I am having big issues with my tune. All was fine (but far from optimised) with my previous combo: L98, TPI, Crane Compucam2032, shorty headers, no VATS or emissions. I put a HSR on and now the idle is doing real strange things... No matter what i try in the MAF tables (I have TunerPro), it either stalls out (drowns i think) if i drag the table down (reduce airflow values per mvolt) or if i drag the line up (increase airflow per mvolt) it ramps up and idles at 1500-1700rpm! Why cant i get a happy medium? Am i missing something? I don't know understand about BLMs or anything like that just yet and cannot currently datalog so my ability to analyse stuff is a bit limited.. I even tryed returning to Arap (stock vette) bin to try and start over but it does the same (idles high). Ignition timing is set at 6deg base. Help! Thanks in advance!!

did you check for vacuum leak?

yeah, it sounds like a mechanical issue rather than your tune.

Is the brake booster vacuum hooked up? What about the FPR and accessory vacuum?

Could be right at the TB too. Just double check everything and listen.....

good luck

-jason

Plug up every vacuum port except for the fuel pressure regulator, i.e. take everything else off the manifold except for the FPR and then run the engine. If things improve you probably have a leak somewhere. If it's still running rough, it could be in the manifold seal to the heads or something else. Also check to make sure your throttle body bolts are tight.

I have checked for vacuum leaks by covering the intake tract with cardboard. It stalls out immediately. This prob isnt 100% reliable since a small Leak may not support idle on its own.
However when i fiddle with the tune, it seems to chenge eithe way.
Perhaps this is masking the hidden leak somewhere?
Would it idle on just the leak only?
I have blown two plenum gaskets so far, and you can hear them go "wiizzzz!" when they go south.
Any other things i should check?
Should the HSR run pretty close with a stock tune?
Thanks for the responses!

A vacuum leak can be large enough to screw around with the engine but not large enough for the engine to run on. Covering up the TB with cardboard wouldn't rule out a leak if I was diagnosing it.

Thats what i was thinking.. The leak wouldnt be big enough to sustain idle.
So thats prob it.
Anuy other suggestions regarding PROM setup? Should it run on a stock tune?

With your setup, it should idle fairly decently with a stock tune I would think. Of course not optimum, but it should be fairly steady and predictable.

FWIW, I recently had a vacuum leak and my car went from normally idling around 1000 to 2000!!! I realized I had installed my SLP runners on the wrong sides

Vacuum leaks can be a bitch and its definetly something you'll want to rule out or fix.

I tryed disconnecting the vacuum booster as a test. the idle went up 200rpm.
So i ruled out a much smaller vacuum leak putting the idle up 800rpm.
Anyway, thanks for the responses. I'm fed up being a laptop slave and i am now looking at converting to carb. Spose i can then be a slave to the fuel bowser...

Cheers!

I have a similar issue, but it has to do with a loose Steering pump pulley. What happens is The car will run fine, idle fine but after it warms up the idle goes up and down like a yo-yo. I thought Vacuum leak, and after listening to the engine I could hear this faint rattle then the engine would slow slightly, then it would race, and then come back down and rattle again. What I suspect was happening is the rattle was triggering the Knock sensor, and the ECM would retard the timing slowing the engine slightly. Once the RPM dropped below 550 it would add fuel to keep it from stalling and then overshoot the other way, While it was slowing the pulley rattles again and the whole cycle started over, eventually it will stall...

Jon

Thanks very interesting. Talk about a strange prob!

you really need to buy a cable and at least watch in real time with Tunerpro or something. just looking at sensor values while the engine is running can halp you rule out all kinds of things.

i would spend the dough on a cable before you slap a carb on there.

I started my 11:1 383 HSR ZZ409 cam with a ARAP bin.
Idled fine once I adjusted the rev v coolant to suit the cam.
Fueling way off , pig rich at idle and way too much spark on stock settings but idle was stable.
Was same tune I used fwith TPI setup ,just changed the injector constant to reflect 30lb used with HSR.
Have got a spare ALDL to serial cable if you want to datalog
vetteoz@optusnet.com.au

Thanks for the offer, i'll shoot you an email now.

BTW Its running now. Fixed it!!
Turns it was the Holley 58mm TB was throwing things out. Idles fine with stock 52mm TB!
The IAC cavity might be bigger to suit bigger venturi diameter or something?
LESSON LEARNED - Keep It Simple Stupid!