UPDATED- AFM induced fuel cut

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UPDATED- AFM induced fuel cut

Postby erolit » Wed Mar 09, 2005 5:58 am

EDITED AND UPDATED, SEE BELOW.


Ok heres the deal, I have installed a 4.3V Zenier diode on the PIM line, i have CUT (Totally severed and capped) the PIM pressure sensor wire DIRECTLY at the ECU based on http://www.alltrac.net/reference/ST185ECU.JPG. I have also disconnected the turbo pressure sensor harness at the pressure sensor.

The results were as follows, on addition of the Zenier diode, nothing happened (Yes, it is teh right PIM line, same colour as the wire going to the pressure sensor), and fuel cut stayed (yes it was the right diode). F

Following this i cut the PIM line with scissors and taped both ends of the tape. The boost pressure gauge on the dash still worked.... and the fuel cut stayed. (at ~12psi)

Following this, i disconnected the wiring harness AT the pressure sensor. This made the stock boost gauge on my dash read max boost when i start the car (it just max's out and stays there)... But i still get damn fuel cut!

ARGH this is painfull. What is going on??! I get codes 34, and 35 every time it fuel cuts - code 35 probably because i have cut the wire hence it detects a short.

HOW THE HELL can it tell what boost to fuel cut at if i dont even have the damn sensor plugged in???????????????????????????????? Something else simply MUST be triggering the fuel cut... but what?

I Don't want any fuel cut at all, ever, not once. Is there any way i can find whatever it is that cuts the fuel (Relay? Solenoid?) And smash the crap out of it with a spade?

This is incredibly infuriating :(:(

Any ideas greatly appreciated,
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Last edited by erolit on Tue Jul 19, 2005 8:13 am, edited 2 times in total.
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Postby erolit » Wed Mar 09, 2005 6:30 am

UPDATED:

HERE is something i have found that, in all of alltrac.net, only one person has ever mentioned - and they left after making this single post:


"THE AFM WILL CALCULATE FUEL CUT BASED ON AIRFLOW, USING THE AFM, IF YOU DISCONNECT THE TURBO PRESSURE SENSOR OR PIM LINE ELECTRICALLY."

I have just tested this, and it is true - If you disconnect the PIM wire, or the sensor, it will fuel cut at between 10 and 12 psi. the ECU will throw code 35, (short in the TPS), and will fuel cut you EVEN IF you plug the sensor back in - ie it will happen till you reset your ECU.

I also have reason to suspect that the stock boost gauge MAY be able to change into a mode where it calculates boost based on air flow, as with the PIM line to the ECU cut, my stock boost gauge will still move to full boost (albiet very slowly, and not at all with the boost curve)

This means that pulling the TPS wireing harness will NOT remove fuel cut, but pulling the pressure line from the sensor WILL, as the TPS will never see boost but it wont be shorted either.

IMO a bleed valve on the TPS pressure line is quite a good method of removing the fuel cut.

BTW the problem is solved now.

Elliott
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Postby Gary » Wed Mar 09, 2005 6:37 am

Try this.
Connect all the wires back together. Return to stock setup.
Then, cut the PIM wire AT the turbo pressure sensor.
Install 4.0V zener diode (yes 4.0V not 4.3V) on the PIM wire.
Make sure you connect zener diode correctly.
Anode side to the pressor sensor
Cathode side to the ECU.
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Postby Gary » Wed Mar 09, 2005 6:47 am

It is common for many control systems that if you remove the connector or if there is an open circuit, the main controller (ECU) will have a failsafe mechanism. In this case, I suspect that it doesn't calculate any fuel cut level. Instead, it will set the cut level back to default level (12 psi or so) That means if you remove pressure sensor plug, the ECU will not get voltage reading from the PIM wire and "could" go into default mode and set fuel cut at 12 psi automatically.
I have no way to prove this. It's just my speculation.
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Postby MrWOT » Wed Mar 09, 2005 8:26 am

You know... I was speculating this exact issue with toayoztan a few nights ago. He's got a friend who did a gte swap and has been having problems with fuel cut, he already disconnected the sensor etc, and I figured that the afm might be able to trigger fuel cut if an open circuit was detected in the boost pressure sensor. He's tried having the circuit both open and closed, but is still hitting cut with the hose disconnected. I wonder if the AFM will trigger cut even with the sensor properly connected once it reaches a certain flow?
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Postby erolit » Wed Mar 09, 2005 9:43 am

Im assuming Gary didnt actually read my second post.

I will write up a post detailing 3sgte fuel cut - it seems fuel cut can be generated in quite a few ways that haven't been documented. As well as this, it turns out the excessive timing retardation many people have been experiencing could have actually been caused by fuel cut...

