Got this truck almost a month ago and have discovered a few issues possibly related to a misfire? Manual 4 speed transmission. Starts up fine but fluctuates when at idle, also makes a small pop every 3-5 seconds and dips my rpm by about 100-200. When I’m driving it these pops occur too, but way more severe and happens every 10-15 seconds. It makes my tachometer needle bounce for a couple seconds. It feels like I hit a big pothole when it happens. Sometimes the misses go away for 30+ seconds and then they’ll start back up on that 10-15 second rotation.

What’s weird though is that this problem only occurs every once In a while. Yesterday driving to work none of this happened other than the rough idle, today is was horrible.

I have replaced spark plugs and wires, which seemed to have helped at first, but a couple days later it was back to doing the same thing. Spark plugs are motorcraft platinums. Supposedly the previous owner did some digging into the distributor, I’m assuming to fix the issue that I currently have, but I’m not sure the condition of it. The truck sat for a summer, and I’ve filled the tanks completely 2 times, so I’m assuming bad gas isn’t really probable.

Honestly any trouble shooting tips would be helpful. I’m planning on digging into this after work today and making sure sensor connectors aren’t corroded or dirty. I’ll update if I find anything.

 

I would start by cleaning and tightening the connections at the battery terminals and the fender mounted starter relay (aka solenoid).
Inspect the main ground cable that goes from the battery negative, to the engine block, or starter mounting bolt and frame. Those connections need to be clean and reasonably tight also. If the visible strands of wire in that cable are corroded, it probably needs to be replaced.
Here's a picture of two power grounds for the computer, that need to be connected to the battery negative terminal. If you have these clamp style battery terminals, they may be your problem.


There are also small gauge grounds located near the driver's side hood hinge, near the windshield washer bottle, behind the driver's side kick panel and the parking brake pedal, and from the firewall to the intake manifold or engine block.
I use antioxidant on my connections before I assemble them, after I clean them. That's my preference though. You can also coat the connections in dielectric grease after you assemble them.

Are your fuel pump and ECM relays laying on your driver's side fender, or are they in a black plastic cover? Mine were laying on the fender. When I looked at the wiring going up into the base of the ECM relay, I discovered a repaired terminal end. Check there for corrosion and loose connections.
The ECM relay base is brown, the fuel pump relay base is green.
Also, when the truck is idling, and hesitating every three to five seconds, see if you can feel the ECM relay clicking. Or listen for it clicking.
Also, you may have a fuel pressure problem. Some parts stores will loan you a fuel pressure gauge and there's a thread on this forum on how to check fuel pressure.
Did you change the fuel filter?

 

Are your fuel pump and ECM relays laying on your driver's side fender, or are they in a black plastic cover? Mine were laying on the fender.

I think they are laying on my fender, but not 100% sure, I’ll check when I get home.


Also, when the truck is idling, and hesitating every three to five seconds, see if you can feel the ECM relay clicking. Or listen for it clicking.

Ok. If I don’t feel or hear it clicking does that indicate a bad or failing ECM relay? And am I able to swap that out for a functioning relay in my truck to confirm that it is failing.


Also, you may have a fuel pressure problem. Some parts stores will loan you a fuel pressure gauge and there's a thread on this forum on how to check fuel pressure.

Yeah I have seen on here a few times that fuel pressure could be the cause of multitude of issues related to mine, was planning on renting one from the parts store on my way home.


Did you change the fuel filter?

I did not change the fuel filter, but while I’m at the parts store I will grab one.

I will definitely clean and confirm that all of those spots you pointed out are in good condition. I will update later on when I get time to look at it.

 

The ECM relay shouldn't continuously click. That would be a potential cause for your hesitation.
When the key is turned to the on position, power is sent to the ECM relay's coil, so it should be powered any time the key is on.
The fuel pump relay coil is grounded through the ECM, so when you turn the key on, the fuel pump relay should activate for about two seconds, then turn off, until you turn the key to the start position, and the computer sees the engine rotating.
Neither relay should chatter.
I think you can swap the horn relay into the ECM relay socket, but I'm not a hundred percent sure. I'll have to check.
If I were going to test a relay, I'd remove it from the base and power it with a 9 volt battery, and use a multimeter to ohm out the normally open and normally closed contacts. But, I wouldn't recommend unplugging relays from their sockets until you establish that all those grounds are clean and tight or replaced if necessary.
The relays are difficult to remove from their bases and the clips are probably brittle.

