Help with AFR reading at WOT

[Damon88]

Need some help with my AFR readings. Here is my engine set up.

1983 911sc 3.0 with Lambda, SSi's and sports muffler. Running 93 octane, All fuel pressure tests are in spec. Currently running Oxygen sensor unplugged and using Wideband Ox sensor. Waiting to see welder next week to have second bung installed on passenger side SSi.

I just installed a PE Wideband and took the car out for a drive. Having some lean issues between 5000 and 6000rpm. Here are the numbers.

Outside temp were 68

Warm idle was around 12.4 AFR (idle set 950rpm)

3rd gear cruising at 4000 rpm 13.8 AFR

3rd gear between 4000 to 5000 it starts jumping around 17.0 AFR then starts dropping once speed is reached. ( AFR numbers are fluctuating quickly )

3rd gear at 6000 RPM to WOT it goes back to roughly 13.2 / 13.8 AFR.

There seems to be this jump to 16.5 to 17.1 when ever I accelerate hard in any gear, then goes back down once RPMS settle and back to cruising. On decelerating I see it go down around 12.9 AFR.


All vacuum lines and hoses are new. When I remove oil cap idle drops.

Any help with some info on this would be greatly appreciated. Cheers

[PeteKz]

Both my WURs (an 033 and an 089) act much the same way. This behavior is the roughly the opposite of what you would expect the WUR to do. I have removed, disassembled, cleaned, reassembled, and checked pressures on them. They still act that way.

What cam are you using?

I've spoken to several other members who have observed the same behavior. I'm suspecting that the WURs just work this way, however, previously people weren't aware of it because they didn't have AFR/lambda meters installed.

I'm probably telling you what you already know, that a 12.4 AFR at idle is too rich. That corresponds to a CO% of 8% or more, which is way above the spec. But, if you lean out the AFR to 14.7 at idle, then it starts to run roughly at idle and cruising throttle. So you run it rich for good drivability.

I am considering doing what several other members have done, which consists of installing a manual fuel pressure regulator in the CP circuit and bypassing or removing the WUR. Then you can dial in your AFR for whatever your driving conditions. For highway cruising and gas mileage you can set it to 15.5 or so, and for power, you can set it to 12.6 or so. Turn it down for cold starts, and turn it back up when warm.

Anyway, let's see if Tony has an explanation. Tony, do you have an AFR meter in your car?

[Damon88]

Peter thank you for responding. You definitely know a lot more than me when it comes to this set up. I purchased a new WUR from K jet couple months back when my engine was rebuilt. Car runs and idles great at 13.8 AFR. But it was lean between the 5000 and 6000 acceleration. So I tried to richen it up at idle to see where out would be at WOT. Cam is stock. I installed a PE Bluetooth Wide band AFR to monitor. Maybe Tony can shed some light as you mentioned. Cheers

[elt0m]

I had some issues with my car last year, even with a rebuilt WUR and fuel head. The issue was way far from my fuel system—a major air leak. Look at all your intake runners to make sure they are all bolted down and secure. Mine were loose, and when the car was at high rpm, the AFR was oscillating and backfiring when decelerating. Also, look at your bonnet on the top of your throttle body. At high rpm, the car will suck air from anywhere so your AFR will become lean.

[PeteKz]

Damon, With a relatively new WUR, I think you can rule that out as a source of the AFR changes. FYI: I'm running an M1 cam, with more duration than stock, so that should reduce my vacuum somewhat. But that still doesn't account for the AFR behavior, because the WUR CP increase is already maxed at 10" vacuum on my 089, and maxed at even lower vacuum on my 033.

When you step on the gas, the intake vacuum drops dramatically, to near zero at WOT. So the WUR decreasing the control pressure should enrich the mixture and AFR reading, but it seems that it does not. A puzzler.

['78 SC]

PeteKz's comments about manifold vacuum would apply to a CIS basic system (early SCs), but Damon88 has a 1983 engine with the Lambda CIS system. That uses the Lambda controller, throttle switches and frequency valve to enrich the mixture for acceleration and WOT. Have you confirmed those components are working? Even with the oxygen sensor unplugged, the system will operate in "open loop" mode.

