whall

Okay, after installing a new LC-1 and then posting for advice, I have everything apparently working correctly. The Logworks tracker and guage are consistent.

Now the question is regarding the mixture. I get fairly consistent 19'ish at idle. I haven't taken it for a ride due to pouring rain but was wondering if the lean could be related to the supercharger or something else. I have a FMU on the car as well. Any feedback would again be appreciated.

Also thanks again to those that provided guidance on the LC-1 install issues I was having.

dprantl

You should be seeing ~14.7:1 at idle. 19:1 is way too lean and probably causes the idle to not be smooth. This should have nothing to do with a supercharger. Is your 928 a US model with a stock O2 sensor? If yes, start looking for the vacuum leaks.

Dan
'91 928GT S/C 475hp/460lb.ft

AO

As you go out for a drive, it will be very tempting to step on the go-fast pedal. But if your AF ratio is higher than 14's under light accel, you will probably start knocking. If you really step on it, you should drop into the mid-12's ideally. If it's much higher... get off the gas before you do damage.

whall

I have a vacuum guage installed and the vacuum is good. Maybe it is time to replace O2 sensor again - did this about 5k miles ago tho. You are right re the rough idle.

Bill Ball

I think you may not have things working right with the LC-1. 19 could indicate no signal. That not just "lean" - the car should not run for very long if that's the real AFR. I would expect it to stumble and die. Superchargers don't affect idle much as they usually are on bypass and there is no boost - like you said, you have good vacuum.

If the AFR is real, what is your fuel pressure? If you are using an FMU, you need to know this.

whall

I am not 100% confident in the LC-1 reading but I believe it is picking up a signal. The mixture changes significantly with throttle - goes as low as 12-14. The car is stumbling so it may not be quite as lean as the reading. I recalibrated a couple of hours ago and the readings went from 22'ish to 19'ish.

blown 87

If it is missing it will show lean, not rich, my guess is you have a dead or missing cylinder at idle.

Bill Ball

OK, so just very lean at idle only. I'm not sure how a dead cylinder at idle only would do this, BUT you can find a dead cylinder easy enough by pulling spark plug caps one at a time until you find one that does NOT make any difference in the idle speed or smoothness. Be prepared to get shocked if you don't do this carefully.

I'd still be interested in your idle fuel pressure.

whall

Idle fuel pressure - just checked - have a guage on the fuel rail - 38 psi.

Bill Ball

38 is low. Stock is ~48. I don't know anything about your SC setup, injectors, etc. Sometimes in SC setups with large injectors and no Sharktuning, the idle fuel pressure is dropped for proper idle mixture. From my observations, injectors shut off at below 30 PSI, and I suppose that could vary with injector model. They fire best at stock fuel pressure. You might have some that are giving up and not firing until the pressure rises off-idle. The car will run (badly) on as little as 4 cylinders. You might find a number are not firing by doing the spark plug cap pull test, but this is all a wild guess since I know nothing about your SC setup or the history of this problem.

Is this a new problem or has the car been stumbling at idle for a while? When did it start doing that? What was the last repair before the problem appeared?

Glen McCartney

Bill, if he has the stock 85 FPR, that reading is good as the preS4 fuel pressure is lower than the 87 and up.

Bill Ball

Right, Glen, that could be part of his setup. Andy used those with his SC kits. The earlier FPR is also boost-responsive.

Need more information.

whall

The SC is a custom setup. Stock injectors, only major mods are Novi GSS and Paxton FMU. I don't recall the idle fuel pressure ever being any different - even before the S/C.

Bill Ball

Oh, sorry, you have an 85. Brain fart. That is normal FP.

Anyway, I'm not sensing whether you have a real problem or just curious about the AFR reading. You say the idle is "rough". AFR of 19 should barely idle if at all.

dprantl

I just went down and tried this. I forced the O2 off and minused the non-cat fuel map until my wbO2 read 19-20:1 AFR. The car still idled well; a little rough but much better than I would have expected. There would be a little oscillation every 10 seconds or so, but it would fix itself.

Dan
'91 928GT S/C 475hp/460lb.ft