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Bucking 86



My 86 aluminum head Z-51 coupe has developed a bucking problem when slowing down. It accelerates fine, cruises fine, and runs well under load, but when I slow down to stop or for any reason it begins bucking. I would say it feels like the ignition is cutting out for a fraction of a second and then coming back on, which sets up a pretty good shake in the car. It does not shut off. I also notice that it starts very well when it's cold, but cranks too long when hot. Sometimes giving it some throttle will make it start better, which I don't think is normal for a fuel injected car. I paper clipped the diagnostic socket, but it just repeated the 12 code over and over way more than 3 times. There are only 18000 miles on the car so I doubt that much was ever done to her. This is why I bought a C4, to start to learn something new about these computerized machines, So here we go. Any help would be greatly appreciated. I have searched the forums quite a bit. I forgot to mention, I love the car.... Thanks



Well-known member
May 26, 2003
1986 Black "Indy 500 Pace car replica"
Sounds like an interesting problem. At what speeds and engine RPM's does this occur?


in 'throttle-closed'' decel, the ecm will not pulse any injectors...long cranking when hot indicates loss of residual fuel pressure in the fuel rail... need to depress gas pedal for hot start indicates ''flooding''.

all signs point to a leaking fuel injector(s).

first check to do with any efi is fuel pressure (that's why the test port is very ez to access on almost all gm efi types)...a fuel pressure test gauge can be purchased at most auto parts store for under $50, harbor fright sells one for $15 (chinese junk but useful)...fuel pressure for multi-port injection cars (vette 85-96) should be abt 40 psi as soon as the ign key is turned 'on', bleed off slowly when the key is turned off, should hold 10psi after 12 hour key-off...tbi's (84 vette, some later firebirds, camaros, trucks, etc) only abt 10 psi max key-on...do not need to start/run to do this test...do not work in enclosed space as a small amount of fuel leakage will occur when connecting the test gauge...do not leave the test gauge on the engine permanently as gauge leakage is not unusual and would be catastrophic if un-noticed for even minutes.

Ruby Fan

Well-known member
Oct 7, 2004
1985 Black Beauty Coupe, 93 Ruby, 96 CE vert
My guess is that you have a fuel problem(duh)- maybe you got a bad batch of gas, or are NOT using premium grade. Maybe lot of carbon build up. Change your fuel filter. Also you can check around the distributor for cracks. Sounds like a lot of things to think about, but that's what goes thru my mind when I have a problem. Good Luck

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