1992 Camaro Z28
L98 5.7L V8 350ci. Mild Cam, no cats. Headers.

BEFORE Nov of last year my avg was 180km/per fill up.

November of last year I have my engine tuned (dealer) up and was getting approx. 235km per tank full (Well 1/4 tank or so left). At the time I had no cats running.

Few weeks ago I put the cats on to get thru aircare and then had them install Flanged straight pipes which are a bit longer and larger in diam. The dealer then retuned the car back up as the car had to have the timing DROPPED to get it to pass aircare.

Ever since I put the straight pipes back on, retuned back up, I've noticed since then my gas plumets to around 180km per fill up like it was BEFORE.

I discovered that the air tube that goes from the PCV valve to the TPI is split around 1 1/2" long at the very end were it pushed into the nipple of the TPI throttle body. It was BARELY even on it, basically hanging there. I pushed it back on and complained to dealer that I was getting the same gas mileage than I had BEFORE they did the timing.

They claim the car is likely running lean because of this tube allowing air in.

[Pic found on google]



Is this true? If this tube is allow air in (passenger side) will it effect my gas usage??

 

IIRC, that is the pcv air makeup tube(ie breather) and yes, that will effect fuel economy, the MAF will read less air then is actually going into the engine, o2s will read lean, and the pcm will add fuel to compensate

Sent from my PC36100 using Tapatalk

 

You can't use km per tank to accurately determine real fuel mileage. You need to use km per gallon. There are lots of people that throw a breather on the valve cover and do away with the hose. I've never heard anyone complain about a fuel mileage change. The O2s will cause the computer to eventually adjust the BLMs to compensate for the small amount of air that is bypassing the MAF. I don't see this as being a cause for a significant change in fuel mileage. I would still fix it and you can see what it does for you.

 

I posted this on another message board as well and part of the replies are YES it will cause the engine to run lean, and the other half NO it will not.

I don't understand.. Are people guessing or really don't know how cars work? (Not trying to sound sarcastic or anything I just don't understand how it can be either a strong YES, or strong NO. Obviously only ONE can be the correct answer and the others are guessing)..

 

As has already been explained, it may cause it to run lean briefly. In an MAF setup, air bypassing the MAF sensor means the ECM has a low number for the mass air entering the engine, and doesn't add enough fuel = running lean. But the O2 sensors pick up the lean condition, pass that info on to the ECM and the ECM adjusts the long term fuel corrections (aka "BLMs") to bring the A/F ratio back to the hard coded stoichiometric value of 14.7:1. Ran lean for a few seconds, but the ECM corrected. No longer running lean.

You don't appear to have an MAF sensor, so your engine is running in speed-density. The ECM calculates the mass air flow based on the engine displacement, RPM/2, manifold absolute pressure and the intake air temperature (also converted to an absolute reference). From that point on, the process is the same. If the engine starts to run lean (and it's less likely with speed-density) the ECM corrects the condition based on feedback from the O2 sensor(s).

And technically, the PCV vent hose is not attached to a vacuum port on the throttle body. The port it connects to goes into the chamber right under the cover plate, and that chamber connects in FRONT of the throttle blades. So it won't affect idle speed, and in a speed-density setup, will be part of the mass air calculation. The MAF setup would still be affected because the air is still bypassing the MAF sensor.

But in the end, assuming the O2 sensors are accurate, you have no exhaust leaks before the O2 sensors, and you do not have excessive misfires, the ECM corrects to the 14.7:1 A/F ratio that the engine is supposed to run at in closed loop (part throttle, low engine load).

 

Just did a fill up (had about 1/4 tank left).

Distance Driven : 210km
Fuel Used: 41.714L
Cost $ 62.49Cdn & 1.496/L

Calculates to 19.9 liters per 100km
fuel cost is $0.3 / km

ALL CITY driving! Rarely drive over 50KM and coast as much as I can.

 

Which converts to 11.8 MPG.

 

What do other people get?

 

I get about 16-17 city driving with my small cammed (CC503 on a 114 LSA), 1.6 rockers, no headers, A4, stock stall, 3.73 gears, 95 Camaro, being nice to it. I should probably get better than that.