Is it TPS, MAF, ECT, IAT, PCM or ...?????

:banghead: Can anyone help? 2000 Ford Explorer 4.0L OHV. A little history on this truck; Over a year ago it was in the shop for tranny not shifting and slipping. Got the truck back and all was good for a minute but then got a P0715 code and getting misfires on various cylinders (but not P0300, random misfire) for over a year. Pull, check and either clean/re-gap and reinstall or replace all 6 spark plugs. Check wires. Runs fine for a couple of weeks and misfire returns. About a year ago, start getting P0171 and P0174, lean both banks. Check for vacuum leak, can't find any. Replace all O2 sensors. Still have various P03xx, P0171 and P0174 codes. Replaced MAF (even though it tested good), tested TPS and ECT (also tested good but did not replace). Last month, a few days after replacing MAF, was running down the road and got a huge backfire. Now, the P0171 and P0174 have been replaced with P0172 and P0175, rich both banks, still have the P0715 and now there are additional codes of P1121 and one for the coil pack (sorry I don't remember the exact code as I'm writing).

Yesterday I quickly narrowed the current misfire to cyl. no. 4 (even though there was no code set) and pulled the plug. It was fully fouled from cathode to anode so I thoroughly cleaned and reinstalled it, ran the engine for a moment and re-inspected that plug. I found what looked like oil but smelled like gas on it So I Ran some tests for spark to that cylinder and from what I can see, that's not the cause. Next, I tested the fuel system and got 64psi with the engine off and had a smoke while I waited to see if the pressure dropped. It dropped to 60psi in 7 min. Started the truck and got a steady 64 psi at both a low and reving the engine. Gunned the engine and the fuel pressure dropped slightly but instantly returned to 64psi each time. I also hooked up a vacuum gauge and found a good steady reading of 19 inches at idle that would drop to near 0 then up to 25 when the throttle was snapped open indicating no vacuum leaks and no valve problems.

So far, about the only things that I can think of, that I have not tested are the compression and the EGR system. This is frustrating the #$%^&* out of me and any help or suggestions that anyone can give is much appreciated.

Your P1121 diagnostic code could be key here. It's defined for your truck as, "Throttle Position (TP) Sensor Inconsistent with MAF Sensor" and a likely cause is a vacuum leak between the MAF and the throttle body. There are other possible causes, but if you have a crack between a couple of folds of the intake air tube flex joints it would be difficult to detect visually. I struggled to figure out the source of a misfire on a co-worker's car a few years back and ultimately a shop identified the cause as exactly this, a crack in the intake air tube between the air filter and throttle body.

-Rod

Thank you, shorod. I will check this out as soon as possible. I'm not convinced that this could be the sole cause for the plethora of problems/symptoms that I'm having. Do you have any thoughts or ideas about the rest? (http://www.automotiveforums.com/vbulletin/member.php?u=194894)

Early morning blahs:

You can clean out an egr quite easily. I would replace all the plugs and wires. I bought the e3 plugs, they fouled out both times, the FIRST set AND the second warranty set.

There certainly could be a sensor issue, or wiring issue for one of the sensors, but carefully checking for a vacuum leak is free. The majority of your codes relate to a lean condition, rich condition, misfire, or fouled plugs, and all of those could be due to a vacuum leak or MAF sensor/TPS sensor that are not providing the PCM with accurate results.

-Rod

Still dealing with this:banghead:. Changed plugs/wires, cleaned EGR, verified ECT, compression test, check for vacuum leaks (none), ...now I'm not getting rich codes on both banks but now I'm back to getting lean codes on both banks. I'm starting to believe that the only thing to be done is to remove radiator cap and replace vehicle.

Lean codes are almost always going to be either a sensor issue or a vacuum leak.

You should also check the fuel pressure with a gauge if you have access to one.

-Rod