Stumbling, Bog-down, like mis-fire Issue

Hi folks,

Mods are in my signature. I replaced my spark plugs with Autolite 3924s, 9mm Ford Racing spark plug wires, MSD distributor cap and rotor, and deleted the EGR. I had a local Mustang specialty shop double-check my work, and welded-off the EGR exhaust manifold bung. They also tried adjusting idle air, idle and verified TPS voltage of .98.

Now, the car stumbles and bogs-down under a load. It sounds/feels like mis-fire. A sputtering, machine gun feeling/sound. The exhaust sounds "flat" and not in "tune", if you can understand what I mean. Everything else seems to be functioning normally, with a very lopey, low idle ~900 RPM. Static fuel pressure is at 55 psi. MAF is calibrated for 24 lb. fuel injectors and the MAP sensor was installed with the A9L conversion long ago. The motor is determined to be mechanically sound. CEL has not illuminated. I've removed the SPOUT connector and drove it, and the stumbling, bog-down is minimized, but it still does not sound like it's in "tune". My narrowband Air/Fuel gauge shows a leaning during this sputtering. I figure the narrowband has some merit when showing lean and fat conditions, the extremes, with a light show bobbing back and forth as the computer tries to maintain stoichiometric ratio. When I modulate the throttle and back off, reducing the load, the stumbling and bog-down subsides and the narrowband AFR indicates fat.



Please give some additional guidance on things to check and verify.

Thank you,
Paul

tstang21 said:

it could be a couple of things according to my understanding. double check your plugs and wires. check for any vaccum leaks. try checking your timing as well and fuel filter.

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1. Base timing was reset to 10 degrees, SPOUT removed.
2. Plugs and wires are secured and gap was checked.
3. I will check for vacuum leaks.
4. I will replace the fuel filter.

Thank you,
Paul

divit250r said:

Do you have any way to monitor fuel pressure while it's acting up? Sounds like this may be the problem. On a side note, why such high fuel pressure? And, since you did the A9L swap, has the car been tuned with a chip? Just curious.

Scott

Click to expand...

1. Yes, I have a fuel pressure gauge in the car, I'll watch it again during the stumbling.
2. Hypothosis: Cranking up fuel pressure is a way to compensate for a clogged fuel filter?
3. I'll double-check fuel pressure when the stumbling occurs and replace the fuel filter.
4. No, the car is not chipped. It ran great prior, with ~280whp/300wtq dyno'ed.

Thanks,
Paul

divit250r said:

I hadn't ever thought of using fuel pressure to try and cure a filter problem, but it seems you might be stressing the pump a bit? What fuel pump are you running?

For what it's worth, my combo is kinda-sorta close to yours (minus the cam and pistons) and we're pretty close on numbers. I got 265/300 out of mine last spring.

Scott

Click to expand...

I have no idea what fuel pump is in the vehicle. I can only assume it's OEM stock. I have nearly full documentation for the car, and I have not come across any document showing the fuel pump getting R&R'ed. Chilton's manual states the static fuel pressure (vacuum hose detached) for 1998 and earlier is 40 - 50 psi.

I'll replace the fuel filter, regardless, and keep digging around.

Thanks,
Paul

Bosko5.0 said:

try seperating the plug wires, and keep them at a distance from the alternator.

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Thanks, but the placement of the plug wires has not changed from when the car was running just fine.

Photos of engine compartment: (fixed broken images)

http://picasaweb.google.com/paulsheehanjr/MustangEngineBay/photo#5216264673429468418

http://picasaweb.google.com/paulsheehanjr/MustangEngineBay/photo#5216264672945865106

http://picasaweb.google.com/paulsheehanjr/MustangEngineBay/photo#5216264678915859154

Paul

Hmmm, after some more research and thought, I will be checking the Ignition Control Module (TFI) tomorrow. It's the "Dyna Mod" brand currently in the car.

Paul