[fallenjedi]

Afternoon -

So my latest problem with my car:

When stopped, and with the car on, the idle will jump from ... say 1-1500, over and over and over and over. Drives fine, no other bad signs, just that. When giving it gas at stop I can hold the idle steady at about 1700ish?

Ideas?

Help.

[j0k3r]

Is it fuel-injected? If it is, check the IAC valve.

[hans5849]

Throttle, my friend had the same problem in his 89 prelude

[Justin3]

There might be crap clogging your throttle body.

[Dustin]

IACV
Coolant temperature sensor
TPS

To name a few. It's a fairly common problem on cars from that era. Each motor has a different culprit but it's usually one of those things

[guy]

TPS...

[fallenjedi]

Silly questions

IACV is?

TPS is?

Thanks

[j0k3r]

Originally Posted by fallenjedi

Silly questions

IACV is?

TPS is?

Thanks

Idle Air Control Valve
Throttle Position Sensor

If the idle constantly goes up and down, it's probably the TPS. If the idle gets really rough when you are stopping or slowing down, it's probably the IAC.

[Chris]

Don't forget to put those new cover sheets on those TPS reports.

[BrentN]

The ACV will NOT cause the throttle to jump up and down. The air controll valve is strictly to allow air into the intake, for a higher idle, due to loads such as the power steering putting strain on the engine.

The TPS could be at fault, but most likely, especially with a car that old, it is an intake leak. Unmetered air is entering the intake and causing the idle to fluctuate becuase the ECU is trying to compensate.

So much mis-information in this thread...

[Dustin]

Actually, if an IACV valve is forced stuck open (cold idle mode) and the ECU detects idle speed is too high, it will quickly cut ignition to try and drop the idle back down to normal. Since the valve is stuck open, still allowing air past the throttle body, the idle will raise up again. The ECU will again try to lower idle. Repeat, repeat, repeat.

I've experienced this several times on several different types of fuel injected cars. My MR2 had this exact problem, so I blocked the IACV valve off so it would always remain closed. Problem solved

Things aren't as cut-and-dry as you always try to make them seem Brent

[BrentN]

Originally Posted by Dustin

Actually, if an IACV valve is forced stuck open (cold idle mode) and the ECU detects idle speed is too high, it will quickly cut ignition to try and drop the idle back down to normal. Since the valve is stuck open, still allowing air past the throttle body, the idle will raise up again. The ECU will again try to lower idle. Repeat, repeat, repeat.

I've experienced this several times on several different types of fuel injected cars. My MR2 had this exact problem, so I blocked the IACV valve off so it would always remain closed. Problem solved

Things aren't as cut-and-dry as you always try to make them seem Brent

Dude, your wrong.
Im sorry to have to say that.

The ACV does controll idle quality, that we can both agree on. What it will NOT do, is pull UNMETERED air into the intake, wich you suggest. Your suggesting this becuase your associating the idle problem with this device. If the valve was faulty, or left open, it has the EXACT same effect as having the throttle open slightly, or a high idle, not a fluctuating idle.
I have never seen an ACV function so irradically, that it will open for a second, then close for a second, and repeat this process. If anything, the valve gets stuck do to the PCV system gunk clogging the valve.

Take a look at your MR2. You'll clearly see that the ACV valve is pulling air BETWEEN the Throttle body and AFM/MAF meter, not after.

If his problem was associated with the ACV he would either have a very low idle (powersteering/airconditioning putting too much strain on engine) or he would have a high idle (ACV stuck open).


I would suggest checking for a vaccuum leak first, adjust the TPS and Timing and your problem should go away if all are done correctly.

[Dustin]

I had the exact same problem as fallenjedi is having. EXACT. And I diagnosed the problem, and then fixed it. The problem was the air control valve being stuck open, so I plugged it to remain closed at all times. Problem solved, perfect idle.

On my room mate's 240sx, he had issues with high idle and jumping idle. He took out the primary IACV and blocked it. Problem solved, perfect idle now. No issues with power steering or air conditioning stalling the engine or dropping idle down below spec. But that vehicle has a secondary vacuum idle advance for accessories...

Either way, I don't really know what to say. It's great that you're giving fallenjedi another point of view, but I only give advice if I have some first-hand experience with the problem. Hopefully he finds out what the problem was and will let us know

[BrentN]

Well true dat

I dont want to fight over it, but I too have delt with that issue many many times and its always usually a vac leak of unmetered air.

Question though dustin. When you fixed those issues, were you able to get the idle to factory spec after?

[Dustin]

Yes, bang-on to factory spec with idle at factory spec and idle screw set to very bottom (factory setting)