Idle control and environmental temps

[spacecad3t]

One concern on the exhaust bleed path — with the throttle held open for access to the manifold side, any cylinder sitting with a valve open bleeds pressure straight through to the exhaust. Will I be able to maintain enough pressure with a hand pump to do a meaningful soap test despite that continuous bleed, or does the exhaust path make this impractical? Is there a way to work around it, or do I just pump continuously while working around the manifold with soapy water?

You’re thinking about it the right way. If the throttle is open, you can absolutely end up pressurizing the cylinders and then bleeding out through an open intake valve and out the exhaust. That makes the test noisy, hard to stabilize, and it can hide small leaks because you’re feeding a big intentional “leak” through the engine.

Workaround is simple: don’t do the test with the throttle open. You want the throttle closed for the pressure test. You don’t need manifold-side access via an open throttle plate; you can still soap-test everything externally with the throttle shut. If you need access around the throttle body area, remove the intake boot/MAF plumbing as needed, but keep the throttle plate closed.

If you must have the throttle open for some reason, you can still do it, but it becomes a “flow test” instead of a “hold pressure” test. In that case you’ll be supplying air continuously and listening/soaping for bubbles, but it’s less sensitive for tiny leaks because the engine becomes the dominant bleed path.

A couple practical tips to keep it meaningful on a 944 NA.

Pressurize from the front of the intake tract (MAF/J-boot side) with the throttle closed and the engine off. That keeps the cylinders mostly isolated. There will still be some leakage past rings/valves over time, but it’s usually slow enough at 2 to 5 psi that you can build pressure and then hunt.

If you’re still seeing rapid bleed-down with the throttle closed, that’s actually useful information. It usually means you’ve got a big leak somewhere in the intake plumbing, a cap/port left open, or you’re venting through your crankcase vent-to-atmosphere path. Temporarily cap the crankcase vent for the test so you’re not just pressurizing the crankcase and dumping to atmosphere.

If you want a more “set and forget” air source than a hand pump, a small regulated shop air feed works better. Set the regulator to 2 to 3 psi and you can keep a steady low pressure while you soap around. With a hand pump you can still do it, you’ll just top it up every so often. If the throttle is closed, you shouldn’t need to pump continuously.

If you report back whether it holds any pressure at all with the throttle closed and the crankcase vent capped, that’ll tell us immediately whether you’re dealing with a real leak big enough to matter at idle.

Last question for now. Could a slipping or binding alternator cause this? Im seeing 14v on the gauge, but I was having some squealing and voltage drops over the winter, it's on my list of things to address, but this might motivate me to bump its priority.

[CarBot]

Last question for now. Could a slipping or binding alternator cause this? Im seeing 14v on the gauge, but I was having some squealing and voltage drops over the winter, it's on my list of things to address, but this might motivate me to bump its priority.

A slipping or binding alternator won’t usually cause a steady warm idle that’s consistently 150–200 rpm high with the bypass screw shut. That symptom still points much more toward an air path/control issue than charging.

Charging problems can affect idle quality, though. If system voltage is low or unstable at idle, the DME and ICV can behave a bit erratically, and the ICV may not respond as crisply. That tends to show up as hunting, dipping, occasional near-stall, or weird behavior when you turn on loads like headlights, rear defrost, or the blower, not a stable high idle.

Since you’ve had belt squeal and winter voltage drops, it’s worth moving up the alternator/belt work anyway. I’d check belt condition and tension, alternator pulley and bearings, and confirm real voltage at the battery with a meter both at hot idle and with electrical loads on. If you see it dropping into the low 12s at idle with loads, or bouncing around, fix that first because it can muddy every other diagnosis.

But if you’re seeing a solid 13.8–14.2 volts at the battery at hot idle and it stays there with loads, I wouldn’t expect the alternator to be the root cause of your high idle.

[spacecad3t]

Last question for now. Could a slipping or binding alternator cause this? Im seeing 14v on the gauge, but I was having some squealing and voltage drops over the winter, it's on my list of things to address, but this might motivate me to bump its priority.

A slipping or binding alternator won’t usually cause a steady warm idle that’s consistently 150–200 rpm high with the bypass screw shut. That symptom still points much more toward an air path/control issue than charging.

Charging problems can affect idle quality, though. If system voltage is low or unstable at idle, the DME and ICV can behave a bit erratically, and the ICV may not respond as crisply. That tends to show up as hunting, dipping, occasional near-stall, or weird behavior when you turn on loads like headlights, rear defrost, or the blower, not a stable high idle.

Since you’ve had belt squeal and winter voltage drops, it’s worth moving up the alternator/belt work anyway. I’d check belt condition and tension, alternator pulley and bearings, and confirm real voltage at the battery with a meter both at hot idle and with electrical loads on. If you see it dropping into the low 12s at idle with loads, or bouncing around, fix that first because it can muddy every other diagnosis.

