I have a JDM TII engine. I am having an issue with the RPM's bouncing with the gas pedal being held still. The motor idles fine at about 650-700rpm. It will allow you to rev and hold up to about 1700rpms without any issues. Once I start to rev above this range, my RPM starts to bounce by about 100rpms. I can rev it all the way to 4000rpm, but if I hold the pedal still, the RPM bounces between 3900-4000rpm. It is almost like the ECU is cutting fuel off for just a second. The car has a S4 TII AFM, ECU, a 255lph fuel pump, open downpipe, fuel cut defencer, and emissions removal. I have two TII ECUs and it does the same with each. The gas in the car has been there since December with about two gallons of high test I added last week. Any help is greatly appreciated.

 

Usually bouncing RPM is because of a vacuum leak, but since it's starting higher, I would doubt that was the issue, but still might be worth checking. Have you checked the secondary throttle plates to make sure they're set correctly?

 

I have had multiple 7's have the bouncing idle problem and they are all usually tied to the same problems.

Misadjusted TPS, BAC or vaccum leak.

Those have always fixed my bad idle problems.

Mike

 

check your tps settings. there is a great write up here: www.fc3spro.com

 

I have checked for vacuum leaks, but haven't found any. I removed and blocked off the BAC. The car idles fine, the problem only happens above 1700 RPM. How do I check the secondary throttle plates.

 

I know I saw a great write-up on it, either here or in the service manual that's linked in the 2nd Gen FAQs at the top. You take the throttle body off and check how far one opens with a feeler gauge before the other starts to open. You adjust them using the small set screw just to the front side of the throttle body (at least in my NA) but I wouldn't mess with them without the procedure. You won't hurt anything permanently, but it's usually easier to mess it up than improve it. Basically that little screw adjusts how far the secondary throttle plates are open at rest. Make sure you take the nut off of it before trying to turn it. I broke that screw off once and that's no fun.

I agree with Rx7onfire though - if you haven't checked your tps sensor yet, I'd check that first, even if it's just to confirm it really is 1 ohm at close with no flat spots.

 

PhantomHP, how did you troubleshoot the bad BAC, or have you ever had any luck cleaning it?

 

My list of general idle issue checks goes like this:

1. Vacuum leaks (a gauge will tell you if you've got insufficient, flucuating vacuum, indiciating a leak)

2. TPS (warm it up and adjust it with a test light, then check with a DMM to make sure the resistances are within the ranges)

3. Idle speed (...hopefully by now you'll be closer to a stable idle, even if it's not high enough or two high. If you adjust the idle speed screw and it doesn't change, you've probably still got a vacuum leak)

4. Timing. (if you've got a very low or very high idle, warm it up, set it to under 900 RPMS if you can, and check the timing. If it's too advanced, the idle will be stronger, maybe too strong. If it's retarded, the idle will be weaker, and you'll run rich...)

If you've done all these things and it still idles wonky, you probably still have a vacuum leak... you might need to use a flammable spray, a mechanic's stethoscope, or a fogger to locate leaks (fogger FTW, but typically only mechanics have these...).

 

Thanks for the tips. The main thing that is confusing is that the engine idles like a champ. It fires and idles by itself. The problem is only if revved above 1700rpms. Bring tt down and it idles fine.

 

In that case I'd say you have a FUBAR'ed TPS.

 

Huh?

 

...a throttle position sensor that is f'cked up beyond all recognition.

 

well if it's that bad, then try just unplugging it completely, and see if theres any difference. All that will happen with it unplugged is there will be no fuel cut on deceleration, and kinda sputters when decelerating, but still accelerates fine. If the TPS is really that bad and has spots where it has open or shorts, (infinate and zero resistance) then it will do all kinds of wierd things. Worth a try anyway.

 

Since you didnt say what year i will assume a S5 T2, meaning you have the full range TPS. TPS is bad.

 

It is actually an S4, but it came with a Twin TPS setup like an S5. My harness and ECU are set up for a single TPS so I split the sensors and am currently only running front sensor(The one with the Green/Red and Black/Red wires).

 

1700 is still well within the range of the s4 TPS :P

 

I had a TPS on my car that at like 1500 and 2500 RPM, the revs would bounce...if I was in gear, it made it look like I can't drive.

 

Thanks for the input. I probably should stop cheapin' it by using half an S4 Twin TPS and just get a single S4 TPS unit. If anyone else has input, I'm all ears.