Still misfiring - ECU graph interpreting

[rmaurer]

Hi all, following on from my previous thread here it still feels like I'm dropping a bank when accelerating at low rpm, despite changing both coil packs and HT leads. I've just been out for a short run but first reset the adaptives. Initially the car seemed to run better but after a while the misfire reappeared. I managed to hook my laptop up and log a short space of time when the misfire occurs, here is the graph:



The misfire was evident between 4-8 seconds before I lifted off and the engine ran cleanly again. If you've got the software, here is the actual trace file from the ECU.

I've got the car booked in but can't get it looked at until 15th May so was wondering if anyone can spot anything obvious in the above that may point to the fault? I was wondering about the earth connections as when the fans kicked in a couple of times, the engine did try to stall...

Any help would be much appreciated!
Ross

[simonsparrow]

Could be worth checking battery voltage, although anything about 11v is ok. Also this thread (which doesn;t show up via the search engine, quite a few don't....) by alinton had some useful info.

alinton said:

Hi all - you might recall a few weeks ago, some postings from me about my '98 4.2 who's odd bank of cylinders weren't firing. Well I thought I'd recount the tale of how I got it fixed, without the aid of a dealer, in case it helps out anyone else. This story includes some lessons I learnt too! Cyliners 1,3,5 and 7 - those on the passenger side - weren't running. I first suspected the coil pack, and it did look a bit iffy, a bit puffy and split. So I got a new one and put it in. Actually I got two new ones - one from Offords and one from the local Citroen dealer as the same part is used in a 1996 ZX 1.6i ! But no change. Then I decided to get technical, and got out the oscilloscope and checked whether the coil pack was getting power and signals from the ECU. Pins 3 and 4 on the coil pack input connector should have 12v; Pins 1 and 2 get pulses to ground (negative) from the ECU. Pin 1 goes to ECU pin 40, pin 2 to 55. (on the other coil pack pin 1 goes to 19, pin 2 to 20) Using the scope I was able to compare the signals arriving at the dead side coil pack with those arriving at the working one, and look at the signals actually at the ECU connector. All those signals looked fine. Then I did what I should have done first, removed the air inlet, removed the spark plugs and checked for a spark. There were sparks! So it wasn't an ignition problem after all. Neither was it a lambda sensor thing, the ECU software showed they were working. So the only other thing that would stop the cylinders firing is no fuel. Each injector has two connections - 12v and a signal from the ECU. The oscilloscope told me that the red 12v wire did indeed have 12v on it, but the yellow signal wires had nothing. On the working side though, the yellow wires had nice negative-going pulses. I started to fear for the health of the ECU at this point. So I looked at the pins on the ECU connector which feed the injectors - strangely all four injectors are fed from the same pin - 38 on the odd side and 53 on the even (so all four injectors squirtnat the same time!) Both mine were OK! So the signals were there at the ECU but not there at the other end of the wire, at the injectors. A ohm-meter showed a very high resistance between the injectors and the ECU - somehow that one yellow wire in the loom had gone high resistance. I'd suspect a splice joint in the loom somewhere. (not in the loom connector - as my car doesn't have one!) I identified some spare wires in the loom, there's about 5 that in the engine bay are labelled 'stepper' but not connected to anything. So I used one of those to replace the faulty wire, soldered it to pin 38 at the ECU and to the nearest splice to the injectors in the engine bay (which necessitated pulling back some of the black sheath to find the first splice). All fixed and running lovely! My thanks to Jerry Plummer for his suggestions and especially to Dave Connolly in Belfast (a great TVR mechanic) for his telephone advice and help with wiring connections and understanding the way the Cerbera's ignition and fuel system works.

[trackcar]

I started replying to this but frankly there's so much wrong about that trace that i gave up, there's also a lot that doesn't make sense ie no lambda increse on accelerating the throttles open and also obvious faults that need addressing before you go any further like the excessive throttle signal noise causing wild ignition advance spiking .. there's just too much to put in writing and too many possible avenues you'd need a day just to type it, read it and understand it in words lol.

[rmaurer]

trackcar said:

I started replying to this but frankly there's so much wrong about that trace that i gave up, there's also a lot that doesn't make sense ie no lambda increse on accelerating the throttles open and also obvious faults that need addressing before you go any further like the excessive throttle signal noise causing wild ignition advance spiking .. there's just too much to put in writing and too many possible avenues you'd need a day just to type it, read it and understand it in words lol.

Thanks trackcar, looks like it's best left to the experts then...
I've booked it in with Tower View next Thursday as they're fairly local to me...

[nemasis]

lookin at the live graph you have posted cannot make out most of the values but of what i can see at time of misfire i would try crank sensor.