Bucking at low RPM

sfaller · 10-13-2009, 05:38 PM

I'm new to this site. I'm a child of the 60's who's reliving some of the fun I had with my '67 Fastback (289/4spd, candyapple red with black hood stripes) and now have a fresh 5.0 motor in an '86 GT convertible project car. The engine has some mild modifications-lost the factory SD and replaced with '89 plus EEC, MAF conversion with 70mm sensor and TB, Explorer intake, E303 roller, PJ timing gears, BBK, CI mild ported heads with 1.94/1.75 dbl springed valves , 1-3/8 shorties, windage tray, roller rockers, underdrive pullies, cat delete w/ O2's intact, Magnaflow SS exhaust and mufflers. I tjhink it sounds pretty good, engine idles well and accellerates quite nicely if I drive aggressivelly with WOT. I do find it necessary to keep the engine CRUISING above 3000 RPM, otherwise it bucks and misses. Although WOT is really fun, I can't do this indefinitely. H-m-m, the tires do need replacing soon! Sorry, I digress.... it really bucks on upshifts at partial throttle, particularly uphill, forcing me to down-shift to get the RPM's back up and put my foot back into it. I've heard of similar problems with Bosch Platinum plugs (currently installed and cheap enough to change to Ford this weekend) Have new plug wires, rotor, cap (going to resistance check the new wires anyway) TPS is still original and new non-adjustable one is on order. Also, I am running a standard fuel-rail w/19 lb injectors at this time.
Any OTHER suggestions from you up-to-date 5.0 guys?

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mattdel · 10-13-2009, 05:55 PM

Well, you're on the right track with scrapping the platinums. The TFI ignition system isn't designed to run a platinum plug. AutoLite #25's are best.

Secondly, try some MAF cleaner and spray down the sensor wires inside the tube. Don't use any chlorinated chemicals though.. Any electronic cleaner or more specifically a can of MAF cleaner should work wonders.

lx5.0 · 10-13-2009, 09:42 PM

i had the same problem could never cruise because it would always buck bad, what i did i replaced was one of my 02 sensors cause one of them was showing lean , maf cause i accidently broke other, and tps. But what i think was the problem was the maf, cause if you have a dirty or malfunctioning maf it will create those problems, when i installed it i quickly noticed the diffrence faster start up and better idle and no more bucking, and i found it at a junkyard for $20

mighty90 · 10-13-2009, 09:53 PM

Had similar problem, Ran the codes (14) and found out i needed a new distributor. Was an easy and cheap fix.

sfaller · 10-14-2009, 06:36 AM

Thanks for the insight. The MAF is new, so I don't think it needed to be cleaned and I didn't expect any malfunction there but its possible. I do not have engine light on and unfortunately I do not have a CODE scanner either, but probably should invest in one. Anyone know how to check a MAF with a multimeter?
The 02's are old and probably SB replaced or get eliminators, especially since I have MT cats. Could reduced backpressure be causing O2's to read inaccurately?

mattdel · 10-14-2009, 04:51 PM

O2's wouldn't contribute to a bucking condition, the variation in the fuel tables they contribute to isn't enough to do so. Your problem is either in fuel supply, air metering, throttle voltages to the computer via TPS, or an ignition problem, but I wouldn't suspect ignition without further problems.

Bubba 95SN · 10-14-2009, 08:10 PM

Filled up a dumpster with Bosh plugs, Fords do not like them. Get some Motorcraft or Autolite plugs and that part of the ignition will not be a issue. Then check for codes.

1fast5OH · 10-14-2009, 10:22 PM

Mine does it below 2k rpm's, but i was told it did that because i still need a dyno tune. the only codes that showed up were for e.g.r which i dont have hooked up anymore.