Cold idle is low, and rpm dips when returning to idle
03-23-2017, 02:43 AM
#1
RL Community Team
Rennlist Member
Rennlist Member
Thread Starter
Cold idle is low, and rpm dips when returning to idle
I'm helping a friend figure out this issue on his 968. It's a tiptronic.
I've driven the car for a few days now to get a feel for the problem. It seems to have 3 symptoms:
1. The idle is low when the engine is cold, and gradually rises as it warms up. When it's cold (ambient temp is about 60F), it idles at about 800. At normal operating temp, it's at about 950.
2. When you rev the engine, and the revs are coming back down towards idle, if the engine is cold the revs will dip below idle and then come back up to idle. So if you rev it high (like 3-4000) it will dip too low and stall, and be tough to start again (but it will start - opening the throttle helps). When the engine is warm, the revs drop down to idle with no dipping. And the transition from problem mode to good mode is abrupt; it does it until a certain temp, and then stops doing it.
3. Disconnecting the ISV will raise the idle by about 250, regardless of temperature. The higher idle makes for a livable workaround to this problem, because then cold idle sits at ~1100 and cold idle in Drive sits at ~850. But, the idle is too high when it's warm (about 1300 in Park, 1050 in Drive). Not to mention, that I've never heard of idle going UP with the ISV disconnected. I tried connecting the cable to my spare ISV while the engine was running, to see what the computer does with it in my hand. It looked like it was keeping it closed, and this was fully warmed up engine, idling at 1300. This seems to point to an ISV which is stuck open... but as soon as I plug the ISV electrical harness back on, the idle drops by 250.
We've already replaced the DME temp sensor and the O2 sensor.
I guess the cold-start features are not working? What exactly are they and how do I test them?
The next things I wanted to try are:
check O2 sensor voltage when cold vs warm
spark plugs (because oddly this seems to fix a lot of strange issues, and they're cheap)
I've driven the car for a few days now to get a feel for the problem. It seems to have 3 symptoms:
1. The idle is low when the engine is cold, and gradually rises as it warms up. When it's cold (ambient temp is about 60F), it idles at about 800. At normal operating temp, it's at about 950.
2. When you rev the engine, and the revs are coming back down towards idle, if the engine is cold the revs will dip below idle and then come back up to idle. So if you rev it high (like 3-4000) it will dip too low and stall, and be tough to start again (but it will start - opening the throttle helps). When the engine is warm, the revs drop down to idle with no dipping. And the transition from problem mode to good mode is abrupt; it does it until a certain temp, and then stops doing it.
3. Disconnecting the ISV will raise the idle by about 250, regardless of temperature. The higher idle makes for a livable workaround to this problem, because then cold idle sits at ~1100 and cold idle in Drive sits at ~850. But, the idle is too high when it's warm (about 1300 in Park, 1050 in Drive). Not to mention, that I've never heard of idle going UP with the ISV disconnected. I tried connecting the cable to my spare ISV while the engine was running, to see what the computer does with it in my hand. It looked like it was keeping it closed, and this was fully warmed up engine, idling at 1300. This seems to point to an ISV which is stuck open... but as soon as I plug the ISV electrical harness back on, the idle drops by 250.
We've already replaced the DME temp sensor and the O2 sensor.
I guess the cold-start features are not working? What exactly are they and how do I test them?
The next things I wanted to try are:
check O2 sensor voltage when cold vs warm
spark plugs (because oddly this seems to fix a lot of strange issues, and they're cheap)
03-28-2017, 05:29 AM
#5
RL Community Team
Rennlist Member
Rennlist Member
Thread Starter
Tried a MAF from a car that runs well - no change.
TPS checks out OK.
I checked the NTC II sensor at the DME plug to confirm that the wiring was intact. 2.75 kOhms when cold (about 20-21 C)
I found out that my friend had adjusted the throttle stop screw to increase the idle. (968s don't have adjustable idle, but that set screw can hold open the throttle plate.) This explained why disconnecting the ISV would raise the idle - the ISV was staying closed, in an attempt to bring the idle down to where the computer wants it.
So I reset the set screw so that the idle would be about 850rpm when cold and with the ISV disconnected and ISV hose pinched. So the ISV should be able to raise the idle over 850, but not go lower. With ISV reconnected and hose unpinched, the idle went up to almost 900, but I'd like to see it higher, especially when cold.
It really seems like the cold start enrichment isn't happening.
TPS checks out OK.
I checked the NTC II sensor at the DME plug to confirm that the wiring was intact. 2.75 kOhms when cold (about 20-21 C)
I found out that my friend had adjusted the throttle stop screw to increase the idle. (968s don't have adjustable idle, but that set screw can hold open the throttle plate.) This explained why disconnecting the ISV would raise the idle - the ISV was staying closed, in an attempt to bring the idle down to where the computer wants it.
So I reset the set screw so that the idle would be about 850rpm when cold and with the ISV disconnected and ISV hose pinched. So the ISV should be able to raise the idle over 850, but not go lower. With ISV reconnected and hose unpinched, the idle went up to almost 900, but I'd like to see it higher, especially when cold.
It really seems like the cold start enrichment isn't happening.
07-24-2020, 09:16 AM
#7
Tried a MAF from a car that runs well - no change.
TPS checks out OK.
I checked the NTC II sensor at the DME plug to confirm that the wiring was intact. 2.75 kOhms when cold (about 20-21 C)
I found out that my friend had adjusted the throttle stop screw to increase the idle. (968s don't have adjustable idle, but that set screw can hold open the throttle plate.) This explained why disconnecting the ISV would raise the idle - the ISV was staying closed, in an attempt to bring the idle down to where the computer wants it.
So I reset the set screw so that the idle would be about 850rpm when cold and with the ISV disconnected and ISV hose pinched. So the ISV should be able to raise the idle over 850, but not go lower. With ISV reconnected and hose unpinched, the idle went up to almost 900, but I'd like to see it higher, especially when cold.
It really seems like the cold start enrichment isn't happening.
TPS checks out OK.
I checked the NTC II sensor at the DME plug to confirm that the wiring was intact. 2.75 kOhms when cold (about 20-21 C)
I found out that my friend had adjusted the throttle stop screw to increase the idle. (968s don't have adjustable idle, but that set screw can hold open the throttle plate.) This explained why disconnecting the ISV would raise the idle - the ISV was staying closed, in an attempt to bring the idle down to where the computer wants it.
So I reset the set screw so that the idle would be about 850rpm when cold and with the ISV disconnected and ISV hose pinched. So the ISV should be able to raise the idle over 850, but not go lower. With ISV reconnected and hose unpinched, the idle went up to almost 900, but I'd like to see it higher, especially when cold.
It really seems like the cold start enrichment isn't happening.
If outside that range either the ISV is bad or the DME is bad.
