running rich SD

If you're not a novice then forgive me for asking this: At what rpm is it running rich, and how do you know? It's amusing to watch guys "tune" their fuel curve based on the smell of the exhaust at idle

People here will likely suggest tuning this by adjusting the fuel pressure. Please keep in mind that even though your car may run eye-wateringly rich at idle, it may be running great or even slightly lean up top. Pulling fuel pressure will temporarily lean it out all the way across the powerband. Then the computer will "sense" where the car went too lean and will add fuel back in at those points. However, the computer has a tuning window of only about 12.5% leaner or richer than it's base tune. So, if you are running more than 12.5% lean, it will not be able to compensate. As you can see, this would really cause a bad problem for a guy who just wanted to stop a rich condition at idle. I'd personally rather live with a rich idle than a lean WOT condition.

You may be able to get it close enough to be happy by just tuning the fuel pressure, but just keep a close eye on what the car's doing up top. If you want to do more and optimize the tune across the powerband, then I reccomend a wideband and a piggyback like the Tweecer RT, or the Quarterhorse. Good plug-reading skills and a lot of elbow grease can substitute for the wideband, but there really isn't another correct way to tune EFI.

vacuum leaks = too much fuel so check all your lines REAL good.

Tom, that's counterintuitive. Vacuum leak is extra air getting into the manifold unmetered, but doesn't speed density account for this by basing injected fuel on MAP and load?

Is it because it raises fuel pressure? i.e. less vacuum = higher manifold absolute pressure = higher pressure at the FP regulator than expected?

On my bigstuff, I've noticed a vacuum leak putting me in fuel cells that I would never reach otherwise... Your post has my tuning curiousity peaked.

Thanks

If a vacuum line is leaking (small leak), it causes low vacuum to the MAP in a speed density car, low vacuum = WOT and more/max fuel pressure is applied by the regulator = runs rich.

Gotcha. Now it makes sense.

Thanks

sometimes in a SD car, the easiest way to ID the problem is to cap off all the lines (except the one to the map sensor), then fire it up. It should run good. If it doesn't, it could be an intake leak.

then start connecting your lines one at a time, and don't re use original lines

In reference to the FPR...On my 87GT my stock one was running 42-43 psi and running rather rich. Once installing the a Kirban FPR I set it to 38 psi and took it to the dyno. After running on the dyno the car liked 37 psi the best with almost perfect A/F ratio across the board. My MPG's went up as well as power. If you can check the fuel pressure easily it might not be a bad idea if the vacuum lines don't do the trick.

I think EVERY FI car should have a fuel pressure guage to monitor - mine is mounted in my cowl.

mc0311grunt said:

My 86 has E7s on it,bbk shorties, and an MSD coil. After I put the e7s on it started running rich. Any ideas on what could be the problem? My timing is at 14.

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'86 with flattop pistons, no valve relieves? If so, did you check for the possibility of PtV problems? Have you performed the KOEO/KOER self tests? Manifold vacuum reading with a gauge is? .... LUK