1GR-FE Diagnosis: Bank 2 Misfire (P0300, P0171)

Hi All,

Got a mystery to solve on a 2005 Tacoma 4.0 V6

Overview: My rig is throwing codes P0300 (multiple random misfire) and P0171 (System too lean Bank 1) after redoing the timing. Compression is normal for bank one / passenger side, low on bank 2 / driver side (cylinder 2: 85psi, cylinder 3: 78psi, cylinder 6: 86psi)

Background: Purchased my 6 speed manual '05 Taco 2.5 years ago at 209k, with a check engine light (P0016 camshaft position). The guy I purchased from is a local Teamsters Union driver for Boeing who had evidently maintained the vehicle well. He had already changed the cam / crank sensors and performed diag narrowing it down to a stretched timing chain, and discounted the price to account for the parts + work needed. P0016 did not affect drivability.. until it did about a year later. It would lose power intermittently every few days which could be reset upon restarting the engine, at which point I stopped driving it. At this point the most I'd ever done was change my own oil. No shops in my area were willing to take on a timing job, rebuild or even pull the engine for me to send it off for a swap. Good thing I have access to a garage and tools.

Procedure: Did the timing job myself, all OEM parts (chains, sprockets, tensioners, guides, etc) following toyota's book procedures + TacomaWorld Torque Guide + youtube (https://www.youtube.com/watch?v=UR50da0sDOc&t=3081s). Found it was pretty gunky on bank 2, and bank 1 had a broken cam phaser that luckily lodged itself in a spot that didnt interfere with moving parts. I replaced both cam phasers with new OEM phasers. Cleaned everything up good removing all debris. Replaced all gaskets, seals, O-rings, spark plugs, and ignition coils with OEM stuff. Any hoses showing wear were replaced. Cleaned throttle, MAF sensor, VVT's, and VVT oil filters with CRC MAF sensor cleaner. Also installed new PCV valve. Replaced coolant with new Toyota super long life and oil with Valvoline synthetic 5w30.

Troubleshooting: When I started the ol gal back up there was rough idle and an audible misfire. P0300 & P0171, running lean. Smoked it out from several inlets checking for vacuum leaks, no dice. Firestone did a general diag / sensor check. Sensors are all good, they diagnosed misfire as result of 1 tooth off on timing, and informed me that the driver side bank had low compression. Took it apart again and reinstalled timing chains at TDC, no change in rough idle / misfires. Took it to an exhaust shop to see if cats were plugged and they said it was also timing. I took it apart AGAIN and the timing was perfect, A-OK, not off. Just to be safe I reset the marks at TDC. Confirmed photos with local dealership's master tech. With the covers off I couldnt do a compression test, and did not have the right tools to do a leakdown test. Instead did a redneck comparative compression check: balloon & tube on the spark plug hole, turn the crank by hand and see how big it gets on each cylinder. Bank 1 inflated significantly more than Bank 2. Confirmed that the compression issue is not timing related.

Hypothesis:
Researched it down to 2 likely causes for one bank to have low compression:

Could be bent valves, but 1GR is not an interference style engine.

Could be a failing head gasket.

Through process of elimination seems like a bad head gasket to me. Given the head gasket recall on the pre 2006 1GR-FE head gaskets and high milage on my rig I think I oughta change the head gasket either way. Parts are ordered, I'll follow up in the thread with results.

Thanks for reading if you made it this far, I've learned a lot here on TacomaWorld and hopefully this helps the next guy troubleshoot. Chime in with any ideas!

 

All the cylinders on 1 bank are low?
It screams timing to me.

I know you said you’ve checked it 2x.
But to have a head gasket fail on all 3 cylinders at once?
That’s very unusual.

 

I highly doubt it's a headgasket, if it was blown that bad it would be puking coolant everywhere and the cylinders would be filling with coolant.
Did you replace the Phaser that was busted?
It sure looks like a timing problem, the chains may be in time but are the cams?

 

Timing will cause low compression. Something is really off.

 

OP did you ever sort this out?

 

@boulderz Yes I did. Took the heads to a machine shop, they said valves looked great. Replaced head gaskets, plenum etc. Put everything back together, same misfire issue. (Don’t feel bad since it needed to happen anyhow with 280k on the original gaskets, plus picked up a new skill)

Back to the drawing board. Since all good ideas had been exhausted, I disassembled timing components again, down to removing sprockets from the cam shafts. Turns out one of the VVT timing gears had been slightly off when I torqued down to spec. You can see where the camshaft punched a new hole in the gear. It was close enough to set TDC, but off kilter just enough to cause misfires. I drilled out where the punched hole crowded into the factory hole, and torqued it back down. Started up like a champ.

Mystery solved. Learn from my mistake, check twice & ensure snug fitment before torquing down timing gears.