My truck has been idling really rough since I got it and I can't seem to track down the problem. It seems to miss every so often at idle and you can really feel it shaking while you're in it. Just earlier today I got on the throttle a little over half way and it misfired really hard. It never misses that hard when at idle. It doesn't miss during normal driving badly, seems more like just rough running. It's not as noticeable when driving at higher rpms like 2k. Not only that, but it runs so rich I get 12 MPG, you can really smell how rich it runs at the exhaust, and the plugs are pretty covered in soot after only 2k miles on them. It struggles to start too. It's not like its cranking and cranking, it immediately fires but really struggles for a couple seconds to reach idle speeds. I've replaced every single sensor in the engine bay except for the TPS sensor. Did a vacuum leak smoke test and found a leak at the lower intake manifold and pcv valve gasket, replaced both of those and that did not help at all. Not sure if I got all the vacuum leaks as I used a cobbled together smoke machine. Did a compression test too, 135-150 on all cylinders. New distributor cap, coil was replaced before I got it, timing was dialed in at 12 degrees BTDC, new oxygen sensor too. I'm just dumbfounded on what could be causing this.

 

Welcome to the froum

You should include year and engine with questions
1986 2.9l

O2 sensors fail after 12 years or 100k miles which ever comes first, the ONLY sensor that wears out, its like tires and brakes, except lasts long but does for sure wear out just like tires and brakes
And failed O2 WILL cause engine to run RICH

On the engine is the FPR(fuel pressure regulator) it has a fuel line AND a vacuum line, check the vacuum line for gasoline, shouldn't be any of course
Raw fuel can be sucked into the intake and cause RICH running

If its an automatic check the modulators vacuum hose for ATF

 

Sorry I forgot to put that haha. The O2 is brand new along with the connector, so is the fuel pressure regular plus the vacuum line going from that to the upper intake plenum. I'll check the line for fuel though just to be sure along with checking the modular lines for atf. I should include that I get just about zero codes with the exception being a egr feedback pressure code I got once then never again (all egr is brand new), so not sure if something is up with the ecu or if that's just obd1 being next to useless. My scanner is only a few months old and specifically made for these years of Ford so I doubt its the scanner itself.

 

Then the PCM/computer is most likely the issue
Pull it out, take off the covers and have a look at it

 

I took it out and one thing I noticed was its a reman already. There's damage all across the board like scuffed traces and solder mask coming off in places, but those aren't bad enough to actually cut the traces or affect functionality at all. What may affect functionality is the corrosion on the crystal oscillator itself and the pins to it plus corrosion on the jumpers with yellow plastic around them. Another thing is it looks like the solder joints on those jumpers are coming off of the actual jumper pin so I can try to reflow the pins of that and the crystal oscillator to see if there was issues with connectivity there. The other thing I noticed is that on the backside, there's a couple bridged components and on the front side next to the two yellow jumpers it looks like there's another two jumpers missing (there's an indent in the silicone sealant and also four very poorly cut pins)

Also, I didn't see any ATF in the trans modulator vacuum line.

 

You could try cleaning it up but the crystal does look finished

Good place to look for used parts: https://www.car-part.com/

1986
Ford Ranger
Computer Box Engine
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Select "Electronic Control Module" that matches what you have, i.e. 2.9l AT(automatic) or MT(manual)
Federal if its not a Calif vehicle
2WD or 4x4 doesn't really matter but select what you have
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Should get a list of available Computers, they will of course be 1985/86 so will be 38 years old, but................so are the "rebuilt" models, they are just cleaned up and the 2 or 3 capacitors are changed
$40-$60 is usual going rate

I didn't think the EEC-IV ever had a daughter board added, but it looks Factory

 

Is there a difference between the E6TF-12A650-P2A and e6pf-12a650-c2a?
My current one is a e6pf-12a650-c2a, but I see a lot more E6TF-12A650-P2A. The closest one thats my part number is 80 miles.

https://www.ebay.com/itm/184531568980 There is a reman on ebay that says it's to replace either E6PF or E6TF so I'd assume they're interchangeable, but not sure.