[boyt911sc]

Originally Posted by '78 SC

PeteKz's comments about manifold vacuum would apply to a CIS basic system (early SCs), but Damon88 has a 1983 engine with the Lambda CIS system. That uses the Lambda controller, throttle switches and frequency valve to enrich the mixture for acceleration and WOT. Have you confirmed those components are working? Even with the oxygen sensor unplugged, the system will operate in "open loop" mode.

CORRECT. People are comparing the enrichment characteristics of a vacuum operated CIS system to a non- vacuum (lambda-O2) which are totally different would only cause more confusion.

Tony

[PeteKz]

He says the lambda is unplugged. With lambda working, that should correct for changes in AFR between idle and WOT and the switches should cause enrichment at WOT. Without, it's back to an open loop system. But, as both of you noted, it doesn't have the vacuum signal like 1973-79 and RoW cars do.

Does the OXS box have any effect on enrichment at WOT or idle when the lambda is unplugged? I don't have that system, so I don't know if it still provides enrichment, or is completely disabled.

From his description of the AFR behavior, it sounds like it is full open loop, and there is no WOT enrichment going on. This could be due to the switches not providing the signal to the OXS, or because the OXS is not operating.

Damon, is the OXS system still connected to power? Fuse still in place? Do you know what part number is on the WUR?

[AndrewCologne]

1. With warm engine and with OXS sensor plugged you should read Lambda 1 (AFR 14.7:1 with no ethanol fuel) during all engine rev states, except at higher acceleration and WOT, here the ECU switches to a fixed and non sensor controlled enrichment state, means 65% duty cycle.

2. IF you read an AFR higher than 12.5:1 at WOT you can compensate that by turning the CO adjustment screw clockwise. But this also affects the duty cycle at idle and and normal driving, but this is no problem as still lambda 1 will be the result with a proper working ECU.
So just turn the CO screw clockwise until a resulting duty cycle of aprox 40% at idle will result, wich results in approx. AFR of 12.5:1 at WOT when the ECU switches to 65% duty cycle, ... so you must try ... it's an approach thing.

3. It's a known problem with the original ECU that at gear changes the mixture leans out for a moment and that's a known problem and the backside of that type of ECU from these years. I developed a CPU based ECU for '80-'83 911 3.0 lambda models wich beside other enhancements it uses a totally different approach at gear changes. Means at gear changing a slight mixture enrichment is done by the ECU so at all gear changes and at small or higher accelerations from "closed to opened throttle" a 1 or 2 sec extra enrichment ist done, ... like known from the cold sensor/temp state where the Original ECU witches from 65% to 75% for two seconds. Everything can be set up via Software, even a Lambda target other than 1.0 while normal driving or special mixture enrichment at the very beginning of cold starting etc etc etc.

https://nineelevenheaven.wordpress.com/optimized-control-unit-ecu-for-the-911-sc-with-lambda-control/

And here info about the 911 SC ECU lambda based mixture control in general
https://nineelevenheaven.wordpress.com/the-lambda-control-of-the-911-sc/

[Damon88]

After more digging and some a couple phone calls with Ian(iCarp) I was able to figure out my issues. Looks like my injectors and seals need to be replaced.Not sure if this is 100% the issue. But a great place to start. I Sprayed some carb cleaner throughout all the area's that have vacuum. Everything was fine until I sprayed injectors. Idle changed immediately. They are fairly loose and wiggle back and forth. All parts have been ordered. I will install next week and report. Thanks again, to everyone that replied! Big thank you to Ian for taking the time on the phone to walk me through a simple process of elimination. Cheers to all.

[boyt911sc]

Damon,

Do a smoke test and you will find more leaks that you missed. The bottom and rear sides of the CIS air box are almost inaccessible using carb cleaners for locating these hard to find air leak sources.

Tony

[Damon88]

Originally Posted by boyt911sc

Damon,

Do a smoke test and you will find more leaks that you missed. The bottom and rear sides of the CIS air box are almost inaccessible using carb cleaners for locating these hard to find air leak sources.

Tony

Smoke test kit ordered. Great call. Thanks Tony!

[don gilbert]

I got so sick of out of spec WUR's I deleted that ancient contraption and went with this setup 13 yrs ago and have converted havle dozen kjet systems to this. Make life so much easier. Just a simple reg with a knob on it. Complete control of AFR at all times. Top gage is afr, bottom control pressure.

