Strange Idle problem...

[CaelanSeth]

originally posted in 240sx technical with no replies so I thought I would try in here...

I know there are several posts on idle problesm but I could not find a solution. OK so my car is a high mileage (225,000 miles) 1989 240 auto. Last week it idled fine, if maybe a little low.I changed my belts a few days ago, while I was at it I sprayed a bunch of cleaner in the throttle body and cleaned the plate.

Put belts back on and everything seemed good, except I noticed the idle was very high when I put it in park or neutral. And when the car gets warm, the idle rapidly bobs up and down from around 1700 to 1200 rpm. I first tried adjusting the screw, it had no effect. I figured the cleaner in the TB pushed a bunch of gunk into the IACV, so I tried cleaning that.

When its in drive it seems ok, except when i let off the gas it seems to drop rpms very quickly, but that coulkd just be my imagination.

I removed the IACV and cleaned it out real well, there was a gasket between it the the TB that broke so I put it back on without it, as many people have said their cars dont have the gasket. Anyways, I wasnt sure where to position the brass screw so i just screwed it in quite a ways, put the valve back on and my idle is worse, now over 2,000 rpms.

I pinched the air regulator hose to see if it would drop, it didn't. Disconnected AIV, but it had no effect. I cannot think of what it could be, except a leak possibly somewhere in my intake, the rubber bellows that connetcs to TB is old and a little cracked, but i dont know and have no idea why it started when I changed the belts... any one have any idea?

I also manually opened the throttle plate to clean, but it looks as if it is closing properly, is it possible it is not and i just cant see it, and is there any way to check?

[vancouverbc]

try spraying carb cleaner to find a leak

[CaelanSeth]

yep i tried that.... couldnt find anything... although I did not off my intake tube and like I said the bottom of the rubber boot that attaches to the TB is quite a bit cracked, i can even see a very small amount of sunlight come through when I look through and wiggle it around, could this be my problem? (edit: just as a possible temp fix, i electrical tape around the cracked areas, wrapped it in sticky plastic, retaped it to make sure no air could get in and it had no effect)

I thought it could be the TB plate, as I manually turned the plate to spray cleaner in, but it seems to close all the way.

The only other thing i can think of is the TPS, someone told me to disconnect the tps and the emissions sub harness, if the idle returns to normal then it is a bad tps, i dont currently have a meter reader so i cant check it. Also, I am unsure of where the emissions sub harness connector is.
Modified by CaelanSeth at 3:16 PM 7/6/2009

[vancouverbc]

meters are 20 bucks. only use digital. back probe the tps connector to be certain the reading is .45 volts when plugged in. you can loosen two small bolts to adjust the tps.

also shut off the air supply to the iacv and air regulator and see what happens.

[vancouverbc]

the air supply hose is in this picture. it connect to the black plastic tube that comes from air filter and connects to throttle. there is a small and larger hose connecting there. the hose you plug is the larger hose.

[CaelanSeth]

Thanks for the pics and replies... I plugged up the feeder hose to IACV and regulator with no effect. I'm guessing this rules those out as the culprits.

Tomorrow I will get a meter and test the tps, if its not that I am guessing there must be a leak of something with my throttle plate.

[vancouverbc]

Thanks for the pics and replies... I plugged up the feeder hose to IACV and regulator with no effect. I'm guessing this rules those out as the culprits.

Tomorrow I will get a meter and test the tps, if its not that I am guessing there must be a leak of something with my throttle plate.

Sounds like your tps might be ok because your aac valve must be completely closed.

The fsm says to race the engine at 2000 rpm. If there is a leak, the rpm will falter.

From experience, I know that just a slight opening of the throttle will give you 2000 rpm of air. maybe when you cleaned it , you increased how open it is. There is a small screw that controls how open the throttle is. you back this off until it no longer holds throttle plate open. then, tighten until it just touches and then twist it another .5 turn. try plugging the throttle with a rag if this doesnt help.

disconnect the negative terminal of battery after you find problem so computer can get rid of its self learning.

[CaelanSeth]

k thank you for the info, i will give that a shot tomorrow. is the screw on the throttle plate itself? I think there are two screws on the plate when I was looking at it, but did not want to mess with them. Again, thanks for the help

[vancouverbc]

k thank you for the info, i will give that a shot tomorrow. is the screw on the throttle plate itself? I think there are two screws on the plate when I was looking at it, but did not want to mess with them. Again, thanks for the help

the screw is on the outside. the end of the throttle linkage rests on the screw.

[vancouverbc]

[CaelanSeth]

K I backed the screw off then retightened, rpm's dropped to about 1800, which was actually a relief, something actually worked to chnage the rpm, afterwards I stuffed it with a rag and it was around 1200. i tried adjusting the idle the conventional way to get it lower, but no luck (infact, i have yet to see any real change at all by using the idle adjustment screw)

i removed the rage and Just to see what would happen, I loosened the screw to where it was no longer touching, slightly pulled on the linkage and started it up, rpms were a little erratic but were right where they should be around 800-1000... the only thing is as soon as I tighten the screw even slightly, as in to where it touches the linkage, rpms go back up to around 1800.(edit: actually i loosened the screw after it was warmed up and had been driving, even with no contact on thr throttle adjustment, it is still pretty high, around 1500)

atleast i know what my problem is, i just dont think it is safe to drive with the screw being so loose, i had also heard that adjusting this screw means you should adjust the TPS as well, but I am unsure about that... anyways, other than the screw, is there any other way to make sure the plate closes flush with the TB? adjusting the linkage in some way perhaps? again, thx for the help
Modified by CaelanSeth at 4:07 PM 7/7/2009

[vancouverbc]

Yeah, be careful about the throttle plate binding. Make sure the screw is touching the throttle linkage. Your throttle is not the problem. you still had 1200 rpm with it plugged.

Your idle screw has plugged the idle hole so any further adjustment will do nothing. I surmise that your aac valve is closed also which means you should have no air when you plug the throttle. Maybe start a thread on vacuum leak to help find the leak. Your tps wont be off by much from playing with the screw but you will have to check. . i dont know the spec for the 1989. Make sure the screw is touching the linkage and that you turn the screw a bit after that. when you depress gas pedal it should not stick.

I could not find anything about the tps specs in 1989 fsm. with the 1991 , you just loosen two bolts and turn the tps until you get correct voltage.

I dont have an exhaustive understanding of mechanics. In my theory , you have shut off all the legit air and yet you still have an idle which means vacuum leak. when you cut off air to the iacv , it made no difference which means the ecu has kept the aac closed and obviously idle hole is closed as well. i only know 2 sources of air. 1. through the throttle plate 2. through the hose that supplies air to aac valve, air regulator, idle hole, and ficd.

Modified by vancouverbc at 5:26 PM 7/7/2009
Modified by vancouverbc at 5:47 PM 7/7/2009