2016 Ford Fusion 1.5 stumbling idle with a/c

[D60]

2016 Ford Fusion with 1.5 Ecoboost, about 160k mi.

The idle will stumble under load -- now, "under load" can mean a/c compressor on or high electrical load (the rear defrost really upsets it).

By stumble I mean the idle drops to ~400 and then recovers to, say, 800-900. It almost starts "bouncing" but if you give it the least bit of throttle then let off it smooths out temporarily until it starts bouncing again.

The owner reports this happened after he had the timing belt replaced. No misfires, but LTFT is at +18 while STFT is +/-4.

I tried smoking where the filter airbox connects but found absolutely no vac leaks. Is there a better/more appropriate place to smoke these turbo engines?

Absolutely no codes except for the AGS (active grille shutter) which I'm going to fix just because. No crusties observed in the AGS connectors and power and ground are present along with a pulsing ~9V which is likely normal signal wire behavior.

Again, zero misfires in the counters.

I cleaned the throttle body and wiped throttle trim, but this did nothing. Plugs are new Motorcraft from O'Reilly and new coil boots. Purge valve tests good.

I don't like the high LTFT, but would this even cause symptoms of a stumbling idle and ONLY under a/c or alternator load?

Also I've found several other reports of EXACTLY THESE SYMPTOMS online, with people sometimes noting it gets worse when the a/c is on and the radiator fan kicks up (ie a/c and electrical load) but no one ever posts a resolution.

Otherwise the vehicle runs great with no codes (except grille shutter which is being addressed).

Ideas welcome. Really would like to find this vacuum leak. These are speed density so I dunno where (ie what areas) vacuum leaks matter??

 

[Brian123]

IAC, Idle Air Control Valve is something to check!

The owner reports this happened after he had the timing belt replaced.

Something was not hooked back up correctly, just a thought! Check all of the vacuum lines.

Also I've found several other reports of EXACTLY THESE SYMPTOMS online, with people sometimes noting it gets worse when the a/c is on and the radiator fan kicks up (ie a/c and electrical load) but no one ever posts a resolution.

We have a Ford expert here, PM me and I can give you his name, he might have an answer to your problem.

 

[JavierH19]

I had this happen to my sierra also with the ac on at idle but to a smaller degree after new plugs, wires, air filter, and fuel filter but not immediately only after a few hundred miles but I read somewhere that disconnecting the battery lets the computer software reset and it worked for me. I left it off overnight so it had plenty of time. Maybe try that, couldn't hurt.

Try disconnecting the maf too which seems odd but many ecu's will still run without it but also switch the way they apply metering and erratic or weird running engines will run smoothly but with a less fuel efficient mixture and slower throttle response and acceleration but usually smoothen the rough idle and in rare cases weird shifts.

 

[bdcardinal]

IAC, Idle Air Control Valve is something to check!

Something was not hooked back up correctly, just a thought! Check all of the vacuum lines.

We have a Ford expert here, PM me and I can give you his name, he might have an answer to your problem.

These are fly-by-wire so no IAC.

 

[D60]

IAC, Idle Air Control Valve is something to check!

Something was not hooked back up correctly, just a thought! Check all of the vacuum lines.

We have a Ford expert here, PM me and I can give you his name, he might have an answer to your problem.

As noted with TBW (throttle by wire) there's no need for an IAC. It can crack the butterfly as much as it chooses at any time.

I will say, watching the throttle position live, it corresponds with the stumble, eg it'll be idling at, say, 11....then drops to 9 and stumbles then responds by opening to 15. Rinse and repeat. Values approximate.

So....I don't know what to make of the low value which causes the stumble -- is the PCM commanding that? Or is something amiss with the throttle body? If the latter, why doesn't it happen at idle with no a/c and no large electrical loads?

 

[D60]

Try disconnecting the maf too which seems odd but many ecu's will still run without it but also switch the way they apply metering and erratic or weird running engines will run smoothly but with a less fuel efficient mixture and slower throttle response and acceleration but usually smoothen the rough idle and in rare cases weird shifts.

Speed density so no MAF. It uses IAT and MAP. Looking at IAT, the values seemed reasonable. I don't think I had time to look at MAP values, and I'd need to brush up on what they should be.

 

[JavierH19]

Speed density so no MAF. It uses IAT and MAP. Looking at IAT, the values seemed reasonable. I don't think I had time to look at MAP values, and I'd need to brush up on what they should be.

I know GM uses all 3 on their v8's like my L87 even if redundant to increase efficiency a bit. I'm guessing ford only does map and iat. I think the early ecoboosts had a maf too but they ditched it by the next revision to save a bit on costs. But those do have the same symptoms as dirty or malfunctioning maf's. Even if the pressure sensor reads correctly if it's gone slow and reads lazy it might bounce like how slow maf's do. I imagine it'll still be able to run with either one or even both disconnected and will resort to some default table for its logic. Try it and see what happens. Can't imagine it bricking the car where plugging it in wouldn't make it run again.