Engine misfire issue (cruising) - Third Generation F-Body Message Boards

FFCam1991 · 09-14-2009, 01:59 PM

Hey,

This problem is driving me crazy. My 91 305 TPI is very smooth when you punch it, but when cruising along at a constant speed it begins to thunk and stutter. It doesn't feel like a typical miss, but something is up. In lower gears the car shakes. On the highway, you can barely notice it. Mostly around 1500 RPM.

All pertinent info is here: Premium plugs, wires, cap, rotor. New Bosch Design III 22lb injectors. Holley adjustable FPR with PSI set at 30 (yes, I know this is low, but I'm looking for better fuel economy, and my injectors are a bit bigger then they should be. Notice no performance diff between 30 and 45 PSI). Idle vacuum is 18 inches, plenum and runners have fresh gaskets. The engine itself is actually super quiet, the injectors firing is probably the most noticable.

Timing is dead on 6 degrees BTDC w/ EST disconnected. I tested each wire individually with a timing light and they are all delivering sparks. When I test timing with EST connected, it jumps around a bit, maybe more then it should? I'm not familliar with EST. You can hear the engine noise change as the timing jumps from, probably, 12 degrees to 22 degrees at idle (that being the total range, not what it jumps to and from immediately).

There is a custom chip but I have no knowledge of what the settings are. Basically, all I can think of is that it is the module or the coil. I tested the coil and the values seemed reasonable. And I hear modules usually fail completely.

I get piss poor economy and I also have a bastard of an exhaust leak at the main clamp.

Any help here?! Thanks in advance.

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