P1506 Code - High starting idle - 99' 4.0 SOHC

[taylorpro]

Alright guys. I'll give you a quick rundown. I have a 99' 4.0 SOHC. 173k. I custom mounted a 3g alt and by making the upper radiator hose work for the time being, the stock IAC to Intake tube hose wouldn't work. (the molded plastic hose that goes under the intake up to the IAC) So i made one with vacuum hoses. Couldn't ever get the size/length correct so idle wasn't correct. Got the P1506 code obviously. It was high in start up and and idled high. I worked the size down until i got it to a manageable idle. Except it would almost die on cold weather start up since the IAC couldnt compensate at that point.

WELL NOW. I re-routed the upper radiator hose today SOLELY so i could put the stock vacuum line. Thinking that I've just had the size of the line was the problem. And it's STILL showing the light, and its still idling pretty high. around 1000rpm. And on startup, thats the worst. goes up to about 2000 then comes back down. this is uncomfortable to me since i have a higher mileage engines with not so perfect timing chains.

Now, I've checked all the vacuum lines I see around the intake and they all seem ok. All of the hoses in the intake tube are in correctly and the clamps on the tube are tight. The ONLY thing that may be leaking some is the rubber boot between the molded plastic vacuum hose and the IAC itself. I will address that tomorrow. The idle Does go down a bit when i unplug the IAC. And it idles smooth either way.

I mostly concerned about the damn start up Rpm. when its cold and the rev's go that high on startup the chains just cry its no bueno. If i can't fix this im going to just use my ghetto-fab line until i can figure it out. at least it doesnt go up crazy on startup.

If any of you guys can help me out or drop me some plans of attack id really appreciate it.

**I should Note too, that before i started this alt insall, the car idled perfectly fine at about 6-700. Nothing was out of the norm till i put my own line on there and it hasnt been right since. thanks

 

[2000StreetRod]

Intake manifold vacuum lines

P1506 Idle Air Control Over Speed Error

Check all of the intake connections: Intake manifold vacuum lines

 

[taylorpro]

pretty sure all of them are fine. i looked over it for a good ten minutes yesterday and didnt find any that didnt look okay.

You think it could just be that i need to reset the battery? Let the computer go back to default now that it has the factory sized line? I haven't done that yet since putting it back on there. And it only has ran for about 25 minutes since

 

[2000StreetRod]

clearing DTCs

If you don't have a code reader to clear the DTCs then disconnecting the battery is an alternate method. Otherwise, it takes quite a while of driving to clear the code. Clearing the code may not alter the idling characteristics. I'm curious why the PCM isn't reporting lean conditions if there is an intake leak. Make sure that your throttle plate isn't sticking open.

 

[taylorpro]

Just reset the battery and let the truck run for 10 mins or so. Light is now out and its still idling high. 2100rpm spike on startup. All vacuum lines seem good. Throttle plate is closed, cable has slack.

 

[2000StreetRod]

Battery voltage?

P1506 indicates that the PCM is unable to lower the engine idle speed via the IAC valve to the desired value. Often when non-metered air enters the engine via an intake leak the PCM sets P0171, P0174 or both. Since "The idle Does go down a bit when I unplug the IAC" that indicates the PCM is electrically connected to the IAC valve. What is your battery voltage when the engine is idling? Maybe it is excessive and affecting the IAC valve in some adverse way.

 

[taylorpro]

P1506 indicates that the PCM is unable to lower the engine idle speed via the IAC valve to the desired value. Often when non-metered air enters the engine via an intake leak the PCM sets P0171, P0174 or both. Since "The idle Does go down a bit when I unplug the IAC" that indicates the PCM is electrically connected to the IAC valve. What is your battery voltage when the engine is idling? Maybe it is excessive and affecting the IAC valve in some adverse way.

its solid, but not high. the system charges at 14.3-14.5 depending on temperature.

Drove home from work and the light stayed off. YET the high idle persists. I went ahead an ordered a new IAC to try. If it doesn't work, ill return it. Not sure what else to look at ATM if the IAC doesnt work

 

[taylorpro]

Got a new IAC. No luck. Still doing the same type deal. 2k RPM on cold startup and drops down to 1100 or so cold. 900-1000 hot.

