Disconnecting MAF while troubleshooting a lean condition....WHY?

[Lee_Burough]

Re: The TTA Service Manual Supplement (page 6E3-A-58).

It says that a bad MAF sensor output can cause the ECM to sense lower than normal air flow causing the system to go lean, hence high BLMs, right?

The manual says to disconnect the MAF with engine in closed loop and at idle. If the lean condition is gone (BLMs go back to normal) then, you have a bad MAF. Am I reading this correctly?

My BLM reading went from 145 to 113 by disconnecting the MAF. Tried the known good MAF too, same thing. Are both MAFs bad? What am I missing here?

Thanks in advance

 

[T-top87GN]

Pulling the MAF out of the loop will make the car run off of hard coded values instead of what it reads from the sensor. These were factory programmed values that are good enough to make the car run. Not run good, but it will get you home in the event of a failure.

So the reasoning is that by taking the MAF out fixes the lean condition, then the MAF is the cause of the lean condition by reporting incorrect values to the ECM. Since you reading changed from 145 to 113 then you have shown that the MAF is the culprit according to the service manual.

However, if you want to see if the MAF is bad simply record a nice blast on the road with your scantool and then review the MAF numbers, THAT will tell you if it's bad or not. What codes have been thrown with your lean condition?

 

[Lee_Burough]

T-top87GN, Thanks for the response

No codes here....Only high BLMs at idle (at 143 now, takes 150 to set a code). Off idle its 127-130. Passed emissions yesterday by idling above 1100 RPM where the BLMs go to 128. Turbolink recordings show normal MAF readings and normal BLMs for all conditions except idle.

Have done all the usual things: Looked for vacuum leaks, inspected every inch of vac line, new PCV, butane at vac connections, blocked the vac block on top of the intake....All led me nowhere. Two different OEM MAFs read identically in all conditions (top out at about 236 gps @ 12 PSI).

I've just reread RAZOR's BLM post on t6p.com and will check and recheck for leaks in the intake systems above the heads. I might have to change injectors (I have a spare set with 20k miles on them), O2 checks OK. EGR valve checks OK. Installed stock FPR yesterday. I'll look again at the MAF hose and see if I can find any air leaks. Might try another chip too.

I think its back to the Thrasher and open loop idle with BLM and INT locked at 128.... ....really.

I think I see what unplugging the MAF does for troubleshooting a lean condition. thanks, any suggestions?

lee

 

[TurboDave]

The numbers you're seeing (143) are not terribly outrageous. A vacuum leak will almost always bump them up to 150 (or whatever the max is in the chip).
Your WOT maf number of 236 is about right for the low boost level you saw it at.
BLM's in the 140's usually (not always) indicate a slightly miscalibrated fuel curve in the chip. Injector constant could be a few # off, etc.

 

[Lee_Burough]

Originally posted by TurboDave
The numbers you're seeing (143) are not terribly outrageous. A vacuum leak will almost always bump them up to 150 (or whatever the max is in the chip).
Your WOT maf number of 236 is about right for the low boost level you saw it at.
BLM's in the 140's usually (not always) indicate a slightly miscalibrated fuel curve in the chip. Injector constant could be a few # off, etc.

Thanks TurboDave, I take special note whenever you post Especially when your advice is encouraging and easy to do. I'll start with another chip. For now, I'll just try another stock chip. When I get confidence in the whole system, and its working within normal limits, I'll prolly put a Thrasher in there until I get the Alchy with a new chip.

Thanks for your help on this

lee

 

[Lee_Burough]

Replaced the chip with a stock version....Reset the PCM and then idle BLM was 135

Satisfied that there is no real out of limit condition existing in the tune, I just put the Thrasher in and will call it good until the next issue comes up.

Thanks to T-top87GN for helping me understand the purpose of disconnecting the MAF during troubleshooting, and to TurboDave for reassuring me that my MAF number at WOT was fine and that the fuel curve in the chip is suspect in cases like mine.

All is well that ends well

 

[T-top87GN]

Glad you got it figured out. I figured it was probably the inj PW in the chip.

Once that MAF dies on ya, get a Translator and an Extender chip and never look back!

 

[Lee_Burough]

Originally posted by T-top87GN
Glad you got it figured out. I figured it was probably the inj PW in the chip.

Once that MAF dies on ya, get a Translator and an Extender chip and never look back!

I bought the whole T+ setup for my first TTA. I enjoyed it, but never used its potential. This time I'm planning on a stock engine and drivetrain with a progressive Alchy system....TTAs are now running low 11's on pump gas an Alchy....

 

[TT/Ameasap]

Originally posted by lburou This time I'm planning on a stock engine and drivetrain with a progressive Alchy system....TTAs are now running low 11's on pump gas an Alchy.... [/B]