Scanning for misfire – Strange anomaly occurs

[and1y]

I’ve been trying to troubleshoot a random misfire. In the process i’ve been learning about and applying the use of a scanner by autoenginuity. I plugged it in to my 2005 Chevy Suburban and started watching live data on a graph with several sensors being monitored. Everything seemed to be normal and then out of nowhere a little blip occurred and it would occur every so often across all sensors. It would occur intermittently, and sometimes as close as 10 sec apart. All readings at an exact moment would drop to a reading of zero for a second and return back to normal. Is this perhaps a bad ECU or an ECU that needs to be reprogrammed? It doesn’t make any sense. I’m told the next step I should do is monitor spark waveform or fuel injector pulse width with an oscope to verify if the blip on the AE scanner coincides with a misfire. I don’t have an oscope though. Any advice? Has anyone seen this before. It’s not the AE software, because it doesn’t occur on my other vehicle. Also, this occurs on the AE scanner with the vehicle running or not running.

[Schrodinger’s Box]

You say that the blip occurs accross ALL sensors and that they drop to zero for a second and return to normal.
In my opinion you are looking for a “common to all” fault. Look and the ground side of the electrical system, start with the battery and move onwards to chassis and block grounds and don’t forget the ECM ground as you obviously have enough power to crank and start, but you could still have a poor ground to the ECM.

You can do this by eye and with the multimeter, then check power feed to the fuses box and then to the fuses to the ECM (engine control module/ computer).

Something I assumed, and I should not have – – does the blip coincide with the missfire. A different senario, but you have enough to go on with at present, if the above does not help come back and let us know how you get on, I still feel that what you said about the blip occurring across all sensors is significant.

[Juiced105]

Is this what you are seeing?

https://youtu.be/PtwbDDt0A4E

This is the problem I’m having hooked up to 3 different vehicles with 2 different devices(PC/Android). I’ve sent my connector back in to have it checked out. Verified with a scope that this blip doesn’t not really exist.

[and1y]

Yes, this is the same thing that is going on with my scanner. However, I’ve checked it on another vehicle and this did not occur. What model/year vehicles was it hooked up to? Somebody else told me this happened to them on some older GM models. Aslo, can you let me know what the manufacturer says about your device? Is it the autoenginuity scanner? Thanks!

[Juiced105]

Yes it is AutoEnginuity scanner. I’ve had it hooked up to 2000 Ford, 2007 Chevy and a 2002 Lincoln. When I called them they said something like this is usually a car or the device(computer) issue. I’ve tried it both with an Android tablet and a PC. Wired or Wireless, does the same. I’ll update you once I hear back from them.

[Schrodinger’s Box]

My feeling is that the focus has moved from the missfire to the blip, the immediate problem being the missfire, keep the problem in the front. From memory Auto Enginuity can count missfires and their source, when you say “random” missfire, are you talking about a missfire from any cylinder or one particular cylinder. If any cylinder it could be from the CKP sensor or if from a particular cylinder it could be in the area of a paticular ignition coil.

I’m not saying the it is the ignition coil for example but something that relates to that particular part of the system that makes the ignition coil working in harmony with the rest of the engine.

[Juiced105]

AutoEnginuity tested my connector and found nothing wrong. I’ve since discovered I only have this data dropping issue in enhanced and not in generic.

[Schrodinger’s Box]

And so back to the missfire – you say you have a missfire OK, is it spark related or injector related ???. In other words what is the root cause of the missfire. Having sorted that out, which cylinder?. Back in the day we could find a problem by starting a cold engine and running it for a few seconds and finding which exhaust manifold port was the coolest, this was a pretty cool way of finding a problem area. Back in the present we have Infra Red Heat Guns and put the red dot on any part of the engine to get a far more accurate result. Your random missfire would I feel have to be pretty constant and on one cylinder only for this to be of any use to you here.

I think it come back to the first sentence – is it spark or injector related.

