Doug Thorley Short Tube Header install causing weird lean issue on my 2007 GX470 (1 Viewer)

[Jinman3]

Hello all,
First time poster! I recently picked up my 2007 Lexus GX470 on March 8th and can't really seem to find anything regarding this issue here with the search so please forgive me if I missed a similar post. I've done some digging through general searches (google, not on this forum) about what could potentially cause a lean (P0174) code to be thrown. My buddies at the shop were generous to take on this install for me, they chopped the cats off OEM headers to weld and install the shorties onto my truck. The install went super well aside from finding that the passenger header had a minor leak by where the EGR tube was supposed to go.

They pulled the passenger side off and fixed the leak with more welding and reinstalled the header. Now the truck has no exhaust leak (thank God) yet it's still throwing the P0174 code. They replaced the o2 sensor and the problem persists, it's a very strange issue where the car will be driving perfectly fine (shrtft2 ~ 0% & longft2 ~ 0%) before it suddenly jumps to shrtft2 ~ 22% & longft2 ~ 30%. Not incrementally, just straight 0 -> ~20. We are stuck scratching our heads as we try to diagnose the issue but we feel the car just needs to have some sort of recalibration on the sensors now that they are seeing much more air than pre-install. Mind you, the OEM exhaust leak on my truck was horrendous, it made lawnmowers sound good. I live in California at the moment and so I need the shorties until I'm back in Alberta, where they laugh at anyone that has to deal with emissions testing, and can replace the headers with the DT long tube headers.

Sorry for this block of text but this is what we are dealing with. The owner of the shop is going to take the truck home today and do some longer drives to figure out what's going on. But I wanted to post here to see if anyone else has experienced this before (or know why it's being funky). (Bonus, I'm eagerly waiting to install the Magnaflow Overland exhaust when I get the truck back, it's currently sitting in the cargo area).

Cheers.

 

[Rednexus]

I've had the same problem on my rig. It was due to the MAF sensor pigtail/wiring. The connectors on these rigs are old. They probably removed the MAF and air intake tube when installing the headers. That may have disturbed the pigtail enough to cause an intermittent wiring issue. I'd remove, clean, and wiggle the MAF sensor pigtail around. If the problem goes away temporarily, you can get a new MAF pigtail for around $15 and splice it in to the MAF harness. The MAF itself could be a problem - they are about $60 for an OEM Hitachi part on Rock Auto.

The other area to investigate is a vacuum leak. It's also possible they unhooked a vacuum hose and didn't properly re-hook it. This can also be caused by a loose clamp on the air intake tube at the throttle body. You can check for vacuum leaks by holding a un-lit propane torch around vacuum hoses, connections, etc and watching short term fuel trims in real time. If they spike negative, you've found the area of the leak.

 

[Jinman3]

I've had the same problem on my rig. It was due to the MAF sensor pigtail/wiring. The connectors on these rigs are old. They probably removed the MAF and air intake tube when installing the headers. That may have disturbed the pigtail enough to cause an intermittent wiring issue. I'd remove, clean, and wiggle the MAF sensor pigtail around. If the problem goes away temporarily, you can get a new MAF pigtail for around $15 and splice it in to the MAF harness. The MAF itself could be a problem - they are about $60 for an OEM Hitachi part on Rock Auto.

The other area to investigate is a vacuum leak. It's also possible they unhooked a vacuum hose and didn't properly re-hook it. This can also be caused by a loose clamp on the air intake tube at the throttle body. You can check for vacuum leaks by holding a un-lit propane torch around vacuum hoses, connections, etc and watching short term fuel trims in real time. If they spike negative, you've found the area of the leak.

Interesting, I passed the information forward to the shop. I figured it has to be some weird electrical gremlin but was hoping it was the o2 from a previous suggestion. I’ll hear back tomorrow about what they found (or didn’t find) and update here.

I appreciate your share of experience, definitely a huge help!

 

[Rednexus]

Interesting, I passed the information forward to the shop. I figured it has to be some weird electrical gremlin but was hoping it was the o2 from a previous suggestion. I’ll hear back tomorrow about what they found (or didn’t find) and update here.

I appreciate your share of experience, definitely a huge help!

MAF measures the air volume. If the MAF reading is off in one direction or the other, it will throw off the computer, which has a default table of required fuel based on MAF-measured air volumes (and other things like temperatures). Since your trims are so positive, the computer is adding much more fuel than what the default table says it should in order to get the desired air/fuel ratio measured by the O2 sensor. The lean code will kick on at plus 25% ST+LT fuel trims, which your rig has exceeded.

You can also watch the MAF reading (at idle) with a scanner. It should be around 1 g/sec per liter - so about 4.7 g/sec for a 2UZ-FE. Since your fuel trims are jumping positive, if you do have a MAF issues your reading at idle would probably be a lot lower than that.

 

[Jinman3]

MAF measures the air volume. If the MAF reading is off in one direction or the other, it will throw off the computer, which has a default table of required fuel based on MAF-measured air volumes (and other things like temperatures). Since your trims are so positive, the computer is adding much more fuel than what the default table says it should in order to get the desired air/fuel ratio measured by the O2 sensor. The lean code will kick on at plus 25% ST+LT fuel trims, which your rig has exceeded.

You can also watch the MAF reading (at idle) with a scanner. It should be around 1 g/sec per liter - so about 4.7 g/sec for a 2UZ-FE. Since your fuel trims are jumping positive, if you do have a MAF issues your reading at idle would probably be a lot lower than that.

Alright so they are replacing the upstream o2 sensor which should arrive today and they should have another answer hopefully this afternoon/evening. Who would’ve guessed 18yr old electronics would need replacing

 

[RamGox]

Alright so they are replacing the upstream o2 sensor which should arrive today and they should have another answer hopefully this afternoon/evening. Who would’ve guessed 18yr old electronics would need replacing

Any updates? I having those headers installed myself and trying to anticipate problems. Hopefully you got everything sorted out. Did you need an O2 sensor extender with the short headers?

 

[CarGuyCarl]

If you continue to have this issue, just wanna put in another vote for a MAF connector. Rednexus gave me this advice, I wish I did it first before dumping money into O2 sensors. For me it started as a lean code on one side and eventually became both. MAF readings jumping around. Would shift at high RPM.

 

[Jinman3]

Any updates? I having those headers installed myself and trying to anticipate problems. Hopefully you got everything sorted out. Did you need an O2 sensor extender with the short headers?

Hey! We replaced the upstream and downstream o2 sensors and it runs like a dream now. Likely the sensors were old

 

[RamGox]

Hey! We replaced the upstream and downstream o2 sensors and it runs like a dream now. Likely the sensors were old

That’s great news! I picked up my GX today after installation, and made about half a mile before CEL and VSC light came on. Same codes as you I think, so I went back to the exhaust shop and thankfully it was just a poor connection. Everything was back to normal after they addressed that

 

[Jinman3]

That’s great news! I picked up my GX today after installation, and made about half a mile before CEL and VSC light came on. Same codes as you I think, so I went back to the exhaust shop and thankfully it was just a poor connection. Everything was back to normal after they addressed that

Phew! I was doing diagnostics with my code reader as I can view live data so it was very odd to see how my shrtft were 0 (+/- 2%) with WOT but regular driving would have 30% on bank 2. I was lead to believe it was a sensor from its behavior.

I have the magnaflow overland exhaust on and now it sounds pretty sweet