the spark plug in top of the motor and the spark is good. It has to be cutting off the fuel to that cylinder. I am at a loss as to where to look next.

 

Welcome to FTE.

Do you have a CEL lit??? If so post ALL of the trouble code numbers.

If your certain the spark is ok to the cylinder in question (& we don't know which center cyl you mean, as you forgot to tell us where/how you were positioned when looking at the engine) any way an inductive timing light hooked to it's plug wire will tell if it is firing, if so, then listen to the fuel injector for that cyl with a stethoscope & detemine if it's clicking.

If it is, then is that cyl's spark plug wet with fuel????

The computer supplies ground switching for both the fuel injectors & coil pack, so make sure their electrical connectors are fastened tight, the pins & sockets & wiring are in good order, all the way back to the computers firewall connector, no bent pins, spread sockets, or corrosion or sknned wiring insulation.

Right now without more detailed info, it sorta sounds like a connector or wiring issue.

Which cyl has the problem????
Cyl's 1-2-3 are on the passenger side, 4-5-6 are on the drivers side, so is it cyl 2 or 5 that your having problems with????

Remember that on the 4.0L waste spark ignition system we only have 3 coils to fire all 6 plugs, so cylinders are paired such that when one plug is firing on it's power stroke, the companion plug is being fired on it's exhaust stroke, which requires less oomph from the coil, so be sure to check the companion cylinders plug & wire.
On V6 engines cyl's 1/5, 2/6, & 3/4 are paired together, so if cyl 2 is the problem, be sure to check cyl 6 plug & wire.
If it's cyl 5 with the problem, be sure to check cyl 1 plug & wire.

Also the passenger side bank of cyl's gets a reverse polarity spark, which eminates from the ground to the center electrode & the drivers side bank of cyl's gets a normal polarity spark, which eminates from the center, to the ground electrode, so we need double platinum plugs for our waste spark system to keep the spark gap erosion in check for long periods.

The plugs resistance is about 4K ohms.
The plug wires resistance is about 2K ohms/inch of length, 30K ohms max, no matter the length.

A bunch of thoughts for cosideration, let us know what you find.

 

Thanks for the help. I will check the connections a little better. We didn't look to much at those. It is cylinder 2 that has the trouble. The plug is bone dry. I am sure the fuel injector is cutting out.

 

Ok good feedback, so it's #2 cylinder & it' plug is dry.

Well imo right now, with #2 plug dry & it acting like it does when you unplug & reconnect the coil pack, or drivers side bank of plug wires & then have it run ok for 20 seconds or so, we don't yet have enough evidence to rule out a coilpack wire, electrical connector pin/socket, spark plug or ignition wire problem with cyl #2, or it's companion, #6 cylinder, so pull the #2 & #6 plug & wire for a physical inspection & do an ohm check on both plugs & wires to the values I posted above.

Have you listened to the suspect fuel injector, to detrmine if it's not clicking/firing. If it's not, then do a "wiggle" test at it's electrical connector.

If it's too difficult to listen & do the wiggle test at the same time, maybe consider installing a "noid light" in the suspect injector's wiring harness & see if it's blinking, or starts/stops blinking with the wiggle test.
Most autoparts stores have noid lights for a refundable deposit in their "Loan-A-Tool" program.

More thoughts to ponder, keep us posted on your results.

 

I borrowed a noid light from work and it blinks. I also hooked up my multimeter to the fuel injector connector and when you put the plug wires on that were pulled off the voltage drops. I thought maybe I have enough juice to run the light but not enough to run the injector. I will try to determine if the injector is actually cutting out. I will have to wait until Sat. to mess with it, not enough hours in the day after I get out of work. Thanks again for the help.

Also I have no CEL lit. Scott

 

As an update I put new plugs and wires on and it still won't fire on number 2 cyl. I made a homemade stethoscope out of wood dowel and I can't hear a difference if the fuel injector is plugged in or not. I still get it to fire if I pull all the plug wires off 4, 5, and 6. The injector sounds the same but the cyl is firing. As soon as I put the other wires back on it cuts out. I am about ready to say goodbye to this truck.

 

OK good feedback, so you believe the #2 fuel injector is working as the noid light blinks & you can hear #2 injector clicking away with your home made stethoscope.

I take it when you say that #2 is firing when you remove the plug wires from cyl's 4-5-6, you mean that #2 then gets spark & fires????

If so & seeing as how #6 plug shares the same coil with plug #2. pull the #6 plug wire & see if cyl 2 will then fire.

If so, suspect something is wrong with #6 plug or ignition wire.

 

Even when all the plug wires are in and number 2 is not firing I still have a huge spark if I pull the no. 2 wire.

I took it to the Ford dealer and they charged me $259 and it still misses. The only thing they could find is a sticky intake valve in number 5 cyl. Could this in any way cause a miss at number 2?

 

Just a thought. These are 'waste spark' systems right? By this I mean that the coil pack fires one cylinder on the compression cycle and also sparks one that's basically empty.

Could there possibly be a problem with the opposite cylinder? I don't know what it could be, but maybe something is causing it to take excess energy.

Also, I thought that SOME Fords used different plugs depending on whether the spark was + or -. (Not real sure about that)

 

IMO now you have a diagnosis that fits your symptoms.

So why are you still so convinced that you have a problem with #2 cyl????

Lets think about this some more.

You haven't provided any evidence that there is any problem with #2.

You've done a compression check on #2 cyl, replaced the computer & #2 fuel injector, listened to it click with your home made stethoscope, hooked up a noid light & it blinks, showing you have B+ to the injector & that the injectors coil has continuity & the computer is providing ground switching.

You've replaced all of the spark plugs, wires & coilpack & confirmed you have spark to #2 cyl

After a $259 diagnosis, the Dealer didn't indicate anything was wrong with #2 cyl, BUT he did find a problem with #5 cyl intake valve sticking & that sure fits your description of it running ok for 20 seconds or so, then running rough again.

SO, has it been using any oil, puffing blue smoke on start up after a over night shutdown????

Or maybe has gum, varnish, pudding like sludge, or carbon deposits under the valve covers???

If so, you may have to pull the heads for service, or maybe if deposits aren't too heavy, there may be some other things you could try that might help unstick the valve without having to pull the head/s for service.

A bunch of things for pondering.

 

The reason I am looking at number 2 is when I pull plug wires there is no difference in RPM when I pull no 2. There is a drop in RPM with all the others including number 5. It has me puzzled to say the least.

 

Well why not focus on the problem the Dealer says you have.

When #5 intake valve sticks, the engine should get noisey on the top end & run rough.

Any signs of heavy deposits under the valve covers, or has it been using oil????????

If the deposits aren't heavy, maybe consider a dose of Auto-Rx in the engine oil & see if it'll free up the #5 intake valve.

It'll also tidy up other parts of the engine & free up the ring lands too, but it does it slowly, so as not to break deposits loose too quickly & clog something important up, like a solvent type flush might.

Visit their site & see if you think maybe it's for your problem.
Auto-Rx Engine Cleaner Transmissions Motorcycles Stop Leaky Seals Oil Burning Increase MPG Better Gas Mileage Trouble Shifting ARX

Let us know how it goes.

 

BluRang4.0 did you ever get your problem solved????