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Paul Workman · 11-05-2015

"Bucking" in NORMAL mode only, but NOT in FULL ENGINE POWER, (key ON) suggests;

one or more primary injectors = "kaput"
primary fuel pump at issue.

Whereas, missing (bucking) at WOT w/ power key ON, suggests fuel issue and/or ignition: which is NOT what you've described.

So, IMO, NOT missing at WOT, but miss-firing in NORMAL rules out ignition (plugs/coils) for now. (FYI, the most severe load on ignition system is at WOT. If coils or plugs or wires were going to fail, that would be when it was noticed - the opposite of what you've detailed fur us here. Just sayin.)

With the secondaries ON and at WOT, the secondary fuel pump is running, AND the secondary injectors are chiming in too, which could be masking a bad primary pump and possibly one or more marginal injectors too (or check-valve issue like I had once).

If the fuel pressure checks out (~45psi in NORMAL, low throttle mode and 52 or closer to 55 psi at WOT) you will want to measure the resistance across each of the injectors while they are HOT (Ignition key OFF).

Resistance should be uniform across the lot, ± an ohm from average. [averages depend on manufacture. The may average, 12, 14. or 16 ohms ± 1.] Typically, when they cause issues, the resistance of a bad injector is 3 or 4 ohms lower than the average. For example, if the group reads approx 14 ohms, but one reads (for example) 11 or less, it is marginal and probably headed "south". But, by the time it is reading closer to 9 (in this example) or lower, it is NOT firing properly; replacing the entire lot is the recommended solution.

For specific instructions on "how to" make these measurements, you can refer to Dynomite's sticky (top of the TECHNICAL section and click on the diagnostics for verifying fuel pump operation, as well as how to measure the injector resistance.

Between pre-93 injectors and old fuel pumps, this is by far the lion's share of the problems you've described If this all checks out, then it is something more involved and we'll have to go from there. (Note: I'm not saying coils are NOT also and issue, but at this point, they are not high on the list, due to the symptoms you describe. We'll see tho, I recon...

Let us know how it goes~

11-05-2015	#5
Paul Workman Join Date: Nov 2007 Location: Squires (near Ava MO in the Mark Twain N'tl Forest) - Missouri Posts: 6,466	Re: Bucking only when Power key is OFF "Bucking" in NORMAL mode only, but NOT in FULL ENGINE POWER, (key ON) suggests; one or more primary injectors = "kaput" primary fuel pump at issue. Whereas, missing (bucking) at WOT w/ power key ON, suggests fuel issue and/or ignition: which is NOT what you've described. So, IMO, NOT missing at WOT, but miss-firing in NORMAL rules out ignition (plugs/coils) for now. (FYI, the most severe load on ignition system is at WOT. If coils or plugs or wires were going to fail, that would be when it was noticed - the opposite of what you've detailed fur us here. Just sayin.) With the secondaries ON and at WOT, the secondary fuel pump is running, AND the secondary injectors are chiming in too, which could be masking a bad primary pump and possibly one or more marginal injectors too (or check-valve issue like I had once). If the fuel pressure checks out (~45psi in NORMAL, low throttle mode and 52 or closer to 55 psi at WOT) you will want to measure the resistance across each of the injectors while they are HOT (Ignition key OFF). Resistance should be uniform across the lot, ± an ohm from average. [averages depend on manufacture. The may average, 12, 14. or 16 ohms ± 1.] Typically, when they cause issues, the resistance of a bad injector is 3 or 4 ohms lower than the average. For example, if the group reads approx 14 ohms, but one reads (for example) 11 or less, it is marginal and probably headed "south". But, by the time it is reading closer to 9 (in this example) or lower, it is NOT firing properly; replacing the entire lot is the recommended solution. For specific instructions on "how to" make these measurements, you can refer to Dynomite's sticky (top of the TECHNICAL section and click on the diagnostics for verifying fuel pump operation, as well as how to measure the injector resistance. Between pre-93 injectors and old fuel pumps, this is by far the lion's share of the problems you've described If this all checks out, then it is something more involved and we'll have to go from there. (Note: I'm not saying coils are NOT also and issue, but at this point, they are not high on the list, due to the symptoms you describe. We'll see tho, I recon... Let us know how it goes~