Cylinder Balance Test - Porsche V8?

928NE · Post by **928NE** » Sun Feb 09, 2014 10:15 pm

I tried to get the cylinder balance test to work on two different 928s. One with a misfire and one without. In both cases the software errors-out with an array bounds index error.

I've tried every useful combination of connections and loading of the electrical system, but the result is the same: array error.

One thing that is very consistent is that the set-up dialog displays the wrong RPM value. On the car without this misfire it displays roughly 1/3 of the actual RPM and on the one with the misfire it displays 1/2 of the actual RPM. In the first case the actual idle is about 1k-rpm (and yes, that is both a correct measurement and a correct idle for this particular 928 in its current state of tune) while the CB-setup shows high-300s. On the second 928 the idle RPM is about 800 with the CD-setup showing low 400s.

Can anyone offer any insight? There's nothing too weird about the 928's V8. It's not a flat-plane crank. It doesn't have a bizarre firing order. It does have a dual-coil/dual-distributor ignition system with each 'side' firing half of a bank of cylinders. One 'side' fires 1,4,6,7 and the other fires 2,3,5,8.

Post by **Steve Smith** » Wed Mar 12, 2014 9:51 am

Hello and thank you for your enquiry and sorry for my late reply.
My first thought here is what version of software do you have in both Pico diagnostics and PicoScope automotive. This will certainly help to establish the build of software and what improvements have been made?

Could you send in a saved file to support@picotech.com?

The error message "array bounds index error" is a message directed towards the software engineers that should not really be displayed to the end user, this will be rectified on future updates.

I strongly suspect the issue surrounds the quality of the AC ripple evident at either the battery or the alternator, and that's not to say there is a charging issue, just that the ripple does not suit the pattern expected by the software and so a cylinder balance or rpm calculation cannot be made.

Could you send in a scope trace of the AC ripple using the pre-set under the automotive menu "charging and starting" "alternator ripple and diode test". This may help to clarify why we have the issues above and may assist you with a cylinder balance evaluation.

One other issue to consider is the fact a misfiring V8 engine may not slow down sufficiently enough during the misfire to effect the AC ripple, as we have 7 other contributing cylinders to carry the engine over the misfire due to momentum and the increased frequency of firing events in comparison to a 4 cylinder engine

I hope this helps, take care......Steve

DebbieSidera · Post by **DebbieSidera** » Wed Mar 19, 2014 6:24 am

Hi guys,

I'm also having all kinds of troubles with the Pico Diagnostics software. This week I tried in a F250_GT/L V12 and it was showing errors. I repeated the test several times and it showed different results.

I always trust with the Amp clamp. It is so much better

Then I pulled a spark plug out to check the real pressure. I hope to get soon my transducer.

I attached 2 graphics. They are from the same engine. The first shows a death hole in the cylinder K... then I repeated the test and you can see that everything it's just fine!

(next time I'll try to reduce the Amperage range)

Steve, what do you thing these spike are? They appear between the bank 1-to-6 cylinders.

The best
Debbie

Post by **Steve Smith** » Wed Mar 19, 2014 5:10 pm

Hi Debbie and thanks for the feedback on this issue. Whilst there is real value in the Pico diagnostic cylinder balance and compression test on a multitude of vehicles we have to concede on the limitations mentioned with these "high cylinder count" engines.

The spikes you mention appear to be evenly spaced and timed suggesting possible induced noise from ignition or injection events. What is interesting is how the spikes miss one compression then appear on the next.

Can you confirm if an ignition or injection event was present during the test on one bank only?

Cheers.......Steve

DebbieSidera · Post by **DebbieSidera** » Wed Mar 19, 2014 6:34 pm

Hi Steve,

No I did not had them connected. The F250 GT/L has two coils and two distributors. I just pulled the center coil wire of them. Also I disconnected the terminal of the fuel pump. May it could be the carburetors residual gas but after repeat the test several times.. i don't think that there was any gas left in the carburetors.

Thanks

The best
Debbie

928NE · Post by **928NE** » Wed Mar 19, 2014 11:56 pm

Hi Steve, thank you for the response.

steve smith wrote: Could you send in a saved file to support@picotech.com?

Can you point me to more-verbose instructions on precisely what kind of saved file you seek and the procedure for saving it? I have looked through the application's on-line help and nothing stands out.

In this specific case, I have moved on to other diagnostics, but I am still interested in assisting you all in getting the cylinder balance test to work.

Post by **Steve Smith** » Thu Mar 20, 2014 10:10 am

DebbieSidera wrote:No I did not had them connected. The F250 GT/L has two coils and two distributors. I just pulled the center coil wire of them. Also I disconnected the terminal of the fuel pump. May it could be the carburettors residual gas but after repeat the test several times.. i don't think that there was any gas left in the carburettors.

Hi Debbie, thanks for the feedback. Forgive my ignorance as I did not know your vehicle was non injection.
Could you send or post the psdata file to enable me to zoom on the spikes and measure accordingly?
I do feel as though these are induced into the signal as HT would still be generated with the king lead removed from the coil tower. (Would this be the “centre coil wire” removed)?
Could we have one leaking coil during cranking (King lead removed) and so generating the noise picked up by the current clamp?
Another theory to ponder over.

928NE wrote:Can you point me to more-verbose instructions on precisely what kind of saved file you seek and the procedure for saving it? I have looked through the application's on-line help and nothing stands out.

Hello and sorry for the lack of information re saving and sending. I have just simulated the error you have received and the save feature is not permitted. I have spoken to engineering and the save option is disabled as there is no relevant information gathered and so nothing to interpret. If a test could be completed however bizarre the results, we could look at the "signal" as this information would have been included in the saved file.

It would be interesting to see the AC ripple waveform from the PicoScope which you could save and post here or send to support@picotech. Whilst it may not answer the questions surrounding the PD error message it may assist with potential cylinder balance evaluation assuming there is some uniformity to the signal

With your waveform on screen, select FILE-SAVE AS fill out all the relevant details select OK-name your file, select CURRENT WAVEFORM ONLY (if this waveform has all the detail required) and save to a location on your PC.

You can then attach this file into an email or post the file here on the forum. Saving CURRENT WAVEFORM ONLY will keep your file size down as opposed to saving 32 or more waveforms within your buffer.

I hope this helps.......Take care......Steve

PicoScope 7 Automotive

PicoScopes
& kits

Noise, Vibration and Balancing

Breakout
leads/boxes

For vehicle manufacturers

Milliohm

Current clamps

EV kits and
accessories

Pressure sensors

Accessories

PicoScope forum

Contact

Training

Automotive guided tests

Case studies

Newsletters

Training

A to Z of PicoScope

Documentation

Waveform library

Videos

Reviews and awards

Automotive Support Forum