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Bad stumble under load
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Post Bad stumble under load 
Hi,
I've been been running Megajolt on my 1970 Jaguar XJ6 4.2 for a few weeks now. During this time I've been twiddling and tweaking on a daily basis, trying to find the elusive perfect tune, without all that much luck. It'll idle reasonably well (it's running some fast road cams on a flowed head, so idle has never really been terrifically smooth), and run ok when on light throttle, but as soon as I accellerate harder from around 2000-3500rpm, it stumbles terribly. Best case, it's stuttering, worst case it's almost bunny-hopping along. I've tried adjusting the mixture, but this was a blind alley. I've tried the simplest map possible, i.e. 10 degrees at idle, rising to 36 degrees at 3000, no modifications for changes in vacuum., just to eliminate a poorly constructed map from the equation. Yet it's still awful, and I'm stumped.

I left the old ignition (Mallory Unilite, Crane HI-6 ignition amp and PX-92 coil) in place when I fitted the EDIS and MJ, and when I switch back to this, it runs smooth and without issues. The mixture setting in the triple SU HD8s is AFR 14 at idle, and is well synchronized (using 3 wideband O2-sensors). The EDIS and MJ share switched 12V from the fuse panel, and share a common grounding to the chassis. Plus wires are brand new, custom made items. The wiring to the sensor is proper shielded cable, as is the wiring from the EDIS controller to the MJ. The sensor bracket I welded up from 6mm flat steel, and the sensor is roughly 1mm from the trigger wheel.

I bought the MJ ready built from Brent, and it's the MAP-sensor version. Running the latest firmware (3.2.1).

I've been reading in the forums looking for tips, but frustratingly I find that those that have had similar sounding problems tend not to post the solutions!

Can anyone offer any advice?

Nick
Norway

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You mentioned setting ignition map to the same advance values- that's a good test. However, what would be best is to run the engine with the MJLJ disconnected all-together, allowing the engine to solely on the EDIS module, setting the EDIS in limp-home mode 10 degrees BTDC. This is an effective troubleshooting step as it removes the MJLJ from the equation and will quickly indicate were the problem lies.

Rough running and stumbling at mid-range RPMs is often attributed to sensor vibration- but what is interesting is that it only shows up under load? So it indeed never happens when under zero load- i.e. while revving in neutral?

Regards,


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Thanks for the input, Brent. I just did another trial run, based on this. I disconnected the MJ and drove my test route. The car ran
a lot better than with the MJ connected, both with and without load. In fact, there was no stumble at all. Almost as good as on
the regular ignition, going by the seat of the pants feel.

I then reconnected the MJ, and the stumble was back. As soon as the revs are up a bit it starts, this time with a nice POP! in the
air filter as the revs pass 3000 or so. I tried revving it without load, in neutral, and it seems to rev cleanly up to 3500 (I'm not
really comfortable going higher with things as they are).

Does this help at all?

Nick

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- Post or paste in your map file and another pair of eyes can have a quick look to see that there's nothing simple that's going awry

- How about running with the MJLJ but without the MAP sensor plumbed in?...could be a useful halfway house test (ie 2D ignition based on rpm only) between EDIS only and full MJLJ

If EDIS only is almost as good as regular ignition then it's going to be fantastic when it's sorted Smile

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I've tried a great many different maps now. I've attached the one I'm currently running, based on the simplest
ignition specs for the engine. A basic 10 degrees at idle, increasing to 36 at 3000. I've tried any number of
variations, with or without advance for vacuum.

The current map is attached.

Nick




simple e-type spec no vac.zip
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Well, the map is "interesting" but I don't see any reason why it should give the symptoms you describe...

The rev limit is well up the scale, but that doesn't mean you're not getting interference that the MJLJ is interpreting as high rpm and so switching into rev limit mode....so...you could try setting it to 9900rpm temporarily just to see if it makes a difference (I don't trust the 10000rpm setting - maybe for no good reason)

Some things to try:
- if you run the engine with the PC connected then as you rev it up and down does the red box move smoothly through the map bins? Are the runtime traces smooth and as expected?
- can you hook up some LEDs to the programmable outputs and the shift light/rev limit outputs? Then set the prog outputs to say 1000, 1800, 1500, 3500 and rev the engine/ go for a drive - what happens? If the LEDs light/flash at lower revs than they should then that indicates some noisy signals somewhere.

I'll keep thinking..

Report back...