[and1y]

Thanks for letting me know about the results from your AutoEnginuity scanner! So, focusing on the random misfire I’ve ruled out it’s not the spark plugs, spark plug wires, coil packs, fuel injectors, fuel pressure, cylinder pressure or cylinder leakage (strangely, I do find a little oil on the threads of the spark plugs. And not just on the misfiring cylinders). Misfires occur mostly on cylinder 2, and a little bit on cylinder 8.

A bit of history: before the random misfire code occured there was an evap leak code on the Suburban. I left it unfixed for several months and sometime during those months a random misfire code turned on to. But because the check engine light was already on I have no idea when misfires started to occur. I’ve since replaced the canister vent valve and the evap leak code has disappeared. And, there is a small rear main seal leak as well.

[Schrodinger’s Box]

I have never seen a 2005 Chevy Suburban, but does you engine have a waste spark type system? Now people will say that as it has Coil On Plug that is does not, but be careful with that one. Can you scan the Crankshaft Position Sensor and synchronise those pulses with No1 and see how many “trigger points” there are, there does not have to be a separate pulse for each and every cylinder. However all pulses should be of equal value or strength. It can be that 4 trigger pulses will do for 8 cylinders. I do not have the time to do this but you could go through the firing order and work it through, but it’s easy to make a mistake……………it’s relevant to something that 2 and 8 mostly missfire, and the rest of the horses are pulling their weight.
IF your scanner is Bidirectional, you can shut off each cylinder in turn and measure the RPM drops each time, could be a pair of lazy cylinders, but I think you would have found them earlier. When I was a child we used to talk about a “V8 beat”, this of course was an urban myth and a good V8 was as smooth as could be when all cylinders worked in harmony.

[and1y]

Thanks for the reply. I will run some more tests and see what I come up with! Also, I forgot to mention the misfiring I found only occurs at idle.

[Schrodinger’s Box]

OH,……………. it is amazing what a chance remark can have in the course of events, the fact that it only occurs at idle pretty much negates everything I said in my previous comment. Get the garden hose, put it on a trickle flow – run the motor and run the water flow around all items that are connected to the cars vacuum intake. Keep the water away from electrical items and your hands away from the fans …. you are looking for a small vacuum leak that is so small that it is overridden by an increase in RPMs and an increase in the vacuum.
Don’t forget the Brake Booster pipe – and Brake Boosters can have an internal leak, this can be checked by dissconecting the pipe and blocking it – if the trouble goes away the leak is inside the booster…

This is not guaranteed to find your problem but is a good starting point, low tech and does not mean the use of the “B” word. (BUY)…………….. WHEN you have done this let the motor run for while to thoroughly dry out the engine. I’m not a fan of riding ’em hard and putting ’em away wet!

[and1y]

I did run smoke through the vacuum system with a homemade smoke machine a few months ago. Perhaps I should check again and also check the brake booster.

[and1y]

Would my scanner pick up a small leak like you’re describing? The MAP sensor says it’s at 15psi with the key on and when the motor is running it’s down to 5psi. From my knowledge that’s within speck. Thanks for all your feedback! Much appreciated!

[Schrodinger’s Box]

Smoke and mirrors, er I mean scanners, are good and I am not decrying them. For now stay lo tech and try the water trick. I keep comming back to it (missfire) being on two cylinders that come on song when the revs increase.
Staying with the Lo Tech, have you tried moving first the Coil on Plugs to different cylinders and putting known good cylinder’s COP’s to numbers 2 and 8? Does the trouble move to other cylinders or does it stay with numbers 2 and 8. Only move the COPs not the plugs at the first test – you need to know if it is coils or plugs and moving both at the same time is going to be too much information.

If moving the COPs around does not move the missfire, repeat and move the plugs around and try again. Knowledge is power and power means control – the above will add to the knowledge and you can move on from there.
If we know what the foregoing does to the problem we are a step closer to fixing it, and, still hav’nt used the “B” word.

[Author]

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