1994 4.0 L all stock, 134K

Just picked the truck up a few months ago, has been running fine until about the past week.

Ive had a rough idle when starting warm
The plugs are old, about 13 years, but only about 40K on them. As soon as i started noticing this issue, I poured a can of BG 44K in a full tank, thinking it was probably dirty plugs.

After that tank there was no change, which make me think low fuel pressure, or a dirty MAF sensor.

Also noticed a slight decrease in mpg since the issue has started from 17-18 to 14-15 mpg

Any suggestions on trouble shooting?
all input is appreciated, Thanks!

 

Welcome to the forum

Make sure plugs are gapped at .054
And if spark plug wires are original change them.

Call Ford dealer parts dept and see if/when they will be running a sale on motorcraft wires, and spark plugs, get price either way.

My '94 4.0l wires lasted 18 years, well 16 really, lol, started getting what you describe then but finally got new wires 2 years later.

Watch the Coil Pack firing order, one side is 1, 2, 3, the OTHER side, is 5, 6, 4 !!!


4.0l engines eats spark plugs, motorcraft last the longest, then autolite, other brands go bad pretty quick.
Had an NGK just go dead one day driving down the highway, steady misfire, maybe a year old.
And either get Double Platinum or just copper, no single platinum with Ford Coil Packs

MPG did go up with new plugs and wires

Fuel filter is only $10 and worth the time to change it every 5 years or so, also clean the MAF sensor, dirty MAF sensor causes more issues with the 4.0l than other engines for some reason.
Generally low fuel pressure would be seen at highway speeds, steady high demand for fuel.
Or a hard cold start if pressure was leaking out when engine sat for longer period.

When O2 sensors start to fail they cause computer to run the engine richer, so drop in MPG is noticed.
I swapped mine out at 200k and noticed a bit of an improvement, 1 or 2 MPG over the next 1,000miles or so, it isn't instant.

Also if you have a vacuum gauge check for clogged exhaust, Cats and mufflers start getting broken off pieces inside which limits exhaust flow, this drops MPG.
No, no 4 stroke engine needs back pressure, lol.

Vacuum gauges are $25 and a good tool to use on any engine.

 

RonD,

Thanks for your help, I ran by the store today to pick up MAF cleaner, a new fuel pump, and had them run an OBD. Code 543 is popping up which is either the fuel pump or the PCM. i will explore the PCM first, and then go after the fuel pump.

 

543 isn't related to the fuel pump itself.

The fuel pump gets power from the Fuel Pump Relay.
The computer(PCM) controls this relay.
When the computer closes the relay power is sent out on TWO wires, one wire goes to the inertia switch and then to the fuel pump.
The other wire goes to the computer, pin 8

So the computer can "see" that the fuel pump relay is closed and power is going out, when pin 8 has 12volts

543 means no power is being seen when relay is closed at pin 8.
If engine is running then obviously fuel pump is getting power so this is a wiring issue.
Wiring from fuel pump relay to computer pin 8
Could be in the computer, but long shot.

This won't effect operation, 543 it is there to tell mechanic either Fuel Pump fuse is blown or fuel pump relay is bad, but only if engine is not running.

 

alright, so next steps are to check the voltage on pin 8 in the computer, and get a new fuel pump relay? they're cheap, wouldn't mind replacing the 22 yr old one anyway..

Looking at my engine wiring diagram, i see another wire from the relay to the solid state, pin 22, would this throw a different code if this was the wiring issue?

could this wiring issue be the cause of my rough idle at a warm start?

that is good news the computer is likely ok, those things aren't cheap.
just out of curiosity, what are the symptoms of a failing fuel pump? does it normally go out all at once, or are there warning signs?

fuel filter, MAF and wires to be replaced/cleaned this week as well, soon as i empty the tank.

Thanks RonD, you know your stuff!!

 

Pin 22 is the Fuel Relay control wire.

When key is turned on the EEC Relay closes and sends power to the EEC(computer), Coil/EDIS, fuel injectors and Fuel pump RELAY, not fuel pump.

Fuel pump relay, like the fuel injectors, get 12volts when key is turned on but are not Grounded so no action is taken.
After getting power and starting the computer will Ground pin 22 for 2 seconds, this closes the fuel pump relay for 2 seconds which sends power to the fuel pump for 2 seconds.
This 2 second thing is a safety precaution, if engine RPMs are under 400 then computer leaves fuel pump off, i.e. in a crash or rollover the last thing you want is an electric fuel pump continuing to pump out fuel, lol.

