MKPatrick

Hello,

I just bought a 2013 5.0 Mustang with the Brembo brakes and an automatic transmission.
This is my first Mustang ever.

I now have about 4k miles on it. I've added a security system and remote start which are aftermarket.

So here is my first technical question about this car:

I watch gauges a lot. (I work in aviation so I'm about instruments, I can't help it.)

The air/fuel gauge when I first got it was rock steady at idle, it sat about 14.3.
Now it moves around. When it used to be around 14 or so, it now goes from 14.9 down to 12.2 or so.
The idle also fluctuates a bit.
Sometimes, the idle will fall down to about 500 or so very briefly and then recover to about 1100 and then settle back to about 800 or so. Almost a stumble. This will happen every few minutes or so.
Incidently, I've only put 92 octane it since I've owned it.
Could this be an airfilter issue? If so, what is recommended for air filters?

Is this normal?

Thanks for any advice,

M

Roush05

I think 14.3 is a bit lean, but I could be wrong. 14.9 definitely is.

BigDinTexas

On my old '99 Eclipse GS that I turbocharged (stage 2, T3/T4 ball bearing) when in closed loop operation the A/F would bounce around like that, but when I got on it, that's when it was rock steady at a perfect 14.7 A/F ratio (stoich), but this was before the widespread use of E85 blends.. Ethanol actually has a different stoich A/F ratio, so maybe some of what you are seeing is because of variations in the E85 content of your gas.

Is what you describe happening when it is just normal idle or driving around at normal speeds? Could you be observing closed loop operation?

As far as the RPM stumble, I'll get that sometimes, but usually when I have my AC on so I always attributed the RPM change to the compressor kicking on/off.

Don

BigDinTexas

As far as whether your air cleaner could be the culprit, I would think that with only 4k miles that it wouldn't but do you live an in an especially dusty area? Do you have an aftermarket intake or filter?

Don

MKPatrick

This has only been at idle.
Not that dusty here, humid but not dusty.

I'll watch for this when I have the AC on and report back.


I expected some variance with different driving conditions such as acceleration and coasting.
Coasting in gear pegs the needle at max.
Acceleration, if abrupt will have the needle dip to around 12 for a split second which I can imagine is normal.

What concerns me is the at the below 2000 mile mark, it was rock steady at idle.
Now its constantly bouncing between 12 and hi 14's.

M

moosestang

You shouldn't be seeing 12 anything at idle, nor should the rpms fluctuate that much. Take it to the dealer.

jpplaw

"Coasting in gear pegs the needle at max"? Are you sure you are looking at an AFR gauge and not a vacuum guage?

Fish05

As for the idle change is the AC on while it is doing this? It could be the AC unit engaging and putting a bit of a load on.
It could be a vacuum leak but you are under warranty I would have the dealer check it out.

MKPatrick

No AC on, I checked for that. I do hear the cooling fan come on from time to time but that doesn't seem to affect it.

OK, I will take it to the dealer.

Thanks for the advice!!

So glad I wrote about this!

Fish05

Remember to post up what/if the problem was.