Quick backstory. I bought an 05' 4.7 4x2 with an idle issue. LOTS of parts replaced, but it still won't hold an idle. The only code I'm getting is a P0068 code. STFT's are pegged rich at -32.8 on both banks and then lean up as you pass 2000rpms. I pulled the plugs and all but 2 and 7 were black as night (2 and 7 were white).

MAP sensor is new and it's good (vacuum tested it). It shows about 20Hg at high idle on my OBD2 scanner (as best as I can tell since it doesn't really idle well and wants to die under 800rpms.). Seems to operate normally though as the pressures dip if you hit the throttle.

5v and ground to the TPS are good.

But here's the question/issue. I have three separate TPS sensors (1 old/2 new), and all of them bench test from about .1v to 3.8v, over a spread of about 120degrees. Problem is the throttle body only rotates 90degrees. So when on the vehicle, they all only go from .2v (idle) to about .8v at full throttle. This isn't correct right?? But all three do the same thing. On my OBD2 data scanner it shows the throttle is 12.7% idle to about 78% at full throttle. This indicates that the ECM is reading the full range it seems?? Do I just have three bad TPS sensors (all different brands). Or should I be looking somewhere else for this problem? I'm desperate for ideas.

New parts
All 8 Injectors, All 02 Sensors, MAP sensor, IAC, TPS, Coils, Platinum NGK plugs. Fuel pressure is 58psi.

 

Maybe this might help

 

 

Thanks Sabre. I've got the service manual but just started reading into the P0068 pages. My MAP tests are good I think and it's operating normally. But in my original post I mention my TPS might not. Instead of reading from .8v at idle to 3.5v+ at full throttle like he manual says, mine is only reading about .2v to .8v. But my diagnostic scanner shows the PCM reading my throttle position correctly still from 12% at idle through about 78% at full throttle. So I don't know what to think. Maybe it somehow learns the two limits no matter what the voltages are??

That said, I noticed that in step 2, it says in bold letters that a "Large vacuum leak is most likely the cause of this DTC". And somehow I missed checking some simple vacuum failure items (like the PCV valve). So I'm excited to test those when I get home today. After MUCH hair pulling I'll be angry and happy if it's a simple $7 PCV valve that just stuck open. I'll report back.

 

Now that I understand what you are saying -- I would also check for opens , high resistance or grounded wiring on MAP and TPS sensors and make sure the wiring is good between sensors and PCM. The steps for these checks are in the shop manual starting at 9-50 thru 9-57

 

Thanks. I'll check that for sure. Then I'm pretty much down to a compression check. I'm starting to seriously suspect a leaking head gasket or pistons. It seems like I've run through the full list of likely suspects and nothing has improved it. Maybe I'll try to rig up a smoke machine to fully test the intake manifold and vacuum system.

With that said, I just removed my PCV and it rattles. Looks old though so I'll replace it anyways. Plugged the PCV hose that goes to the manifold to simulate a closed valve at idle and no change. Ran a tiny bit better for a minute or so and held a weak idle but then reverted back to the old symptoms again and glug glug glug... and died below 500rpms. No change when I clamped the brake booster vacuum line and the vapor canister line so I don't think those devices are leaking. Couldn't clamp the EGR as it's a hard line but I'm not getting any EGR codes. Got a P0135 02 code (maybe because of the plugged PCV hose and I revved a few times?), but also the P0068 code came back too.

I'll check resistance on the two sensors formally, and compression check all 8. But any other help would still be gold.

 

My opinion I think the .8 volt at full throttle is trying to tell you something. I would unplug the TPS and C2 Black-Orange connector of the PCM and check pin 2 of the TPS to ground. Check for any broken wires at TPS. Make sure the signal wire is good between TPS and C2 connector of the PCM.

 

I've got a "Blue Driver" OBD reader and it gives me both Absolute (Raw) TPS % and Relative TPS % (interpreted). The relative is based on the calibration procedure when you install it, or after disconnecting the battery to clear learning, then doing it again. I'll have to check the high end, but I've had 3 different TPS installed and the absolute value at idle was 10-12% on all 3.

 

It is strange that the reading are off obviously, and I'll check the wiring FOR SURE and report back.

However I get the same type of results on the bench. In other words, with 5v and ground connected on the bench (not on the truck), and when I twist the TPS sensor dial roughly to the point of where it is when it's installed at idle (it rotates 10deg or so to line up the bolt holes after engaging the TPS splines), the signal wire reads about .2v. And then when I continue rotating it 90deg from that point (the max the throttle plate would turn on the truck), I get .8v. On all three sensors. Only when I rotate it past the 90deg from idle do I start getting readings over a volt and up to 3.8v. So maybe the potentiometer in all these TPS sensors is not rotated and glued in correctly from the factories? As in, when installed, to line up the bolt holes, it should be rotated more than 10% in order to get .5+ volts. And then 90deg from THAT point would max out the potentiometer and be a correct WOT reading. In other words, if I could install this at 30deg instead of 10deg from relaxed, it would operate normal I think. It's just weird that all three bench test the same. One looks worn (possibly NGK/NTK by design, looks identical), another is new Duralast from Autozone, and the third is a cheap new Ebay special that I bought just to see. Perhaps it's time to throw a MOPAR part in here but damn they are expensive for a parts toss here. Amazon has free returns though, lol.

 

I would try the TPS unmounted and use screw driver and rotate by hand and see what kind of results you get.