MSII behaving differently in the Car vs. Bench?

[KLattig]

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!

[Bernard Fife]

KL,

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

Lance.

[KLattig]

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!

[KLattig]

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!

[Matt Cramer]

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.

[KLattig]

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

[Matt Cramer]

Removing Q14 and Q15 should turn overcurrent protection off.