how's it going? over a month ago, my truck began surging a little here and there. a week later, my check engine light came on and threw P0171 & P0174 codes. after a little research, i found this can range anywhere from a vaccuum leak to a dirty MAF, etc. i haven't really had a chance to look into it, but it's gotten so bad that i've decided not to drive it until it's fixed.

my RPMs started going crazy. they would suddenly drop down under 1000 and then come back up. sometimes this would go for on for several seconds which is not fun when you are on the highway. it mostly happens when i'm going around 60-70mph, but it also happens while simply driving around town. also, it's almost guaranteed to happen driving over a bumpy surface. i took an offroad shortcut last week and had to slow down to a 10mph crawl to make it stop.

i'm gonna take a look under the hood tomorrow, but i wanted to ask if you guys had any thoughts or suggestions. i will probably clean my MAF/TB and start inspecting hoses. i hear it's common for the PCV hose at the back of the intake manifold to go bad so i'll check that out too. what's the best way to find a vaccuum leak? any other ideas? thanks.

 

My guess is PCV hose. Can replace or use fuel line, good idea to replace PCV valve (or spray out with brake cleaner).

Clean your MAF. There are threads on how to do it. I just spray down with throttle body cleaner, but there might be better ways to do this.

Remove black air intake at the throttle body and spray cleaner on front side and through hole on the butterfly valve. Prop BF valve open and spray/wipe down the bottom and top side. that helps with idle and sticking feeling when accelerating from a stop

Check for air leaks starting at the air filter back to the intake. Sometimes i use a mechanics stethoscope (a few bucks from Harbor Freight). Check the few vac ports on the intake manifold, one on the left side (EGR, Fuel regulator), one in the back, I think it is the brake booster (hard to get to), and usually rotten.

Could also use vac gauge and check system vac. I do this sometime, and may be good to check as is, then after checking each item... My usual is about 20 in.. Haynes manual or internet has the explanations of varying vac conditions. This "old school" method works pretty good for me.

Check your IAC and EGR port..

 

Thought I'd post my results. I took one afternoon out did the following:

-Cleaned TB
-Cleaned IAT
-Cleaned IAC
-Cleaned MAF
-Replaced Fuel Filter
-Replaced PCV Valve
-Seafoamed intake manifold

Plugged everything back in and the problem has stopped. It's runs much better, and my check engine light hasn't come back on. Luckily all it needed was a lil love.

 

Glad you got it right now.
Mine is doing the opposite. Sometimes for no reason the idle speed goes up when driving. It makes it real hard to stop when in drive. If I put it in neutral it revs up to over 2K rpms. I shut it off and wait then it goes back to normal for a short time.
I cleaned the TB, IAT and MAF. It runs better overall now but still does the rev thing with no warning. Please help if you have a clue what to check?
It's a 98' E350 5.4L with 354,000 miles.

 

free ideas.... and probably worth what you are paying for it....;-)

* Does taping the gas pedel help fix it ?
* Lube gas pedel and throttle cables up top the butterfly valve ?
* Did you clean the butterfly valve ?
* EGR function (vac pump or 3 ft fuel Line and mouth suction) and apply vac, engine should almost stall.. if not your EGR passages may be clogged ? or EGR valve was be rusted open or shut. clean , scrap crap off..
* Crusie control box going crazy? that cable needs lube ?
* Fuel rail pressure going up and down ?
* Fuel filter ?
* Search this site ?

WOW, 354k miles.... what is your secrets????

 

My 2002 Expedition did the same thing 2 winters ago. Suddenly the RPM's would stay at a level while driving (2500-3000rpm) and the only way to stop the truck was to put the trans in neutral. My issue came back to the throttle linkage under the hood at the throttle body. I found out the gunk that built-up had become "gummy" from the cold weather and wouldn't allow the throttle cable to return. Cleaned it up with some WD-40 and never had an issue since them. I now clean the cable every fall before the cold weather sets in...just to be safe.

 

The cruise and horn both quit working at the same time more than a year ago. That's another problem I can't figure out. Changed the clockspring to no avail.
I'll try the other suggestions and keep an eye on it. Thanks guys
No secrets for high miles other than religious oil changes (every 5K with MC filters and Castrol GTX 10 40) and other maintenance (complete trans fluid changes every 40K).
The trans has never been worked on either.