From what i have found out this afternoon:

1. Fuel cut can be triggered by the AFM
2. If there is a "short circuit" in the PIM wire, or Turbo pressure sensor, then the ECU will switch to detecting fuel cut through the AFM
3. The stock boost gauge will switch to reporting "boost" through the airflow meter - ie the more air that flows through the higher the boost gauge will go, this happens in situations where the turbo pressure sensor pr pip wire is shorted (this can also happen when you screw up a fuel cut remover)
4. MOST IMPORTANT: A FAILURE SOMEWHERE IN THE CONNECTION BETWEEN THE ECU AND THE BOOST PRESSURE SENSOR CAN CAUSE THE ECU TO RETARD TIMING MASSSIVLY WITHOUT FUEL CUT BEING ENGAGED. If you have short in the PIM, screw up a fuel cut/zenier diode installation, or disconnect the turbo pressure sensor, the ECU will retard timing, seemingly at random? based on the ammount of air flowing through the AFM (more on this tomorrow). I replaced my knock sensor, because i was randomly having a timing retardation issue, only to find that one i re-connected the turbo pressure sensor and PIM wire, and simply disconnected the pressure hose from the sensor properly that the tiiming issue went away.

From this i learnt that:
you should NEVER, UNDER ANY CIRCUMSTANCES, ELECTRONICALLY disconnect (via the harness) the turbo pressure sensor. Doing this will cause the stock pressure gauge to start reporting boost via airflow, and retarding timing and generating fuel cut at random.

If you want a quick 1-step removal of fuel cut, simply LEAVE THE STOCK BOOST PRESSURE SENSOR SYSTEM IN PLACE, but disconnect the vacuum hose from the pressure sensor. (this will cause the sensor to read 0 (on atmospheric pressure). This will totally remove fuel cut, and any surrounding issues of timing retardation.

Excuse me for this post and any errors it contains, i have tucked into the alcomohol quite severly before the post.

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Postby Stubble » Thu Mar 10, 2005 1:05 am

Excuse me for this post and any errors it contains, i have tucked into the alcomohol quite severly before the post.


A man after my own heart! 8)
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Postby RedCelicaTRD » Thu Mar 10, 2005 2:43 am

Doesnt the stock gauge have its own signal wire from the pressure sensor? The stock gauge reads 0-4v while the ecu can read 0-5v. This is to help get the gauge to the + side even though boost levels can be different. and I also found that just clipping the ecu wire will cause boost cut. I dont think the stock ecu can calculate boost cut via airflow but it goes into a failsafe mode like gary said. And the only way to really know if the ecu retards timing when the pressure sensor hose is off would be to test it with something other than a butt dyno.
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Postby erolit » Thu Mar 10, 2005 4:08 am

Yes, but if you remove the connection from the ECU to the pressure sensor, then it can't actually detect what boost pressure the engine is running. I have no doubt that it goes into a failsafe mode if you do this, and it has to detect fuel cut some how - and that way is thruogh the ammount of airflow going into the engine (im pretty sure).

At 7psi with the PIM wire cut, it would not fuel cut even at WOT from 0-150km/hr. At 12psi, it would fuel cut almost as soon as i went over 10-11psi, every time. I would get diagnostic codes 34 and 35, so it was definatly turbo pressure induced.

Somehow this is also related to timing retardation. With the PIM wire cut, on WOT even at 7psi, it would randomly pull timing HARD. Im talking car juttering, boost but no power kind of timing retardation. On reconnection of the PIM wire there was no problem. It was DEFINATLY a timing issue, and it definatly was related to the turbo pressure sensor being electronically disconnected. At a guess i would say that with it disconnected, the failsafe mode the ECU was in made it far more responsive to small negative changes in air flow temperature, knock, etc.

Just a note, that once i hit fuel cut for boost with the PIM disconnected, i then had to pull an EFI fuse as it would go into safe mode 2000rpm limited, just as if i had hit fuel cut normally.

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Postby Mafix » Thu Mar 10, 2005 4:24 am

wow...lots of fun here. how come hardly anyone uses a FCD? because the diode works just fine kind of deal?
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Postby Gary » Thu Mar 10, 2005 5:56 am

erolit wrote:Im assuming Gary didnt actually read my second post.

I will write up a post detailing 3sgte fuel cut - it seems fuel cut can be generated in quite a few ways that haven't been documented. As well as this, it turns out the excessive timing retardation many people have been experiencing could have actually been caused by fuel cut...

From what i have found out this afternoon:

1. Fuel cut can be triggered by the AFM


How do you explain why people can successfully install
HKS FCD and other homemade FCDs and make it works without
touching the AFM?
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Postby erolit » Thu Mar 10, 2005 2:20 pm

Because the FCD's dont physically sever the connection between the turbo pressure sensor and the engine, they just restrict and modify the input.

This post is detailing why not to ever electrically disconnect the pressure sensor.
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Postby Gary » Thu Mar 10, 2005 3:25 pm

Sorry, I didn't realize the title was editted. :doh:
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Re: UPDATED- AFM induced fuel cut

Postby alltracman78 » Sun Mar 13, 2005 2:47 am

erolit wrote:I Don't want any fuel cut at all, ever, not once. Is there any way i can find whatever it is that cuts the fuel (Relay? Solenoid?) And smash the crap out of it with a spade?


:rofl:

I've been thinking about using a second bc for fuel cut. I think the setting it up will be kind of a pain.
I will get around to it one of these days....
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Postby makarov » Mon Mar 14, 2005 9:41 pm

So, could someone exlpain how to disconnect the vacuum line (which one etc...) and why this would not be advisable? I suppose unregulated the car could over-boost. On an otherwise stock car, how much boost will it generate?

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