Misfire at idle 300 i6 f150

[Orangepeel]

At idle I’ll get a hiccup every now and then on my 96 f150. New tune up (complete) just done and no change. Seems to affect all cylinders when I watch misfire counters it’s 3 and 5 most often but they all miss every now and then. No vacuum leaks, fuel pressure is good and it runs good and doesn’t miss when driving it. Only sputters at idle every several seconds. Where would you go from here?

 

[Rand]

How often is this driven?
Does it "sputter" with AC on?
How old is gas?

 

[twX]

The fact that it happens on all cylinders pretty much rules out plugs, coils, or injectors.

The MAF and O2 sensors would affect all cylinders. A dirty throttle body would tend to affect idle more than anything, so I'd check that too.

 

[D60]

Still don't know if the 300 got MAF in '96? The V8s did -- first year in the fullsize trucks despite being the last year (mostly) for the body style

 

[Orangepeel]

The fact that it happens on all cylinders pretty much rules out plugs, coils, or injectors.

The MAF and O2 sensors would affect all cylinders. A dirty throttle body would tend to affect idle more than anything, so I'd check that too.

Throttle body is clean. Maf appears to be reading correctly I cleaned it just to be safe but no change.
Gas is fresh and driven regularly. Front or rear tank doesn’t make a difference either.

 

[meep]

camshaft timing from a stretched chain or belt will do this, especially at idle. Check valve lash, especially exhaust. Overheating valves can also do this, though more common while underway.

 

[OVERKILL]

camshaft timing from a stretched chain or belt will do this, especially at idle. Check valve lash, especially exhaust. Overheating valves can also do this, though more common while underway.

The 300 I6 has timing gears, no chain. However, on some years they were nylon and the teeth broke off, but then it wouldn't run at all, if that was the case here.

 

[OVERKILL]

The fact that it happens on all cylinders pretty much rules out plugs, coils, or injectors.

*coil, this should be EEC-IV, single coil w/distributor and TFI module.

The MAF and O2 sensors would affect all cylinders. A dirty throttle body would tend to affect idle more than anything, so I'd check that too.

A sticking EGR valve could also cause it to misfire.

 

[wlk]

Another thing to check is your ECU, pull it and look for any swollen/leaking capacitors and resistors or corrosion on them.

 

[clinebarger]

Don't put much faith in the misfire data

 

[Orangepeel]

*coil, this should be EEC-IV, single coil w/distributor and TFI module.

A sticking EGR valve could also cause it to misfire.

I can actuate it with a scan tool and it does respond. Not sure if that’s a fool proof test but I can increase the duty cycle and make the truck run worse and worse or back it off to zero and it’ll run “normal”

 

[Lubener]

At idle I’ll get a hiccup every now and then on my 96 f150. New tune up (complete) just done and no change. Seems to affect all cylinders when I watch misfire counters it’s 3 and 5 most often but they all miss every now and then. No vacuum leaks, fuel pressure is good and it runs good and doesn’t miss when driving it. Only sputters at idle every several seconds. Where would you go from here?

How about some figures. You say fuel pressure is good, no vacuum leaks. What exactly have you done so far to conclude that and what is your fuel pressure? Since it's a 96, you should be able to list your fuel trims. MAF "appears" to be reading properly, what is the reading? No data and just speculation. You might want to do a compression test. If the cyllinders aren't within 20% of each other, you will never get a really smooth running engine.

 

[dnewton3]

It's been a long time since I owned one of these; same year in fact. So my memory is suspect, for what it's worth ...
I'm just chiming in with the hope of helping. I cannot say if my info is accurate for sure.

1) aren't these "map" sensor (barometric) systems, and not MAF? I don't think the 4.9L EFI units ever got MAF, but I could be mistaken

2) these engines are old enough to have the TFI (thick film ignition) modules. Those were suspected to have issues as they age; the ECU would essentially misfire occasionally because of internal signal leakage in the unit itself, or something like that. And they have external components which also can cause issues.
See these:
https://www.f150forum.com/f10/gray-black-module-327933/
https://www.ford-trucks.com/forums/1484093-do-you-have-the-right-icm-gray-vs-black-93-96-a.html
https://fordsix.com/threads/understanding-standard-and-signature-pip-thick-film-ignition.81515/


3) OBD II should have been present in the F150 starting in 1996; so this would be a first year for that feature. The systems back then were very rudimentary; not a lot of info or bi-directional control options.

 

[OVERKILL]

3) OBD II should have been present in the F150 starting in 1996; so this would be a first year for that feature. The systems back then were very rudimentary; not a lot of info or bi-directional control options.

I know on the passenger car versions, EEC-IV was surprisingly capable in terms of diagnostics, you could even do a cylinder balance test for example and they provided a pretty considerable list of codes, the issue was, as you note, data, trying to get things like voltages, trims...etc.

 

[OVERKILL]

I can actuate it with a scan tool and it does respond. Not sure if that’s a fool proof test but I can increase the duty cycle and make the truck run worse and worse or back it off to zero and it’ll run “normal”

How well can you see the pintles for it through the throttle body? Do they look fully shut?

 

[ka9mnx]

A sticking EGR valve could also cause it to misfire.

^^^ This.

 

[ka9mnx]

Another thing to check is your ECU, pull it and look for any swollen/leaking capacitors and resistors or corrosion on them.

^^^ Or this.

 

[Orangepeel]

How well can you see the pintles for it through the throttle body? Do they look fully shut?

I will try to look and report back.

 

[D60]

I can actuate it with a scan tool and it does respond. Not sure if that’s a fool proof test but I can increase the duty cycle and make the truck run worse and worse or back it off to zero and it’ll run “normal”

Try blocking it off entirely. IIRC a quarter or an electrical box KO might work -- don't remember if 1/2" or 3/4"

 

[Joba27n]

Another thing to try is to remove your spout jumper and see if it changes. Sometimes the "cam sensor" in the distributor would get defective. Unplugging the spout removes computer controlled feedback and fixes ignition timing to base.