07 5.3 vortec misfire has me stumped

[DFBrews]

My little sisters 5.3l In a Tahoe started running like crap last week overnight it's an 07 with 170k throwing faults p300 and once a p303 new plugs and wires all the way around cleaned the maf new coil and injector on 3


watching live data with my scanner #3 and #7 are misfiring up to 100 % and mirror each other as far as misfire percentage.

I cant find a wiring diagram to see if they share a common return or something similar.


She did get fuel at a gas station in Bailey The day before this happened

[BushMasterBoy]

Check fuel pressure at the fuel rail schrader valve. Notorious for fuel pump failure. Pressure should read 55 psi. Could be failed intake gasket. Spray some carb cleaner around gasket and see if idle changes. We have a 2000 GMC with the 5.3L Have fixed both problems mentioned above. Does it idle good? Could add 1 gallon of E-85 if maybe water contamination in fuel. I just hope the I/E valves are OK. Could do a compression check.

http://tahoeforum.com/showthread.php?t=4474

[ray1970]

Ignition coils maybe? Doesn't the 5.3 have a coil on each plug?

[68Charger]

Not familiar with the 5.3, but I recently had a misfire on cyl#4 on my 5.7L Hemi... only on hard throttle, tho...

swapped coil between #2 and #4, swapped injectors between #4 & #6... but tapped the injectors upside down on a paper towel while they were out- #4 had some crap come out (there's a pre-filter/screen in each injector)... was going to see if the problem moved or stuck with cyl#4... but instead it went away...

flow was too low with the crap that was in the screen is my guess... runs good now, no codes- 207k miles

whenever you have misfires, there are the 3 things needed for reliable ignition- fuel (in the proper ratio), spark, and compression...
Though I have heard of some engines that will throw general misfire codes when the crank sensor is failing... (doesn't give reliable data to the computer)

[ray1970]

whenever you have misfires, there are the 3 things needed for reliable ignition- fuel (in the proper ratio), spark, and compression...

I will add a fourth thing. Ignition timing. I had an engine once that fought me. Fuel was good, compression was great, and it had spark. Took me longer than I wanted before I figured out my spark was firing on TDC on the exhaust stroke. Might not apply in this situation because the ignition system I was dealing with had a cam position sensor that could be installed two ways- the right way or 180 degrees out. The guy before me had pulled the sensor and cleaned it and wasn't paying attention and installed it incorrectly. I didn't realize it had been removed and didn't think to check it.

[BushMasterBoy]

Another maintenance frequently item is the "idle air control solenoid". Clean the nose of the plunger where it meet the throttle body. It cakes up with crud a lot.

[Martinjmpr]

07 was the first year for the AFM (Active Fuel Management) system. This is the system that would deactivate 4 cylinders for better MPG. I have heard (on message boards) that the 07 and 08 5.3's, in particular, had problems relating to the AFM malfunctioning. Not sure exactly what the symptoms are (my 5.3 is an 04) but it may be something to check.

[MED]

My little sisters 5.3l In a Tahoe started running like crap last week overnight it's an 07 with 170k throwing faults p300 and once a p303 new plugs and wires all the way around cleaned the maf new coil and injector on 3


watching live data with my scanner #3 and #7 are misfiring up to 100 % and mirror each other as far as misfire percentage.

I cant find a wiring diagram to see if they share a common return or something similar.


She did get fuel at a gas station in Bailey The day before this happened

I would start with a fuel pressure test. If that checks out, then swap the coils of the cylinders that are missing with ones that are not. If your codes jump with the coil pack then you know it's defective. If the miss stays with the same cylinder, then you will need to diagnose fuel, spark, leak at that cylinder.

Edit: I saw that you replaced parts already. I don't have AFM on my older Yukon, but that might be causing your problems too...I am not familiar on how to diagnose those issues. If I was driving one of these, I would definitely use one of these: DiabloSport Sprint Active Fuel Management Module - S1000

I thought this description of AFM might be helpful too:

Cylinder Deactivation (Active Fuel Management) System Description

System Operation

General Motors Active Fuel Management engine control system has the ability, under certain light load driving conditions, to provide maximum fuel economy by deactivating 4 of the engines 8 cylinders. The engine will normally operate on 8 cylinders in V8 mode during starting, idling, and medium or heavy throttle conditions. When commanded ON, the engine control module (ECM) will direct the active fuel management system and deactivate cylinders 1 and 7 on the left bank and cylinders 4 and 6 on the right bank, forcing V4 mode. Refer to Lubrication Description and Cylinder Deactivation (Active Fuel Management) System Description.

