Vx - Does knock detection cause spark misfire?

Holden/Delco Tuning. ALDL, OBD 1.5. Circa 1989 to 2004.
Post Reply
JohnDee68
Posts: 249
Joined: Fri Feb 02, 2018 7:05 pm
cars: L36 Vx Series 2 8/2002 Acclaim Wagon 3.8L ECOTEC II.
Location: South Australia

Vx - Does knock detection cause spark misfire?

Post by JohnDee68 »

Hi everyone,

I am trying to diagnose what possibly could be an intermittent misfiring on my Vx series V6 3.8L.

Can anyone tell me when a detection of a knock is made by the ECU from a knock sensor, does the ECU only makes adjustments to the spark timing. It doesn't actually prevent a spark plug from firing does it ?

Here is why I ask:
As I have been trying to diagnose a possible misfire, using an induction timing light on each of the HT leads in sequence. While raising the revs during each lead test and seeing if the light shows electrical miss. Some were fine but 3 seemed to miss (Cyl 1, 2 & 4) as the timing light had voids in illumination at times. I replaced the (1,2 & 4) plugs and lead with a spare (good one) for the suspecting cylinders. But the issue still seemed to be happening of the timing light blacking out intermittently with revs increase. (I can't rule out a poor induction pickup of the timing light either but it wasn't happening on all cylinders).

I then changed a single coil pack relating to Cyl 1 (again with what was as I believe a working spare I have) to see if it resolved the issue. As the issue is intermittent, it seemed to make no real change for the better, just a series of missing flashes. I know that there is a DFI module under the coil packs, but i think they usually just stop working rather than causing intermittent firing, maybe ?

So I am wanting to ensure that the ECU doesn't actually turn off the spark to a cylinder at all when a knock sensor activates before i replace parts with new. As that would make diagnosis maybe a little different method and path to take. ie; Knock sensors failing and not actually a HT electrical component failing ?

Appreciate any shared experience and knowledge.

Kind regards
John.
User avatar
vlad01
Posts: 7780
Joined: Mon Oct 08, 2012 6:41 pm
cars: VP I S
VP I executive
VP II executive
VP II executive #2
VR II executive
Location: Kyneton, Vic

Re: Vx - Does knock detection cause spark misfire?

Post by vlad01 »

They just retard the timing when it detects knock and once it clears it slowly adds the timing back in. As far as I know, none of the factory Delco tunes or even most of the ECUs as far as I recall can do spark cut.
I'm the director of VSH (Vlad's Spec Holden), because HSV were doing it ass about.
immortality
Posts: 3416
Joined: Thu Apr 09, 2009 12:31 pm
cars: VH, VN, VS, VX

Re: Vx - Does knock detection cause spark misfire?

Post by immortality »

Each coil fires a pair of cylinders, if the fault occurs on cylinder 1 than it should also show the same fault on cylinder 4 (See diagram)
Image
In which case it would suggest the coil for that pair of cylinders is suspect. If replacing that does not fix the problem and you know the plugs and leads are good than the next would be the DFI module.

Seeing as cylinders 3 and 6 seem to be fine, swap that coil with the coil for cylinders 1 and 4 and see if the problem stays the same. It it does than look at the DFI module.
JohnDee68
Posts: 249
Joined: Fri Feb 02, 2018 7:05 pm
cars: L36 Vx Series 2 8/2002 Acclaim Wagon 3.8L ECOTEC II.
Location: South Australia

Re: Vx - Does knock detection cause spark misfire?

Post by JohnDee68 »

vlad01 wrote:They just retard the timing when it detects knock and once it clears it slowly adds the timing back in. As far as I know, none of the factory Delco tunes or even most of the ECUs as far as I recall can do spark cut.
Thanks, I was thinking it would be an inefficient and possibly harmful way to prevent knocking by cutting spark to cylinders. I have been told that newer V8 (BMW's) do cut cylinders on highways to save fuel (not sure if that is true though).
JohnDee68
Posts: 249
Joined: Fri Feb 02, 2018 7:05 pm
cars: L36 Vx Series 2 8/2002 Acclaim Wagon 3.8L ECOTEC II.
Location: South Australia

Re: Vx - Does knock detection cause spark misfire?

