4.0 rough idle question

[Roger Jolly]

1996 Ranger Supercab 4.0 5-speed 4x4. The truck currently has 234,000 miles on it. At 152,000 miles the lower intake gasket gave way and I replaced it. Luckily It happened in my driveway getting home from the store. Since I fixed it the coolant level stays the same.
Recently the EGR valve diaphragm broke off so I had the EGR valve and pipe above the exhaust manifold replaced. My EGR solenoid was broke so I replaced it. I also replaced the DPFE sensor with the updated version. While doing this I also checked all of the vacuum lines on the engine for leaks. About a year ago I replaced the IAC as it was inoperable.
It runs rough seemingly when it warms up to operating temperature and throws a P0300 code. Occasionally it will also throw a P0301 or a P0305. Power is also down at highway speeds and fuel economy is down approximately 20%. Obviously it idles rough but it also doesn't quite feel right while driving at highway speeds either. I'm not sure how to explain it. There isn't really a hard miss, but a slight roughness.
I replaced the thermostat as it wasn't getting up to operating temperature this winter.
It has new plugs. Autolite APP105.
The plug wires check within specs with an ohmmeter.
I back probed the TPS and voltage is .93v at idle and around 5v WOT.
I also back probed the MAF sensor and it reads within specs at idle and at approximate road speeds.
For good measure I also cleaned the MAF. Twice.
Despite the fact the IAC appeared clean I cleaned it anyway.
The coil checks good on the primary side with an ohmmeter but slightly high on the secondary. The book I have says no more than 15K and all three of mine read 15.6K.
Vacuum at idle stays between 17 and 18 but dips 1 to 2 during the rough idle episodes.
Fuel pressure is around 28-32 psi.
I have not done a compression test as of this posting.
The truck, when started cold, idles around 1000 rpm for a few minutes then dips to around 800rpm. This is when the P0300 code appears, especially with the added load of the A/C compressor.
Does anyone have any ideas? Thanks.

 

[SHAKE&BAKE]

CHECK YOUR IDLER VALVE?

 

[Roger Jolly]

From above

About a year ago I replaced the IAC as it was inoperable.

Despite the fact the IAC appeared clean I cleaned it anyway.

 

[Nhaz]

I've had nothing but problems from the 2 Iac's I've put into my truck and eventually I wired the second one right to a toggle switch.

I'd check the IAC again.

 

[Roger Jolly]

The IAC is within specs checked with an ohmmeter. It is very clean inside. Unplugging it with the truck idling results in very low idle speed, almost dying.
Additionally, the IAT and ECT sensors both check within spec.
This morning I drove it 30 miles and it did not throw a P0300. It only threw the P0301 and P0305.
I double checked the resistance on the coil when I got home. It was in the mid 14K area on the secondary. The primary was around .8 as I remember. I later checked the secondary after a drive and it read 15.6K hot. Still not sure if this is good, but all three are consistent with each other.
I'm considering pulling the upper intake and checking the gaskets above and below the fuel rail.
Has anyone had trouble with these gaskets? IIRC I didn't separate the fuel rail and the lower intake when I did the lower intake to head gaskets. I think I also reused the o-ring style gaskets between the upper intake and the fuel rail.

 

[Nhaz]

Have you applied 12 volts dc right to the IAC or checked its incoming voltage?

Mine checks out fine but still doesn't work. Its not getting the voltage from the computer. I have no clue as to why. And I currently don't care since I wired it direct. sidestepping the entire problem.

 

[Roger Jolly]

I forget if I checked the incoming voltage, I'll check it today. I do know it does something as it energizes when I turn the key on. I have a new IAC ordered, it should be in today.

 

[Roger Jolly]

Just checked voltage at red wire to ground. It measures 12.18 volts. I read somewhere it should be 10.5 volts. Anyone know for sure?

 

[Roger Jolly]

I see a trend. This morning I got a P301 and P305 from a cold start, same as yesterday. It's around 72 degrees here in the morning and warms up to mid 90s during the day. This is when I get the P0300 code.

 

[Roger Jolly]

I'm still stumped....

 

[Nhaz]

I would pull the plugs from cylinder 1 and 5 and eyeball them. and then backtrack from there. Ive seen bad new plugs and wire. its possible the wire isn't seated correctly on the plug. yes ive seen that too. right out of the box and the end falls off.

 

[dangerranger911]

have you checked o2 sensors or cleaned MAF sensor? if MAF is dirty you get a rough idle same with o2.

 

[Roger Jolly]

The MAF was cleaned twice. I have not checked the O2 sensors.

 

[HndMeDnBroncoII]

i believe the 300 is cylinder misfire and the 301 and 305 is jucst indicating which cylinder
so try swapping the plugs fromother cylinders around and see if the missfire shows up in thise cylinders instead

 

[Roger Jolly]

So I swapped the plugs from #1 to #2 and from #4 to #5. I still get the same codes 300,301,305. My neighbor bought me a coil pack this weekend and it didn't change a thing. Not sure wher to go next.