Valve Lifter Oil Manifold Assembly

The valve lifter oil manifold assembly is bolted to the top of the engine block beneath the intake manifold assembly. The oil manifold consists of 4 electrically operated and normally-closed solenoids. Each solenoid directs the flow of pressurized engine oil to the active fuel management intake and exhaust valve lifters. The active fuel management oil pressure relief valve, located in the oil pan, regulates engine oil pressure to the lubrication system and the oil manifold.

When enabling conditions are met for active fuel management operation, the ECM will ground each solenoid control circuit in firing order sequence, allowing current to flow through the solenoid windings. With the windings energized, the solenoid valves open and direct pressurized engine oil through the valve lifter oil manifold into 8 vertical passages in the engine block lifter valley. The 8 vertical passages, 2 per cylinder, direct pressurized oil to the valve lifter bores of the cylinders to be deactivated. When vehicle operating conditions require a return to V8 mode, the ECM will turn OFF the ground circuit for the solenoids, allowing the solenoid valves to close. When the solenoid valves are closed, remaining oil pressure is exhausted through the bleed passages of the valve lifter oil manifold into the engine block lifter valley. The housing of the oil manifold incorporates several oil bleed passages that continually purge trapped air from the manifold and engine block.

To help control contamination within the active fuel management hydraulic system, a small replaceable oil filter is located in the valve lifter oil manifold oil inlet passage. The oil pressure sensor monitors engine oil pressure and provides information to the ECM.

Active Fuel Management Valve Lifters

When operating in V8 mode, the active fuel management valve lifters function similar to the non-active fuel management valve lifters. The active fuel management oil manifold solenoids are in the closed position, with no pressurized oil directed to the valve lifters. The pushrod travels upward and downward to actuate the rocker arm and valve. The spring loaded locking pins of the lifter are extended outward and mechanically lock the pin housing to the outer body of the valve lifter.

When the active fuel management system is commanded ON, the ECM will direct the solenoids of the oil manifold to open and direct pressurized oil to the valve lifters. Oil travels through the valve lifter oil manifold and engine block oil galleries and enters the inlet port of the valve lifter.

When operating in V4 mode, pressurized oil forces the locking pins inward. The pushrod remains in a constant position and does not travel upward and downward. The outer body of the lifter moves upward and downward independently from the pin housing. The valve lifter spring retains tension on the valve train components to eliminate valve train noise.

When the active fuel management system is commanded OFF, the ECM directs the solenoids of the oil manifold to close, stopping the flow of pressurized oil to the valve lifters. The oil pressure within the lifter will decrease and the locking pins will move outward to mechanically lock the pin housing and outer body.

The active fuel management engine block incorporates additional features to support active fuel management system operation. Engine oil pressure is routed to the valve lifter oil manifold assembly from an oil gallery in the rear of the cylinder block. Cylinders 1, 4, 6, and 7 each have 2 vertical, cast-in-block oil passages . The vertical oil passages permit oil flow from the manifold assembly to the valve lifter bores.

[DFBrews]

Thanks for the insight so far guys.

So far I have done

plugs and wires.

strongest miss is on #3 so has a new coil and injector. verifed it has spark.

#7 closely follows #3 in the misfire percentage on the scanner I have which varies from 40-100% I can not read injector pulses with the scanner but pretty much every thing else

no other cylinders have any misfire readings on them.

cleaned MAF

checked fuel pressure 55-60 psi

drained all the fuel and replaced with shell premium.

dropped the left bank cat to make sure it is not plugged.

spraying starting fluid around the manifold does not change the idle at all


my O2 sensor readings are all very rich voltage wise.

on the short list to check before i reseal the intake manifold and potentially pull the valve covers is to

check compression

throw some sea foam in the oil to see if maybe she has a couple stuck lifters. I know the last oil change interval was a little long.

[BushMasterBoy]

I might add that after I replaced the intake gasket, the idle was still not right. I changed the throttle position sensor and all was good again. Spraying the intake gasket with carb cleaner did not affect the idle either. Even so all the car guru forums said the gasket was leaking according to the symptoms. Apparently the OEM gasket is a rubber coated plastic gasket. The plastic cracks and then the rubber leaks. Was replaced with a FelPro brand intake gasket.The TPS literally fell to pieces when removed.