Vx - Does knock detection cause spark misfire?

[JohnDee68]

Hi Guys,
Ok, so last night after a 12km drive, I removed the vacuum hose from the fuel pressure regulator. There was no signs of liquid fuel in or even smell of ever being in the vacuum hose, that I could tell.

I did some more thinking and researching in to my cars issue. This leaves me to question now that could it still be a fuel regulator, maybe just stuck open slightly or fully and therefore I am just seeing the fuel pump line pressure in the line pressure test? (approx 40psi) Along with either a dirty or worn MAF unit (0.9g/sec at idle). Maybe, though the MAF seems to respond accordingly but is showing a lower than previous readings 2 or more years ago.

I will track down a new can of MAF cleaner spray, then clean the MAF sensor with it. Apparently, a dirty MAF sensor can also cause negative fuel trims, though not as low as what I am seeing.

[kojab]

I do recall a few years back having on more than one occasion where the 12 Volt supply to the maf sensor was missing.

I can't remember now what the cause was. Should bring on engine light one would think. No just can't remember.

Fractured wire on the MAF sensor plug maybe. Probably worth checking.

[JohnDee68]

Hi guys,

I thought I would just run some further basic checks this morning while I can.
On the software this morning, it was showing -38's STFT and -11's LTFT in Closed loop mode (apparently that makes it a total of nearer -50's.). Still, not throwing up codes at all.

Looking in to the plenum chamber, there is no signs of neat fuel coming in to the chamber (as fuel hasn't cleaned the area near the vac line). The only really noticeable cleaner area is near the PCV entry. (pic available).

I also tried disconnecting the MAF connector while watching the software for changes. I know in the past, when issues weren't an issue, that this disconnection would have stalled the engine almost instantly. So any noticeable change to that would be a sign something is not the same now.

Firstly, the engine didn't stall, nor did the MAF readings on the software show any changes to the g/sec. at this time it was around 6-7 g/s at idle (I dont think I have ever seen it that high on my car). I disconnected the connector, and nothing changed. (that's not what i would have expected). Upon reconnection of the connector, for a brief 10th of a second the engine wanted to stall but recovered back to the same as if disconnected and connected.

So I am tending to think there is something not quite right with the MAF. I am waiting on some CRC MAF sensor cleaner to arrive later this week. Which will be first on the list towards resolving the cars issue of the rich condition.

With this particular type of MAF sensor on the V6 3800 (vx) , it does have part of its sensing outside of the intake flow, which could easily become victim to foreign material or even damage? Not that damage is likely with my car in the engine area. But for sure, part cleanliness and failure is likely.

[hsv_senator_sig]

have you have any luck with this misfire, and did you end up trying your replacement dfi module?

This probably wont help, but...
I bought a vx wagon v6 with a known issue of intermittent misfire that the original owner gave up on trying to resolve.
I threw new leads, plugs, cas, cam sensor at it and it would still misfire, sometimes misfiring 400m up the road since the previous miss, sometimes 20minutes later, there was never a pattern... The misfire was quite a jolt.
I had SOE on my laptop for my v8, so i bought AED as a chance it might help with the diagnostics (until my laptop died soon after ) but that never showed any codes being thrown, so i was almost ready to give up as throwing stuff at it just wasnt working.

I ended up getting a cheap second hand dfi module from u-pull it (just out of curiosity) as new ones were expensive, and its not misfired since.
If you havent put that dfi in, I think its worth a check.

[vlad01]

The Delphi DFIs are garbage, fail very quick and often not all the coil outputs. My nephew went through 2 or 3 in less than a year. Chucked one one of the old ones from one of my VP motors and it's been fine since.

As above, could well be the DFI and chances are if it was already replaced it's likely to be worse than the original one on there.


I looked up genuine NOS AC Delco units and there are only a few total available online and they are over $400 and something.

[JohnDee68]

Hi guys,
Thanks for the suggestions with the DFI. I have been in touch with the Delphi techs, now Borg Warner. They have advised that to test the DFI modules, it is able to be done with a multimeter. I am not sure if it will be good enough to confirm any intermittent issues though, but mine tested fine as suggested.

Just a little update. I am not even convinced that I am having a misfire. How can one confidently confirm it. When testing with a timing light again, the missing flashes never happened again.