Martin

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I'm not sure how the rev limit features work, and I'm not knowingly using them... The engine redlines at 5500, so I'm
never anywhere near going off the map. I created the map with the idea of having just a very simple advance curve,
not affected by changes in vacuum (I've expended enough days and hours creating all manner of maps with vacuum
advance and retard). You'll see from the map that the BINs go off my scale in both load and RPM, hence the simple
map should cover all real world eventualities. I think!

When driving the marker moves around on the map exactly as I'd expect it to. The tach is still connected to the old
ignition system, so I can check the RPM against that. I'm also running an Innovate DL-32 datalogger, which logs AFR
for each carb, in addition to MAP and RPM, again this appears to check out fine.

Nick

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It sounds like you may have too much advance with the MJ connected. Have you / can you verify the 10 degrees BTDC with a timing light when running on EDIS only? Is your VR sensor set to 60 degrees (6 teeth) before the gap at TDC?

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I'd be hesitant to say with 100% certainty that it's absolutely right, but I did spend some time making sure
I set it up right. I know I did the 6 teeth setting, after bolting in the sensor to get the starting point. If there
is an error, I don't think it can be too large. I even checked the direction of the engine rotation when I tried
driving on EDIS only earlier this evening!

The idea is good though, and it's something I've been considering as a possible problem myself. I've tried
really retarded maps as well though. Even with 8 degrees advance at all RPM-values at loads from 80 to
100 KPa it stumbles. I've looked at a lot of maps, including a rolling road tuned map from an almost identical
engine, and that was running a lot more advance at full load (around 30).

Nick

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OK, but still definately worth checking with a timing light, comparing the EDIS system (both with and without the MJ connected), with the original ignition system. There's a difference somewhere!

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Excellent troubleshooting tips guys.

Agree that you should observe the advance using a timing light- rev it past where you normally have the stumble, even if it's ok at idle. Also verify that the advance is changing as you expect it. In any case, it's interesting to watch the timing change in real time as you adjust the software.

Secondly, drive around and do some data logging with the configuration software and see if you can observe any RPM spikes. Logging is useful so you don't have to drive around with one eye on the runtime gauges. RPM spikes likely indicate the MJLJ is interpreting noise on the PIP line as legitimate signals.

Let us know what you discover!


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Thanks for all the input, guys. When I get home from work, I'll try following:

Using the timing light, verify that the EDIS is giving a straight 10 degrees advance.

Using the timing light, see if I can get an impression of what the MJ is doing.

Log a session of driving with the MJ.

Lying awake thinking about this, it would appear that the MJ is receiving the correct
signal from the EDIS controller (from what I can see on-screen). Is it possible that
what it is sending back is either jumbled, or becoming jumbled en-route to the EDIS?

Another point that struck me, it could be the stumble is there even at low load, just
not as noticeable, as it does appear to be amplified by load.

Again, thanks for the ongoing help!

Nick

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Ok, results of further troubleshooting are in:

Timing with EDIS only looks spot on, and rock solid through RPM.

Timing with Megajolt controlling makes no change at idle. Feeding throttle the timing advance steps up
and looks like it's doing the right thing. No obvious problems.

Started logging and went for a drive (needed some shopping, so I had an excuse). Idled down to the
main road and gave plenty of throttle in 2nd gear. The engine was fully warm already. Rapid acceleration
gave plenty of stuttering and spitting from the carbs. Ran it a bit up and down through the RPMs. Also
logged the return trip. Both logs are included.

I'm properly confused now!

Nick




megajolt_log.zip
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Hmm.....a few further ideas:

I'm concerned that you said it ran OK on EDIS only, but poorly with 8 degrees in the MJ map, which is very close to the EDIS-only 10 degrees. Construct a map with 10 degrees in *every* cell, and drive the car both with and without the MJ connected. In theory the timing should remain at 10 degrees in both cases - if there is a difference in the running, then the EDIS<->MJ communication is going awry somewhere.

Engines 'missing' more under load than when stationary can be a symptom of a poor quality spark. You haven't mentioned your 12v supply feed to the coil pack - this will be the highest current part of the setup, and thus needs a good quality 12v supply. This current is returned via the 3 lines (in your case) to the EDIS on pins 10,11 & 12, then to ground from pin 9, so all these need to be good quality too. (Apologies if these are all fine!)

Rich.

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The coilpack is supplied by the same switched 12V from the fuse panel as the rest of the EDIS and MJ gear. All supplied
from the same, and grounded to the same.

Is the power draw greater under a large load than at a smaller load (almost closed throttle). Could the 12V be faltering
under heavy load?

At this point I'll try anything...

Nick

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