light load misfire

ozi_milhouse · Postby **ozi_milhouse** » Mon Aug 20, 2012 11:41 am

Ok all,

So it wasnt the cas or coil plugs and cables. they have all been stripped back and rewired with fresh cabling. No more cracked cables. Still have a miss.

Spoke to DIY auto tune who have been great and they suggested re loading firmware. did this and still no result, still have a miss.

Hey Jarrah, by Tips, i guess you mean TPS ie. throttle position sensor. Have tried that too and still have a very light miss.

I'm stumped. ANy other guesses? armchair tuners out there with a clue?

Justin

bruce · Postby **bruce** » Mon Aug 20, 2012 3:04 pm

Could it be something mechanical - burnt valve?

ozi_milhouse · Postby **ozi_milhouse** » Mon Aug 20, 2012 4:23 pm

I guess but i did a compression test a little while ago and it was ok. Might try another comp test / leak down test and see how it goes. But she's not blowing smoke on acceleration or under load. I might just have to grin and bare it, its such a light miss, but it makes tuning at cruise almost impossible.

plohl · Postby **plohl** » Mon Aug 20, 2012 4:29 pm

dirty injectors? maybe???

bruce · Postby **bruce** » Mon Aug 20, 2012 4:46 pm

Dirty fuel, dirty fuel filter?

I want to say 'spark plug leads', but you've changed them.

ozi_milhouse · Postby **ozi_milhouse** » Mon Aug 20, 2012 5:24 pm

doubt it is dirty fuel or filter as there is no problem at WOT or at higher revs. The car runs like a train in those conditions.

Injectors, not so sure its those either, swapped them around and systematically unplugged each injectors whilst trying to hold the miss point. wasnt that either. did the same with the plug leads and still a miss. Im gravitating towards wiring or noise affecting the ecu because it doesnt appear to be spark or fuel related...............

i noticed the pulse width dropping momentarily at the miss point in my logs..... its got me stumped.

bruce · Postby **bruce** » Mon Aug 20, 2012 5:36 pm

Faulty ECU? I doubt any electrical noise would affect it as it is inside a tin box (to avoid noise).

toppertee · Postby **toppertee** » Mon Aug 20, 2012 5:47 pm

I had a bit of a miss at around 2-3 rpm. GSlender had a look on sunday. TPS had a dirty signal.
Switched to full MAP all good. Have you tried running yours on 100% MAP?

Postby **NitroDann** » Mon Aug 20, 2012 6:06 pm

G is a legend.

Im going with noisy signal.

If the cas wire goes near the ignition this can happen, for example.

Dann

ozi_milhouse · Postby **ozi_milhouse** » Mon Aug 20, 2012 11:32 pm

Yeah already on full map only use the VTps for overrun cut settings.

All the megasquirt gauges run smooth no jumping so noise can't be the issue.

I'm stumped

gslender · Postby **gslender** » Tue Aug 21, 2012 9:48 am

email me a log of the problem and tell me exactly where in the log it is missing. gslender at gmail

ozi_milhouse · Postby **ozi_milhouse** » Tue Aug 21, 2012 11:24 am

Thanks G Slender. Will do when i get home tonight and have access to the log. Email you then.

cheers

justin

gslender · Postby **gslender** » Tue Aug 21, 2012 8:43 pm

Just looking at the log and I'm initially very concerned.

1) AFR is mostly see-sawing between 13 and 15 - odd behaviour?

2) MAP is very odd and flat... not what I'd expect either... max is 30kPa and min 15 - WTF?

3) PW is 1.0 - !! :shock:

1.0 ms... that can't be right... but yup it is, max logged PW is 2.0 - so I can only assume this old firmware is broken in logging the correct PW

4) GVE rocks between 57 and 58.... that's it...

This log is bonkers and I can't make sense of much of what I'm seeing.

The tune MSQ looks ok.

It is going lean, and the map sensor isn't really showing the correct true manifold pressure - it can't be only between 15 and 30 kPA... impossible. There is a period in the log where RPM starts at ~1000rpm and then engine revs climbs to nearly 3000rpm in 6 seconds and MAP goes spastic hitting 30kPA in very a few spots, then settles at 22kPa where it remains mostly flat for the entire rest of the steady rpm (where you begin to have misfires and RPM oscilations).

Something is sick - I'd start with the MAP and get it reporting correctly. Take more logs and study them.

G

ozi_milhouse · Postby **ozi_milhouse** » Tue Aug 21, 2012 11:16 pm

Hey G,

Sorry should have explained the log was taken steady state in neutral with a constant throttle position to instigate the miss. I will drive to work and log the full trip tomorrow and reload. As the issue is at light load at cruise at around 3000rpm it is easily repeatable in neutral where the MAP is consistantly low.

Will reload a log with a wider range of parameters.

cheers

Justin

gslender · Postby **gslender** » Wed Aug 22, 2012 2:36 pm

Checked the log you emailed this morning - I'd say it is also concerning.

You have 106kPa as being the highest MAP value reached. Unlikely in a turbo, and the curise manifold pressure is mostly in the low 30s.... again very unlikely.

My guess is that you have either a faulty sensor, or the ECU isn't calibrated for the sensor you have. Either way you need to get that fixed as we are trying to fix something that is already got something else broken.

Also, the AFR is generally all over the place. Really messy and as if you have major tune issues (it needs to be way, way more stable) so maybe it is spark (as that would defintely show up that way) but if it is pulling strong then that is odd and I would say it isn't - also if it idles ok, then I'd be against it being spark.

My vote is that it is probably running too lean all over, or you have a vacuum leak that is 1) making it run lean and 2) mucking up the MAP signal (but normally that goes in the other way, with values centred around atmos 100kPa).

Keep looking and fix up that map value.

G