 

[taylorpro]

ARIGHT I noticed something else upon checkin over the car a bit more for vacuum leaks and the like. The car's idling about 1000rpm. If i flick the throttle blade and jump the rev's up to say 1500rpm, the throttle does NOT come right back down. It will float at 13-1400 for a few seconds and then slowly make its way back down to 1000. The throttle blade is closing all the way and on time it seems. And again I have no CEL.

Could this be a sign of a TPS maybe? I would think a vacuum is probable too, BUT the car idles smooth as glass. If it has any variation at all, its so little I can't see the needle moving on the tach. Open to any and all suggestions guys.

 

[2000StreetRod]

Ford Explorer - Ranger TPS Test Procedure

Sounds like an intake leak to me but here's a link to check your TPS: Ford Explorer - Ranger TPS Test Procedure
The PCM has a "dashpot" routine that keeps the engine from stalling when closed throttle is detected.

 

[taylorpro]

Sounds like an intake leak to me but here's a link to check your TPS: Ford Explorer - Ranger TPS Test Procedure
The PCM has a "dashpot" routine that keeps the engine from stalling when closed throttle is detected.

could it just be carbon build up in the throttle body perhaps??? An intake leak doesn't seem impossible by any means it just seems odd that it'd start as soon as I added that first big vacuum line. ever since then I've never been able to get the idle back to where it's supposed to be

 

[taylorpro]

Checked my TPS. It was reading 1.043 at idle (triple checked the butterfly was 100% closed and it was. not THAT dirty either) and 4.92 at WOT. From what I read it should be around .98 and the difference of .03 can put the TPS into "partial throttle" mode; at least how the PCM recognizes it. That being said, I put my custom IAC vacuum line on there that works perfect (minus very cold days) and I'm going to try a New TPS this saturday. I work at Advance auto parts so I'm just going to try it and see if it works really quick. Really hoping this fixes my problem. These idle issue's seem like an absolute nightmare

 

[2000StreetRod]

ratchet routine

The PCM reads the TPS voltage when the ignition key is switched on. The PCM assumes the throttle is closed at that time and sets the value as the minimum. The PCM periodically checks for a lower minimum (ratchet routine) and if found sets that as the updated value. I doubt that a reading of 1.043 volts for closed throttle at ignition on is a problem. I'm surprised that your WOT value was greater than 4.8 volts. Did you measure to the return line or chassis ground? The PCM reads between the high side and the return side. The return side is not connected to chassis ground.

 

[taylorpro]

I used the grey/white and grey/red wires to measure it. the key was "on" when i did the tests. and yes it only went to 4.62 at WOT. From what i read on the other thread 1.04 seemed a bit high. like i said i'm going to try a TPS for *****. But am unsure of where to go next. the thing idle's too smooth for me to think that there is a prominent vacuum leak. and more so what would have caused in the first place

When i measured the Blackish/white and grey/red wire i got a constant reading of 4.92

 

[drdoom]

Try cleaning the throttle body, and also check the hard idle screw, yes the one you're not supposed to mess with, maybe somebody adjusted it up at some point. Just a thought.

 

[drdoom]

You should also be checking the cables, mostly just the throttle, but you can try disconnecting the speed control cable, to eliminate it as a problem. Basically what I mean is assure you have free movement in the throttle system from the pedal to the plate, properly lubed, with no binding.

 

[drdoom]

Pull the intake hose. Check the throttle body and plate.

 

[taylorpro]

throttle cable has plenty of slack in it. the throttle plate is opening and closing properly. plenty of spring tension and no binds. The plate has been cleaned best i can on while its still mounted on the car. not too bad IMO. I may be suspect of the screw if it ALWAYS did this. but it hasnt. i started when i started messing with the vacuum hoses and has been incorrect since. even since going back to the OE IAC hose.

 

[2000StreetRod]

flexible elbow

Check the flexible elbow between the IAC valve and rigid tube. It may have split when being removed or installed.

Is the main intake to right valve cover hose installed?

 

[taylorpro]

Check the flexible elbow between the IAC valve and rigid tube. It may have split when being removed or installed.

Is the main intake to right valve cover hose installed?

The little rubber cuppler looks like its it fine shape. Ill double check in a bit. The passenger side valve cover vent hose is installed