AFR fluctuations on idle when engine is below operating temperature

[matthewr87]

Recently I've noticed rhythmic fluctuations in AFR at idle while the engine is still warming up. Every 4 or 5 seconds the AFR jumps from ~14 to ~15, and results in a corresponding drop in RPM at the high end of the AFR. This seems to disappear when the engine has warmed up. Any ideas?

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[SnowFox]

Could be a tiny Vaccum leak. Could be you have your A/C or defrost on.(Though you'd probably know that)

Some more details might help narrow it down. The AFR as you fluctuates some, especially when the engine is cold.

Any other symptoms?

 

[EFI]

To me this sounds like a leak somewhere in the exhaust if it goes away when warmed up.

Vacuum leaks on the intake side usually don't go away with temperature, while leaks on the exhaust side can go away since metal pipes expand alot more than plastic. Alot of cars or trucks can have bad exhaust leaks at the manifold when cold, but then go away with temperature as the iron expands and seals back up.

I would check for exhaust leaks upstream or right at the first o2 sensor.

 

[JAJ]

Recently I've noticed rhythmic fluctuations in AFR at idle while the engine is still warming up. Every 4 or 5 seconds the AFR jumps from ~14 to ~15, and results in a corresponding drop in RPM at the high end of the AFR. This seems to disappear when the engine has warmed up. Any ideas?

The O2 sensors don't operate until they're hot, so while the engine is warming up, the ECU operates open-loop, delivering fuel according to the signal from the MAF sensor and some special programming that compensates for fuel not fully vaporizing in a cold intake. Generally it works pretty well. Something, maybe a CAI or perhaps a faulty PCV valve, seems to be causing a little bit of error, which the ECU only learns about when the RPM drops. Once the O2 sensors warm up, all is well and the engine is running closed-loop.

 

[matthewr87]

Hey everyone. Some good tips here, thanks. AC is not on because it actually doesn't work (started blowing hot a couple of months ago) and I haven't gotten around to getting it fixed.

Interesting angle about the PCV valve. I need to check my catch can. I haven't emptied it in a while. Maybe it is full and causing issues.

 

[EFI]

the ECU operates open-loop, delivering fuel according to the signal from the MAF sensor and some special programming that compensates for fuel not fully vaporizing in a cold intake.

I'm pretty sure that Coyotes don't run in open loop ever as they have true wideband sensors. I think the fueling logic is slightly different when cold to take into consideration that the upstream sensors may be cold, but it's still not true open loop like alot of the older engines.

This still points to something in the exhaust side throwing off the readings when cold.

 

[JAJ]

I'm pretty sure that Coyotes don't run in open loop ever as they have true wideband sensors. I think the fueling logic is slightly different when cold to take into consideration that the upstream sensors may be cold, but it's still not true open loop like alot of the older engines.

This still points to something in the exhaust side throwing off the readings when cold.

You must have a different version of SCT Pro Racer than I do.

 

[Pistol_91]

They will run in open loop for about ~30 seconds from initial start up and immediately go to closed loop. While in open loop lambda is set to 1 value. There is no fluctuation. If you're seeing fluctuation, it is in closed loop.
It could be the cams moving if you're only experiencing this when the car is initially turned on and it is warming up (going from emissions reduction to optimal stability). 14-15 is not anything id worry about unless you have other issues involved.

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