Post by JohnDee68 »

immortality wrote:Each coil fires a pair of cylinders, if the fault occurs on cylinder 1 than it should also show the same fault on cylinder 4 (See diagram)
Image
In which case it would suggest the coil for that pair of cylinders is suspect. If replacing that does not fix the problem and you know the plugs and leads are good than the next would be the DFI module.

Seeing as cylinders 3 and 6 seem to be fine, swap that coil with the coil for cylinders 1 and 4 and see if the problem stays the same. It it does than look at the DFI module.
Thanks for the advise. I will do that today, as the coil i did use, came from an older Vt (working) engine I have.

I might try swapping the DFI with a spare I also have today. See if any issue moves, hopefully stops playing up. :thumbup:

Will get back with my results.

Thanks again guys.
User avatar
vlad01
Posts: 7780
Joined: Mon Oct 08, 2012 6:41 pm
cars: VP I S
VP I executive
VP II executive
VP II executive #2
VR II executive
Location: Kyneton, Vic

Re: Vx - Does knock detection cause spark misfire?

Post by vlad01 »

Pretty much as Immortality said. Usually when a DFI output faults, it dies forever but it could be possible to have an intermittent fault, although I have never seen it. Though a coil itself and intermittent faults go hand in hand and therefore I'd try as he suggested.

Also, lots of engines (mainly V8s) these days cut cylinders in various ways, including fuel and spark to save some fuel when cruising.
I'm the director of VSH (Vlad's Spec Holden), because HSV were doing it ass about.
JohnDee68
Posts: 249
Joined: Fri Feb 02, 2018 7:05 pm
cars: L36 Vx Series 2 8/2002 Acclaim Wagon 3.8L ECOTEC II.
Location: South Australia

Re: Vx - Does knock detection cause spark misfire?

Post by JohnDee68 »

vlad01 wrote:Pretty much as Immortality said. Usually when a DFI output faults, it dies forever but it could be possible to have an intermittent fault, although I have never seen it. Though a coil itself and intermittent faults go hand in hand and therefore I'd try as he suggested.

Also, lots of engines (mainly V8s) these days cut cylinders in various ways, including fuel and spark to save some fuel when cruising.
Thanks, I am hoping I can get to swap over one of the better coils to the location of the suspect one and see if the fault follows. I have also been advised to measure the resistance of the newish leads, as they can break down even if new. I personally have had experience (in my youth) with new spark plugs breaking down at frequency which was a pain to find at the time. As they measured and performed fine at bench testing levels.
I am hoping my current issue with he car is coils, leads or plugs this time.

Just a side note, (as I have not experience multiple cylinder misfiring before) would a misfire of 3 cylinders, make the vibrations through the car severe enough to feel like a car driving on corrugated roads at times ?
User avatar
vlad01
Posts: 7780
Joined: Mon Oct 08, 2012 6:41 pm
cars: VP I S
VP I executive
VP II executive
VP II executive #2
VR II executive
Location: Kyneton, Vic

Re: Vx - Does knock detection cause spark misfire?

Post by vlad01 »

Yeah the car will vibrate as you describe, the more cylinders not firing the worse the vibration (simplification).

I've had intermittent misfire for a split sec every few weeks. Until it got worse enough to be once or twice a day. Ended up being the plugs, they were unknown age and must have been at least 200K old as I never changed them when we got the car and it had already done close to 200k since.

I had a miss on cold start that lasted a few sec that started last week. Looks like it was the coils (or one of them) as I did all the timing cover gaskets, pump, chain etc... and I had to pressure wash the crap out of the engine before starting the job, so no crap would fall into the sump. I tried covering the coils but water still got on them. The miss didn't start until a week later. I initially thought coolant was getting into the cylinder again as I had the same symptoms when the intake gaskets perished, but they were changed only 14 months ago.

