Another P0172 thread

My P0172 has been resolved. It was a combination of the charcoal can being faulty, and me having it connected to the wrong port on the throttle body that sees vacuum 100% of the time.

I realized I posted to the wrong forum so I moved the thread here.

I've been trying to find the root cause of this P0172. If you give it a little gas the rpm's will return to idle, then stumble down just a bit. No hard starting has been observed.

This is a 2003 2RZFE engine swapped into a 1993 Pickup. The engine has been in the truck since 2012, but this is only a recent issue.

Short term fuel trim is ok
Long Term fuel trim at idle is like -23% so it's pulling fuel. The AF is at 14.8 and soon as I get under any load and moving the LTFT moves into the acceptable range

The service manual states it could be anything below


Air induction: Not finding any vac leaks. Tried soapy water and cigar smoke. Tightened clamps. I replaced the PCV with new OEM. I cleaned the throttle body and the IAC and used new OE gaskets. I've eliminated the brake booster by capping the vac port and the issue persisted. I pinched the charcoal vac line and didn't make any difference in LTFT fuel trim.

Car Scanner app with Bluetooth says -12psi vac at idle. That's about 21in/hg

Injector leak or blockage: I had a spare set of OE 2004 pink injectors that I sent off to be cleaned and checked. Replaced those with all new seals and cups. Runs a little smoother but issue still there

Mass air flow meter: I bought a new one just to compare. yes I threw parts at it
warm idle MAF 3.3-4.7v
calc load is about 19-20% so the MAF is reading within spec.

Engine coolant temp sensor: I haven't messed with the ecu temp sensor yet . I did replace it with new back when I got the swap completed. I may replace this part soon since OEM is $50

Ignition System: New spark plugs=no change. I don't have any misfire cels. The coils are original OE parts though

Fuel Pressure: Verified FPR function and fuel pressure are correct using the LCE fuel pressure gauge setup. Fuel pump is Denso and only a couple of years old.

Fuel pressure runs at 45psi when connected before the throttle body(like a stock truck).

Fuel pressure runs at 31-37psi when FPR is connected to intake vacuum(per the FSM)

Fuel pressure remains for at least 5 min after I shut off the engine which you would think means the injector arent leaking, right?

Gas leak in exhaust: I don't think this is it. I checked the exhaust manifold and exhaust connections for the manifold and the downpipe

Open or short in MAF or A/F sensor 1: The A/F sensor is a 5month old Denso part.

Wiring is correct to the AF sensor. Going by the factory service manual you ohm out pins B+ and HT1.

At 68deg the reading I got wasn't good according to the fsm. I haven't tried it with the sensor at 1472 deg. The easiest way is to get the engine good and hot and shut it off, then disconnect the sensor and check but I haven't done this yet

I did call Denso support and they sent me a document that says to Ohm out the 2 Heater pins(which is different from the FSM). When I did that the sensor was very slightly out of spec.

With the issue seeming to start right after fueling up, and the Ohm readings I've ordered a fresh Denso A/F sensor from Summit.

EFi main relay: not it...as the truck runs fine. If this is bad the truck won't stay running (I've had this go bad once before)

A/F sensor relay and heater circuit: could be......

I did perform the Active A/F test using TechStream to flip flop between +25% and -12.5% and output voltage of the Oxy sensors looked ok.

It could be a bad cap in the ECU but usually the way Toyota/Lexus ecu caps fail cause them to run REALLY bad. I still might pull it and check for damage.

I haven't adjusted the valves or even checked them so maybe that's it, but I suspect the AF ratio sensor or ECU coolant sensor is slightly out of spec causing this.

It's got to be something common though because other people have the exact same issue but I'm not seeing a smoking gun in any of those threads.

 

All grounds in engine bay verified good with test light

The MAF readouts
VG 'maf signal line' in spec when blowing through MAF and probing VG and E2
THA 'iat power' idling 2.63V
EVG 'ground for VG' continuity to ecu (still need to check)
B+ 'power to maf' is 11.3 key on, and 13V idling
E2 'ground' continuity to ecu (still need to check)

 

It wasn't the ECM coolant sensor. I replaced it with new OEM and it wasn't the AF sensor as I tried a new one as well.

