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Missfire help!


John Dixon

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OK, I have a missfire on my engine at all revs including idle. Cyls 2 and 4 not running at all - see vid here http://uk.youtube.com/watch?v=xwE1LD4XrO0 You can hear the difference in the exhaust bypass too.

Diagnostics so far:

 

Compression test - all cyls 16bar

 

Removed sparkplugs from engine and grounded them. Turned engine over and all plugs sparking.

 

Manually shorted ECU outputs to check each coil on correct output - OK

 

Manually shorted ECU outputs to check each injector on correct output - OK

 

Increased coil dwell time - no change.

 

Ran new heavy gauge +ve/-ve lines directly from battery to coils. No change.

 

Swapped coilpacks from 1/3 to 2/4 - missfire still on 2/4

 

Swapped injectors from 6/8 to 2/4 - missfire still on 2/4

 

Swapped ignition modules from bank to bank - missfire still on 2/4

 

Swapped coil sub-harnesses from bank to bank - missfire still on 2/4

 

Changed sparkplugs - no change

 

Any ideas greatly appreciated, it's really annoying me now!!!!

 

I'm beggining to wonder if it's something physical, maybe the injector angle is off on these 2 cyls and the fuel is running down the manifold wall instead of entering the cyl as a mist or something. Will need to investigate more when I get time.

If I enrich the mixture on all cyls then re-connect the injectors on these 2 I can get some spitting back like you normally get through the intakes with a too-lean mixture.

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so youve determaned your getting spark....are you positive the ignition timings right? if all that determaned then its either air or fuel... id say you got something clogging those 2 injectors or theyre stuck closed...check your fuel filter and check your fuel pressure then make sure your injectors arent clogged and lines to them arent clogged either...

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Have checked and substituted injectors with no change. Pressure etc is fine as all other cyls are running OK and gauge is reading 3 bar steady.

Ign timing checks out OK with timing light. It's wasted spark too and the other cyl off each driver is running fine.

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John,

 

First off, that is one FRIGGIN COOL Hybrid Z-32! VERY nice, very nice… ave.gifvictory.gif

 

Now to your issue.

Looks like you have, without a doubt, verified that fuel and spark is happening at the offending two cylinders, i.e. the swapping of coils, swapping of injectors, swapping of harness, verification of spark at the plugs, etc.

 

Having fuel, and in the correct amount means the fuel issue is covered 100%, (injectors that function correctly in one cylinder should work correctly for all cylinders). Regarding the sequential injection sequencing/timing, see my tid-bit below.

 

 

At this point, the only two reasons I can think of that would cause the #2 and #4 cylinders not to fire are;

1) Ignition timing of those 2 cylinders is not taking place at the correct time, (which could cause the popping out the intake with more fuel added).

 

2) The offending two cylinders are not building compression, i.e. leakage past the valves, head gasket, or rings/pistons. (bent intake valves could cause the popping out the intake with more fuel added...)

 

#1) first. (My gut feeling tells me this is the problem...)

The ignition event of each cylinder needs to take place just prior to TDC during the end of the compression stroke. Being as you are using a DIS ignition system, (you mentioned wasted spark, MS, SDS or Electramotive?) the ignition timing of each cylinder is controlled by the ECU telling which coil to fire when, pending the coils are wired up correctly. wink.gif Is there any chance that the the plug wires for cylinders #2 and #4 are crossed? My source tells me the VH firing order is 1-8-7-3-6-5-4-2, which for a wasted spark ignition system would mean the following cylinders are paired to the same coil; 1-6, 8-5, 7-4, 3-2

 

 

#2) If those cylinders are getting the fuel, (we know they are), AND if they are getting the spark at the correct time?, (see #1 above), then those 2 cylinders are not mechanically "sealing" allowing compression to build which would allow for combustion. A few things that could be the cause of this issue are bent/stuck valves, broken rings or busted/holed piston, or a severely blown head gasket. The quick and dirty highly accurate way to check this is to perform a cranking compression check. If the 2 offending cylinders produce comparable compression to the other cylinders that run, then all is fine and the issue is most likely improper ignition timing, i.e. high tension leads for #2 and #4 need to be swapped. If the cranking compression is less than 100 PSI, (or metric equivalent), that would explain the lack of combustion in those 2 cylinders. From there you would want to perform a leak down compression test to verify where the compression loss is taking place, i.e. intake or exhaust valve, piston/rings, or head gasket. With the faulty cylinders pressurized, listen in the crank case, (oil dipstick tube or oil fill in the valve cover) for hissing indicating broken pistons/rings, listen in the exhaust pipe for leaking exhaust vales, listen in the intake inlet for leaking intake valves, and listen in the radiator for bubbling/hissing from a blown head gasket.

