Misfire at low loads

[Brewster]

IF you guys wouldn't mind taking a minute to look over this file and logs. Working on a 1990 GT with a 347, Fixed a bunch of customer install issues and started up a base file. Running and starting good but cruising on the dyno at low loads its somewhat bucking and seems like it has a misfire. I feel like its either a lean miss or an actual miss. If I get into the throttle a bit it clears up and runs good, will have wideband hooked into EEC today for future logs. Maf looks a little unstable, Just trying to use the trims to get part throttle worked out. Also to mention I found a Capacitor in the EEC that leaked its guts out and had a broken leg. Had Cap replaced and other x2 caps looked mint. EEC seems to be working well.

1990 GT 347
24lb EV1 injectors ( Purple )
Professional Products 87mm MAf
New plugs Auto lite 3924
New wires ( Taylor Thunderbolt)
Mallory Ignition box
MSD blaster coil
Timing set at 10 with spout out
GT40 Intake
Comp 224 cam
Edelbrock E street heads- 202

A9L2-CL-Mech-Idle.bin

(56 KiB) Downloaded 247 times

[dleach1407]

bypass the aftermarket ignition box and see if the problem is still there.

[Brewster]

Yea I'm starting to think the aftermarket ignition is an issue so far. Last couple hours been working with the MAF and FN035A to get the fueling dialed a little closer. Runs good with no miss if your hard into it but now it acts like its hitting a rev limiter right at 4700 rpm every pull no matter what I do. The data looks pretty good to me but maybe someone else can chime in incase I've missed something. Maf isn't very stabile but I don't think that's the issue. 3rd gear pull log attached with newest .bin

[Brewster]

Also to note the Wideband is a touch fat but hoovering around 11.5, and when it hits the brick wall at 4700 it sounds just like a rev limit so I let off instantly. Fuel pressure is maintaining 43 psi

[Brewster]

Here is a log of a steady cruise and then a slight accel through a few gears and then decel back down to idle. Who thinks this maf is super unstable and causing some issues?

Why is bank 1 and 2 lambse apart from each other ? Car has short tube heads but look like they have some length to the primaries.

[Brewster]

Exhaust leak or vac leak ? I sprayed some brakleen around the intake and what not and am not hearing anything or seeing any trim changes. Exhaust seems sealed up but will smoke it for the hell of it

[m-barans]

Exhaust leaks could explain the bias in fueling for bank 1 vs bank 2. If you have a misfire(s) on one bank it can throw off that banks fuel trim (rich) as well. If I had to guess it’s probably an ignition system related misfire on that (richer) bank.

[Brewster]

Just found a crack in the intake manifold! Smoked the exhaust and intake system with a Mac smoke machine and sure enough the intake manifold is leaking. I'll report back once I replace the intake manifold.

[bad86capri]

Did this fix your issue? My car is having a similar issue. Between 1300-2000 rpm at light throttle is goes straight lean and starts bucking. if youre on the throttle around halfway its seems to be fine. My issue started when i let a guy help me with my tune. i switched from the A9L2 bin to binary editor. after making tune adjustments thee issue popped up. im going to move all the data from back into A9L2 and see if it goes away.

[Brewster]

Still haven't had a chance to finish this one, customer opted to come grab his car and replace his intake manifold himself. When he's done with that I'm sure he ll bring it back and I can continue onward and upward.

I've noticed any cammed fox's I've played with usually have a bit of a miss while cruising or low loads. I've always inspected the ignition systems and never find any issues there, not sure if there just running a little lean and don't like it, a characteristic with a cam or what it could be.

Hows your trims when yours starts messing around?