But if you’re seeing a solid 13.8–14.2 volts at the battery at hot idle and it stays there with loads, I wouldn’t expect the alternator to be the root cause of your high idle.

No Im sorry, the original issue I mentioned, "Warm 70° day → engine fully warm → DME commanding ICV relatively closed to maintain idle → engine shut off → ICV stays in that relatively closed position → next morning at 30°, engine cranks with ICV already partially closed → not enough air for a cold start at that temperature → rich bog at 650-700 actual while the DME figures out it needs to open the ICV more → recovers within a minute as the DME responds to the cold temp sensor reading and opens the ICV to where it should be for that temperature."

[CarBot]

No Im sorry, the original issue I mentioned, "Warm 70° day → engine fully warm → DME commanding ICV relatively closed to maintain idle → engine shut off → ICV stays in that relatively closed position → next morning at 30°, engine cranks with ICV already partially closed → not enough air for a cold start at that temperature → rich bog at 650-700 actual while the DME figures out it needs to open the ICV more → recovers within a minute as the DME responds to the cold temp sensor reading and opens the ICV to where it should be for that temperature."

Got it. For the cold-start bog/low idle that clears within about a minute, an alternator/belt issue is still unlikely to be the direct cause, because the alternator isn’t really doing much during cranking and the DME/ICV are mostly living off battery voltage until the engine is running.

Where charging can matter is the first 30 to 60 seconds after start if voltage is low or unstable at idle. The ICV is a PWM-driven valve and it can be a bit lazier if system voltage is sagging, and the DME can be more sensitive to marginal voltage right after start. That would usually show up as worse behavior when you add electrical load right after a cold start, like headlights or blower, and it may improve as RPM comes up and the alternator starts charging harder.

If you want a quick way to confirm or rule it out, put a meter on the battery and watch minimum voltage during crank and then voltage for the first minute of cold idle. If it’s dropping very low during crank or sitting in the low 12s right after start, fix the belt/alternator/battery connections first. If it’s cranking at a healthy voltage and then quickly stabilizing around the high 13s/low 14s, I’d keep looking elsewhere.

Given your earlier description, I’d still suspect something like a slightly sticky ICV (even new ones can be finicky), a small air leak that changes with temperature, or a DME temp signal/connector issue rather than the alternator. If you can share what the cold idle RPM does if you lightly crack the throttle for the first 10 seconds, that’s a good clue whether it’s primarily “not enough air” versus “too much fuel/weak spark.”

[spacecad3t]

No Im sorry, the original issue I mentioned, "Warm 70° day → engine fully warm → DME commanding ICV relatively closed to maintain idle → engine shut off → ICV stays in that relatively closed position → next morning at 30°, engine cranks with ICV already partially closed → not enough air for a cold start at that temperature → rich bog at 650-700 actual while the DME figures out it needs to open the ICV more → recovers within a minute as the DME responds to the cold temp sensor reading and opens the ICV to where it should be for that temperature."

Got it. For the cold-start bog/low idle that clears within about a minute, an alternator/belt issue is still unlikely to be the direct cause, because the alternator isn’t really doing much during cranking and the DME/ICV are mostly living off battery voltage until the engine is running.

Where charging can matter is the first 30 to 60 seconds after start if voltage is low or unstable at idle. The ICV is a PWM-driven valve and it can be a bit lazier if system voltage is sagging, and the DME can be more sensitive to marginal voltage right after start. That would usually show up as worse behavior when you add electrical load right after a cold start, like headlights or blower, and it may improve as RPM comes up and the alternator starts charging harder.

If you want a quick way to confirm or rule it out, put a meter on the battery and watch minimum voltage during crank and then voltage for the first minute of cold idle. If it’s dropping very low during crank or sitting in the low 12s right after start, fix the belt/alternator/battery connections first. If it’s cranking at a healthy voltage and then quickly stabilizing around the high 13s/low 14s, I’d keep looking elsewhere.

Given your earlier description, I’d still suspect something like a slightly sticky ICV (even new ones can be finicky), a small air leak that changes with temperature, or a DME temp signal/connector issue rather than the alternator. If you can share what the cold idle RPM does if you lightly crack the throttle for the first 10 seconds, that’s a good clue whether it’s primarily “not enough air” versus “too much fuel/weak spark.”

Normally the cold idle RPM is fine and the car is responsive to slight throttle input. However when this event happened, it would big down, but slowly climb RPM, but not stall. Seems like a too rich condition to me, not a too much air condition? I still mix things up tho

[CarBot]

Normally the cold idle RPM is fine and the car is responsive to slight throttle input. However when this event happened, it would big down, but slowly climb RPM, but not stall. Seems like a too rich condition to me, not a too much air condition? I still mix things up tho

If it’s normally fine and only occasionally does the “rich bog but won’t stall, then slowly cleans up,” that does sound more like transient over-fueling or weak ignition during that first minute, not an air shortage. If it were simply not enough bypass air, a small crack of throttle would usually snap it clean immediately. A rich condition often feels like it’s loaded up and takes a few seconds to clear even if you add a little air.

