After installing my CAI I filled up my tank and calculated mileage. I averaged right around 21mpg combined city/highway which is what I was seeing with the stock air box. After refilling I calculated mileage again and it had dropped to around 17mpg. On my last fill up it has now dropped to an average of 12mpg. I am calculating mpg by dividing miles on the trip meter by the amount of fuel needed to fill the tank. My driving style hasn't changed, my tires are inflated to 32psi and a complete tune up was done to the car last May. I'm not getting a CEL. I'm thinking that the 02 sensor in the manifold could be the cause (it is the original), but the last couple of mornings I noticed the car smells like it's running rich for a few seconds upon initial fire up. It doesn't smell rich when restarted during the day...only after sitting overnight. Leaky injector maybe? What are your thoughts on this? I also read that the PCM may need to be reprogrammed after installing a CAI, but would the mileage continue to drop off like it is?

 

Do you have an oiled filter? Try cleaning the MAF.

Also colder air = denser air and the engines solution to all that air is dumping fuel at it.

 

I pulled and cleaned (CRC MAF Cleaner) the MAF the day after I installed the CAI. I do believe the air filter that comes with the CAI is oiled though so I'll pull and clean it again. Wouldn't the 02 sensor detect a rich exhaust and tell the PCM to decrease the amount of fuel, or is that where having the PCM reprogrammed comes into play?

 

I believe the stock o2 sensors just operate within a range, not an exact. So it would have to run really rich for that to happen. But I'm not 100%

 

I've heard unplug the battery after modifications so the computer can reprogram its self. Maybe put the stock airbox on and check mpg to decide if its something other than that cai? And like chibi said cold climate can kill mpg with a cai.

 

Unless he disconnected the battery's negative terminal during installation. I know K&N tells you to do that when installing one of their intakes.

 

I pulled the MAF sensor and cleaned it again even though it didn't appear to be dirty. The battery cable was disconnected when I installed the CAI, but I'll pull it for 30 minutes or so then reattach to see if it makes any difference. The rich smell in the morning is overpowering. I mean make your eyes burn rip your lungs out overpowering. That's why I was wondering about a possible leak in an injector, but wanted to try and eliminate the possibility of a faulty 02 sensor first. I also noticed that the car no longer pulls hard when I accelerate. It now bogs and lays down...something it has never done before. The computer compensating for the additional air by dumping more fuel makes sense also, so I just might put the stock air box back on this weekend and see if it resolves the problem.

 

Seems like a logical plan, go back to what you had prior and see if it reverses the situation. Honestly though, a CAI should not cause this much of an issue, but this is a good first diagnostic step.

 

That's what I've been thinking also. The car ran great right after I installed the CAI with the exception of the mileage. At first I was thinking the PCM just needed to adjust itself. When I saw the mpg drop continue at each fill up I raised an eyebrow. The rich smell in the morning and power loss started a couple of days ago. With the way the mileage has been dropping and now a rich condition in the morning I'm starting to think there's a leak somewhere (maybe an injector) that is getting progressively worse.

 

Hi `Mike,
Sorry to read that you are having a problem with lower MPG's How many miles on your Monte ?

When is the last time you replaced your PCV valve ?
They can cause lower MPG's if they are bad or clogged..
That's my only suggestion 4 now

Quote:

Decreased Engine Performance & low MPG's

Quote:

A bad PCV valve may stick or not close properly. This allows an excess of oxygen to enter the combustion chamber. Excess oxygen dilutes the air/fuel mixture ratio, causing a "rich" fuel mixture. A rich mixture can lead to a build up of back pressure in the exhaust system, which can cause engine stalling and a decrease in overall engine performance...

Good Luck on finding your problem...Keep us updated `ok.