Need 2nd set of eyes... setting idle AFR

[tirwin]

Short version:
I’m trying to set idle AFR after changing exhaust to freer flowing setup. Getting strange behavior. Need a 2nd set of eyes. AFR is rich and doesn’t want to change.

Solved: Shop removed the male O2 sensor connector exposing bare wire. Bare wire apparently touched the chassis, thereby grounding the O2 sensor input to the K-Jetronic lambda ECU. This caused the ECU to read a lean condition from the "phantom" O2 sensor input and ECU enriched the mixture to compensate. Found exposed wire, taped it off and was then able to set idle AFR.

Longer version:
I bought a set of M&K 1.63” headers and 2-in/2-out exhaust. I decided to take the car to a local shop to do the exhaust work and a couple of unrelated things.

The active exhaust needs vac lines plumbed to the pots. They seemed perplexed by this so I said I’ll take care of it. No big deal to me but ok.

Picked the car up right before they closed on July 2nd before the holiday. Once the car was warmed up it wanted to stall at idle. Ok, new exhaust. Probably too lean. If it wasn’t already 5pm before a holiday I would’ve turned around and driven back. With no vac to the pots, the exhaust was wide open - all 4 ports. Sounded glorious but not enough backpressure at low rpm. It’s a short drive to my house so I got it home. Once home I realized that the narrowband O2 sensor was not connected. Strange because the headers have O2 bungs on each bank.

Had the day off Friday so I plumbed the vac lines. Took 10 minutes. Adjusted the idle speed a bit and went for a test drive. It was in the ballpark but I wanted to re-check idle speed, timing and AFR.

Today I got everything set up to tune it up.

I have an Innovate LC-2 and AFR gauge that I bought to install one day but just never got around to it. Connected it to 12v power. I connected my DMM to read duty cycle. Red to green/white wire on the test port and black to ground. Had my handy dandy 3mm allen in the adjustment port and timing light ready. Dizzy vac lines disconnected and plugged.

Fired up the car. Duty cycle read 63% so I think that checks out - default for cold start. Let it warm up for 4-5 minutes.

Duty cycle crept up to 94%. Wasn’t expecting that with no CIS O2 but proceeded anyway.

(Forgot to mention that fuel pressures were checked within the last year and they were bang on. Car was running great.)

Once engine was warm AFR was reading around 11.4-11.7. Too rich. Was not expecting that. I was thinking it would be too lean. Tried to adjust idle mixture and managed to get it to around 12.3 but I kept feeling like it was fighting me. I didn’t want to change the adjustment screw too much.

Then I thought if the O2 is disconnected, why would the duty cycle be so high? Shouldn’t it be default value in open loop with no O2?

Does it seem like the ECU is reading a lean condition and increasing the duty cycle to compensate?

I tried to find the O2 sensor port and strangely I couldn’t find it! At this point I was aggravated and sweating buckets so I decided to step away and cool off. I will look further later.

Taking a break and seeking other opinions to see if I’m missing something obvious.

[tirwin]

Well, I partly answered my own question. The male O2 sensor bung is totally gone. I found the bare wire in the engine bay.

I had replaced the bung with one of the ones made by Louie85 a few years ago. Maybe the bare wire was grounding and that’s why the duty cycle was so high. Will check again later.

Every freaking time I let someone else work I my car I regret it. Serves me right.

[911obgyn]

I have to learn that lesson every once in awhile also.
I think, time? Money? Go for it!
Then I have to spend a bunch of time re checking and getting frustrated with the shoddy workmanship.

[tirwin]

Ok, theory was proven correct.

I put a piece of electrical tape on the bare O2 wire. Started it up, duty cycle was briefly 65% (normal cold start) and then went to 50%.

After that I was able to set the AFR to ~13.4 and reset idle RPM. I am going to let it sit overnight and see how it cold starts tomorrow. Then I’ll verify timing and should be good to go.

Will be giving the shop a call too.

[tirwin]

Originally Posted by 911obgyn

I have to learn that lesson every once in awhile also.
I think, time? Money? Go for it!
Then I have to spend a bunch of time re checking and getting frustrated with the shoddy workmanship.

Yep. I just need to realize that I am my own best pit crew.

[Discseven]

Originally Posted by tirwin

Yep. I just need to realize that I am my own best pit crew.

.

[Funracer]

Thanks tirwin for posting how this was resolved. It would be awesome if more people remembered to post their fix! So many threads end without a published resolution.

[gtihop]

so the grounded ox sensor wire made it read 94% duty cycle?

[tirwin]

Originally Posted by gtihop

so the grounded ox sensor wire made it read 94% duty cycle?

Yes, the grounded wire caused the ECU to think the exhaust was super lean and it tried to compensate by increasing the duty cycle.

[911 SLANT]

I will also be setting AFR at Idle today. Just installed the Innovate DL-1 duel lambda kit. I also have Brians M&K headers and a free flowing 997 GT3 muffler. What should my AFR readings be at Idle 900 rpm ? What are you running for fuel management? I'm running 46mm PMOs.

[tirwin]

13.1 - 13.4 AFR with CIS

I will be switching to EFI & ITBs this fall. I’m using PMOs and MS2 ECU with COP.

Not familiar with the dual lambda kit. I should check it out.

[911 SLANT]

Tirwin Here's a picture At 900rpm A-driverside carb.
B-Passengerside carb. Idle

[tirwin]

Is the dual lambda left/right banks? If so that seems like a big variation.

[icarp]

911slant, John it looks like you have an air leak in the intake or the exhaust to get that much difference from the 1,2,3 to the 4,5,6
Ian

[911 SLANT]

I will spay some carb or starting fluid to see if Idle changes. May be the jetting

[911 SLANT]

adjusted the air mixture screw and was able to bring down the passengerside carb. My gauge reads.

[tirwin]

Good job!