Stock 1996 F150 5.8l MAF ~152,000 miles on the rig.

Looking for some guidance as far diagnosing this code further than just looking for vaccuum leaks (which i have done extensively). Both propane and brake cleaner (i know the danger of brake clean and testing for leaks).

Only code present is P1506.

Hooked up a scan tool and poked into the live data streaming, O2 sensors seem to be functioning properly both front O2 sensor are jumping around (voltage wise) which is normal, rear o2 sensor is steady (voltage wise). I have no noticeable driveabilty symptoms just a high idle, rpms do hang a bit when coasting so i guess maybe a slight driveability concern. Fuel trims look acceptable with LTFT at bank 1 being around 3.4% and bank 2 being around 4.6% respectively.

"Ohming" out the IAC valve i come to 10 ohms which i believe is in spec. How would i go about testing the wiring for the IAC valve? How would i go about testing the TPS?

Unplugging the IAC valve when truck is warm and at idle has no affect. (is that normal)? Coolant temp sensor via live data is around 195-197 when warm so I dont think that is an issue. MAF is reading around 1.001 Ibs/hr which coverts to about 8 g/s. (I believe this is normal) When resting the tps sensor reads at 18.8% and at full throttle 92.5% via live data, Is there a spec on what percentage the TPS should be when the throttle plates are closed?

Throttle body and iac valve are as clean as can be. I literally mean that; you probably could eat off of both of them. I am kinda stuck as far as what else i should be looking for at this point. I am really hoping i dont have a PCM issue.

 

So my idle issues seemed to have cured itself, not sure exactly how but my idle is back to where it needs to be. Will update if anything else weird comes my way.

 

Glad that it seems to have cleared itself up.

I'm not an EFI expert, but I have dabbled with it & wanted to share some things.

I remember hearing repeatedly that 02 sensors can get lazy before they even throw a code. I don't believe a scan tool will pick up on the laziness (since even the relative "lag" is still happening very quick) of the o2, but the tell-tale sign of the laziness is usually a decrease in mpg.

As far as LTFT, I personally don't pay much attention to it. Since you could have a scenario where vehicle parts were recently replaced & the computer was never reset. In a case like that, the LTFT could lead a person on a wild goose chase.
On the other hand, taking a minute to watch the STFT (at idle, engine at full temp, no accessories on) fluctuate is what will tell me if there is a vacuum leak or other problem. If the STFT fluctuates more than 10%, then there is an issue (although, I prefer it to be even less than that). If the STFT fluctuates rapidly, then there is also an issue.


Personally, I like to warm up the car, get screenshots of everything, THEN do a thorough ECU reset/relearn (sometimes this alone will "repair" issues with the car) & from there on out focus on the STFT, instead of the LTFT. This way, I know I'm not chasing issues that aren't actually there.

 

It's all normal.

I never saw a TPS at 100% at WOT on any car & others said that if you do get a 100% reading, then it's a sign the TPS might be bad (it has something to do with the fact that a TPS isn't suppose to max out on values, but rather give a voltage difference or some crap like that that I don't understand exactly, lol). I believe the MAF reading is actually what's more important for WOT readings, not the TPS reading.

The idle on our 96 4.9 is at 18% at idle. I believe this is because even though the throttle body blades might be closed, the ECU programming is calculated to take into consideration air passing through the IAC, EGR, PCV, etc. into the engine. I've heard this is why it's problematic if people remove or modify these things on OBD2's & don't tune the ECU for it: the computer is still calculating that it's being fed air through sources other than the throttle body.

 

Closed throttle TPS output voltage is supposed to be between 0.6 to 1.0 VDC. 0.6 equals 12%, 1.0 equals 20% so it looks like the closed throttle value is in acceptable range. Your WOT percentage, 92.5%, is equivalent to 4.625 VDC. That is acceptable in my book. The computer interprets anything greater than ~2.71 VDC above the closed throttle TPS output as WOT anyway.

 

Going to dive into more of what you said regarding STFT, just because a code is gone doesn't mean i couldn't have any other underlying issues, fuel economy is pretty average at 11 mpg with 33x12 tires and a 3.55 rear end. With o/d. Actually surprsied its even that high.

 

Definately awesome info you threw my way! Thanks!!