OK, I have worked on an intremittant P1506 for a few months and am running out of options. I can get the code to come on when I drive a long haul at around 2 hours of driving, usually when I let up on the gas, at around 70 mph.
About half the time I also get the P174, P175 Lean Banks 1&2, Its like I get a big slug of air into the system.
Stats
1997 F150 180K 4.6L
New Engine (and upper end) & Tranny - 8K miles (Dealer Reman installed myself)
New intake, EGR, EGR TUbe, DPFE, MAF, 75mm Throttle (Oversize),

Checked the Intake System for cracks, holes, ect...

I did find a torn boot at the main vacuum outlet of the throttle adaptor, and fixed that, and the problem decreased by 100% to what it is today, The troubleshooting book says start looking at the Vapor Management System,
I can pull and hold a vacuum from the Hose at the Intake to the Vapor Management valve, But so far I cannot get the hose of the inlet to the VMV off. That hose goes back to the spare tire area and theCarbon Canister Control Valve, and should hold a vacuum for 20 seconds, although that is suppossed to be always open at above 150F engine temperature, and the problem always occurs when the engine has been running for awhile, I got the stethoscope out and listened for intake leaks, ect,,,,and done my best to check the vacuum lines, could it be a bad PCM ? today I will disconnect the battery and let that reset.


frustrated, at least there is nno performance difference

 

The 1506 code is an overspeed error by greater than 200 rpm.

The codes you give of 174 and 175 I believe are in error.

You either have a 171/174 = both banks lean or 172/175 = both backs rich.

The 1506 code could be supported by the codes 172/175 combination (rich).

This means excess air is raising the idle (1506) and being sensed by the OX sensors, sending the fuel correction tables too far in the rich direction trying to compensate for the extra air getting in somehwere and causing a lean code combination.
The lean or rich codes doesnot mean the engine is running that way but means the tables in the PCM are at their rich or lean limits and set the codes.

Said another way, the codes are set from what the PCM has to do 'internal' due to a fault 'external'. I know it confusing and most people take the codes for this literal when in reality they mean just the oppisite because they are thinking in terms of just the engine alone when it's a PCM internal action..

I would believe you have an air leak at some point from the TB on, with either hoses or an actual crack in metal or some part is not tightened enough that may open wider on longer run times, by expansion.
Good luck

 

I had system lean codes for a long time, ended up being the vacuum line to the fuel pressure regulator were cracked

 

Remove the IAC, put some tape over the ports, then cover the throttle body with your hand. Have someone start the truck, if it idles, you've got a vacuum leak somewhere.

 

Is the new MAF an OEM or is it aftermarket?

JMC

 

The MAF is a NAPA,

 

Oh yeah, only time I've ever seen this code in my personal experience is when someone's jacked with the throttle body manual stop.

 

Along those lines check the throttle body to see if the butterfly is completely closed at idle. It should be. If that checks out I am thinking that the MAF has the wrong calibration for the application. It is flowing more air at idle that it is registering and the PCM is can't correct properly.

JMC

 

OK Guys, Alot of good information here, I was running through sensor (o2) readings today and noticed that bank 2, sensor 2 is, in general, reading zero. The harness is fine and power is fine, I think I may be getting an air leak in the exhaust, because after a run to New Mexico to junt Javelina, the truck is generating alot of water condensate. Interesting that I do not get the P1506 when I have my pop-up camper on, then after 50 miles without the code returns. I will start at the TB and block off the AIC, then work my way down through the exhaust, and double check the line to the fuel regulator. I will let you guys know what I find. This weekend I am also doing all the ball joints, Upper and Lower ( A little to much dessert action at 200K) But my books say the lower ball joint is integrated into the control arm, although my NAPA guy says on 4WD the ball joint can be pressed out after removing a "c" clip. You guys have any experience.

 

Yes. The lower can be replaced using an aftermarket replacement. Ford's replacement is the whole LCA.

You can "borrow" a ball joint press kit from AutoZone with appropriate deposit.

Steve

 

Just a little hint.
If you play with the throttle stop don't back it down and completly 'hard' close the plate with the engine warmed or the throttle body will shrink after cooling down and grab the plate.
Next time you go to start and drive off the throttle will not open because it is stuck fast until the motor fully warms and the TB housing expands again.
There is a procedure for this but just be awhere of it.