Erratic Idle after Smog Test??
05-10-2005, 11:01 PM
#1
Racer
Thread Starter
Erratic Idle after Smog Test??
My 944 has always had a steady idle (assuming A/C is off), until I took it in for a smog check a couple days ago. Ever since the smog jockeys put it on the rollers and "played" with it, revving it up and running through the gears, the idle has been very erratic, RPMs jump from 400 - 1000. Checked under the hood, no signs of vacuum leak or anything else. Is there anything else that could have been screwed up by them "driving" it at higher RPMs than it is used to, such as the O2 sensor getting out of synch? Will the idle eventually go back to the way it was?
05-10-2005, 11:54 PM
#2
Hey Man
Rennlist Member
Rennlist Member
Did the smog monkeys 'play' with your TPS during their testing? You might want to check that, also maybe they thought the bypass screw on the throttle body was the idle adjustment and tweaked it.
05-11-2005, 01:50 AM
#3
Three Wheelin'
You need to contact the inspection station's governing body (In NC this would be the NC State Police, so in your case it may be the AZ Highway Patrol), and find out EXACTLY what the inspection requirements are.
Typically a safety/emissions inspection is just that, an inspection, and you either pass or fail. By "playing" with your car the inspector(s) may be in violation of state law.
As for your problem, I suspect someone may have indeed "tweaked" the by-pass on the Throttle body, which by the way just happens to be the idle speed adjustment. Another possibility is that someone "tweaked" the TPS, trying to get the engine to idle at a higher RPM.
What may be happening is the DME "thinks" the engine is in a "Trailing Throttle", or "Over Run" condition. This happens when the engine RPM is above a normal idle when the throttle plate is closed (the TPS sends a "Throttle Closed" signal to the DME), just like when you lift off of the throttle or engine brake.
In this condition the DME will cut off the injectors to save fuel, and the RPMs drop. As the RPMs approach a normal idle, the DME turns the injectors on, the RPMs hit the high set point, and the process repeats.
The easyiest way to prove this is to unplug the TPS and then start the engine. If the "Hunting Idle" goes away, the idle speed is set too high so either the by-pass is open too far, or the TPS is holding the throttle open more than it should be. BTW: The idle spec for an early car is 850 +- 50 RPM.
If you can prove the inspector caused the problem, and by doing so he/she did in fact violate state law, then lower the boom, drop a dime, squeal, rat them out, call the cops, and then find another inspection station.
Personally, I think safety and emissions inspections are a good thing, mainly because I don't like sharing the road with rust buckets and rattle traps that spew clouds of noxious smoke while they run down kids in a school crossing because the windshield was too cracked/pitted to see through, even if both headlights were working and aimed properly, and they couldn't stop anyway because even though the brakes were "just a little weak", bald tires have no traction "in the wet."
What really gals me is when an "inspector" makes things worse and the "inspectee" has to fork over the green to pay for a "repair".
Sorry, end of rant....
Fianl BTW: I'm an AZ native, born in Williams, lived in Winona, Seligman, Phoenix, and Flagstaff where my mother, brother and younger sister live.
Typically a safety/emissions inspection is just that, an inspection, and you either pass or fail. By "playing" with your car the inspector(s) may be in violation of state law.
As for your problem, I suspect someone may have indeed "tweaked" the by-pass on the Throttle body, which by the way just happens to be the idle speed adjustment. Another possibility is that someone "tweaked" the TPS, trying to get the engine to idle at a higher RPM.
What may be happening is the DME "thinks" the engine is in a "Trailing Throttle", or "Over Run" condition. This happens when the engine RPM is above a normal idle when the throttle plate is closed (the TPS sends a "Throttle Closed" signal to the DME), just like when you lift off of the throttle or engine brake.
In this condition the DME will cut off the injectors to save fuel, and the RPMs drop. As the RPMs approach a normal idle, the DME turns the injectors on, the RPMs hit the high set point, and the process repeats.
The easyiest way to prove this is to unplug the TPS and then start the engine. If the "Hunting Idle" goes away, the idle speed is set too high so either the by-pass is open too far, or the TPS is holding the throttle open more than it should be. BTW: The idle spec for an early car is 850 +- 50 RPM.
If you can prove the inspector caused the problem, and by doing so he/she did in fact violate state law, then lower the boom, drop a dime, squeal, rat them out, call the cops, and then find another inspection station.
Personally, I think safety and emissions inspections are a good thing, mainly because I don't like sharing the road with rust buckets and rattle traps that spew clouds of noxious smoke while they run down kids in a school crossing because the windshield was too cracked/pitted to see through, even if both headlights were working and aimed properly, and they couldn't stop anyway because even though the brakes were "just a little weak", bald tires have no traction "in the wet."
What really gals me is when an "inspector" makes things worse and the "inspectee" has to fork over the green to pay for a "repair".
Sorry, end of rant....
Fianl BTW: I'm an AZ native, born in Williams, lived in Winona, Seligman, Phoenix, and Flagstaff where my mother, brother and younger sister live.
05-11-2005, 11:07 AM
#4
Racer
Thread Starter
I'm not a mechanic, but I'll pull out the Haynes manual this weekend and see if I can figure out if the by-pass needs adjusting. I don't think the smog guys did anything under the hook other than gawk and point, but maybe they did. My primary concern is that they appeared to "over rev" the engine when they had it on the rollers - I'm sure they had the RPMs pretty high. The good news is that the 944 passed the tests with flying colors, with only 25% - 50% of the allowable amounts in all emissions categories, which I thought was pretty good for a 21 year old car.
