MSefi.com read only forum (Jan.15,2018)

I've been running fuel-only MS on a 350 SBC with Throttle-Body Injection for ~3000miles, with only the usual first-timer problems. A couple weeks ago, the engine began to run very poorly - barely idles, sounds like major misfiring requiring about double the usual throttle to maintain speed. Interestingly, it runs well when floored - almost like there's an on-off switch at about 70% throttle. And, occasionally during this 2 weeks of de-bug, it'll run perfectly - for a few miles, then suddenly reverting to crap again.

I started with the ignition, checking timing, replacing the coil & module (4-pin in a GM HEI cap). Checked all the cap, plugs, & wires - OK (plugs not wet - no signs of flooding). Compression is 165 +/-10 on all cylinders. Vacuum at "idle" (if you can call it that) is only ~20". Verified that the brake booster & dizzy pot aren't leaking. I had the injectors tested & rebuilt (tested good before & after, and 'look' OK when running). Fuel pressure at the TB is good at 13psi. Voltage stable at 14.1V.

I've bench-tested the MS II (V3) board extensively (including the bits in the troubleshooting page, especially item #11), and all looks well. When hooked to the stim, everything looks good, and I've 'scoped the injector circuit & tach circuit extensively. I re-washed the board with IPA (haven't been confident enough to conformal-coat it yet). I'm tempted to drive around with the 'scope attached, but worried I'll crash due to too many things to look at!

I'm wondering if I could be experiencing a partial failure of some of the injector circuit components. Could the operating environment of the vehicle be different enough to cause marginal parts to act up? Any other ideas???

Thanks!

KL,

Board version? Processor version? Code version? Datalog? MSQ? All of that information is necessary to provide informed help.

Lance.

Board V3.0
Processor V3.01(C)
Code ver 2.891 (attached)
MSQ attached
Datalog attached

Note that the current datalog looks like crap - the problem has degraded to the point I can barely keep the engine running. If I'm lucky, whatever it is will die altogether & I can track it down.

Another important bit: I can swap the leads to the TBI Injectors, and measure the temperature of the header pipes. In the 'normal orientation, cylinders 2, 3, 5, 8 are ~350F, and the others are ~180F. Swapping the Injector leads inverts the measurements: Cylinders 1, 4, 6, 7 are hot, the others are cooler. I have a dual-plane manifold; this implies to me that one of the injector circuits or Injectors is not performing the same as the other.

Thanks!

OK, I've definitively ruled out all other components of the system by removing the TBI & MS, and re-installing the old Q-Jet carburetor. Runs fine now. Why was it I decided to use MS in the first place? Hmmm...
Anyway, still interested in any ideas anyone has about this problem.
Thanks!

Yes, the bench test is a bit different from running on the car - real injectors will run a lot more current through the drivers than the Stim does. The first thing I'd try is checking if the passive current limiting circuit is damaged by removing Q14 and Q15 - these can shut down the injectors if the current goes over a set threshold, which should be pretty high but if Q14 or Q15 are damaged they can shut down early.

Thanks for the input, Matt.
I can easily try this on the bench, but what should I look for? I can run megatune or TunerStudio, and can probe any point with a 'scope. I'm not enough of an EE to know what the circuit should do with Q14/15 missing. What should I be looking for to help me know if this is /is not the problem?
Thanks,
- Kelly

Removing Q14 and Q15 should turn overcurrent protection off.