 

The first 4 numbers are year and vehicle and department
E6 = 1986, E9 = 1989, E1 = 1981(F = 199, so F4 = 1994)
P = passenger car
T = truck
F = the department/division at Ford responsible for this part

P or T wouldn't matter in 1980's
(in the late 1980's 7 replaced T, 7 = Ranger)

12A650 = the part number of any Ford Computer for any model or any year, a 2023 Ford will have a 12A650 computer in it, this number is often left out when looking for Ford computers because it is the same for all Ford computers
So you would see
E6TF-P2A
e6pf-c2a
Listed without the 12A650 in between

Last digits are software/firmware versions, these matter the most, as they denote engine size, emissions, manual or automatic transmission
And Ford NEVER published any cross reference for these
But over the years there have been cross references put together, I don't have them, but it seems many wrecking yards have kept track of what came from what so are fairly reliable, if they say it matches then it usually does
Also Computer rebuilders are usually correct

 

Just got a replacement that looked brand new on the inside, do you have to drive it for the ecu to learn? Cause it's idling exactly as it did before I changed the ecu.

 

Should idle high on cold start, 1,000+ RPMs
Then as coolant warms up idle should slowly drop to about 750 for automatic or 650 for manual, V6 engines, 4cyl in the 800's

New Computer or reset computer needs to learn IAC Valve settings so can wander a bit but not very long maybe 2 drive cycles

Re-describe idle issues

 

I took it out for a 10 mile or so drive and it was kinda weird. When you first start it up it idles like it used to, but sitting at a stoplight its not nearly as rough and actually almost perfect. As soon as you shut it off and start it up again it idles like it used to until you start driving it (even when fully warmed up). I just followed the procedure here: https://www.therangerstation.com/for...stment.196011/ and it idles rough enough to shake the entire hood when its open. It seems like it's misfiring a LOT less though now. I set it to idle at about 800 in neutral because that's just what the sticker under my hood says it should idle at. It shakes more often the lower the rpm. I attached a video of the engine bay so you can see it shaking (the camera makes it look less shaky than it actually is) and also a video of the exhaust to hear how its running.

 

You can't/shouldn't set the idle on any fuel injected engine, that's the computers job

If you are using the anti-diesel screw then don't

Warm up the engine and let it idle
Unplug IAC Valve
Turn anti-diesel screw counter clockwise until engine is barely running, but running, usually 600rpm
Plug IAC Valve back in and RPMs should go back up to "target idle", the RPM level the computer has in memory

If you manually set the idle it does two things
First, it can make TPS(throttle sensor) voltage too high
Second, computer can't set low idle and becomes confused, not "confused" like a person, confused in that it has a parameter it can not control, so that can effect other things


Drive it a bit more and see how MPG does
Clean spark plugs, if it was running Rich like you said then they need to be cleaned

135 to 150 compression is not great, that is at 10% difference, especially if 4 were at 150 and 1 or 2 were 135
Do the Dollar bill test on tail pipe
Google: Exhaust dollar bill test

See if you have a burnt exhaust valve, watch the videos, easy test
A spark or fuel misfire will not cause the bill to move out as far
Burnt exhaust valve misfire will pull the bill back towards tail pipe

If you have another vehicle that's not misfiring test on that as well

 

Just did what you said to do and then drove around a bit. It idles at about 800 rpm when in park but when I put it into drive it idles at 600-650 according to my tach. I also stalled while pulling out of the FedEx parking lot. Seems to idle smooth and rough when it feels like. I can definitely tell that something is happening when I unplug/replug the IAC but it's not significant. Before I replaced the ECU it never idled smooth so we're getting somewhere at least

