light load misfire

ozi_milhouse · Postby **ozi_milhouse** » Sat Aug 11, 2012 5:50 pm

Hey guys and gals, after a little help. im stumped.

I have a mx5 mk 1 1600 stock motor running a gt 28 motor, 550cc rx7 injectors and megasquirt pnp ecu.

The car has some kays on it but good compression, starts first go, and on boost goes like the clappers. the motor is strong and sound. The problem is i have this light load misfire from about 1500-3000 rpm and i cant track down the cause. I have new magnacor leads, tried new plugs, new coil, another good cas, rewired the stiff cracked coil and cas wire harness and still no joy..... im stumped.

cleaned all the grounds on the engine i could find.

there's no noise on the coolant temp sensor (the one at the rear of the engine)

tried either unplugging or making the following items redundant on the megatune.
the VTPS, the front coolant sensor, the wide band 02 and still no luck.

Any suggestions?. anyone had a similar problem before?

and before anyone suggests it i have checked the megasquirt board and it already has the js8 capacitor filter installed.

thanks in advance.

Postby **NitroDann** » Sat Aug 11, 2012 5:59 pm

What air fuel ratio?

Dann

gslender · Postby **gslender** » Sat Aug 11, 2012 6:03 pm

Dwell set correctly? Spark gap enough? Afr too lean? Is it AE decel or truly a misfire? Have seen AE decel look like misfire. Turn it off by setting to 100%

G

ozi_milhouse · Postby **ozi_milhouse** » Sat Aug 11, 2012 6:09 pm

Its hard to judge because of the misfire. Before the misfire its at about 13.5 but because of the miss it spikes lean then goes back to about 13.5. if i hold the throttle just right i can instigate multiple misses and make it sound like a rotor or boxer engine. has me stumped.

have for pronged audi plugs at the moment but tried iridium gapped to .7 and tried again at .8 had the same issue.

ae decel has already been switched off. no accelerator enrichment either. dwell is set to 3.5ms but have also tried 5ms with no joy.

if i am accelerating moderatly there is no miss. but if i am cruising on light load the miss is there. At full throttle there is no issue either.

ozi_milhouse · Postby **ozi_milhouse** » Sun Aug 12, 2012 12:23 pm

Anyone else care to help with a desktop analysis. I've attached a data log. the 2nd mark ive inserted illustrates the really light miss. just before the second mark you can see the spark angle oscillate up and down a fraction, same with pulse width and duty cycle. THink these are just symptoms but not the cause. Any help would be appreciated.

It also seems to happen more after warming up.

ozi_milhouse · Postby **ozi_milhouse** » Sun Aug 12, 2012 12:23 pm

Anyone else care to help with a desktop analysis. I've attached a data log. the 2nd mark ive inserted illustrates the really light miss. just before the second mark you can see the spark angle oscillate up and down a fraction, same with pulse width and duty cycle. THink these are just symptoms but not the cause. Any help would be appreciated.

It also seems to happen more after warming up.

datalog201208120915.xls

manga_blue · Postby **manga_blue** » Sun Aug 12, 2012 12:34 pm

ozi_milhouse wrote:Its hard to judge because of the misfire. Before the misfire its at about 13.5 but because of the miss it spikes lean then goes back to about 13.5. if i hold the throttle just right i can instigate multiple misses and make it sound like a rotor or boxer engine. has me stumped.

If there was a miss caused by no spark then wouldn't that show up as a momentary rich condition instead as raw fuel went down the exhaust?

ozi_milhouse · Postby **ozi_milhouse** » Sun Aug 12, 2012 12:44 pm

MY understandng is that a miss due to no spark would show up lean as there is more oxygen in the exhaust due to it not being burnt up.

happy to be corrected on that one.

manga_blue · Postby **manga_blue** » Sun Aug 12, 2012 12:49 pm

Dunno, worth researching, but if you're right then both an ignition miss and a transient lean mixture problem would show as a lean O2 result?

plohl · Postby **plohl** » Sun Aug 12, 2012 9:56 pm

a missfire will show lean as the lambda sensor does not detect unburnt fuel. I have seen this in my car a billion times lately :roll:

Postby **NitroDann** » Sun Aug 12, 2012 10:02 pm

Oxygen sensors sense oxygen.

Any missfire of any variety shows lean.

Dann

manga_blue · Postby **manga_blue** » Mon Aug 13, 2012 1:50 am

You live and learn.

I asked the question in the first place because your problem sounded as much like a rolling lean mixture miss as an ignition miss.

Postby **NitroDann** » Mon Aug 13, 2012 2:03 am

I agree, which is why I asked air fuel ratio as a first question.

Its hard to diagnose past these basics from behind the innanet.

Dann

ozi_milhouse · Postby **ozi_milhouse** » Mon Aug 13, 2012 5:35 pm

thanks guys,

this is why i ask as i have exhausted my knowledge. that is until yesterday when working on the car i unplugged the coil to work on the dry and brittle harness that i had rewired previously due to a cracked cable only to find another crack in a different location, at the base of the plug............

waiting on mania to send me a new plug and length of cable so i can resoldier. hopefully this is it. Just seems off cause when i checked continuity at the igniter plug there was no resistance and good continuity......... maybe just had the cable at the right angle to make contact. know more in the next couple of days.
keep you posted.

jerrah · Postby **jerrah** » Sat Aug 18, 2012 7:50 am

Unplug the tips and see if it stops. My mysterious miss was caused by noise from my coil pack upsetting my tips signal. This triggered random accel enrich and decel which presented as a miss.