Autonerdz forums

brettless02 wrote on Jun 11^th, 2010 at 2:33am:


Hi Brett. I would at least request from you Year, Make, Model, Engine size(with engine code) and other info to help put me where you are at in your testing so far.

Why was the ignition parts replaced? Testing or parts shotgun?

What gives you an impression of something pointing to the fuel/injectors?

Spence

Hello,
        The car is a VX Commodore in Australia.Its a V6 wasted spark system similar to the Buick motor in USA.If you rev the engine from idle you can hear the miss in the exhaust and when driven with slight open throttle you can also feel it miss.I have checked the plugs and leads and all appeared ok.To make absolutely sure that they were not  the problem i replaced the spark plugs and leads but problem still there.I have also replaced the coils and DFI module from a know good working unit and the miss is still present.Today i removed the plenum chamber and checked the injectors for operation and all appear to be spraying ok.I have also checked the crank and cam angle sensors and they too appear ok.Also checked EST and EST bypass as there was a fault present for bypass circuit but this appears to be an old fault.Checked its operation with scope and looks ok.Was thinking if the bypass was taking control of spark timing it may not have been advanced enough,also i get a backfire through air cleaner on revving and that is intermittent.Was thing it had a blocked injector causing a lean mixture but doesnt appear to be the case.

Cheers Brett

Hi Brett,

before you start replacing any parts under the bonnet, it is best to start with a good look at all the basics.
The 3.8 litre ecotec engine runs the Delco waste spark system.

The KV voltage and spark line can be observed on each cylinder, but the best starting point is with a compression test.You need to check the condition of the engine in the first place.

Go to supa cheap autos or Repco & grab a compression tester.

Brad

Brettles02,

Could you clarify about the misfire is it a complete cylinder misfire that can be felt at idle or one that only takes place in certain conditions?

If it is a dead hole you could do a low tech cylinder kill.  There is some risk involved both to equipment and person.  The process is to use a back probe inserted into one ignition cable at a time.  Then using a jumper lead ground the cylinder.

IF NOT DONE RIGHT IT WILL SHOCK YOU.  IF NOT DONE RIGHT IT COULD DAMAGE EQUIPMENT. So be careful if you try it.

The idea is with a cylinder not contributing there will be no engine speed drop on the bad cylinder.  The problem is if it is a partial misfire it will not work as well especially if the miss is not at idle.

Have you replaced any GM fuel pumps with in tank pressure sensor?
If you have you have a low cost pulse sensor for the exhaust you can build yourself.  That is of course additional equipment you would need to fabricate.

The technique with the least effort would be to observe secondary ignition.  Based oh the post I am assuming you do not have a DIS adapter such as the Mixmaster or the Ferret.  You should have at 2 or four KV probes that came with your scope.  Connect the two probes to the two coil towers and duplicate the symptom.  Do this two times if you have 4 probes or 3 times if you only have two.

So you will be connecting to 1/4, 2/5, 3/6 if that is similar to a Buick V6.

I am posting a link to a video I uploaded on Google video some time ago.  It was from a Ford, although it should give you an example of what the set up is.  I also used Pico Diagnostics.  I did not mention that because there is not a good way to determine the cylinder that is missing through Pico diagnostics. at this time.  The video clip I posted had a broken spark plug, I suspect from a head gasket leak.

Here is the video:



Once you record you files with the misfire present post the psd files here and we will try to help further.

Good luck

Sam

Hello,
        Here is the pics of the plenum chamber

Here are the compression and cylinder balance test results

Question for ya...  Was the miss only at idle? In other words did the miss go away when you reved the engine to 2000 to 2500?

Spence