These were the compression test results:
Cyl 1: 135/140
Cyl 2: 140
Cyl 3: 145
Cyl 4: 135
Cyl 5: 145/150
Cyl 6: 145

I forgot to do the dollar bill test while I was out so I'll do that some time soon

 

Those numbers are OK
Rule of thumb leave out the highest and lowest cylinders, add up the remaining cylinders, then divide that by 4 in the case of V6, to get an average PSI
140
140
145
145
142.5

Didn't include cylinders 4 and 5

10% of 142 is 14.2, so 14
142 - 14 = 128
142 + 14 = 156

So any cylinders lower then 128 or higher than 156 may have an issue, 10% away from average
Although the 135psi is still on the radar, lol



Its odd you have 800rpm in Park idle(warm engine) then lower "in gear idle" just the opposite of what it should be, computer should detect dropping idle RPM, instantly, when "in gear" and increase voltage to IAC Valve to compensate

I would expect 800rpm in gear, with automatic, 700-750 in Park, so odd

Computers with IAC Valves are +/-4rpm accurate so a very precise setup, and it reacts very quickly
I see why you had to adjust the anti-diesel screw to prevent stalling, but you really shouldn't have to

There is no "in gear" status for the computer on the A4LDs, that didn't start until the 1995 DTR sensors
EEC-IV Computer does have a "neutral" sense, pin 30, but in Rangers all it got was 12v when starting the engine

 

What do you think that means though? The IAC valve was replaced before I got it, no idea how long ago that was though. I think the guy that sold it to me made it idle higher to mask some issues, he also screwed up the timing and I had to fix that too. Every car I've driven has had the RPM drop when putting in gear though.

 

With computer control it will drop then come back up, that's the point of computer control, with a carb it can only drop unless it had a small computer operating an electric solenoid to kick up idle when in gear
Those were the first IAC(idle air control) Valves, lol, they opened up throttle a bit more letting in more air

Does current IAC Valve look like a Motorcraft or Hitachi brand?
Those are the only 2 IAC Valves that will work correctly with Ford software, they are true solenoid valves

If you want you can put a volt meter on the IAC valves 2 wires while its plugged in, warm engine
With key on/engine off you should see close to 12volts
Then start the engine, all EFI engines should surge above 1,000rpm on startup, warm or cold, its because computer has opened IAC Valve all the way, with 12volts

Should then see voltage drop, there is no "correct voltage" the computer "learns" the voltage to set, for say 700rpm idle, with this one IAC Valve on this one engine
But 7 to 8volts maybe
Then have someone shift it into gear
RPMs should drop and then come back up as computer increase voltage, should be instant, you should see a voltage jump, may not be much but should go up as should RPMs
If you see the voltage "bump up" but idle doesn't go up then could be IAC Valve issue

 

There is absolutely no branding on it. The only writing is four numbers "2697"
looks exactly like this one front to back
At cold startup it sits at around 1,650 RPM for a couple seconds then drops down to 1.2K and slowly drops to idle from there while it warms up.
At warm startup it does that but drops down to idle immediately.

I'll measure the voltage there tomorrow when I do the dollar bill test too.

 

Sounds like its working like it should
Except for the bump up when idle drops too low, that shouldn't happen

Not motorcraft or hitachi brand
If there is a wrecking yard near by see if you can find an original motorcraft IAC Valve, they were the same over many years just the connector changed
I wouldn't buy a new one just yet since you can avoid stalling by adjusting the anti-diesel screw

 

I just drove around the neighborhood for 45 mins trying to adjust the idle to how it used to be, which was a bit high in park (950 or so) but still like 850-900 in drive, and I can't get it to idle over 700 RPM in drive without the idle rpm in park being 1K. No wrecking yards near me still have rangers of this age. I went to every single pull a part in Ohio and the closest was a 89 2.3 Ranger. I did the dollar bill test and it never got sucked in at all. Also got 11.73v at the IAC valve with key on engine off. I can't measure the voltage while turning the truck on and putting into gear, all the wires/connectors are covered up so I can't get multi meter leads in there

