VTX1800N1

Feels like a misfire, only at idle.

Is usually worse the colder the weather. Will sometimes go away after the engine is good and hot.

Can only be felt at idle.

New plugs and coils, all genuine Motorcraft and Visteon parts.

Had dealer check for leaking injector, they replaced #5 under the factory extended warranty.

No codes or pending codes.

The cold weather part makes me think this is a vacuum leak. I have cleaned the TB and MAF. Replaced the air filter gasket. 83k on the truck. It does have the cam phaser noise, which may be the cause of this issue. Plenty of power and good fuel economy.

Now, here's the weird part. If I turn on the A/C, the rough idle pretty much instantly goes away. I think it has to do with a small additional load being placed on the engine. The rough idle is more prominent in park or neutral but can also be felt in gear, so that somewhat negates the small load theory. That points me to the TPS as a possible issue.

Any ideas?

edwin108

I agree with the vacuum leak theory since the roughness only occurs at idle (when vacuum is highest). Try spraying a small amount of water or brake cleaner around the vacuum hoses, ports, intake manifold, etc. and listen for changes in the idle.

Especial86

iTrader: (1)

Whats your idle speed in neutral, and what does it jump up to with the AC on?
I'd also be looking for a low voltage issue, possibly from a weak alternator or battery.

VTX1800N1

I got a minute to test this today. A/C had no effect whatsoever on idle speed, cold or hot, neutral or in drive.

Cold engine start = 1500 RPM
Quickly dropped to 1000, with a slow, steady progression all the way down to 650 in Neutral, 600 in Drive once at full operating temperature.

Curious that I now get a gray code- P1233, FPDM Module Off-Line. Well, it's running... I think the FPDM simply isn't telling the computer what it's doing. That might be the cause. I did inspect it a while ago and it had some corrosion. There were no holes or cracks. I cleaned it up with a wire brush, painted it, and constructed my own stand-offs to prevent further corrosion. I guess I'll start by replacing it and seeing what happens.

Steve@Tasca

I seem to recall something about a seal or gasket in the airbox that can cause an issue like this, try spraying around the airbox as mentioned above and see if you get any results.

Do be careful with brake clean though as some is highly flammable and has a low flash point that a hot engine can ignite.

-Steve

VTX1800N1

It's the foam seal around the air filter tray. It disintegrates. I've already replaced mine and unfortunately it made no difference. I just ordered the FPDM from you guys.

VTX1800N1

I replaced the FPDM and the code for it being offline is gone, however the low grade miss at idle continues. I'm becoming more convinced that it's the cam phasers. Still have to go around the engine bay with a spray bottle and look for vacuum leaks.

08screwCA

I had my phasers vct timing done. Still have a bit of a rough idle. Im going to clean maf t/b and re seal airbox and see if it helps for men did you try cleaning yours?

Especial86

iTrader: (1)

With not even a slight variation in idle with the AC on I might suspect the TPS sensor has a slight dead spot..

Martian

That is pretty weird... you say it's worse in colder weather which immediately makes me think that your intake is leaking somewhere... but since your A/C is somehow affecting it, I wonder if the compressor's clutch or bearings might be shot somehow and engaging the clutch releases whatever tension was on it. That's what was wrong with mine.

If you want to check that out, go spend the $20 on a belt for your engine with no A/C and put that on and run it for a few days to bypass the compressor entirely and see what happens. If the problem is solved, you know your compressor is causing the drag that's causing the problem. Just an idea, maybe it'll help.