On trips 200 miles or more, usually towing, the check engine light will come on. The truck ('97 LD F-250 4x4 5.4) continues to run fine. The codes I get are P0171 & P0174... Bank 1 too lean and Bank 2 too lean. The truck is stock except for a custom dual exhaust system and a Airaid air filter system. The small exhaust tube going into the drivers side exhaust manifold has been cut. I think it goes to the EGR valve. We had to cut it to remove the manifold. I never repaired it after having the manifold resurfaced. I just plugged the manifold. Do you think the tube is drawing in fresh air and causing it? I've looked for vacuum leaks. Everything seems fine. Any ideas? I'm going to plug the tube next and see if that eliminates the problem. Dave C.

 

Since these lean codes are very much related to the O2 sensors, I would say it is a very good possibility the cut/missing tube is your problem. Also, just so you know, except for the highest performance vehicles, your truck will run BETTER with the EGR hooked up so re-connect and see if that helps. I bet it is also illegal in your state to disconnect the EGR, just in case you have to smog test.

 

More than likely the cut EGR tube is your problem.

If that doesnt fix it then you might want to look at the O2 sensors.

 

The EGR draws inert (exhaust) gasses from the exhaust manifold from that tube and injects it into the intake manifold. Normally this exhaust gas is inert and does not participate in the fuel burn at all, i.e., it doesn't make it richer or leaner. The MAF does NOT measure this exhaust gas or include it in the fuel calculations.

Now that you've cut this tube, the air the EGR draws into the engine is just unmetered air. A LOT of unmetered air. There is no fuel added to make up for it, and the result is a very lean mixture. EGR does several very good things for your engine. We would have it today even if we didn't care about emissions. I would reconnect it.

This doesn't matter, but to be more clear. Your truck runs using the MAF to calculate fuel until the O2 sensors warm up. In this timeframe, it won't open the EGR valve. After it warms up, the O2 sensors control the fuel calculation so the added air should be detected and corrected for.

That's true, but instead of the O2 sensors directly controlling the mixture, the O2 sensors simply apply a factor to the MAF's fuel calculations. These are called the fuel trims. There are a short term and long term trim for each of an 8 RPM ranges by 10 Load range matrix or spreadsheet. The trim is the % increase or % decrease in fuel from what the MAF would have calculated. There is a limit to the size these trims can get before the engine management says 'whoa, these trims are so far out we have a lean (P0171 P0174) or rich (P0172 P0175) condition. The limit on these factors for Ford is 25% and adding still more fuel or -25% and substracting still more fuel.

Said differently, the EGR is adding 25% more air than is coming in through the throttle body. The PCM is trying to correct by watching the O2 sensors, but it is limited to correcting 25% or less. Then it sets the lean codes. That's a lot of pirate air.

 

Thanks. I will be repairing or ordering a new tube. I didn't order it before due to its cost. I think it was about $125. I guess I'll be testing my new TIG welder soon. It seems as though my mileage has dropped off a couple. This is probably why, I hope. Dave C.

 

David,

The best way to see if it is drawing in air is to spray some starter fluid into the open pipe during idle and see if you get a surge. If you do, then you are drawing in air and is the cause for the vacuum code.

runnert

 

Good news! I replaced the EGR tube and took it on a 600 mile trip (towing a light 4000 lb load). The light never came on and my mileage improved. Things are back to normal. The price of the parts went down and my dealer now stocks them. I got out of it for $92 with my discount. Thanks everyone. Dave C.

 

The PCM program is very smart. You cannot get away with or ignor very much.

 

My 97 F150 spends alot of time in the water and mud, I have replaced O2 sensors three times over 180K, but the problem you have I am familiar with, my MAF sensor would drop out of range
intermittantly if I took a slug of moisture or even in some cases a chunk of mud would find its way around the filter. Try cleaning the MAF with some electronics cleaner (dont touch it) but for
$100 bucks just replace the unit, its easy, two screws and a plug.

Towards the end I would get those codes a few times a week,
usually coming off speed to an idle, or first thing in the morning.
I did very my air temp input was good before replacing MAF, but since
repalcing no P171,174 codes

 

This site has some helpful info on OBD codes, including the P0171 & P0174:
https://www.obd-codes.com