I bought a spark testing tool, which allowed me to see that all coils (even the old ones) were all firing and with good strength. I have now, one by one, replaced the following with brand new parts. Taking computer logs between each replacement to note any improvements, if at all.
I have now replaced: Coil packs, spark plugs, SDM module (to resolve airbag error) & fuel pressure regulator. I have cleaned the MAF sensor hot-wires with dedicated CRC MAF cleaner spray and none have made a squat of difference to anything other than now the car runs a little rough at idle. I am suspecting the idling rough could be related to the extra strong smell of fuel inside and outside the car which has gotten worse in the last few days.

I have noticed that disconnection of the MAF sensor, makes no difference at all. I read somewhere that the VX PCM can go in to "default" mode should a device become faulty. I am now believing that the MAF sensor is such an issue. Hopefully I can confirm it in the next few days.

Tomorrow I am hoping to get to check the fuel filter (direction of fitment) is correct. Also if there are any fuel lines blocked or restricted. As i am having the issue with filling up my car where it cuts off before being able to fill. Then takes small bits over time to fill it up fully to the 75 litres. I have also, due to my suspicions previously, ordered a new charcoal canister this week.

I have put my little oscilloscope on to the MAF signal sensor and got what I feel is not a good signal from a MAF sensor.

I will still update as I continue to research the issue with my car.

Thanks.

[immortality]

There is a huge discrepancy between banks looking at some of your above numbers. I found this, this morning on our VX.

My difference between banks would fluctuate sometimes only a couple of percent, other times I think up to 13-15%. Can only wonder how long it's been like that.

[JohnDee68]

Thanks immortality,

Unfortunately that isn't similar to my case this time. My fuel trims are both in sync and totalling near the -ve 60's. Which leads my issue to a common element of either a fuel rail supply or an air flow measurement issue.

I am nearly 100% confident that the MAF sensor shouldn't be producing such a varied waveform. as far as I understand what they do, it should be more level rather than voltage variant spikes.

I am going to have to try and locate an old MAF sensor (600+km away) so I can at least see if mine is just doing something weird, different or similar to another one. But of course, using an old one won't guarantee it is any better but I feel it should show a differebt waveform, as it is unlikely for two units to have identical faults. I would hope anyway.

I wonder, has anybody had success with the tempting cheaper Chinese MAF sensors that seem to be overwhelming on the eBay site? I notice they use a standard resistor rather than a titanium resistor as part of their air flow monitoring.

thanks again.
PS: I managed to get a LHS O2 Rich code finally come up today while testing further on the MAF sensor. That is in a strange way somewhat comforting to know I am on the right track of rich issues and the software is showing correct if not close to true values.

[antus]

Im a bit late to this so sorry if I am repeating others suggestions. But from that last post nothing sounds like a spark problem, it sounds more like fuel. Testing revs in the driveway is not the same as testing revs on the road because engine load is so low, therefore fuel is also low. Therefore if you have too much fuel flooding a cylinder and causing flooding in that cylinder and misfire you might not be able to recreate it in the driveway. At this stage I would be suspecting injector (have seen new parts be bad just through bad luck), or fuel regulator leak / problem. I'm just an IT guy though who reads this forum a lot, not hands on hardware like most here. I have read posts where people say fuel regs fail and fuel leaks down the reference pipe. That could be your source then that could be flooding the closest cylinder causing missfire.

[JohnDee68]

Thanks antus,

Nothing wrong with being "just an iT" guy. Everyone is of value when help is needed

I have thought it is fuel/air related rather than spark. I have replaced a fuel regulator, which was basically a waste of money as the original was of no different and actually in hindsight wasn't faulty at all.

As the fuel trims are equally balanced with its attempt to reduce near 50% over fuelling. I am suspecting MAF sensor is at fault. Only because I have pressure tested the fuel rail, no leaks in the rail from the injectors or new regulator. I can't see a set of 6 relatively new genuine Bosch injectors failing all at the same time and equally. (though not impossible in reality). From the oscilloscope signals I am getting from the MAF it seems to not be putting out a square wave that the unit should be putting out (according to Y-Tube vids). So with the MAF, being the original (2002) nearing 21 years old now and some 539,000km on it. I am laying my suspicions on the MAF sensor. Other than bumpers, there isn't much more I could think would need replacing to resolve this issue lol.

I cant see a crank angle or cam sensor being at fault either as that would cause other signs and error codes.

Thanks guys. I am always happy for suggestions.