Idle Bounce Mod'ed 968 New Engine Build

[JivenJim]

So New modified N/A 968 Engine is in. The Idle is bouncing say 400 Rpm's Only after about 2 minutes cold/ or right away when warm. Is very smooth before that.

I have a split (BROKEN) exhaust pipe about 3 inches past the O2 sensor. (to be fixed)
I have adjusted the Throttle idle screw on outside of TB to idle closer to 1100 RPM’s.

Engine has hot CAMs and modified Springs.
Has all new hoses and I gave the Vacuum lines a good check.
Idle Stabilizer was worker great before. Same for Throttle Position Sensor.
Swapped Fuel Reg.
Injectors were service not to long ago.

O2 Sensor being corrupted by fresh O2?

Would you Play with Set screw. Swap idle Stabilizer, or TPS electrical tests.
Or just tow to the exhaust shop?

I have a second complete engine ready to strip any parts.

Where to start?

Thanks fellow Gurus!

[flash968]

well, these engines are very particular - the dual resonance intake manifold makes things very complicated - exhaust size and back pressure are extremely critical too - too big and everything goes wonky

which cams did you choose? there are a bunch out there that don't work right, and cause problems

having to adjust the idle screw that high tells me that you might have either HUGE cams, or something is wrong - it sounds like the computer is stuck in between modes

what software are you running? are you on stand alone?

[JivenJim]

Hey Flash,

Using Web Cam Re-profiles on Stock Cams. Increased Lobs from Stock, but not the super agressive ones that require notched pistons.

Stock DME management. Adjustable Fuel Pressure Reg.

Lighten Crank and flywheel. I know this alone can cause low idle 50-100 rpm bounces.

I think the key is that it idle bouces only when Warm. Even close to 2500 RPM's. I think it levels out above that but can't do much testing with the loud noise (ie cracked exhaust) and neighbors right now.

I've been runing the 968 engines for a couple of years now, but this has many little mods.

The Exhaust is really split a few inches past the O2. Looks like the Bat Cave.

[flash968]

well, first off, the stock software won't work for squat with bigger cams

also, what is the fuel pressure set at? i frequently see WAY too much fuel pressure being thrown at this - i've played around with it, and found that it doesn't take much to throw things out of whack very fast

also, depending on the cams, you may need bigger injectors

i know this seems counter to what i just said above, but it's a flow rate versus flow pressure thing

first order of business is to get the exhaust sealed up and verify the O2 sensor or change it out

then, call pete at rs barn and see what he recommends about software, injectors and pressures for those cams - he's really the guy when it comes to modding these engines - he has spent a LOT of time working out cams and such - i've done a lot of engines over the years, but am really just beginning to work on big mods on these, and am working with pete on most everything engine related - i'm really more of the "suspension and brake guy" on these cars

but, you're not far from me, and i'd be happy to help you out if i can

[fast924S]

I dont know too much about the 968 engines but it might be those cams causing the problem, I know the software on the 8v 944 engines has problems with aggresive cams, Maybe your not making enough vac. or the injectors arent firing right due to the different derration of the cam. ALL just guesses

[flash968]

yup - the stock software is VERY lean - no way it's going to work with big gulpy cams

[JivenJim]

Pete at RS Barn is sharp. I have one of his Balance shaft delete kits. He's also been a good friend when I was looking for some spare parts. Nice guy... I first had the stock 3Bar FPR in there. Then I tried putting in my Variable FPR. It's running only 3 lbs. above stock. Say 51PSI. No difference.
Racer X 968 chip. To me that all sounds like fine tuning stuff. I'll check the plugs but the exhaust smells like a good mix. Since I didn't increase the 3.0 displacement I'm sure I'm not over running the stock injectors over duty. We're also talking low idle issues so that's even less squirts.

02 sensor should be good. If I disconnect it should go full rich just like in warm up mode right??

Yes I should get that exhaust done first. I just don't have time to take off work to get it done till next week.

Here’s the question. What changes in the engine operation after cold start about 2 minutes in?
Still to early for the Thermostat to open. I also bleed the coolant screw.

So what's changing making the smooth idle starting to hop.
If it was the Idle control valve wouldn't it bounce from the go. Same with any vac leak or low vac right.???

appreciate your brain power guys. All ideas welcome.

[flash968]

sounds like you are going lean - start up is pretty rich - i'd need to know a lot more about the cams to know for sure, but that's what it sounds like

i'd call pete

[JivenJim]

Looks like this was simple. The Set screw on the TB was too high. I wanted a little higher idle to aviod closed throttle bounce. And this is what was causing the computer to get comfussed.

On to the next issue now...

Thanks for the help guys.