The P0172 That Won't Go Away...

Hi all,

I've got a '99 5VZ-FE and I'm looking to get some help with an issue that has been plaguing me for a couple of months...a dreaded P0172 that won't go away...

Bit of back story...
When i first bought the truck, I had a misfire (P0303) and rough idle. I swapped out coils, plugs, and wires. The misfire went away but the rough idle stayed. A couple weeks later the misfire returned (P0303).

I then purchased some matched re-manufactured fuel injectors (figured it may be a failing injector, why not swap em all), cleaned out the throttle body, cleaned out the (very gummed up) IAC, and replaced the fuel pressure regulator. The misfire went away but the P0172 still remained. I finally swapped in my friends 4runner MAF and that didn't do anything...

Overall, the things that have been replaced/cleaned so far:
1. New OEM Coils
2. New OEM Wires
3. New Plugs (NGK)
4. Fuel OEM Filter
5. Re-Man OEM injectors
6. Cleaned Throttle Body
7. Cleaned IAC
8. New OEM Fuel Pressure Regulator
9. Swapped in friend's MAF

So, here I am now with a P0172 and a rough idle still...What should my next step(s) be? Change the IAC? Can those things fail and cause the symptoms that I have?

Many thanks for the help in advance!

 

Have you ever hooked this up and read your fuel trims To see if this is a real issue or is tripping from damage someplace

What are the O2 heater circuits like and the wire harnesses ?


When you say Rough idle what do you mean since it seems The PO172 is the only code you have .

Bad injector keeping the plug to wet to fire but not tripping a ignition miss.

Do you have a California or Federal package ??

 

P0172 is too rich Bank1. I would inspect the air filter. (Not enough air, too much gas = rich?)

 

How about your air/fuel sensor? Ever been replaced? I had a lean condition. Negative fuel trim. I replaced the MAF, air fuel sensor and it's been running like a champ ever since.

 

Hi guys, apologies for the late responses...replies below. Thanks for the questions and feedback!

@Wyoming09 - I read my fuel trims this morning...at idle both long and short term are around -20%. While accelerating the both go to about -5%.
I haven't checked the O2 circuits yet, but the voltage readings of Bank 1 sensor 1 and sensor 2 are both about 0.9V.
Rough idle meaning a slight shudder while in idle, it's a rhythmic vibration but it kind of makes the cabin shake a small bit. Noticeable only at idle. The engine surges ever so slightly, the needle doesn't often reflect it though.
I swapped out all of the injectors and the P0172 stayed, so I don't think it's a bad injector...
Not sure what package I have, likely California as the vehicle was sold in and only ever been in California?

@zach141b - just bought a new air filter for it the other day...still no luck. Thinking it might be an air related issue too since I have done a decent amount to the fuel system.

@djohn24 - I am not sure if the AF sensor has been replaced...is this the one on the throttle body?

 

1999 Tacos have 2 O2 sensors
No a/f sensor
Vac hose leaks?

 

I don't think there are any vacuum leaks, I had a shop run smoke and compression tests to check for leaks and nothing showed...

 

Smoke test is good for EVAP lines.
I'm talking about manifold vac hoses that need vac gauge testing or just replace them.

 

Got ya, I haven't checked the intake hoses, I would imagine the shop I took it to checked for them as well if I brought it in for this issue? I will take a look at them today to see if there are any cracks or leaks. Any suggestions as to which one to start with?

 

EGR, Modulator, Intake,Throttle body. Cracks in large intake from air filter box to Throttle body and loose or missing clamps.

 

Many thanks! I will take a look at all of these today. Really appreciate the help!

 

https://www.700r4transmissionhq.com/p0172-toyota-tacoma/

 

I didn't see a new air filter on your parts replaced list.
If you pop open the air box and want to kick yourself please post the fix

 

HA! I didn't know you could walnut blast your air filter!

I replaced the air filter yesterday as suggested by @zach141b with no change...thankfully no nuts in there! It was, however, backwards so whoever put the last one in there screwed up (probably me).

 

You must have great luck to have never gotten a bad part of the box .

That seems to happen quite often as of late .

 

good point, I can re-check all the parts that I purchased...I always assume new = works fine, but have also seen it mean "Never Ever Worked"

 

Okay, to OP, from my experience five years ago. I replaced the air filter. The CEL came on and stayed on. I worried and fretted and cleaned the MAF. CEL stayed on for 100 miles, truck ran fine. WTFO, right? What's the big deal here? So then I cleaned the PCV valve, and the CEL went out and stayed out. As I cannot connect exactly how that would've solved my problem, I put it down to the ECU just finally "learning". Whether this combination of events is accurate or even possible, I cannot say.

 

Ok, I stand corrected. I didn't know that the 99s' did not have an air/ fuel sensor. That is the first sensor below exhaust manifold before cat. Apparently, you only have oxygen sensors on yours. The -20 even at idle is pointing toward your issue. Suppose to be -10 to +10 for normal operation. I do know that high positives readings can point toward vacuum leak. I would replace your o2 sensors

 

Here are some more troubleshooting steps.

 

bump

Both Up and Downstream O2 sensors replaced...tried spraying carb cleaner on intake hoses to check for a leak, but nothing happened.

I started pulling vacuum hoses off to see what changes might happen, and when I removed the air cleaner "Vacuum Line" circled in red on the attached photo, my fuel trims both went up to about -5%, and STFT cycled between +/- 5%...

Any thoughts?? Thanks for all the help in advance!