Help! Misfire/stumble @ WOT *PROBLEM SOLVED*

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Digger
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Help! Misfire/stumble @ WOT *PROBLEM SOLVED*

Post by Digger »

Hey guys. I'm still fighting a near WOT/WOT stumble / misfire / hesitation. Microsquirt computer, Ford 351W roller block, mild cam, 9:1 compression, FAST 4150 throttle body, 4x 71lb injectors, 85 octane fuel (87 equivalent here in Denver) Here is run down of things I've done/learned in the last 6 months trying to solve this issue.

For starters, the engine runs great in all conditions except WOT. Spark plugs are clean with a slight tan color. Fuel mileage is up ~40% vs the old carb. I have run the engine from 1000 ft ASL to 12,000+ft ASL. Baro corrections, cold start, all good. The truck is daily driven year round. The VE map is dead on in open loop operations. At any point where I can hold rpm/load, there is <.2 AFR variance at anytime. Most times its dead nuts on commanded AFR. I cannot stress enough, VE is very good. I have confirmed this with logs and manual checks at many reference points from idle to 4000rpm and 30kPa to WOT.

- I asked for some help before and took other poster's advice to rich'n up the top of the AFR table. It is now at 13.5. I have swept WOT AFR from 12.0 to 14.5 looking for a sweet spot. AFR is having little effect. There is slightly less stumble around 14:1 @ WOT. This engine doesn't seem to mind running leaner.

- On another poster's advice, I switched to %baro strategy from MAP. I live in Denver and max ambient baro is 85 kPa. I don't know if it helped, but it does give me more cells to play with, so I'm sticking with it. It made no change to the stumble.

- Timing is as best as I can figure right now. Throttle response is snappy and the engine pulls hard until near WOT, say 90%+baro. I have swept the timing table to the far extremes of what the engine will tolerance. At WOT, I have run as low as 8 degrees all the way to 40 degrees in 1 degree increments. Right now, it is happiest with the current table, but the stumble persists.

- AFR during the stumble is very near commanded, but during stumble, you can see the AFR gauge flutter slightly

- I have played with time based and EAE enrichment, thinking this may be to blame. I have forced very rich transients and lean transients. The stumble does not change. Again, I cannot stress enough, I highly doubt this has to do with enrichment. I can approach WOT very slowly, moderately, or hard stab, it does not matter. As soon as MAP approaches WOT, the stumble begins and gets worse the further I push the gas pedal.

- I am suspecting this may be hardware related, but I cannot find anything else wrong so far. This problem did not exist with the carb, but I'm extracting more power with EFI, so take it for what's its worth. I have confirmed fuel pressure from idle to WOT. It holds a perfect 40 psi differential pressure. I have changed spark plugs from non-resistor race plugs that came with the engine to conventional plugs. I have also varied plug temperatures looking for pre-detonation. I have switched out the MSD CDI box for my spare, no change. I have checked wires, plumbing, air, exhaust, nothing. I started running premium fuel, thinking maybe I was getting compression ignition, no change. I am going to swap computers, but can't see this being a problem.

I have not attached a log file. They are too large and no one has found anything useful so far beyond what I've already stated. I'm desperately looking for ideas at this point. I don't want to give up, but the truck is not really useful unless I can drive it under all conditions and not risk engine damage.

Has anyone has experience with a bad injector? They seem okay, but I can't rule anything out.
Last edited by Digger on Tue May 09, 2017 11:04 pm, edited 1 time in total.
'69 Bronco, lifted, coiled, linked, locked, caged, ZF5, Atlas II, BB 9inch/HP44, 37s
MicroSquirt, roller 351W .030"over, FAST throttle body, alum heads/intake, header, forged rotating bits. 370HP / 440 lbft daily driver
Digger
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Re: Help! Misfire/stumble @ WOT (Not enrichment issue)

Post by Digger »

One thing to add. I just was reading that spark plug gap could be an issue. What are most people running for a small block 350 chevy or 351 Ford?
'69 Bronco, lifted, coiled, linked, locked, caged, ZF5, Atlas II, BB 9inch/HP44, 37s
MicroSquirt, roller 351W .030"over, FAST throttle body, alum heads/intake, header, forged rotating bits. 370HP / 440 lbft daily driver
elutionsdesign
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Re: Help! Misfire/stumble @ WOT (Not enrichment issue)

Post by elutionsdesign »

As soon as MAP approaches WOT, the stumble begins and gets worse the further I push the gas pedal
I assume you mean the longer to keep it at WOT? Have you checked that the throttle body butterfly(s) fully transition open and nothing odd is happening there? Other than that look for ignition wires that may be moving around, at WOT the engine could be twisting on the mounts and shorting a plug lead.
Graduate of EFI University.

