Wierd dip in AFR's

1986911 · 1986911's Garage

I get a dip in AFR's after I lift off the throttle? Is this normal or a leaking injector or something?

Hard to read, on photo but rpm is at 6460 and afr's at 11.7 when I lift. As soon as I lift, rpm's start to fall followed by a brief peak then the afr's go down to 10.3??? I confirmed it with 2 more runs, same result every time.

JFairman · 12-30-2012, 05:21 PM

If you have a blow off valve vented to atmosphere it will do that because you're releasing metered air to atmosphere instead of recirculating it back into the turbo intake.

By metered air I mean air that has flowed through the air flow meter and the fuel injection is injecting fuel to go with it on the intake stroke. If you vent the BOV air to atmosphere instead of recirculate it back into the intake like the factory does then the AFR will go rich like that on deceleration.
Some guys like that because the car will throw impressive flames out the exhaust on deceleration.

Or, if the recirculation BOV manifold is stock or a C2 recirculation type is on there now then maybe it's not working properly.

Tippy · 06:34 PM

Deleted OP

There's also a lag of the WBO2 displaying AFR's.

JFairman · 12-30-2012, 05:46 PM

He mentions a leask WUR down at the bottom of his post so if thats current he's using CIS, not EFI.

And, regarding what you said about EFI it all depends how the ECU is programmed. Usually OEM EFI is programmed to cut fuel completely on deceleration to get better fuel mileage and reduce exhaust emissions and it doesn't start injecting fuel again until you open the throttle. Or if you leave the throttle closed all the way down to idle speed then the ECU wil start injecting fuel again around 1500 rpms so the motor doesn't die out at 1000rpms or idle speed.

With aftermaket EFI you can program it to keep injecting fuel on deceleration or cut it completely on deceleration or anywhere in between and this happens instantly because the throttle position switch/sensor on the throttle body and the MAP sensor on the intake manifold react instantly to the closed throttle and the ECU cuts out the fuel injection instantly if it's programmed that way. With todays fast ECU processors there is no latency or lag time in response to the sensors.

1986911 · 1986911's Garage

I have the stock bov, it was just rebuilt. I am running the Fabspeed air filter with the little filters on the oil breather and lines that were plugged into the stock air filter system.

Did you say flames

No efi, just a tuned BL wur.

I am running a ton of fuel pressure, the shop that dyno tuned my car originally shimmed my fuel head because I was running out of fuel up top.

I recently replaced my rear fuel pump and checked my system pressure. My gauge goes to 100 psi. It blew past 100 psi and went up to what should be 110-115 psi.

Any issue with running that much psi? Would not bleeding a cis guage correctly give my these crazy readings?

Check out the plunger before and after.

Tippy · 06:27 PM

Quote:

ECU cuts out the fuel injection instantly if it's programmed that way. With todays fast ECU processors there is no latency or lag time in response to the sensors.

That reminds me, I need to set the fuel cutoff for decel on mine. It's % based, not 100% though. I have X-Tau and fuel overrun shutoff setup for cutting fuel when letting off but you made me realize fuel decel cutoff I have yet to turn back on.

Anyways, MegaSquirt/TunerStudio has lag settings for almost every sensor. You sure there is no latency???

JFairman · 12-30-2012, 08:28 PM

Well you deleted your post I was answering to so now mine looks like.. where the heck did all that come from.

anyway.. I don't know megasquirt other than some things i've been told that I understand.
As far as deceleration and the origonal posters question and what you had posted as if he has EFI, my post relates to my experience with bosch L-jetronic 20 years ago.
It has no map sensor like modern systems but the throttle position switch is instant and fuel is cut off imediately when you lift off the throttle all the way so there is no lag in that one paremeter.

And there are alot of diferent EFI systems these days. I have no idea what all the differences are and with computer processer speed these days I'm sure they react very quickly to changes in throttle position, temperature, and map sensors.

And compared to EFI or other bosch mechanical fuel injection systems like the early 911used, CIS is the s l o w e s t reacting fuel injection system ever made.

Tippy · 12-31-2012, 03:45 AM

Hijacking this thread but I just read that for MegaSquirt, you do not want to set fuel decel "on" for boosted engines because the slightest pull of throttle will cut fuel obviously momentarily leaning mixture. Not good!

So, I guess my EFI will continue to have a momentary rich area right after boost as the OP's when closing off throttle.