 

If you don't have cruise control, you don't have a horn relay.

 

Alright little update. Fuel pressure to my rail is a solid 50 psi with nothing abnormal. I cleaned up a couple grounds, not all of them, with no change. I then cleaned every connector I could find/reach and no change. There are a couple I still haven’t been able to get to. Got home late and was working in the dark. Finally I half took apart my IACV and sprayed carb cleaner through it. After that when I started the truck the idle did seem to be a little more steady, and the little “misfires” I got seemed to lessen, however they were still there. Took it for a 10 ish minute drive and only experienced minor “bucking” in first gear around 1500 rpm, which it was already consistently doing. 2nd and 3rd gear seemed normal, with no “bucking”, however It could have just been luck. I did not change the fuel filter tonight, however with the fuel pressure being good and the fuel looking and smelling fine, I don’t think the filter is the issue. I will however be replacing it soon.

Tomorrow I will have a lot more time to work on it and make sure all of my grounds and connectors are clean.

Since after I crudely cleaned the IACV and the idle seemed to smooth out, would it be a bad idea to try and fully clean/replace it? Could a bad IACV give me symptoms related to what I’m experiencing?

Also, both relays seemed to work fine and there was no abnormal clicking. They were however laying on my fender with a bunch of electrical tape around them and coated in what appeared to be dielectric grease.

 

I would finish cleaning all the grounds, especially the main ground cable to the engine block or starter mounting bolt, and frame, then unwrap the relays and visually inspect them, to see if there's corrosion or repairs to the wiring.
Here's a thread on testing and cleaning the Idle Air Control valve https://www.fullsizebronco.com/threa...ur-iac.206960/
Also, you could run the truck up to operating temperature, and check for engine codes. There's a thread on that on this forum somewhere.

 

Disconnecting the ground cable at the negative battery terminal, will clear the engine codes, so check them first, if you haven't disconnected it yet.
https://www.f150forum.com/f10/how-re...es-obdi-10907/
If you already disconnected it, run through the trouble code process anyway. You should at least get results from the engine running part of the test.

 

Little update. I wire brushed every ground you mentioned, and fully cleaned the IACV and nothing changed. The idle may have cleaned up a little bit from cleaning the IACV, but I’m still hearing what I can only describe as a misfire or a popping noise. I also just discovered that when my foot is barely on the gas pedal, my rpm bogs down by about 150 revs and then revs back up. I’m not sure if that’s normal or not, just discovered it today.

While driving to the parts store to return the fuel pressure gauge, I noticed that most of the “misfires” occur when the truck is going uphill or under a heavier load. The “misfires” in first gear still happen at about 1000-1500 rpm and doesn’t matter if I’m on flat ground or going up a hill.

I’m pretty stumped at this point and honestly don’t really know where to go to next. I don’t want to blindly buy 100 dollar parts without confirming that’s the part that I need, but I don’t know how to narrow my problem down.

Next step is going to be reading engine codes and hoping that gives me some more clues as to what’s wrong, but I’m not too hopeful. I’m gonna try and get that done today but I don’t know if I have time.

Any advice on where to start next will be extremely helpful.

 

Did you have a chance to investigate the ECM and fuel pump relays?
Engine codes may give you a direction to look in.
When they start flashing out, have a paper and pen ready, and make tally marks, then when there's a longer pause, that's a new number. That way you don't get confused trying to figure out the numbers as they're flashing.

The two 11's, for instance, were two single flashes about a second apart, with a three second pause before the next 11.
On the above example, you can see that the 11 repeated, then the 41, 33 and 29 repeated.
The 11's are the active codes, then the 41, 33 and 29 are the continuous memory codes, that could be weeks old.
Another option is a code reader, but I prefer counting flashes.
Then, do the key on engine running test, and the computer will test the sensors in real time, then flash out more codes.

There's a thread here somewhere on testing the throttle position sensor.
What condition are your vacuum lines in?
Is your EGR system still present and functional?