I build, repair, install and tune Megasquirt systems in North Dakota and beyond!
slow_hemi6
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Re: Help! Misfire/stumble @ WOT (Not enrichment issue)

Post by slow_hemi6 »

I have the same throttle body on my 349 SBM. Single coil, no MSD, runs sweet to 6K limiter. My plug gaps are 1mm. Similar set up but I run 4 squirts, Simultaneous and 36psi fuel pressure. Mine also has a 36-1 crank trigger.
Find the Manuals up top under Quick links: Manuals. :RTFM:
Cheers Luke
Digger
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Re: Help! Misfire/stumble @ WOT (Not enrichment issue)

Post by Digger »

slow_hemi6 wrote:I have the same throttle body on my 349 SBM. Single coil, no MSD, runs sweet to 6K limiter. My plug gaps are 1mm. Similar set up but I run 4 squirts, Simultaneous and 36psi fuel pressure. Mine also has a 36-1 crank trigger.
Slow - Thanks for the reply! Would you mind posting up a .msq? I would love to compare settings for a similar engine, especially since I'm likely to go to a 36-1 crank wheel in the future. Right now I'm triggering off the MSD distro's 8 pt wheel and controlling the CDI box with the computer.
'69 Bronco, lifted, coiled, linked, locked, caged, ZF5, Atlas II, BB 9inch/HP44, 37s
MicroSquirt, roller 351W .030"over, FAST throttle body, alum heads/intake, header, forged rotating bits. 370HP / 440 lbft daily driver
slow_hemi6
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Posts: 4122
Joined: Fri May 07, 2004 3:33 am
Location: Australia

Re: Help! Misfire/stumble @ WOT (Not enrichment issue)

Post by slow_hemi6 »

Sure, it's fairly basic. I have just changed from running staged on the TBI, back to all 4 injectors sim and replaced my fuel reg and dropped it couple of psi. That's slightly thrown out my AE and Veal has been adding a bit of fuel to the table, so it is a bit of a mess. It's got a 288/292 cam in it so idles in the 65 to 70kpa in gear. Might go out tonight and do some tuning.
2017-05-05_10.49.11.msq
Find the Manuals up top under Quick links: Manuals. :RTFM:
Cheers Luke
Digger
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Re: Help! Misfire/stumble @ WOT (Not enrichment issue)

Post by Digger »

SUCCESS!!!

For starters I went through the whole ignition system changing out parts for spares and back again. I found nothing wrong. Tried re-gapping all the plugs, no change. Re-checked wiring etc, no change. Basically confirmed ignition was okay, leaving the door open for fueling issues.

Slow's post was the key, I went out one night and switched to "Simultaneous" injection instead of "Alternating" thinking that I might have an imbalance in cylinder-to-cylinder fueling. The stumble all but disappeared. I went back home and ordered smaller injectors (54 lb/hr vs 71 lb/hr). Tonight I installed the injectors and first thing I noticed was the orifice had 4x tiny holes instead of 1x giant one like the old injectors. Once installed I performed an injector test and the fuel pattern looked like it was better atomized.

Changed injector size in the .msq and went for a spin. AFR was unchanged globally aside from a couple of cells where I think atomization changed perceived engine VE. So tune appears to have a solid base. Stumble is gone at WOT. I'm getting a very occasional hiccup here or there, but I firmly believe it is a brief transient lean-out. The EAE and timing need refinement. Steady state and slower transients show absolutely no signs of stumble.

Thanks again for help!
'69 Bronco, lifted, coiled, linked, locked, caged, ZF5, Atlas II, BB 9inch/HP44, 37s
MicroSquirt, roller 351W .030"over, FAST throttle body, alum heads/intake, header, forged rotating bits. 370HP / 440 lbft daily driver
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