10,000th P0171 Thread? With a twist.

[WhiskeyRiver]

Ok. I've got the dreaded CE light on my 99 4Runner 3.4L. Code P0171. Oh crap.

First check: vacuum. Leak? Well, yes. A loud one. Only when the engine is cold.

It's at the back of the engine. Somewhere.

Now for the odd part.

As soon as the engine drops out of rich mode the vacuum leak is gone. I can clear the code and it stays gone until the engine sits overnight. Next morning, loud leak, CE light again.

I have my theory but I'd like to hear from folks that have a lot more experience than me. I'm a computer network engineer, not a mechanic.

Hoses all look fine. Nice and pliable. No apparent problems. Removed obvious ones back there, inspected and reinstalled. No change.

Before discovering the leak I cleaned the MAF sensor. Pulled the EFI relay before cleaning it. Reinstalled, plugged in relay and let the engine relearn it's idle. Engine runs fine. Idles great. Runs smooth. Gas mileage seems fine. 19 highway and mostly driven that way. Maybe a little too fast on the interstate but I'm not here to confess.

I have had another issue over the last year. When climbing a hill, anything past half throttle produced a negative return. It bogs down over half throttle.

Are these two issues related? Is Batman a transvestite? Who knows?

All help most appreciated.

WR

 

[hambob]

I would start at the throttle body, check the IAC valve and all the hose's and gaskets. Thats all I got. And yes, he's totally a transvestite.

Good luck

 

[WhiskeyRiver]

I would start at the throttle body, check the IAC valve and all the hose's and gaskets. Thats all I got. And yes, he's totally a transvestite.

Good luck

I've always wondered....

 

[Paperjam]

So the leak goes away when hot? Sounds like it's sealing up as everything expands. The lean code says vac leak to me too. Try smoking the engine cold, I use a special emissions smoke machine in The shop but there has gotta be a good DIY way to do it. Make sure you do it dead cold, you basically pump the top end of the engine full of smoke and it will leak out where the vacuum leak is.

Works like a charm and saves a lot of guessing!

 

[gamefreakgc]

Or you can do it the cheap way, just make sure you engine is cold! Done this a ton myself and has never failed to reveal the vacuum leak.

https://www.youtube.com/watch?v=9CPqbaSgcok

And yes, a vacuum leak would affect your power steering when turning. So would a bad IAC valve, which is located on the underside of your throttle body.

What is your resting RPM when idling cold, and hot?

 

[Paperjam]

Or you can do it the cheap way, just make sure you engine is cold! Done this a ton myself and has never failed to reveal the vacuum leak.

https://www.youtube.com/watch?v=9CPqbaSgcok

And yes, a vacuum leak would affect your power steering when turning. So would a bad IAC valve, which is located on the underside of your throttle body.

What is your resting RPM when idling cold, and hot?

Burn down your engine is right

And sometimes you can't see or get to where your leak is, so this isn't nearly as foolproof.

Works in a pinch though.