High idle in neutral only

[Davo1111]

Hi, i've been trying to adjust my timing on my 1998 Pathfinder. The problem I'm having is that when I unplug the TPS , start the vehicle and put it in neutral the Rpm jumps to about 1500. A while back I cleaned the throttle body and had the high idle issue that seems to follow. I resolved that problem and a few others. I cannot figure this one out. I've convinced myself it's something simple. (Well, simple to someone other then me)
I've checked and cleaned the charcoal canister vacuum lines because of a bad canister. Replaced the purge valve, ghetto blasted the iac valve, adjusted the tps, did the relearn process and replaced the maf. The truck idles fine with the tps plugged in. Without it plugged in it, idles fine in park, but in neutral it jumps to 1500 rpm. What's also strange is if I move the shifter from neutral into different gears then back to neutral, sometimes the idle will remain at 750ish. Could this be the IAC valve? Can I adjust timing in park? How much difference is there between neutral and park? Thanks for any help and sorry for the long boring post. Dave

[AZhitman]

P and N should be the same with regard to setting timing. In fact, I;d think doing it in N would be unsafe.

Almost sounds like you have a vac leak somewhere... Easy to check using a can of carb cleaner or starting fluid (which will increase or decrease idle if it gets sucked into a leaking vac line).

[Davo1111]

Thanks for your response. I also suspected a vacuum leak, even though having a high idle in neutral and not in park didn't make sense to me. I checked twice, once with propane and once with brake cleaner. I also check the lines for any cracks or looseness, and found nothing. I still might try pressurizing the engine somehow and see if i can hear a leak that way. I hooked a vacuum gauge to the line going to the EVAP purge control valve and got 19.6-19.8 inHg at idle and 20.5 inHg at 2500 RPM. My OBDII scanner said 21.1. I installed the new MAF and let it warm up, unplugged TPS, restarted and checked the rpm in neutral and it was normal. All looked good. As I was playing with the OBDII scanner, graphing the O2 sensors, the truck had been idling maybe 20 or 25 minutes and it suddenly just died. I plugged TPS back in and tried to restart. Finally i got it to take. It sounded horrible, defiantly running rich. Black smoke from exhaust, shuddering, barely running. I put old MAF back in and it ran good, I put in nuetral and rpms went 1600. I knew I shouldn't have bought that cheap china MAF off Ebay!(I know,I know... multiple people on this forum said not to) My OBDII scanner shows that the EGR test is always incomplete. I may check into this, but I didn't know if that test took multiple starts and stops to complete. The OBDII also shows that the Bank 1 #1 O2 sensor is constantly fluctuating from .1 to .7 after warming up at idle. The Bank 2 #1 is mainly a flat line between .2 to .6 fluctuating only so often and then steady? I am not sure how a normal O2 sensor should look on a graph after the truck warms up or if there is even "normal". Thanks again for your input.

[AZhitman]

Those O2 sensor readings sound fine. Wonder if the MAF or TPS is sending a different signal in N than P (I'd think the TPS would).

[Davo1111]

Just wanted to say thanks to you AZhitman. I don't know why I didn't think of the TPS. I guess that is what it kind of does is send signals for the transmission... Anyways I replaced the MAF and readjusted the TPS and all is good. Thanks for your help and sorry it took so long to reply again.