Idle Dip
05-16-2005, 09:52 PM
#1
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Idle Dip
Hello, I'm new to the board and a new Porsche owner, thanks in advance for any advice.
My questions is about the idle. It idles great generaly around 1000 AC or or off. Only issue is the idle drops to 500-600 when shifting really slowly (like in traffic) or going into neutral as if coming up on a light. It also drops if I blip the accelerator, but then returns to 1000.
What should I check?
My questions is about the idle. It idles great generaly around 1000 AC or or off. Only issue is the idle drops to 500-600 when shifting really slowly (like in traffic) or going into neutral as if coming up on a light. It also drops if I blip the accelerator, but then returns to 1000.
What should I check?
05-16-2005, 10:14 PM
#2
Burning Brakes
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You need to let us know what kind of 944 you have.
ISV- Idle stabilizer valve located under the intake manifold
Vacuum leaks- replace old plastic tubing and rubber connectors
ISV- Idle stabilizer valve located under the intake manifold
Vacuum leaks- replace old plastic tubing and rubber connectors
05-17-2005, 10:04 AM
#3
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84' 944 2.5L Its not a turbo or anything like that.
When I add to the collection the next one will be a turbo. O wait, I guess it takes more then one to have a collection. Anyway.....
When I add to the collection the next one will be a turbo. O wait, I guess it takes more then one to have a collection. Anyway.....
05-17-2005, 02:22 PM
#4
Three Wheelin'
This behavior is inherent with the early cars.
The issue revolves around the age of the engine. Basically what is happening is the engine is slowing down faster than the DME can react.
When you let off the throttle and the throttle plate closes, the Throttle Position Switch (TPS) sends a "Throttle Closed" signal to the DME. If the engine RPM is above about 1200 RPM, the DME turns the fuel injectors off. This is done to save fuel, and an engine in this condition (throttle closed, RPM high) is said to be in a "Trailing Throttle" or "Over Run" condition.
When the engine RPM approaches the spec (850 +- 50 RPM for an early engine like yours.), the DME turns the injectors on so the engine can idle.
If the RPM drop is too quick the DME falls behind and the engine "shudders" as it returns to idle.
To verify this, just unplug the TPS (Square black box under the Throttle Body).
I also recommend removing the Throttle Body so you can check/adjust the mechanical stop and the TPS.
With the TB out, and the TPS screws loose, loosen the 7mm jam nut on the mechanical stop screw. Back the stop screw out until it does not touch the stop arm. Verify that the throttle plate is fully closed. Turn the stop screw IN until it just touches the stop arm. Turn the stop screw IN an additional 1/2 turn, and then tighten the 7mm jam nut. Verify that the throttle plate returns to the fully closed position.
Now set the TPS so that it makes a soft click just as the throttle plate comes off of its seat and tighten the screws. Verify that the TPS clicks when the throttle is opened and closed.
Install the TB, connect a Dwell/TAch to the ignition coil (Green is Positive, Black is ground) and start the eingine. Use the idle by-pass on the TB to set the idle at 850 +- 50 RPM.
NOTE1: If the TPS or the mechanical stop were holding the throttle plate open (I've seen this more than once!), the engine may not start or it may start and then stall. To "fix" this you need to open the idle by-pass a turn or two.
Note2: Pay close attention to the vacuum lines on the throttle body. One goes to the evaporative emissions control valves, and the other goes to the fuel pressure regulator and the fuel pressure damper. If you get these lines reversed the evaporative emissions system will be in a constant cannister purge mode, and your fuel pressure will be maxed out.
The issue revolves around the age of the engine. Basically what is happening is the engine is slowing down faster than the DME can react.
When you let off the throttle and the throttle plate closes, the Throttle Position Switch (TPS) sends a "Throttle Closed" signal to the DME. If the engine RPM is above about 1200 RPM, the DME turns the fuel injectors off. This is done to save fuel, and an engine in this condition (throttle closed, RPM high) is said to be in a "Trailing Throttle" or "Over Run" condition.
When the engine RPM approaches the spec (850 +- 50 RPM for an early engine like yours.), the DME turns the injectors on so the engine can idle.
If the RPM drop is too quick the DME falls behind and the engine "shudders" as it returns to idle.
To verify this, just unplug the TPS (Square black box under the Throttle Body).
I also recommend removing the Throttle Body so you can check/adjust the mechanical stop and the TPS.
With the TB out, and the TPS screws loose, loosen the 7mm jam nut on the mechanical stop screw. Back the stop screw out until it does not touch the stop arm. Verify that the throttle plate is fully closed. Turn the stop screw IN until it just touches the stop arm. Turn the stop screw IN an additional 1/2 turn, and then tighten the 7mm jam nut. Verify that the throttle plate returns to the fully closed position.
Now set the TPS so that it makes a soft click just as the throttle plate comes off of its seat and tighten the screws. Verify that the TPS clicks when the throttle is opened and closed.
Install the TB, connect a Dwell/TAch to the ignition coil (Green is Positive, Black is ground) and start the eingine. Use the idle by-pass on the TB to set the idle at 850 +- 50 RPM.
NOTE1: If the TPS or the mechanical stop were holding the throttle plate open (I've seen this more than once!), the engine may not start or it may start and then stall. To "fix" this you need to open the idle by-pass a turn or two.
Note2: Pay close attention to the vacuum lines on the throttle body. One goes to the evaporative emissions control valves, and the other goes to the fuel pressure regulator and the fuel pressure damper. If you get these lines reversed the evaporative emissions system will be in a constant cannister purge mode, and your fuel pressure will be maxed out.