 

Just as a testing method for wires, I use sewing pins to pierce wires and then have alligator clip add-on's for volt meter probes, so I can test wires at different places
Sewing pins won't hurt insulation or the wire inside, put pins in at an angle so they slide in farther, and not close to another pin so they could short out, lol
They actually sell needle probes, but I am cheap, and the separate "needles" with the clips don't need to be held in place so I can put the meter where I can see it from cab or other side of engine bay as needed, and have both hands free

Good that there is no sucking at tail pipe

 

Just tried that, 12.28v before starting, once started and idling in park at about 800rpm was 8.95v and kept climbing slowly until it plateaued at 9.34v, once I put it in reverse (rpms are the same in reverse as they are in drive) which was around 600rpm it varied around 9.6v

 

Good data
That is odd that the RPMs did not go up with voltage, i.e. 8.95v to 9.34v, ain't much but ain't 0 either

"at about 800rpm was 8.95v and kept climbing slowly until it plateaued at 9.34v"
Shouldn't have stayed at same RPM
Computer will vary the volts a little to keep a steady Idle, 200rpm drop should have spiked voltage to keep idle up, more than 0.3v, but it did respond a little
I assume you have an onboard tachometer?
Computer has onboard tach tied to the same wire so should have the same data from TFI module

I am puzzled

 

I noticed that when I put it into gear it drops to 500 for a split second then to 650 so it's not straight to 650. Also, its an aftermarket bosch tach tapped into the negative side of the coil. I was supposed to be measuring the gray wire on the IAC right? Because I'm pretty sure thats the signal the IAC gives out. The other red wire is a constant 14v. I'm sure it's been doing this all along and I just haven't noticed, but if I listen closely I can still hear it misfiring or what sounds like misfiring.

 

Yes, Ford uses PWM(pulse width modulation) on the Ground wire of the IAC Valve
So the red wire always has Battery Voltage(system voltage) with key on(engine on or off)
The computer pulses the ground wire(grey wire) which in effect lowers the voltage inside the IAC Valves solenoid

A Solenoid is a coil of wire around a metal tube/core, when voltage is applied the metal tube/core becomes an electro-magnet, so works the same as a Relay, except relays are on/off, solenoids provide movement so can have more than just on/off, open/closed
There is a spring in the IAC Valve that holds the valve closed
As voltage is applied, the electro-magnet over comes spring pressure and pulls open the valve to let in more air, raising RPMs
With feedback from the tach(TFI module/coil -) the computer can Pulse the ground wire to slightly move the valve position in or out, accurate to +/- 4rpm


There is another popular method to do this, called a Step or Stepper motor, basically a digital positioning motor, it turns to a specific position if it gets a specific voltage pulse/level
3rd party IAC Valves can have this setup to be more "universal"
Motorcraft and Hitachi are the only brands that don't, they are true solenoids

Another test you can do with volt meter connected to IAC Valve ground wire and battery positive with warm engine idling
Remove a small vacuum hose from intake, idle will go up of course, and voltage should go down instantly in response
Use your finger on vacuum port to vary the leak to see computers voltage response, how quick it reacts

And then remove the air tube(from air filter) on the upper intake
With engine idling, use a gloved hand to cover up the throttle body to reduce air flow and idle, to see computers response to lower RPMs


IAC Valve would have nothing to do with misfires, that is a separate issue

 

When I take of a small vacuum line and vary the leak, the voltage goes up pretty slow. Not a substantial amount of voltage but it went up maybe 0.20v or so, took a couple seconds to reach it though. Seems to start climbing voltage soon after taking the line off but takes a bit to actually reach the final voltage it climbs to.
Covering up the throttle body didn't change the voltage at all. It just sat around 8.53v until I accidentally choked it out and it stalled.