Mystery of the Missing Duty Cycle - Solved!

[JimW337]

All this recent talk about F.I. got me to thinking (Caution, this condition can lead to dangerous situations). I just thought I should check out some things on the still new to me Yellow Rabbit. Also, I was down at Sebring last weekend attending my second school and I the engine just wasn't pulling as hard as I thought it should in third gear, might have been the 110 heat index and the car original came from the Northeast.

This is what I found...

Control pressure is 49 hot and system pressure is 74, the WOT switch is zipped tied closed, no thermo switch, not sure about the thermo switch wiring if they are open or closed so I will just assume they are where they should be. When I go and check the duty cycle I can't get a reading at the "duty cycle plug".

Question 1: Is it common practice in the Hot VW racing ranks to bypass the WOT switch, thermo switch? Does this pretty much take the OX sensor and freq valve out of the equation and you are just left with a purely mechanical F.I.? Probably not a bad thing. And leads me to Question 2.

Question 2: Is there another way to check the air/fuel mixture without the duty cycle? I'm thinking I have seen where you can take a direct ready off of the OX sensor. Something in the neighborhood of 400-500 mili-volts sound about right? Maybe do this check at 4000 RPM? Or is there somewhere else to do the duty cycle test, at the "brain box"?

I am really liking the idea of taking the car to the dyno and getting the air/fuel and timing checked under load. Probably a good investment of time and money.

P.S. I'm signed off to go racing, so look for me at CMP on 9/16, I'll be the Yellow Rabbit with the big 'X'!

Thanks,
JimW
#74 Wraycer Wrabbit

[Bildon]

>> Question 1: Is it common practice in the Hot VW racing ranks to bypass the WOT switch

No. You can adjust the switch with the 2 screws so that it engages sooner.
But there is no reason to bypass it.

>> Does this pretty much take the OX sensor and freq valve out of the equation

Yes. You run open loop after switch engages.

>> Question 2: Is there another way to check the air/fuel mixture
>> I have seen where you can take a direct ready off of the OX sensor.

Use a logging wide band sensor in place of your OE sensor when tuning. (Innovate is best)
Dont try to read anything into the narrow band values. You can not tune your engine with narrow band voltage readings.

>>Probably a good investment of time and money.

Very smart move :023:

[JimW337]

Update on the Duty Cycle readings or lack there of..

It turns out the multi-meter is over-ranged in duty cycle mode and 4 cylinder dwell mode but I can get readings in the 5,6,8 cylinder dwell mode. I guess this means something in the system is making the duty cycle go over 100 percent.

I installed a new OX sensor and tried to get some mili-volt readings but these readings were going negative, again something has the system wonkers.

Next things I'm going to try...

1. Take the zip-tie off the WOT switch.
2. Chase down what the PO did with the thermo switch wires after the thermo switch was removed.
3. Change out the "Brain", maybe there is damage from my T-Bone experience at Summit Pt.
4. Test the Freq Valve.

I think I need to get this thing resolved before taking it to the dyno. I would like to spend the dyno time on fine tuning not general trouble shooting.

After this, I am open for suggestions.

[msogren]

One of my customers' cars wound up with a fuel test port in the cold start injector. Thus the thermo switch does not need to work in FL. My control pressure was /is lower than 49 tho and that does make a large differance starting the car and on the back straight at Sebring .(lower goes faster). Also try to keep the intake air cold. Duct from the headlight area if you can.
I fuel tuned just with EGT and plugs. and it went the same speed with the frequency valve unplugged and retrimmed to run without it. The frequency vavle does not like the vibration and can quit at the worst times. It's advantage is the car runs nicer at low power setting and idles nice. So, the car can run as fast with no computer at all and will work better, more often,IMHO(20yrs VW racing)

Dont foreget to balance the injectors. MM

[JimW337]

The mystery of the missing duty cycle has been solved!

The freq valve and brain box checked O.K., so I started chasing wires. While looking at the wiring diagram in the Bentley what do I see but something called the OX Sensor Relay. How come nobody ever told me about the OX Sensor Relay? We all know how much fun it is trying figure out which relay is what on our Rabbit's. But, maybe it's the one with the big red dangling power wire NOT hooked to anything!!!

So, after reconnecting the OX Sensor Relay power wire I had a duty cycle reading. Played around with the adjustment a little bit, I was pretty close just setting it by ear. And with the car sitting in the garage and free revving the engine, it sure felt and sounded like it was running better.

I think what I was missing at Sebring was the mid-range power, especially in the third gear corners. I will hook the WOT switch back on the TB and set it to open as soon as possible. I still plan on some dyno time and will try to get a run in with and without the WOT switch, just to see if it effects the mid-range like I believe it does.

Anyway, I hope this helps anyone else who ever has a missing duty cycle.