random drop in RPM at idle

[Dizzy_ax]

I dont know whats causing it. The EGR was deleted about a year ago and i instantly got the check engine light and that random idle drop. i have had two tunes done so far and its still hear! Also the cars idle drops down pretty low when coming to a stop.

Here is what happens when i come to a stop. While coming to a stop the RPM's come down with the speed and sometimes drop down to about 300 rpm, they pop back up once i am completely stopped. the car idles around 900-1000 (as low as i can get it with the IAC plugged in) and every 15-30 seconds the rpms randomly drop off to about 350, it jumps around from 350 to 1100 then it goes back to 900-1000 and holds fine. I can sit there for 10 minutes and it will keep doing it.

I took the spout out once and let it idle and it didnt do it.... if i could get this issue out of the way id be done with this surging idle!

Any ideas what this could be would be greatly appreciated.

 

[HISSIN50]

If you haven't, pull the codes.

Is there a reason to wonder about the IAC function?

I kinda wonder if you have a vac leak. That would cause surging and would screw with the IAC in trying to maintain the commanded idle.

 

[Dizzy_ax]

i replaced the IAC, it now stops the car from dying so i think the old one wasnt helping the situation. Im without a code reader, but im going to go buy one. I was getting an EGR code and i havnt checked if they tuned it out yet.

 

[HISSIN50]

You should have pulled codes and then disconnected the battery while replacing the IAC. The old one and new one are not functioning the same and the computer has no clue you have a new one. It is going to take some time for it to learn how to act.

You can pull codes with a peice of jumper wire if you need to. Your repair manual and the internet have tons of links for doing so.

 

[Dizzy_ax]

Sorry this took so long, tons of over time and sleep ;p

I checked the codes and found i had a bad o2 sensor, replaced them and my car still does the random drop in rpms.

Also said my TPS was set too high, so i re adjusted that back down and nothing is working.

Would i be able to see a vac leak with my vac/boost gauge?

Its just so random when it happens, the biggest thing for me is coming to a stop the rpm's drop off with my speed, once at a complete stop they jump back up.


Thanks for your help in advance

On the code reader i watched the IAC's % and it was holding around 43-47% if that helps.

 

[HISSIN50]

How do you know the O2's were bad? A bad vac leak will cause O2 codes (but that doesn't mean it's the O2s' fault), and the leak can cause the stalling issues and surging. What does your vac gauge show at idle?

 

[Dizzy_ax]

They were not moving on the code reader, just sitting at one number. Not only that but i think 119k on them is a tad bit much..Id like to know where this vac leak could be, id have to go check but at idle the vac gauge doesnt move around, holds steady and when the car almost dies at idle it shoots down a tad and then rockets back up, at the same time my AFR leans out then it goes back to idle like nothing happend lol.

 

[HISSIN50]

Did you check the O2 output at the wiring harnesses? For both O2's to crap out would be weird. When an O2 doesn't send a signal, the reading at the EEC will be frozen at one value.

BTW, what code scanner do you have that lets you see the output of both O2's? I'm just curious (mine doesn't do that).

The only vac reading you care about right now is your steady idle reading (or a reading at like 1K if your actual idle wanders).

As for finding a vac leak, checking over every single vac line and connection is how to do it. You have to check them all manually (or use a smoke machine). Some people spray stuff on the lines but I don't recommend it for fire hazard concerns.

 

[Dizzy_ax]

I went to all ford proformance, had my tuner there helping me out and it was showing everything, IAC%'s what my TPS was set too my speed sensor ect. i have no clue what one they were using but im going back to get my trans checked out so i can check. just the right one was frozen, i'll check to see if it was reading both. the biggest thing was he made the car lean to see if it would change the reading and it didnt. so he said lets try that out.

I'll start checking the vac lines! sounds fun

 

[sublimobile]

It surprises me that I'm always the first to mention this. It has solved the same problem for 2 or three people now. I had the same issue. My turned out to be a clogged PCV valve.

Easy fix. Check it out. Located behind the lower intake.

 

[Dizzy_ax]

replaced it, car still does it


I think my car hates me!

 

[Dizzy_ax]

The vac/boost gauge is holding around 17.5-18, my motocraft PCV looked almost brand new! lol....im pretty sure it was changed out about 3k miles ago when i got the new intake.

 

[Dizzy_ax]

Still doing it, i think i might drive it off a cliff. that should fix all of the idle problems!

 

[Dizzy_ax]

Update, it was throwing two codes this time


123 TP is/was high or short to power - TPS
The code reader showed it at 1.00 (thought that was in range? going to set it back down to 98-99)

and

511 No power to PCM pin 1 or bad PCM (processor)

Is this because i have a SCT chip in the car? or is my PCM taking a dump?

thanks for the help

 

[Dizzy_ax]

anyone have an idea what that 511 code means?



Jeff B.

 

[HISSIN50]

511 can indeed be your chip.

You'd need to test the TPS while it acts up to really see what's going on. On a safe, closed course, you could disconnect the TPS and see how that affects the symptoms.

 

[Dizzy_ax]

Have not tried that out yet, ill go do that

 

[Dizzy_ax]

With it unplugged it stoped doing the random RPM drop. ofcourse since it wasnt plugged in when coming to a stop it tried to die, does that mean the sensor itself is out or the harness is messed up somewhere?

thanks for the help!

 

[Dizzy_ax]

Next thing for me to do is try the TPS i guess! Only thing i have not tried. its moving out of range and i know i have it tightend down pretty good. Would that cause a random drop in RPM at a stop light along with trying to die while coming to a stop?

thanks for the help!

Jeff B.

 

[Dizzy_ax]

I re adjusted it back within range, even though it was still in range ;p And its still throwing the 123 code. Should i try a new TPS or can this be an short somewhere from the wires?