 

To be honest, my first guess is the #2 and #4 spark plug wires are crossed. :2thumbs: My second guess is bent valves from either a busted timing chain or rotating the crank with no timing chain installed.

 

 

As for the injector sequencing. That will have absolutely NO effect on this issue. Remember, the original EFI 280-Z had batch fire injection. ALL the injectors open and closed at the exact same time, once per crank revolution, regardless of where each of the 6 cylinders were at in the 4 stroke cycle, (some had open intake valves, some were exhausting, etc), and it ran fine. If your injector sequencing was not correct, you wouldn’t even be able to feel the difference, let alone cylinders not fire at all. The benefits of properly timed sequential injection is slightly more stable idle and better emissions at idle. As I recall, I "think" the VG30DE and VH45DE EFI will actually switch to batch fire under WOT?

In short, if the injector is opening and closing at the same pulse width as the others, and it isn’t clogged, that cylinder is getting the correct amount of fuel and will run, if compression is being made AND the spark plugs are firing at the "proper" time!

 

 

Hope that helps. Please let us know what you find.

 

Paul

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Paul,

Cheers for the reply, glad you like the car, been hard work but nearly there!

 

In response:

1) My gut was that this was the problem too initially. I've point-to-point checked the wiring from the coils back to the ECU and they're all paired in the correct sequence like you showed. That's why I'm pretty confident that the ign timing is not the issue as the paired cyls are running great! Not sure what you mean by MS/SDS/Electromotive? I'm using original nissan coilpacks and ignitors then the ECU outputs are each connected to 2 of the ignitor inputs to fire the 2 coils at once. This is working fine on 1-6 and 8-5 so I know the theory is OK!

 

2) This is a fresh built 5.0l engine (see http://www.300zx-club.com, members projects area for more details). I've done a compression test and all cyls are 16bar dead on (235psi in paegan units!) ~12:1 static CR. Do you think it's possible head gasket gone between the 2 cyls so coolant leaking in and quenching the combustion?!

 

 

I'm thinking my next steps to diagnose are:

1) Get oscilloscope on all the injectors and coils and see if there is a difference between the good/bad ones - maybe loom induced inteference?!

 

2) Spray some easy start into the bad cyls and see if they come to life - eliminate or confirm igniton as the problem.

 

3) Strip off intake manifolds and check machining the same on all injector bosses (size bored to, geometry etc.)

 

You can see why this is driving me crazy now!

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Jared,

Scope is next stop (as above post).

Haven't done a leak down test yet but I'm thinking with 16bar comp it should at least fire even with some passing?

 

John,

I think you are on right track with your next diagnoses. The thing I might add is to connect a timing light to the #2 & #4 coils and verify they sparking at appropriate time, (#2 should illumiate the same spot on the damper as the confirmed runnig #3 cyl, same goes for #4 and its match#7.)

 

When I read wasted spark, I was asking which aftermarket EMS are you using, MefaSquirt, SDS, or electromotive? I didnt realize the OE VH ECU ran the COP ignition in wasted spark. :wink:

 

With that much cranking compression, a leak down is not necesary as that is plenty to support combustion.

 

Keep us posted on what you find.

Paul

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Cheers, good idea on the timing light. Dual traces on the 'scope should confirm the same thing too hopefully.

 

I'm using an Emerald K3 ECU, just retained the nissan ignition system as it was neater and (should!) be up to the job. Couldn't be bothered trying to get the stock one to work with ITB's etc, aftermarket is much better for my setup.

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  • 3 weeks later...

OK, got the 'scope on it today.

Looks like the last firmware upgrade I did to the ECU to change a PWM output table to a digital has screwed up the firing.

Compared a coil firing signal off a 'pair' of cyls where both were running with a pair where only one was. Guess what, the pair with the missfire was only firing 1/2 as often!

Guess that no 2, 4 cyls are only being fired on the exhaust stroke! Will get in touch with emerald on Monday, am sure they'll fix it quick but annoying how much time I wasted finding it. Hadn't appreciated just how uesful a 'scope can be on a car.

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  • 2 weeks later...

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