Syonyk

I'm trying to troubleshoot one more little issue with my S4 NA. Every now and then, at a stop light, the idle will settle at 1000 RPM or so. Most of the time it settles down to 750 RPM (nice and solid... +10% at idle on the SAFC makes a nice difference).

The strange thing is that if I drag the engine RPM down slightly by slipping the clutch with the brakes on, it settles down to a solid 750 RPM. I don't have to drag it all the way down - it seems like getting it much below 1000 RPM is all that's needed for the engine to remember that it's supposed to be idling lower.

I've adjusted my TPS repeatedly, and decel fuel cut works fine, and the fact that it idles normally most of the time seems to indicate that it's working properly. I'm also not worried about it being my thermowax - if that were acting up, I'd never see a nice low idle.

Also, when the AC is on (more load on the engine), it never has this problem - it always drops down to the proper (though sometimes slightly higher due to the AC) idle.

Is there anything specific I should be looking at? I haven't cleaned my BAC, so I was going to clean that out this weekend (how does one do that, anyway?) and see if it helps. The erratic high idle isn't a huge issue, but it would be sweet to have it always drop down to a solid 750 RPM.

-=Russ=-

B-ROB

This might not be the case for you, but when my OMP started to go bad...oil pressure was dropping too low, so I think the ECU went into a limp/semi-limp mode to keep the rpms higher and keep oil pressure up. As soon as the OMP was replaced the high idle (1000rpm) went away.

Syonyk

I have a S4, so it's a mechanical MOP. However, I blocked that off and am running pure premix.

That's an interesting theory about the ECU detecting a low oil pressure & raising the idle to compensate... I'm not sure the ECU is that smart, honestly. Plus, I don't have anything that strikes me as very low oil pressure - 20-30 lbs at idle. I'll keep an eye on it though, and see if I notice anything.

Was it a constant high idle, or just erratic?

-=Russ=-

Wanked_FC

iTrader: (1)

far as I know the oil pressure on the s4 is entirely mechanical, so i doubt the ecu is doing anything to the idle for a low oil pressure. oil pressure should be around 30psi at idle like yours.

Turbonut

iTrader: (1)

Check the BAC valve for internal leaking past the seal. Make certain TPS is set correctly. My S5 TII would do this. Idle @ 1000/1100 and slightly rough every once in a while. Let the clutch out slighly to drag the engine, and idle would come down. The BAC was functioning properly electrically, but if you blew into the air tube, the air would bypass the inner seal. They can be cleaned, but have found that it is only short lived. No air should bypass that seal. If it's a A Spec BAC cover the small air passage on the intake side of the valve before blowing air into the large tube. If J Spec no need to worry about that, as no idle adj. available.

ilike2eatricers

Wow I've never heard of this before. I'm also surprised that our ECU would know to do this. Anybody confirm or deny the above?

HAILERS

Dittos on the bac. I did have that happen to me ONCE. I removed the bac and cleaned it with carb cleaner. Just pour it in and let it sit for a couple of hours or over night and flush it out with carb cleaner again.

An option is to remove the bac. Leave the electrical connector on it. Invert it so the large hole with diaphram is straight up. Then turn the key to ON. The bac should vibrate. Spray the carb cleaner in it and dump it several times. Then reinstall the bac and see if that corrects the problem.

ilike2eatricers

I have the same problem with the throttle slightly sitting at 1000rpm when coming to the stop sometimes. After about 5 seconds normally it will drop back down. I have the BAC still and tapped a 1/8" NPT for the BAC air feed from my greddy IC pipe. The air feeding it is only the size of a regular nipple. You think the BAC isnt getting enough air through that small fitting?

hugues

to be sure where it's coming from (the 1000 rpms instead of 750), I would check the voltage at the ECU pin for the BAC (or at the BAC itself) and confirm whether or not the idle up is coming from the ECU ... if the voltage at the pin is lower when you idle at 1,000 rpms than when you idle at 750 rpms, the ECU is forcing the idel up for some reason .. if it's coming from the ECU, not much you can do ... if it's not due to ECU signal bac, then follow the other's advice in particular the one about leaking bac.

The dashpot could be sticking also and keep the idle high sometime ... something to consider. You can easily tell if it's a mechanical problem at the throttle plates level by cheking if the plates are all the way down, or by measuring the TPS voltage (Asuming you know the voltage when the plates are completely closed, should be 1.0 V, if properly set).

I have noticed this myself:

the intake air temp switch affects the idle speed ... if nice and warm, it's 750 ... if it's a tad colder, the idle is higher at about 1,000 rpms. that's some time when you come to a stop after a good run on the highway, the idle might be higher than 750 ... I am talking when the outside temp is not super hot.

not saying that's your problem but that's something to be aware of.

Hugues-

Therotaryrocket85

MIne does this too however im a noobie to the second gens so can someone help point me in the direction of the BAC so i can take it off and clean it. Thanks a bunch!