Hot compression test.
175ish 175ish 175ish 190ish

I had one exhaust valve tight at .007 and I adjusted that out with an OE shim to get .011 clearance

I tried a new Denso downstream 02 sensor because the old one was pretty old and worn-out looking. (installed the new one and it's still rich)

I also replaced the gas cap with OEM

I'm starting to lean towards brand new injectors because there are more than a few documented p0172 threads on the internet that were resolved with brand new 'OE quality' injectors. The injector opening and closing drives the injection pulse which in turn drives the AF sensor STFT swing from stoichiometric, which affects the LTFT. If you have a set of 20year old injectors. They may not leak and they may have a good spray pattern but if they aren't accurately opening or closing you can run lean/rich

 

A mechanic friend told me that I should perform an injector drop test before replacing injectors so I ordered the OTC injector pulse tester(I do enough wrenching that it's not a bad investment) and will be doing that tomorrow.

Outcome: injectors all dropped pretty much the same amount

I did find if I pressurized the rail and then removed the rail and injectors together I could check the injector nozzles and all of them had some amount of fuel on them, but they weren't saturated with fuel

 

I checked the throttle body per the FSM. The TPS is testing ok. The throttle stop valve is ok.

The IACV is questionable to me. If you apply power to the center pin then ground the other pins one at a time the iac door is not opening or closing. But when I did the active test with Techstream the Iacv controlled the engine rpm which is good.

 

So far

Fuel pressure is in the right range

MAF is clean and I tried a new one and the issue didn't go away. The calc load and GP/s are within the correct range at idle

IAT sensor is working. I watched it change while driving down the highway. It's not way out of spec or stuck maxed out

Fuel lines aren't pinched and there are no fuel leaks. I actually replaced the Gas cap with new OEM as well

Exhaust leak: I'm focusing on this now. I don't hear any obvious leaks before the AF sensor, but I'm going to step up the effort to verify this by having someone plug the muffler while I listen for a leak

Intake leak: I've smoked out the intake side and didn't see anything obvious. Intake and Exhaust manifold bolts are tight

Valves are all within spec

Engine compression is good

I ordered a refurbished ECM to try out so I can eliminate it from the equation. If it doesn't make a difference I can send it back. When I first did the engine swap I just bought a random ECM

 

Pretty sure I figured out what was causing this issue. I had read some replies that mentioned blocking off the charcoal canister since it basically sucks fumes into the intake richening the mixture.

The better way to test this is to unhook the battery and completely clear fuel trim from ECM memory and then start the vehicle with the charcoal can port completely blocked off. What tipped me off is that I was putting the throttle body back on the car and I heard the charcoal can making weird noises. I picked up the hose(that normally goes to the throttle body) and it wreaked of fuel vapor. I then checked the gas cap and there was a lot of pressure in the tank, so I left the cap completely loose to vent through the fill tube. (I know not good but it's a test right)

I fired it up and STFT went up to about -16% for a short bit(open loop fueling) but started coming down as it warmed, then LTFT hit at about -7.8% and stayed there. I let it idle for a while to watch the trims, and then drove it. I immediately noticed the throttle was snappier and it felt like it had more power. Fuel trims never went very far negative during the drive.

I think another way you could narrow it down to a charcoal evap issue is if you don't have fuel in your oil. If you had a bad leaky injector etc. fuel could end up in the sump. Well, in this case, it's just vapor being sucked through and there isn't really fuel going into the sump so that's why my oil didn't smell like fuel during this.

 

I took this a bit further. Even though my fuel trims are way better I wanted to see if I could get them even closer to zero.

As a test I plumbed the FPR to a manifold vacuum port. I now see 35psi of fuel pressure at idle and when I snap the throttle I see 40-45psi. This is how the FSM states it should be, but for some reason they didn't deliver Tacomas this way.

Anyways, this engine is running smoother and idle is rock solid. You can also see that short and long term fuel trims are actually zeroing out sometimes

 

I also replaced the charcoal canister with a new AC Delco canister since the older Toyota canisters aren't available anymore