Stall from fast rpm drop
08-17-2013, 01:44 PM
#31
Rennlist Member
I'm staying tuned to this. I'm on a bit of a personal mission to see how far I can push the AFM. Mine is ported to hell and back and I'm interested in using something like this to add fuel based on a MAP reading to compensate for the MAP levels which exceed the AFM's ability to control fueling. I spoke with Joshua a while ago and he told he could accomplish the same thing on chip, but that it would of course be an iterative process to get the variables correct. Something like this would make it easy to keep twisting a dial ( simplifying it I know ) to get my fueling correct. I think a true 400 rwhp is doable with an AFM-MAP tune.
08-17-2013, 04:26 PM
#32
The Impaler
Rennlist Member
Rennlist Member
I’ve struggled with this several times. You definitely need to confirm all the basics are good – TPS, ISV, Base Idle, vacuum leaks, grounds, temp sensors, etc. What usually happens is that the MAF signal voltage is remaining too high when the rpms get to idle speed, and the motor is flooding as a result. Normally, at idle, the MAF signal voltage (as measured on pin 7 of the DME) is around half a volt, plus or minus. When you floor it and let off, the voltage needs to go from whatever it was (say 2 volts) down to half a volt or so before the rpms drop to idle and the injectors kick back on. Normally, this isn’t a problem as most MAFs react quickly. However, if you have a vacuum leak, it will act much like a blow off valve vented to the atmosphere – i.e., it will throw off the air fuel ratio because the MAF is measuring more air than ever makes it to the cylinders and as a result the DME will deliver too much fuel. Also, if the MAF is too close to the bypass valve (or turbo) or you clock the MAF in the wrong way, the bypass air (or turbo) can create a windstorm in the MAF and generate a mistakenly high voltage. Early MAFs were notorious for this. Speedforce racing made an early name for itself by figuring out to move the MAF further away from the bypass valve. You can test all this by watching the voltage on pin 7 of the DME via datalogging or a friend with a multimeter in the passenger seat. If you floor it and let off at low RPMs, you will probably see the voltage remaining too high (e.g. twice your steady-state idle voltage) when the RPMs come down to idle speed. If your fuel controller has the granularity, you can mask the issue by strategically taking lots of fuel out under and around where it idles (but preserving the normal idle itself). As for a real cure, every time this has happened to me, it’s either been a vacuum leak (even though I would have sworn on a stack of bibles I had none) and/or a base idle set too low.
So here’s what I’d suggest. First triple check all the basics. Then check again for vacuum leaks – pull the I/c couples off and bend them in your hands looking for slits that might open under pressure, etc. If that’s fine, then I’d set the base idle (the screw on top of the throttle body) as high as you can while still allowing the system to return it to 840+/- when warm. If you still have the issue, try clocking the MAF differently and/or extending it as far away from the bypass valve and turbo as possible. If none of that works, chuck it and buy a Vitesse MAF.
So here’s what I’d suggest. First triple check all the basics. Then check again for vacuum leaks – pull the I/c couples off and bend them in your hands looking for slits that might open under pressure, etc. If that’s fine, then I’d set the base idle (the screw on top of the throttle body) as high as you can while still allowing the system to return it to 840+/- when warm. If you still have the issue, try clocking the MAF differently and/or extending it as far away from the bypass valve and turbo as possible. If none of that works, chuck it and buy a Vitesse MAF.
I had this same problem with an OLD style vitesse MAF for a loooong time on my old car. You hit on it in this post.
MAF turbulence. That ended up being the problem for me. A lot of care has to be taken to ensure proper MAF positioning and design. From what I have seen in other aftermarkets, the possibility for turbulence seems high in the design most commonly used in 951s.
The clocking of the MAF, positioning of the sensor, shape/length of inlets, placement of recirc system, etc all have an effect on it...
I tinkered with the positioning for days until I got it right.
08-22-2013, 05:06 PM
#33
Instructor
I was having this problem after installing the abaco maf using an aftermarket adjustable blow off valve; i switched to the billet bypass by paragon products and it immediately went away. Just fyi.
08-25-2013, 04:08 PM
#34
In 2006 I had the same turbulence problem with my LR MAF. My mechanic worked closely with LR and we ended up changing some of the tubing and moved the MAF sensor farther away from the engine. Getting this right was a big hassle and my mechanic ended up designing a custom cold air intake system that would work with extra long MAF tubing. The problem is completely solved and the car has not stalled since.
08-25-2013, 07:03 PM
#35
Pro
Is it possible that changing the factory DV to a billet DV could increase/create MAF turbulence? My problems occurred some time after I "upgraded" to a billet BOV. Unfortunately, I sold my Bosch DV in a clean-out sale so I can't jut put it back to test that theory.
Last edited by jmj951; 08-26-2013 at 05:30 PM. Reason: changed BOV to DV
08-25-2013, 07:51 PM
#36
Is it possible that changing the factory BOV to a billet BOV could increase/create MAF turbulence? My problems occurred some time after I "upgraded" to a billet BOV. Unfortunately, I sold my Bosch BOV in a clean-out sale so I can't jut put it back to test that theory.
08-25-2013, 11:23 PM
#37
Rennlist Member
The BOV is part of the equation for sure, and a different version can contribute to the issue I have seen. The MAF signal voltage just takes too long to return to idle levels after a heavy load (and BOV blow off), so instead of being around .7 when the motor gets back to 840rpms, the MAF signal voltage might be well over 1 volt. That in turn sends way too much gas to the motor and floods it. The problem is only worse with lightened flywheels/cranks, etc. since the MAF has less time to normalize. My Arduino-based fix clips the idle voltage to a specified max whenever the throttle switch is closed. It's working great, but must be programmed and adjusted based on the idle voltage of each car. I'm working on a variant that will self-calibrate to the car's idle. When that's done, I'll post up details in case anyone wants to make one.
Last edited by Tom M'Guinn; 08-26-2013 at 08:05 PM.
08-26-2013, 12:23 AM
#39
Rennlist Member
It's fairly simple. The MAF wire headed to DME pin 7 is cut and sent to the new box, along with a tap off the idle contact wire. DME Pin 7 is then fed a new wire from the box, which normally carries the original MAF voltage but, if the throttle is closed and the voltage is too high, it substitutes and idle-friendly voltage instead. 5 wires in all -- power, ground, DME7 IN, DME7OUT, and idle contact.
08-26-2013, 11:24 AM
#40
Rennlist Member
It's fairly simple. The MAF wire headed to DME pin 7 is cut and sent to the new box, along with a tap off the idle contact wire. DME Pin 7 is then fed a new wire from the box, which normally carries the original MAF voltage but, if the throttle is closed and the voltage is too high, it substitutes and idle-friendly voltage instead. 5 wires in all -- power, ground, DME7 IN, DME7OUT, and idle contact.
