I’m at my wits end. Truck runs rich with CEL for P0172, P0300 and P0306. It failed emissions because the CEL was on.

Replaced coils, plugs, wires, and upstream air/fuel with Denso or OE Toyota parts.

Checked compression… 150-160 across all cylinders. Not amazing but relatively equal.

On the monitor, it starts out okay and then fuel trims climb to -30 short and long term.

Leaked down held. Fuel pressure was high at 55psi, replaced the fuel pressure regulator and it went down to spec @ 42. (I thought maybe the high fuel pressure was pushing passed the closed injectors)

Clearing the computer and restarting the truck runs smooth until the it loads up and tries to lean out. Leading me to believe this is an electronic issue versus a mechanical one.

I have a used ECU that is currently having the immobilzer codes cleared in hopes that’s it.

any thoughts or things I’m missing?

’99 Limited 225k 3.4L Auto engine and trans are stock.

 

humm -30 STFT is as extreme as it gets, your compression is terrible but likely unrelated, although I'd do a valve adjustment asap in hopes your compression numbers improve, took me a week to sort mine out checking changing checking and rechecking and waiting on shims to come in while sitting in my shop, mind you I went beyond what a normal shop would do to get it perfect, some cylinders went from 170psi to 190psi. the main fuel controls is ETC(the ECU temp sensor is at the rear fuel rail ~ not the front gauge sensor), front and rear 02 sensor for Federal are the same just different wire length, Cali has a wideband 02 before cat, MAF sensor, Ignition module, and TPS, after that I would look at all related connectors to those components and then look into a SR5 non-immobilizer ECU as I'm sure there are some pin differences you would need to swap so not a good replacement. Could be weak spark/ignition not burning fuel so the ECU is trying to pull fuel to reduce the rich condition the 02 sees, kinda spit balling ideas..

***and it looks like #6 is misfiring, it could be a stuck open injector, you should have replaced all injectors after this many years if they are original, I never had one fail stuck open though, usually shut down closed dead.

 

@Malcolm99

thanks for the reply. I know my issue is deep into the woods and no one wants to read it all.

All new coils, wires, injectors, and plugs and swapped the injector around to make sure it wasn’t a defective new part.

Here’s an update from the Toyota specialist that has been trying to sort things out:

“Quick update. I did get the used ecu programed and working with your car. However, it did not resolve the fuel trim issue.



Good news is it did lead me to the solution. I’m working on verifying this now.



Looks like one of the signal wires maybe open from the air fuel ratio sensor to the ecm. When I check reference voltage its zero when it should be 3 volts with car running.



This may have shorted out your old ecm. When switched between old and new ecms noticed the a/f ratio sensor voltage was different by 1-2 volts which is a lot.



I’m working on verifying the open wire first, then I will need to either repair if its readily accessible or may be easier to run a new wire to the a/f ratio harness.”