j.kenzie@sbcglobal.net

Hi guys,
As I make progress on one front, other problems arise. I think unrelated, I added an alternator cooling duct with fan. Nothing much else new. Seems to work fine.
However, over the the past few days I have noticed my wideband AFR gauge gradually reading leaner. First, I was idling around 14 and would go down to 13 or so with throttle. A day or two later, the idle AFR was 14.6, then 15, then 15.5, and now 16 or more. This happened over the course of a few days.
Mine is a 1984 supercharged L-jet, with rising rate fuel pressure regulator in addition to the stock FPR's. I adjusted my idle mixture screw all the way in, and got the AFR down slightly, but the car really wants to die if I let the rev's drop to 800 or so in gear. I have to drive two footed, and feather the throttle just to get the car to move away. Once moving at 2000 RPM or so, the AFR settles around 14.7. Take my foot off the throttle and it goes to 16-17.
Something has changed over the last few days. This car has really been very reliable in all weather. I suspect this has nothing to do with the SC directly, since it does little at the engine speeds involved.
I will attach a fuel pressure gauge on Friday morning. I hear no obvious vacuum leaks. No other engine plumbing has been altered.
I sure would appreciate any advice from those who may have insight.
Thanks very much, in advance,
Dave

j.kenzie@sbcglobal.net

I wanted to add that I am running open loop, no cat, and O2 sensor to WB for info only.
Dave

j.kenzie@sbcglobal.net

OK here's my plan: Too lean means not enough fuel or too much air or incorrect AFR measurement. Since the AFR corresponds with poor running, I assume it is indeed too lean. The AFR does respond to the idle mixture screw on the AFM (it gets even leaner) so I don't think I have just a massive manifold leak. I have to think I have fuel shortage. I will measure fuel pressures on Friday. I will go from there. I am looking for 36 PSI at idle. If too low, I will jumper fuel pump relay, and maybe replace fuel filter. If still too low, I will try to check FPR's and maybe fuel damper. If fuel pressure is not too low, I am not sure where to go. Probably have to look at the AFM. It is a unit rebuilt about 10 years ago, and otherwise seemed trouble-free.
Any ideas or suggestions?
Thanks,
Dave

j.kenzie@sbcglobal.net

OK guys, I have some fuel pressure data:
1000 RPM Idle FP = 30 PSI
No change with FP relay jumpered, running.
FP relay jumpered engine off 36PSI.
These sound like pretty nominal values.
Either way I am running AFR of 16 at idle.
Since I have an adjustable rising rate FPR, I turned the screw until the FP at idle was 36 PSI. This gave a nice 14.4 AFR, which dropped into the mid 13's with part throttle, but down to 11 under full throttle. No dying under load either. I am not sure I want to leave the system like this since the RRFPR is meant to be in series with stock base pressure FPR's. I wonder if my L-Jet ECU is dying, by not giving me normal duty cycle. Again, nothing has changed recently, just a gradual rise in AFR over the past week or so.
Any insights?
Thanks again, in advance,
Dave

j.kenzie@sbcglobal.net

BTW,
Temp II ohms out OK. Green wire replaced 1 1/2 years ago.
Dave

j.kenzie@sbcglobal.net

Hi guys,
I tried a few more things today. I borrowed an L-jet brain just in case mine was faulty, but no difference. I still get an exaggerated range on my WBO2. I can set the AFR so I get a 14.5 idle, and 11 at full throttle, or I can set my idle AFR to 16, and get 12 at full throttle. Just for fun, I tried reconnecting my L-jet's O2 sensor, and I was able to improve about 1/2 point at each end of my AFR ranges. I left it at 15 at idle, and 111.5 full throttle. I guess I can live with this, but I am not sure what caused this problem. OTOH, since I am now running a little rich, I added a litle more timing, and the throttle response is really very nice.
Dave

j.kenzie@sbcglobal.net

Well guys,
I've been going through my AFR adjustments. I disabled my 928MS breather system, and just use a dump tube to atmosphere for venting. This helped a bit because this represented a vacuum leak of sorts.
But what REALLY HELPED was tightening my 8 spider rubbers' 16 hose clamps!
They were loose! Real loose!
Let me back track. About 3-4 months ago, I was also trying to adjust my AFR, because it was too rich. Not really that bad, but I had to fiddle. I adjusted my mixture screw on my AFM all the way out, and it fell out. I had to remove my spider to retrieve it, and reinstall it. Anyway, in the course of reinstalling the spider, I forgot to tighten the 16 hose clamps. The were all lined up, and ready to tighten, but I never did it.
Today as I tightened the clamps, I heard the idle change. My AFR settled down around 13, and holds between 12 and 14 in all throttle positions. I currently have my O2 sensor hooked up, but that may or may not remain so, since I was running without, before this fiasco.
Anyway thanks for reading. It's nice to know there was a logical explanation, even though I overlooked it when it was staring me in the face.
Dave

j.kenzie@sbcglobal.net

BTW, now it is incredibly fast!
Dave

Chalkboss

Good job figuring it out. I'm not familiar with the older intake but I have heard of those clamps causing problems before.

The Fixer

Hi Dave,

i'm glad you figured it out and good to see you back.

i thought we may have lost you to the 911 forum

wish i could take a ride in that 928SC of yours.

j.kenzie@sbcglobal.net

Hey, Matt,
Missed you too. All the cars are like kids (to some degree) and need constant attention and develop new problems all the time. As you know there is always something that needs fixing, and that is the pain, but also the fun of ownership if you are an amateur mechanic.
Take care,
Dave

SMTCapeCod

Dave,

Thanks for keeping the thread updated despite silence on the line for a bit, someone will learn from your experience!
I was betting on the RRFPR or the signal to it, so good thing I didn't pipe up..

Wonder whether t-bolt clamps and/or silicone might be good for your application. I'm envious, my Hbronze/Brown never made it all the way to F/I under my care. Bet it rocks.

Mrmerlin

For others reading in the future...
its always a good thing to ask yourself .........
what was the last thing I fixed?

When new issues crop up seemingly out of nowhere.
this answer will usually bring you to an Ahaaa moment.


Dave , Glad you got the engine running as it should, sounds like it riotous beast

ltoolio

Dave - to echo above, I'm currently troubleshooting a rough idle and have been struggling to figure out why. Your thread might nudge me towards solution.

My AFR floats at 20 (max on my gauge) when I'm at idle. When I'm driving it gets up to low teens. And this made me wonder, as I'm still very new to this, maybe there's the source of my problem. I've done zero work under the hood since I took ownership (other than draining the coolant this week), so not sure what my issue is. Maybe something vacuum as suggested in my related thread.

So, off I go later this week when I return from SFO to do some more research