P0300

[PaulDSL1C4]

I have a 05’ tahoe 5.3L 4x4 w/>300,000+ miles. Just recently its giving that code and running horrible. Several years ago the low oil pressure warning appeared and everything online said to change o-ring on pickup tube which i did successfully but still had the warning. Water pump went out a few months after and went ahead and changed the timing chain oil pump and sprockets at that time. Engine operated normally with no ticks, rarely ever had to add oil between changes and truck ran better than ever but still had low oil warning. Operated fine for several years until about a month ago. Roughly four months ago i did a tune up with new plugs coils and wires. Chronilogical parts list includes:
-o-ring, timingchain kit, water pump, fuel pump, oil pressure sending unit, new battery 23’, plugs coils & wires, camshaft sensor, front o2 sensors both sides, and just today the fuel injectors were replaced.

Motor is rough trying to start cold, but once idling it runs normal. Once in gear if the pedal is pushed further than 1/4” it coughs and back fires. If slight pressure is applied then truck will fairly quickly reach highway speeds but slows down drastically uphills. Runs better with the tow haul button pressed on but if pedal is pressed too much it starts flashing the check engine light until at the top of the hill or if pressure is relieved from the pedal. Multiple Oreilly trips for scan tool only show P0300 codes. 2 weeks ago i did a compression check and every cylinder had 190 psi with minimal drop after 5 min. I have sprayed carb cleaner around intake and vac lines to check for cracks, cleaned mass airflow and throttle body with proper cleaners, i have checked ground connections and disconnected cleaned filed and replaced bolts, ran several bottles of berrymans chemtool through gas tank.If anyone could tell me another avenue to check before i tear it down to replace the timing chain again just to confirm its jumped a tooth or not.

Thanks for any and all feedback

 

[Joseph Garcia]

Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

Pics of the truck, please.

I cannot personally assist you with your generation truck, but many members of this Forum have GMT800 trucks, and they will chime in.

 

[Fless]

I hope you're saving your original parts -- they may be better than what's been installed to replace them. Do yourself a favor and do some diagnostics before loading up the parts cannon again.

What type (how much ethanol) fuel is it running? Ethanol free, E10, E15, E85? Is this a Flex Fuel engine? Is there a fuel filter on the driver's side frame rail and, if so, has it ever been changed?

When you changed the cam sensor, was a CASE relearn done? If not, I would suggest having one done with a capable scanner. You may also be needing a crank sensor (after replacing that, another CASE relearn to sync the two sensors again). Quality parts for these sensors are required, OE or AC Delco, etc. only, if they can be purchased. Buy once, cry once.

Does the cluster tachometer reading match the RPMs as read with a scanner, and do those readings seem accurate?

Throttle body been cleaned lately?

What is the running fuel pressure as read at the passenger fuel rail? Tape the pressure gauge to the windshield and watch it while driving. Is it consistent?

Digging a bit deeper, what do the short- and long-term fuel trims look like?

What is the MAF reporting at hot idle, in g/sec (please don't give us lb/min -- change the scanner setting so it reads in metric for this).

 

[RET423]

Unplug both forward 02 sensors and go for a drive, if the issue improves then one or both of your new 02 sensors is reporting a false lean condition

I can't overstate how bad new 02 sensors are these days, every brand regardless of where they're purchased are failing very quickly & often right out of the box

When disconnected a code will be thrown but the PCM will stop using those sensors to fine tune the AFR and just use the fuel tables and other pre combustion sensors to regulate the fueling; which isn't optimum but much better than the PCM chasing a false lean condition

I always do that first when I hear someone changed the front 02 sensors recently, if nothing improves after that test I resume testing by checking fuel pressure & looking for exhaust leaks with a P0300 code but disconnecting the front 02's has become my 1st step over the last couple of years due to the abysmal failure rate of new 02 Sensors