Engine’s an ‘85 3.2 in an ‘80 911. City mpg is at 10. Highway 27. These results have been double checked. City used to be 18. Drop to 10 mpg occurred at some point during engine's rebuild. Don’t think there’s any need for engine or rebuild details given the wideband data clearly speaks for itself. The issue in hand here will be immediately apparent---O2 delta between 1-2-3 and 4-5-6. Wideband = Bosch 4.9 sensors, Innovate controllers, LogWorks software. To rule out hardware error, sensors were swapped side-to-side as were controllers. Test data remained consistent.
My prior thinking as well as that of others was this being a fuel injector issue. On seeing the wideband data here, and given the injector testing as was done, the finger (in my mind) is now pointing to either the ECU or the injector harness cable. Am interested in alternate views—please see what you think.
I’ll put rebuild notes at the end as I know this matters to some.
Warm engine at idle tests:
Green trace = 1-2-3 side of engine
Blue = 4-5-6
Vertical axis = Lambda
Horizontal = Time in seconds
Idle mixture screw out 5 turns
Lucas injectors
W6DPO Bosch spark plugs
Idle mixture screw out 5 turns
Bosch injectors in 1-2-3 / these are original to engine #0-280-150-158
Lucas injectors remain in 4-5-6
NKG Iridium spark plugs installed
Same config as prior setup only with…
Idle mixture screw now turned out total of 7 ¾ turns
Here's interesting part… The remaining Lucas injectors in 4-5-6 are removed and replaced with Bosch injectors. Engine now has all Bosch injectors… idle mixture remains at 7 3/4 turns out...
Expectation at this point was to see both O2 trace lines nearly overlapping, that being on the potential that the Lucas injectors failed to some degree—and—there being fresh Bosch injectors with even flow now installed. Instead, the wideband pattern of the all-Bosch injectors is uneven side-to-side just as with the all-Lucas injectors. Bosch are indicating richer than Lucas.
More injector detail to consider… Bosch injectors were removed from the engine years ago and shelved. While the wideband system was being installed here, the Bosch injectors were serviced by Mr. Bill. His report…
The Lucas injectors that are now removed from the engine were installed and in service for a number of years prior to the engine’s rebuild. They did deliver 18 city mpg. This fact together with O2 wideband profiles for the all-Lucas and all-Bosch injector config looking similar, and the combo wideband of them being "unusual" in this case… a smoking gun as I see this. IOW.. Injectors don’t seem to be at fault—something else is afoot.
Spark plugs cannot be the cause of the delta anomaly seen but am noting them changed as a matter of record. The switch from W6DPO to Iridiums was advocated by Sal and Ivan---who I credit knowing far more than I about this stuff.
While switching plugs, made sense to test compression. Was done with roughly a 2 second crank and the fuel injection harness disconnected. I suspect cranking longer would result in higher psi. In any case, purpose was to rule out the presence of any cylinder failures. Was good to confirm...TY Sal.
W6DPO plugs that just came out. These were pulled, cleaned, and reinstalled about 500 miles ago when I started into this 10 mpg puzzle. In reality, the color here is more brownish than is seen on my screen.
Engine profile for the interested…
Mods from stock during rebuild:
964 Cams
1.45 matching overlap
SSI Exhaust
2-in-1-out Dansk muffler
Clewette ignition wires
Blaster coil
W6DPO spark plugs
Stock:
Heads rebuilt to spec by Xtreme - new valves & guides, seats faced
Pistons & cylinders were well in spec so all were recycled with new Goetze rings - break in with fossil then replaced with Motul syn 20-50 and new filter.
Polished journals
New journal bearings
Rod large ends rounded and finished to spec with new bearings
New bushings on rod small ends bored to spec
New squirters
New timing chains
Injectors are not stock—are Lucas low impedance—but were installed years ago and so were with the engine doing 18 mpg city. They were “serviced” during rebuild under oddball circumstances. In short, injectors are under suspicion here. ECU is also questionable as the one installed now is NOT the one I used to have pre-rebuild. Won’t get into the saga of these parts as it’s just noise compared to solving the MPG puzzle in hand. Suffice it to say the ECU is also on the suspicious list. Will see about these parts as puzzle-solving unfolds.
After the rebuild and once on the road, power to redline was there through all gears. Were it not for fuel & mileage gauges staring at me, I’d not be writing this. Immediately, and with little Motronic experience I admit, I molested fuel-things hoping for a quick fix. AFM disk, ECU fuel setting screw, mixture and idle screw being included in my initial forray. Varying degrees of performance resulted. Obviously no quick fix was had. As the engine now stands, everything that was molested has been returned to original settings with the exception of the idle speed and idle mixture screws. Warm idle is a steady 850 rpm. Mixture screw is 5 turns out—now changed to 7 ¾.
Did city and highway wideband recordings. That collection is largely redundant so am not positing that stuff. Does exist as good baseline fodder should it be needed.
WOT to redline may be of interest...
This is with Bosch-Lucas injector combo. Did not get to redline in 4th and no 5th at all.
For you who suggested my getting into wideband... you were absolutely right. I regret my hesitation as it now stands. Thank you for egging me on-and-on!
.