I checked the coils and sure enough, the post terminals were absolutely rusted to shit, evidence water got in. It even eroded the lead terminal away from arcing too much that No. 1 would not clip back on, leads were less than 2 years old too. So I put on some good used coils and super glued the seams at the bottom of the posts to water proof them (design flaw with them), new leads yesterday and the miss seems to be gone on cold start.

Ignition can sometimes be finicky as faults often are intermittent and not obvious all the time, like the issue I had with the plugs.
I'm the director of VSH (Vlad's Spec Holden), because HSV were doing it ass about.
immortality
Posts: 3416
Joined: Thu Apr 09, 2009 12:31 pm
cars: VH, VN, VS, VX

Re: Vx - Does knock detection cause spark misfire?

Post by immortality »

If the VX has traction control then the PCM has the ability to cylinder cut but it's a function of traction control and not spark knock.
JohnDee68
Posts: 249
Joined: Fri Feb 02, 2018 7:05 pm
cars: L36 Vx Series 2 8/2002 Acclaim Wagon 3.8L ECOTEC II.
Location: South Australia

Re: Vx - Does knock detection cause spark misfire?

Post by JohnDee68 »

Ok, so here is what happened today (for those of you who are interested - Still not resolved issue).

This was all undertaken while stationary in the driveway.
I thought I would connect the timing light again to each spark plug lead to find out which cylinders were having the timing light off issues under revs as it did the other day. The plan was then to swap what possibly would be a good known coil pack currently in the car with another that was playing up in the car and see if the failure followed. Well, there wasn't any missing timing light issues today (checked while cold and through till at running temp). Though I was and am still getting the excessive smelly exhaust, which I am to believe it is unburnt fuel at this point.

So I thought I would then use an oscilloscope (little basic hobbyists kit unit) to check things at the ECU connectors. I got some lovely expected square waveforms going to the DFI module (pics available) which leads me to feel that it (at the time) was getting signals ok. (this doesn't mean the DFI, coils, leads or plugs are working though either).

I also checked a few other things while at the ECU, like knock sensors with a multimeter. These both only registered a slight change rarely under increased revs. Which I would put as normal and maybe due to normal operation.

The recently fitted (last week) O2 Sensors are not fluctuating as much as my originals did a year or more ago (150mv - 800mv swing), but they are moving around the 500mv to 950mv depending on throttle. With the smells from the exhaust, it is understandable that the sensors are reading rich and the STFT and LTFT are showing around negative 30's as the ECU tries to lean the fuel mix back down from rich. This was the same smells and issues I had on and off for a while and most recently permanent which lead me to changing the original O2 sensors. At the time last week my original O2 sensors were also around the same voltages (500mv to 950mv) as viewed with Envyous Advanced Engine Diagnostics software.

So, I am at a little bit of a loss. Could the new O2 sensors be faulty? Brand is Goss, purchased from AutoPro. Though the ECU seems to see the Rich condition, it just isn't able it seems to reduce the fuel to run right.

The Knock sensors seem to be little to no variants in the voltages read at the ECU, which also don't record in the software as any knocking. So I can't see the ECU making any adjustments to timing if the sensors aren't reporting a knock. Which also makes me think there isn't any knocking or misfire due to that. So i am more confused tonight than yesterday.

The engine idles rather smooth considering, so I don't think there is anything like an injector sticking open. They all have been renewed about 6 months ago with Original Bosch ones from Bosch Australia.

My thought are that if I was getting a spark failure causing rich O2 sensor readings, then the engine would surely be unstable, rough and not idle very well at all. It is almost like the tune in the ECU, or the ECU itself is faulty. But I have also tried another spare ECU with another tune and it still has the smell and large negative fuel trims.

The other thing of concern, is there isn't any codes being thrown up. Infact, I never really see any codes that aren't expected from this car ?

Any more thoughts are (always) appreciated.

John.
Post Reply