7747 - higher fuel consumption with VSS installed

[Dr_Grip]

Hi guys,
the problem I am facing is just what's described in the title - since I installed a VSS on my car, the fuel consumption has gone up eventhough DFCO, Decel Enlean and HyWay Lean Cruise should mean a net decrease in fuel consumption. Cruising at 75-ish, I went from 13.5mpg down to 12mpg, which is more of a difference than what I'd chalk up to chance, weather or similar...

Any ideas?

[steveo]

could just be a coincidence, and some external factor appeared at the same time you installed the vss.

i wouldn't consider it a correlation until you unplug the VSS and your fuel economy improves again, or you see evidence in your fuel trims or something like that.

got a log?

you're talking about over 10% loss in average fuel economy here, which means well over 10% more fuel burned, 10% less power produced, 10% less combustion efficency, etc.. so you'd be looking for a really large error somewhere.

[damanx]

This is an interesting question.

A couple of weeks ago, I converted my analog instrument cluster to a digital. The digital requires a 4000 pulse per mile signal and the ECM, 1228062, (same pinout as 1227747), requires a 2000 ppm signal.

After getting a Novak VSS, a DRAC and then building a D-Type flip flop to get the 2k pulse, everything is good, but, there was a day where the VSS signal was not reaching the ECM and speedo, and I saw no effects on it other than the lockup not engaging.

I've always thought that the ECM uses the VSS just for determining when the engine was at idle or moving, but I don't know for sure. 10% change, I think that's more than just the VSS.

[Dr_Grip]

I got something close to a theory. When installing the VSS, I kicked out the speed sensor for something Ford called "ESC" (electronic spark control"). Basically, it switches and swaps vacuum load to the vacuum advance and vacuum retard connectors based on the speed of the car. These are not connected anymore anyways, but maybe I still managed to introduce a vacuum leak into the system by removing the ESC's influence. Will triple-check that.

[damanx]

Why are you not running a GM HEI setup?

ETA: I had to go back and see what you had. 7747 ECM running on top of a Ford 400.

So, in light of that, I don't think I can help you because I don't know anything about ford ignition systems.

[Dr_Grip]

I am running a GM HEI setup, in fact. The "ESC" from Ford is a bad California emissions hack from the 70s (http://forum.grantorinosport.org/spark-control-switch-d2af-9e740_topic13615.html). And, of course, deactivated.

[Dr_Grip]

So, I re-checked the vacuum lines and removed some questionable stuff that only was plugged before. I attached a log of a short test run. Here's a log and the bin from before the vacuum changes. (http://www.gearhead-efi.com/Fuel-Injection/showthread.php?5158-7747-on-a-Ford-400-still-not-getting-a-backfire-issue-sorted&p=55963&viewfull=1#post55963) If there were no leaks or anomalies in the vacuum system, there should be no differences.

Changes to the bin between the two logs
-Turned on HyWay lean cruise
-Minor VE table tweaks

Things of note in the log:
-I think the HiWay lean cruise might be a tad too enthusiastic in adding spark advance. I'll tune that down a degree or two.

[Dr_Grip]

I removed a little HiWay spark and mostly cleaned up all the vacuum line mess. Seems like I found something there, fuel consumption's back down to pre-VSS levels. Not lower, though, eventhough Decel enlean and HiWay lean cruise should have an effect. Time to break out the WB sensor.