So I have a weird problem. Suddenly both P0171 and P0174 have constantly started to come on.

The car feels fine, and there is nothing obvious so that is why it is hard for me to diagnose. I have started to pay more attention and I finally do start to notice that there are sometimes where during idle say in neutral or park I feel it is not as smooth as it should be idling but most times it feels fine.

When I clear the codes with my cheap obdii reader, I can drive for 300+ miles, and the code will not come back on. As long as there is load in the engine and Im driving it won't come on. Even if I'm stopped at a light as long as I keep it in drive obviously with the brakes pressed it doesnt come on. I always have a thing to put the transmission in neutral once I'm at a stop, and when I do this that is when the code will come on, or go to pending until it goes to actual fault code. Same codes all the time p0171 and p0174.

Has anybody got these codes and suggest where to look first?

This is on a 2003 Mitsubishi Montero (not sport) USDM model Limited

 

You have a vacuum leak somewhere in the intake. Clearing the code will not fix it. It just makes computer to do a re-calculation and determine AGAIN that there is an issue in the system. The reason you can drive for 300 miles without any drivability problems is when the engine is under load the vacuum has less of an effect on the system. It's still there, but the effect is much much smaller. However, when you come to a stop and engine idles, that is when it develops maximum vacuum in the intake and any leak shows its effects very prominently. Check your intake hoses for cracks or or gaps. Check vacuum hoses to make sure none of them had fallen off. If you had the intake manifold off before this started to happen - its possible that a gasket was not installed properly.

 

I have used spray start (ether) to lightly spray around gasket joints, hoses, etc, while engine is running to find source of vacuum leaks. The vacuum pulls in the ether and the engine RPM's momentarily jump up.

 

I was not able to find the leak myself, so I ran with these CEL codes going on and off for over a year. Since I made this post.

When going on long drives, usually soon after a few other stops and starts lights go off no more code. But as it becomes more stop and go driving sitting at lights idling, my code reader starts to see 1 or both of the codes in pending then eventually full fault and lights go back.

I tried to find leak myself but 1) i suck 2) im lazy

I had 3 shops check it out, first shop said "oh looks like there was a hose unplugged or "loose" didnt fix it, wasnt charged

2nd shop, its your MAF sensor, gonna cost a lot to replace. We did a "smoke test" didnt find any leaks. i was like man,, maybe it is the maf. i knew i couldve found a maf on ebay much cheaper and do it myself but i didnt just yet.

3rd shop, found a leak valve cover gasket. replaced, and looks like problem finally gone.

just updating this for reference to help others in the future who might run into something similar.

 

Thank you for the update. Appreciate it very much. To come back to the original post - the reason you only had issued when at idle is that's when the vacuum in the intake manifold is the highest and therefore the vacuum leak is more pronounced. When you step on the throttle (engine under load) there is less vacuum and less of extra air intrusion. If you had a MAF issue, it is likely you'd have the opposite problem (more pronounced under load). Glad you got it resolved.

 

I always try to come back to my original post. When I first got my Montero years ago I saw a lot of your posts and they were very helpful. So I try to also do my part and update for future owners

 

Thanks for the kind words. I recall finding lots of helpful info myself before feeling confident enough to answer posts. That's what community is good for - we're all in this together

 

Well it came back!

The only thing left to do was change the MAS air flow sensor. I picked up those new ones from ebay for $60.

That fixed it, thank goodness because my smog was coming up.

I already knew how the code comes on, it never comes on when driving or cruising. So after i put the new MAS I let it idle and do stop and go driving because thats when it comes on if it will come on. Drove it 300 miles, pass smog too, so I guess I'm safe for 2 more years.