Need some help with a LQ9 misfire
08-30-2015, 06:56 PM
#1
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Need some help with a LQ9 misfire I have a 2000 Trans Am automatic that was thrown together with spare parts from other builds. Car has a 60k LQ9 with all factory internals, the oil pan and intake were swapped to fit in this chassis but the long block is untouched. Has LTs, ORY, borla catback, LS1 intake, inj and TB, SLP lid and 85mm maf, no cats, air, EVAP, or EGR. The car was tuned by frost and ran great for about 5,000 miles. Recently it started developing a misfire at cruise but seems to clear up at WOT and idles wells. The car throws a P0300 code. I plugged my scan tool in and cylinders 1 and 6 show 14k and 16k misfires in the history, all the other cylinders show single digit misfires. I have tried moving plugs, wires, inj, and coils on 1 and 6 and the current misfire count still shows 1 and 6 misfiring. Also cleaned the MAF and even tried a truck MAF fuel trims went crazy but the misfires stayed. Fuel pressure is steady at 58-60psi idle to red line. I checked for vacuum leaks and cannot find any.
Only ideas I have left is the cranks sensor due to 1 and 6 being companion cylinders and both at TDC at the same time. The car has never had the crank relearn done after the swap, but it ran fine for 5k miles before developing this issue. Any ideas?
Also can anyone explain how the PCM uses the crank and cam sensors to determine fireing the injectors and coils? I understand the mechanical side of how the sensors work was looking more for how the PCM uses that info.
Only ideas I have left is the cranks sensor due to 1 and 6 being companion cylinders and both at TDC at the same time. The car has never had the crank relearn done after the swap, but it ran fine for 5k miles before developing this issue. Any ideas?
Also can anyone explain how the PCM uses the crank and cam sensors to determine fireing the injectors and coils? I understand the mechanical side of how the sensors work was looking more for how the PCM uses that info.
08-30-2015, 08:47 PM
#2
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Here is how I understand it...
The crank sensor indicates to the PCM the relative position of the crankshaft. However since it cannot tell the PCM whether e.g. cylinder 1 is on the intake or power stroke, the cam sensor is needed to indicate which is which. The crank sensor sends 24x or 58x signals to the PCM so that it knows quite accurately where it is and can thereby properly time the injector pulses and the sparks. Each time a cylinder fires, the PCM expects to detect a slight acceleration of the crank via this pulses. It it does not detect one or more cylinders accelerating the crank, it sets a misfire code.
If you change the rotational mass on the crank, such as different converter or even a different balancer, it changes the acceleration and upsets the misfire detection.
(The following I understand less well.) In some cases a crank relearn will take care of misfire codes; in other case the misfire tables need to be de-sensitized.
Corrections and additions to my post are much appreciated.
The crank sensor indicates to the PCM the relative position of the crankshaft. However since it cannot tell the PCM whether e.g. cylinder 1 is on the intake or power stroke, the cam sensor is needed to indicate which is which. The crank sensor sends 24x or 58x signals to the PCM so that it knows quite accurately where it is and can thereby properly time the injector pulses and the sparks. Each time a cylinder fires, the PCM expects to detect a slight acceleration of the crank via this pulses. It it does not detect one or more cylinders accelerating the crank, it sets a misfire code.
If you change the rotational mass on the crank, such as different converter or even a different balancer, it changes the acceleration and upsets the misfire detection.
(The following I understand less well.) In some cases a crank relearn will take care of misfire codes; in other case the misfire tables need to be de-sensitized.
Corrections and additions to my post are much appreciated.
08-30-2015, 09:28 PM
#3
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Need some help with a LQ9 misfire I think you're onto to it since #1 and #6 share the same location on the crank reluctor.
Can you peak at crank reluctor thru the sensor bore using a mirror/flashlight as you slowly spin the crank (yes, easier said than done) to see if the reluctor is bent or chipped.
If you know someone with a high resolution automotive oscilloscope, get them to capture waveforms of CKP, CMP, and #1/#6 secondary (using capacitive probe) to see if the CKP signal is missing a pulse... a good repair shop should be able to do this.
I would have said do compression test, but #1 and #6 sre so far from each other.
Can you peak at crank reluctor thru the sensor bore using a mirror/flashlight as you slowly spin the crank (yes, easier said than done) to see if the reluctor is bent or chipped.
If you know someone with a high resolution automotive oscilloscope, get them to capture waveforms of CKP, CMP, and #1/#6 secondary (using capacitive probe) to see if the CKP signal is missing a pulse... a good repair shop should be able to do this.
I would have said do compression test, but #1 and #6 sre so far from each other.
Last edited by joecar; 08-31-2015 at 05:56 PM.
08-31-2015, 01:03 PM
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I will try to get a look at the reluctor, believe my bore scope should provide the best view. Has anyone ever had one break during normal driving? I cannot see how it would get damaged when the motor was sealed.
As far as an oscilloscope I don't have access to one anymore and I don't know of any shops that has one and a tech that knows how to use it. The local GM dealer just changes parts to diagnosis.
As far as an oscilloscope I don't have access to one anymore and I don't know of any shops that has one and a tech that knows how to use it. The local GM dealer just changes parts to diagnosis.