Synopsis
Once in a while the car will idle very, very low but drives fine with good power and behavior as a normal FD. At cold startup, it will idle perfectly with 18+ inches of vacuum. After some driving, it will idle at < 500 RPM with almost no vacuum. And if the car is warmed up and shuts down, the next hot startup will result in a perfectly running idle pulling correct vacuum. This happens every other time the car is driven or so...


The Engine
Street port 15k miles
Twin Power
New Temp 9 Plugs all around.
Fuel filter changed last June.
No ISC (been running without it for 3 years)


Observations

1. When it is idling poorly, holding the throttle open to achieve the correct idling RPM does not result in the correct vacuum, which is 16 inches, but instead gets 8-9 inches.
   
2. Even when idle is below 500 RPM, the engine does not struggle, knock, jerk, or behave like a single rotor. Neither vacuum nor idle are bouncing when this is happening. The engine just seems to become quieter and smoother. At one point, idle vacuum was almost 0 but the engine kept chugging along very smoothly.

My Guesses (in order)

1. Ignition or timing
2. Fuel Injectors
3. TPS
4. Some heat related sensor on the fritz?
5. Grounding

So does anyone have any really profound guesses that I have not covered or have even better suggestions? I want to fix it but my main goal is to know why it is intermittent. Thanks!

 

possibly a boost leak. if its small, it might not always change your idol at startup. could be a vac hose, y-pipe, or a gasket, check them.

if you can comfirm its not a b-leak i would say just play with the tps bleed screw and see if that solves the problem. i had a similar problem just recently..it would idol at startup at around 800rpms then drop to about 500 at stoplights.

also how full is your gas tank when this occurs?

 

So you had a boost leak and it caused this? Where was the leak?

I wonder why it doesn't occur after hot startup but still occurs from a cold startup- even once the engine is fully warmed up.

 

Yeah, I don't see this as viable advice, since we have a MAP system, a "boost leak" will not effect idle. For one, he is not even getting out of vacuum and into boost when this happens.

Though, I digress. Before you go around and look at some more difficult stuff, check simple stuff.

The clutch switch (a little rectangular and silver metal box located near the clutch pedal) is a big culprit in situations like these. An easy way to check it is to press the clutch pedal, then press the clutch switch with your finger to see if the plunger goes in an out smoothly. It's very common in these cars for a bad clutch switch to cause erratic idling issues. The clutch switch is very easy to rebuild yourself with a spring from a pen, a thin flathead screw driver, and a set of vice grips.

This is what worries me. Either you may be pressing the throttle more at the relative idle RPM, causing the vacuum to be lower than you expect it to be. On the other hand, it could be something more complex as you mentioned in your other list of things.

But before you begin tearing into things, some easy things to check are your TPS voltages, clutch switch, etc. (basically anything that is a major player in the car's idling behavior). Good luck!

 

that is not true. if you have a leak idol will be abnormal and can serge back and forth when the car is stationary

 

I'm sorry, but this is untrue. Unhook your intake elbow that goes from your SMIC to your plastic elbow. This would essentially simulate a "huge boost leak". At idle, your throttle plates are closed and small amounts of air are being metered in, so I don't see your logic.

 

Thats true.

I had my intake elbow off one time to adjust my bleed screw on the TB and the idle was fine (well aside from the change i made to the screw lol).

 

I will look into the clutch switch. It wouldn't be the first time coincidence produced false consistency (I'm referring to the idle always returning to normal after a hot startup).

Now will a faulty clutch switch cause the idle to behave like this in the presence of compensated throttling and a deleted ISC? What does the ECU do with the signal?

 

I'm only posting this to provide "an answer" to whomever may search through YABIT (yet another bad idle thread) in the future. A thread is pointless without some form of conclusion after all....

It was either a loose or dying plug wire. I replaced the wires and put some dielectric at the contacts and she's all good again. The glowing downpipe gave it away....

 

I'm not en expert but unhooking elbow will not do anything because its before the pressure sensor. leting the air into the engine bypassing this sensor may cause something but again, this is my guess.

I have the same problem sometimes my car has bad idle, sometimes its too high. i checked FSM about TPS but it looked like you need some sort of special device to test it dont you? if its possible withought that device, just with voltmeter, please tell me how.

 

Yes a voltmeter is fine.

http://www.rx7.org/Robinette/tps_adj.htm