[PeteKz]

Don: Nice setup. As mentioned above, I'm considering the same thing because the AFR goes all over the place with the WUR installed. I already have the AFR gauge.

Q's: What manual FPR did you use?

I wasn't planning to install a CP gauge, but seeing yours, I might do it. What did you use for your CP and how did you wire/route it?

[Damon88]

Back with an update.(work has been busy) Smoke test kit arrived and sure enough I had some leaks where the Brake vacuum splits off to other connectors to air box. There were no other signs of any leaks, so that is great. Everything fixed and back together. Smoke test was perfect.( Thank you Tony for the suggestion). Also installed new fuel Injectors, sleeves and O rings... wow, what a job. Also cleaned all the throttle housing.

Went for a drive and my problem still exists...after reading Andrews site, I figured I would try and set the idle to 13.2. Idle surges a little when engine is warm. Assuming, because it's set rich. Cruising in any gear & any RPM, I show roughly 13.4/ 13.6 AFR steady. When I accelerate hard and hold it to the floor it starts jumping to 15/16.9/18 AFR but quickly comes down once I reach speed.. The AFR gauge fluctuates so quickly.. If I stay in 2nd or 3rd gear and slowly increase speed on highway and stay around 5500 rpm it's steady at 13.3 and when I decelerate it goes down around 12.3 /10.5 quickly then back to 13.4. Driving around town it stays between 13.2 and 13.8

I did a fuel pressure test a month or two ago. My cold pressure was in spec. Warm, I did have to run the fuel pump pressure test on for up to 10min, to get a proper spec. then checked the residual pressure for the next 20 min and it was in spec. I think the Bentley Manual says to run the fuel pump for 3 mins and then off and watch the pressure for up to 20 min..This is a new WUR from Kjet and have spoken with them to make sure it is in spec... Sorry if this sound confusing, just trying to paint the clearest picture of what's going on. The car starts perfect warm or cold and absolutley rips on the highway..

If anyone has any suggestions on what I should check next, that would be great.

Cheers Damon

[Damon88]

Originally Posted by PeteKz

He says the lambda is unplugged. With lambda working, that should correct for changes in AFR between idle and WOT and the switches should cause enrichment at WOT. Without, it's back to an open loop system. But, as both of you noted, it doesn't have the vacuum signal like 1973-79 and RoW cars do.

Does the OXS box have any effect on enrichment at WOT or idle when the lambda is unplugged? I don't have that system, so I don't know if it still provides enrichment, or is completely disabled.

From his description of the AFR behavior, it sounds like it is full open loop, and there is no WOT enrichment going on. This could be due to the switches not providing the signal to the OXS, or because the OXS is not operating.

Damon, is the OXS system still connected to power? Fuse still in place? Do you know what part number is on the WUR?

Pete Thank you for your responses. Engine is a 83 with Lambda functioning and working. O2 sensor is unplugged.I have a wide band Plugged into SSi bung instead.

[PeteKz]

Damon, let me clarify a few items. You say the lambda in installed and working, but if you have the sensor disconnected, it can't be fully working. I assume the Wideband O2 sensor goes to the AFR gauge, not the OXS system, correct?

Follow up: What are the part numbers on your WUR and FD?

[Damon88]

Originally Posted by PeteKz

Damon, let me clarify a few items. You say the lambda in installed and working, but if you have the sensor disconnected, it can't be fully working. I assume the Wideband O2 sensor goes to the AFR gauge, not the OXS system, correct?

Follow up: What are the part numbers on your WUR and FD?

Pete, yes you are correct. Sorry for the confusion.

WUR 090
FD 0 438 100 077

83 3.0 engine.

Cheers

[AndrewCologne]

@.Damon
If your wideband controller also provides a narrowband signal/wire then connect this to the orig OXS wire instead of the orig narrowband sensor.

Also check your air sensor plate initial setting!

[Damon88]

Originally Posted by AndrewCologne

@.Damon
If your wideband controller also provides a narrowband signal/wire then connect this to the orig OXS wire instead of the orig narrowband sensor.

Also check your air sensor plate initial setting!

Thank you Andrew. Unfortunately mine does not have a narrowband. I will have to do some reading up, on how to check the sensor plate setting . Still learning how this system works.

Cheers
Damon