When engine starts, above 400rpm the computer Grounds pin 22 and starts fuel pump again, and it stays on full time, until shut off by key or engine rpms below 400.

You can turn the key on and off as many times as you want to listen if fuel pump is running, it will come on for 2 seconds each time, you can't over pressurize the system.

Each time computer Grounds pin 22 it watches for 12volts at pin 8.

New Fuel pump relay isn't a bad idea, New EEC relay as well, they are the same part number, same relay, if either one fails you will be hitchhiking, lol.
But "new" used to mean "it works"
It now means "never tested, but we give you a warranty", lol, so "they don't build them like they used to" seems very true now-a-days.


Fuel pumps are electric motors, so usually don't give signs of failing, just one day you can't hear it starting up anymore, and engine doesn't start up anymore.

My '94 still has original Motorcraft fuel pump, knock on wood, 350k on it now

 

Good to know about your fuel pump! i know the rangers have some notoriously bad years (newer)
Cleaned the MAF, replaced the fuel filter, fuel pump relay, and EEC relay.
MAF was visibly dirty, noticed a difference as soon as i turned the key. Much smoother cold start and idle. did not have time to take it for a drive to see if the rough warm idle still persists, but will soon.
If it persists, is the next step checking the wiring? I had a heck of a time finding the ECU, and did not want to dig into it with out knowing what i am doing first.

 

Clean IAC Valve would be next.

And run a can of Seafoam or similar injector cleaner in the gas tank.

I do it once a year on all my vehicles, on the Ranger I can tell I get a smoother idle, by the next tank.

 

Rough Idle returned...
I usually do an injector cleaner about every other oil change, along with the MAF clean.
Didn't have any problems until they switched over to the higher ethanol winter blend gasoline... Popped off the IAC and that thing was FILTHY lol.
Also, I started burning premium since the prices at the pump have been so cheap lately, that helped a lot as well.

 

I was having similar issues in my 96 4.0, the rough idle did not go away until the upper intake gaskets were replaced. It was never a problem when cold, but as it warmed up I assume the gap from the warped gaskets increased; allowing more air in. My mileage would tank and I was huffing gas every time I came to a stop!

 

Did you replace the gaskets yourself? The guys at the auto parts store told me that could be my problem, but I hear no leaking, and when I bought a can of carb cleaner and sprayed it along the edge of the gasket, no change in RPM.

 

I did not change them myself, and I did not hear any leaking before I had mine done; but I did trust my mechanic enough to let him go ahead with it. It has been a couple months since the work was done and I have not had the problem return! Granted I also had the spark plugs, wires, and 02 sensor replaced along with a thorough fuel system cleaning.

Did you try spraying when the truck was warm/idling rough? If the heat is causing the gap to expand as suspected than your spray may only cause a noticeable change during this time. My engine knowledge is extremely limited, but it seems that the same mechanism could occur at the lower intake? I wish I could be of more help diagnosing the issue but hopefully someone with more knowledge will be able to intervene.

 

Yeah, I've done that carb cleaner check on a warm start as well as cold. I actually did it again this morning, on a cold start and did notice the engine rpm change a bit so I think I might have a failing gasket. It's a weird thing though, because that rough idle on a warm start only lasts a minute or so. I think if the gasket was truly leaking it would be more persistent.. Was your rough idle persistent or only on start up?

About how much did the O2 sensor and gasket run you? Recently did plugs, wires, fuel filter, and TB and IAC clean. I'm considering doing it myself but if it's reasonable to get it done at the shop I might just go ahead and let them do it.

Random question- what state are you located in? Here in Colorado we have a pretty high ethanol content in our gasoline 10% in summer and somewhere around 12-15% in the winter, and I know that our years of rangers were being manufactured right when they started adding ethanol to the gas. I have filled up with ethanol free gasoline a time or two, and have never really had the problem when running that stuff through my tank (plus +2-3 MPG!!). I have a suspicion that the ethanol might have something to do with the rough idle but who the hell knows.

 

I'm in the Dakotas, couldn't tell you anything about the ethanol content in our gas. For the plugs, wires, fuel system clean, fuel filter, o2 sensor, and new gaskets I was charged $1,000. Which I was quite happy with.

My rough idle initially was only present after the engine was actually warm, which almost never happened during the winter unless I was driving out of town. Then it started to happen more frequently until it was idling rough almost instantly after start up.