Strange Idle

[SwimDude0614]

Car:
1988 900S (bone stock)

Problem:
-When cold, the car idles at no less than 1000rpm.
-When warm, it idles around 1400. On a good day it will stick to 1200.
-When hot, or after running errands (starting the car, turning the car off, starting again, etc), it will raise from 1400 to 1600, and then rapidly fall back to 1400 where it will once again climb back up. Once cycle of this takes about a second or two.

Attempted Fixes:
-AIC thoroughly cleaned out
-Checked for vacuum leaks
-Checked Throttle position sensor


So, any ideas? Thanks for the help.

David

[euromobile900]

Which injection system do you have? If it's LH2.2, you might want to investigate the air mass meter. Townsend has a good page of diagnosis for this.

http://townsendimports.com/Web/tuneup_folder/amm.htm

[tsksaab]

I have an '89 900 S (bone stock also). My car was doing something similar to yours...except my idle would run high and then fall well below normal. When cold, the idle was fine...the engine would turn on with no problem and it was steady.

When the engine was warm, the idle and start-up would be very temperamental.....idle would go up and down, from around 1500 to almost dying and sputtering. When trying to start a warm engine, it would start for a second and then die often as well.

My indie took the same steps you did....cleaned the AIC, injected smoke to check for vacuum leaks, etc. The problem turned out to be the fuel pressure regulator. Once replaced, the problem went away.

[SwimDude0614]

thank you both. i'll see about finding an FPR and AMM to test with. hopefully it's just the FPR.... i don't have the money for an AMM!

[900t]

Originally Posted by SwimDude0614

thank you both. i'll see about finding an FPR and AMM to test with. hopefully it's just the FPR.... i don't have the money for an AMM!

Don't even bother with the FPR

[SwimDude0614]

Originally Posted by 900t

Don't even bother with the FPR

huh??? if i can find another FPR, that's a quick and easy check.

[900t]

Originally Posted by SwimDude0614

huh??? if i can find another FPR, that's a quick and easy check.

Actually try running it with the AMM unplugged (limphome) that one is really fast

[Rodentmaster]

I would be scrutinising the ground leads.........

near the thermostat..... - may not be the direct cause but I have seen subtle improvements in my car afterwards. Check the valve cover breather (integral part of idle path), esp. the small tap at the top. It gets clogged.

[SwimDude0614]

can i unplug the amm while the car is still running or should i turn it off before plugging in or unplugging everytime?

[SwimDude0614]

Test: Unplug and replug AMM

Temp: Normal operating temp

Before: idled steadily at 1400 rpm

After AMM is unplugged: Rapidly jumped between 1100 and 1200

After plugging the AMM back in: Rapidly jumped between 1500 and 1900

I then turned the car off, got back in, and drove into the driveway. At this point it returned to it idling at a steady 1400rpm.

Pehaps I missed a vacuum leak? It seems like all my readings are a little higher than everyone else's.

[stickman]

Originally Posted by Rodentmaster

Check the valve cover breather (integral part of idle path), esp. the small tap at the top. It gets clogged.

I highly recommend this suggestion.

Also clear out the semi-clear plastic valve connecting two vacuum lines near the breather...remove this and spray it out I,ll bet your idle settles...it does get clogged easily.

[SwimDude0614]

Originally Posted by stickman

Also clear out the semi-clear plastic valve connecting two vacuum lines near the breather...remove this and spray it out I,ll bet your idle settles...it does get clogged easily.

i'll check the breather as soon as i type this.

what is this semi-clear valve you're talking about though? can you take a picture?

-Edit-
Thanks! I cleaned both the transparent breathers atop the intake manifold - the one closer to the firewall was FILTHY. propably clogged. the carb cleaner came out black for about a minute before it finally got clean. off to go drive now...

-Edit #2-
Just finished throwing her around the streets pretty hard (that's what usually makes it sputter) and she did fine. still idles up at 1400 when warmp, but the sputter is gone

[SwimDude0614]

well, turns out it was the the throttle position sensor. guess i threw you all off a little when i said i checked it. by "checking" it, i meant it was placed correctly. of course... it doesn't do much good if it's broken.

if anyone reading this has one used that they're willing to sell cheap, let me know. otherwise, i know where to buy them new.

[SwimDude0614]

I was wrong. It is not the throttle position sensor. After unplugging it, the idle did not change. I thought that was the problem. After using an ohmeter to TEST it, i found out i was wrong. the TPS is working perfectly.

so, next idea, O2 sensor. i snapped the wires to the O2 sensor by accident when working on the exhaust and yet again, NO change in idle at all. (before this, i got 22mpg when driving like a grandma around town on a darky rainy night. in the snow. uphill. both ways )

new thought: scary thought... if disconnecting the TPS did nothing to change the idle, even though it was working, is it possible my O2 sensor is (was) working correctly too and the ECU is busted? although, disconnecting the AMM does change idle and makes it run terribly

[SwimDude0614]

I've now added some more things that are NOT wrong with the car. Sunday, I replaced (one at a time) the air mass meter and AIC valve with no luck.

on the other hand... i think i fixed it!!!
i was showing my gf how to drive a stick and when we were all done, i decided to be all cool and such and i did a burn out in the parking lot. after that, car ran great. immediately after that. and for the rest of the day too!!!! I haven't driven today yet though. before burn-out: idle jumping between 1600 and 1900 like i said in the first post. after burn-out: idle steady at 1000.

so apparently, my car is just tired of being driven like a grandma car (i typically go for gas mileage while driving). i think maybe if i do another burn out, my busted headlight brackets will automatically readjust themselves

[900t]

don't do burnouts just drive at WOT on some backroads to 120 mph or whatever (okay you don't need to do that much unless you know the police are at the donut shop)

[SwimDude0614]

so our cars really do need to be thrown around from time to time? i never put less than 91 octane fuel in the car (93 when available) and i thought just so long as i kept the right fuel in there it would be fine.

[900t]

In my experience, no... that said I drive relatively hard. It's fun though

I don't know why you would bother with premium on a non-turbo model, put the 87 she won't mind :P

[SwimDude0614]

when i had the 2.1L, i had terrible knocking anytime i put less than 91 in. turbo has lower compression AND a wastegate - after reading through the bentley, i thought the turbo models were the ones that didn't need higher octane fuel?

[Max Blast]

Have you checked for a ruptured vacuum advance module on your distributor?