On the 944 NA, the most common “intermittent rich for 30–60 seconds” causes I’d look at next are the DME coolant temp signal intermittently reading colder than reality (even with a new sensor, the connector, pins, or wiring can be the issue), fuel pressure being too high right after start (regulator/damper behavior, or a return restriction), or an injector dribbling after shutdown that makes the next start temporarily rich. Since it’s intermittent, I’d also keep “DME relay/voltage during crank” in the back of your mind, but that more often causes no-start or sputter than a consistent rich bog.

A practical way to separate “rich” from “weak spark” is to watch what happens if you hold the throttle slightly open at a steady small angle for 10–15 seconds when it’s acting up. If it still feels soggy and won’t clean up until time passes, that points to mixture/ignition rather than idle control. If it immediately smooths out and revs cleanly, it was more likely idle air control.

If you can, the next time it happens, grab two quick data points: battery voltage at the DME (or at least at the battery) during that first minute, and the resistance/reading of the blue temp sensor circuit at the DME plug when cold. The sensor itself can be new and still have a flaky connection that only shows up at certain temps.

Also, since you mentioned the previous owner leaned the AFM to mask leaks, if your current setup is on the rich side at cold start, it may just be closer to “correct” now and occasionally tips over the edge when something else is slightly off (fuel pressure, injector drip, temp signal). If you have access to a fuel pressure gauge, catching cold start pressure when it’s misbehaving is very telling.

[spacecad3t]

Normally the cold idle RPM is fine and the car is responsive to slight throttle input. However when this event happened, it would big down, but slowly climb RPM, but not stall. Seems like a too rich condition to me, not a too much air condition? I still mix things up tho

If it’s normally fine and only occasionally does the “rich bog but won’t stall, then slowly cleans up,” that does sound more like transient over-fueling or weak ignition during that first minute, not an air shortage. If it were simply not enough bypass air, a small crack of throttle would usually snap it clean immediately. A rich condition often feels like it’s loaded up and takes a few seconds to clear even if you add a little air.

On the 944 NA, the most common “intermittent rich for 30–60 seconds” causes I’d look at next are the DME coolant temp signal intermittently reading colder than reality (even with a new sensor, the connector, pins, or wiring can be the issue), fuel pressure being too high right after start (regulator/damper behavior, or a return restriction), or an injector dribbling after shutdown that makes the next start temporarily rich. Since it’s intermittent, I’d also keep “DME relay/voltage during crank” in the back of your mind, but that more often causes no-start or sputter than a consistent rich bog.

A practical way to separate “rich” from “weak spark” is to watch what happens if you hold the throttle slightly open at a steady small angle for 10–15 seconds when it’s acting up. If it still feels soggy and won’t clean up until time passes, that points to mixture/ignition rather than idle control. If it immediately smooths out and revs cleanly, it was more likely idle air control.

If you can, the next time it happens, grab two quick data points: battery voltage at the DME (or at least at the battery) during that first minute, and the resistance/reading of the blue temp sensor circuit at the DME plug when cold. The sensor itself can be new and still have a flaky connection that only shows up at certain temps.

Also, since you mentioned the previous owner leaned the AFM to mask leaks, if your current setup is on the rich side at cold start, it may just be closer to “correct” now and occasionally tips over the edge when something else is slightly off (fuel pressure, injector drip, temp signal). If you have access to a fuel pressure gauge, catching cold start pressure when it’s misbehaving is very telling.

So back to my original question, it seems you may agree that it could have been cold soaked coolant temp or coolant temp higher than ambient? Or maybe even failing fuel pressure regulator, etc?

[CarBot]

So back to my original question, it seems you may agree that it could have been cold soaked coolant temp or coolant temp higher than ambient? Or maybe even failing fuel pressure regulator, etc?

Yes, I’d put “coolant temp signal not matching reality at that moment” and “fuel pressure behavior on that particular start” ahead of the ICV-stuck-closed theory.

A cold-soaked engine should have the DME coolant temp reading very close to ambient. If, on that one morning, the DME saw an artificially cold value because of a marginal connection or wiring issue at the blue sensor circuit, it would over-enrich and you’d get exactly the rich, lazy, low-ish idle that slowly cleans up as the engine actually warms and the signal stabilizes.

The opposite case, coolant temp higher than ambient, usually makes it leaner and can cause a stumble or flare, but it’s less consistent with a “rich bog” feel unless something else is going on.

Fuel pressure is the other big one. A regulator/damper issue, a restricted return, or an injector that dribbles after shutdown can make the next start temporarily rich. That can be intermittent, which matches your “normally fine, occasional event” description.

If you want to confirm direction without guessing, the two most decisive checks are reading the DME coolant temp value when stone cold (at the DME plug, not just at the sensor) and putting a gauge on the rail to watch cold start pressure and residual pressure after shutdown. If both look normal when the symptom happens, then we pivot back to ignition strength during that first minute.