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Windsor · 01-05-2009, 04:20 PM

Generally fuel delivery problems get worse at higher RPMs.

Vacuum leaks, OTOH, really show during low RPMs because the leak could equal the amount of air passing your AFM. Also, most ECUs built around this time use pre-built maps for idle and WOT, so the O2 sensor is out of play. At higher RPMs, most of your air is coming through the AFM and the closed-loop mode of the ECU can compensate for the minor offset.

First off do a visual check of everything after the AFM. Get a flashlight and mirror and look on the bottomside of the piping and vacuum tubing. The obvious places to look are at joints and bends.

If you find something, you can verify with a tiny shot of WD/40 or starter fluid while idling -- if the idle improves, you found it.

If you can't find anything, then do the WD/40 or starter fluid test anyway, picking key locations again (especially blind spots that you couldn't get a mirror into).

01-05-2009, 04:20 PM	#8 (permalink)
Windsor Registered User Join Date: Dec 2008 Location: DFW, Texas! Posts: 794	Generally fuel delivery problems get worse at higher RPMs. Vacuum leaks, OTOH, really show during low RPMs because the leak could equal the amount of air passing your AFM. Also, most ECUs built around this time use pre-built maps for idle and WOT, so the O2 sensor is out of play. At higher RPMs, most of your air is coming through the AFM and the closed-loop mode of the ECU can compensate for the minor offset. First off do a visual check of everything after the AFM. Get a flashlight and mirror and look on the bottomside of the piping and vacuum tubing. The obvious places to look are at joints and bends. If you find something, you can verify with a tiny shot of WD/40 or starter fluid while idling -- if the idle improves, you found it. If you can't find anything, then do the WD/40 or starter fluid test anyway, picking key locations again (especially blind spots that you couldn't get a mirror into). __________________ '87 T4R Deluxe, mall crawler every non-SR5 option installed (sans AT), as well as: * Cruise Control (dealer installed) * Michelin LTX A/T2 31s on 1st-gen alloy rims * SmittyBilt granny step bar * SR5 Instrument Cluster Last edited by Windsor; 01-05-2009 at 04:22 PM.