Strange idle behavior
06-16-17, 01:31 AM
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Strange idle behavior
I know there are a zillion idle-related threads, but most were either different than mine, or point to issues I don't believe make sense in my case.
In any case, here we go!
As a preface, this has occured in some capacity since I bought the car about a year ago. The symptoms come and go seemingly at random.
To describe the "worst" behavior...
Cold:
- High idle immediately after starting (revs up to >2k, back off to 1k, revs back up, then eases down to rest.
- cold Resting rpm: 1100
- Idle will slowly wander/bounce/oscillate +/-50
- Undershoot if put into neutral from gear (x000 rpm -> 500rpm -> ~1500rpm -> 1100rpm
Warm:
- If driven to warm, settles in at the expected 750rpm
- will occasionally "float" at 1500 for a second before dropping to idle
- Low idle after being shut off then re-started started warm (~500rpm)
- Long float at ~1700 for 5 sec if the AC is on, eventually settling to 1100rpm. Worth noting the AC doesn't actually work on this car, I mostly use it as a fan override to cool the car down before parking.
Work done in the car since (Mostly unrelated to the problem, but mentioning them since they remove some possibilities:
- New coolant temp sensor
- FC thermoswitch
- Engine rebuilt 800 miles ago. Possibly worth noting the symptoms were mostly gone after the rebuild, but crept back up since. However, given the "come and go" nature of this issue it's likely just coincidence.
- Vacuum lines replaced (During the engine rebuild)
Possibly relevant mods on the car:
- Downpipe (Unknown brand)
- RB dual tip catback
- Various cooling system upgrades
Idle vacuum is between .6 and .8 depending on the idle. I tried smoke testing it with carb cleaner before I had the lines replace, and found nothing.
So at this point I'm eyeballing the O2 sensor and TPS, but neither seem to really fit the symptoms from my understanding of their function.
So, thoughts? I might just replace the o2 sensor anyway since it's an unknown, but I'm hoping for more targeted/useful troubleshooting info than the old "Throw your wallet at it until it works" method.
In any case, here we go!
As a preface, this has occured in some capacity since I bought the car about a year ago. The symptoms come and go seemingly at random.
To describe the "worst" behavior...
Cold:
- High idle immediately after starting (revs up to >2k, back off to 1k, revs back up, then eases down to rest.
- cold Resting rpm: 1100
- Idle will slowly wander/bounce/oscillate +/-50
- Undershoot if put into neutral from gear (x000 rpm -> 500rpm -> ~1500rpm -> 1100rpm
Warm:
- If driven to warm, settles in at the expected 750rpm
- will occasionally "float" at 1500 for a second before dropping to idle
- Low idle after being shut off then re-started started warm (~500rpm)
- Long float at ~1700 for 5 sec if the AC is on, eventually settling to 1100rpm. Worth noting the AC doesn't actually work on this car, I mostly use it as a fan override to cool the car down before parking.
Work done in the car since (Mostly unrelated to the problem, but mentioning them since they remove some possibilities:
- New coolant temp sensor
- FC thermoswitch
- Engine rebuilt 800 miles ago. Possibly worth noting the symptoms were mostly gone after the rebuild, but crept back up since. However, given the "come and go" nature of this issue it's likely just coincidence.
- Vacuum lines replaced (During the engine rebuild)
Possibly relevant mods on the car:
- Downpipe (Unknown brand)
- RB dual tip catback
- Various cooling system upgrades
Idle vacuum is between .6 and .8 depending on the idle. I tried smoke testing it with carb cleaner before I had the lines replace, and found nothing.
So at this point I'm eyeballing the O2 sensor and TPS, but neither seem to really fit the symptoms from my understanding of their function.
So, thoughts? I might just replace the o2 sensor anyway since it's an unknown, but I'm hoping for more targeted/useful troubleshooting info than the old "Throw your wallet at it until it works" method.
06-16-17, 10:37 AM
#2
RX-7 Bad Ass
iTrader: (55)
The cold behavior makes me wonder if it could be the clutch switch. Bad clutch switch will cause a lot of idle weirdness. Search on that, most times it just has a bad spring and you can actually fix it from a spring from a cheap/free "click" ballpoint pen.
First step for me on idle troubleshooting is cleaning the ISC. They gum up and will do all sorts of weird stuff. Spray it out with carb cleaner, hook up jumper leads to the 2 electrical posts on it, hook those wires up to a battery and click it open and closed a few times. Then hit it with some WD-40 and click it a few times, that will lube things up.
Start there.
I'm assuming you have a stock ECU?
Dale
First step for me on idle troubleshooting is cleaning the ISC. They gum up and will do all sorts of weird stuff. Spray it out with carb cleaner, hook up jumper leads to the 2 electrical posts on it, hook those wires up to a battery and click it open and closed a few times. Then hit it with some WD-40 and click it a few times, that will lube things up.
Start there.
I'm assuming you have a stock ECU?
Dale
06-16-17, 12:59 PM
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The cold behavior makes me wonder if it could be the clutch switch. Bad clutch switch will cause a lot of idle weirdness. Search on that, most times it just has a bad spring and you can actually fix it from a spring from a cheap/free "click" ballpoint pen.
First step for me on idle troubleshooting is cleaning the ISC. They gum up and will do all sorts of weird stuff. Spray it out with carb cleaner, hook up jumper leads to the 2 electrical posts on it, hook those wires up to a battery and click it open and closed a few times. Then hit it with some WD-40 and click it a few times, that will lube things up.
Start there.
I'm assuming you have a stock ECU?
Dale
First step for me on idle troubleshooting is cleaning the ISC. They gum up and will do all sorts of weird stuff. Spray it out with carb cleaner, hook up jumper leads to the 2 electrical posts on it, hook those wires up to a battery and click it open and closed a few times. Then hit it with some WD-40 and click it a few times, that will lube things up.
Start there.
I'm assuming you have a stock ECU?
Dale
ECU is stock as far as I know, unless one of the previous owners modified the original in some non-obvious way. It doesn't have any stickers on it that would indicate modification.
AC situation is unknown. I live in WA so I don't really care, but the compressor clutch doesn't engage so I assume the system has just leaked.
Last edited by Vectortrex; 06-16-17 at 01:02 PM.
06-16-17, 09:45 PM
#5
RX-7 Bad Ass
iTrader: (55)
The clutch has 2 clutch switches, one for the cruise control and one for the ECU. I think the ECU compensates the ISC to change for the change in load when it's not spinning the guts of the transmission.
I've seen broken clutch switches do some weird things with idle.
The clutch switch that's easy to see is the one for the ECU, the one way in the back that looks like a black box with a white peg coming out of it is the cruise switch. Also, sometimes the rubber stopper that the clutch switch hits disintegrates and will cause the switch not to be tripped.
Dale
I've seen broken clutch switches do some weird things with idle.
The clutch switch that's easy to see is the one for the ECU, the one way in the back that looks like a black box with a white peg coming out of it is the cruise switch. Also, sometimes the rubber stopper that the clutch switch hits disintegrates and will cause the switch not to be tripped.
Dale