abt4x4

Hi, I've got a 5.7 ls1 in my 4x4 race vehicle, been running with it for about 18 months now and it goes really well and has never really let me down. However I have a strange problem that is really intermitted. Baically my idle hunts pretty badly up and down, sometimes bad enough to stall. Strange thing is though I drove it all day the other day with hardly any drama then parked it up at home, next morning got up to move hit the start button fired up then shut down straight away, start button again fired up revs went up then down almost stalled and wouldn't run without help from the throttle, till it got a little warm then will sit there idling too low I think. Few things about the motor it is freshly rebuilt by me (I'm a mechanic), it runs a MAFless tune, it has brand new O2 sensors, brand new IAT, removed the inlet manifold replaced all the O rings, replace PCV complete setup. Now I know this is a bit long winded but the only thing which I can find that has any sort of effect is if I start it while the fuel pump is in its 3 sec prime when you turn the ignition on it will rev up correctly and idle down to a respectable idle, check fuel pressure seems fine and use a corvette filter/regulator that I replaced thinking it could be effecting it. I know its a lot to read but I'm struggling to diagnose this problem any help would be great.

Cheers

TransAmBlue

1st thing I would check is the fuel pressure at the rail while this is happening.

abt4x4

Unfortunately I have already checked this and it stays steady, I replaced filter and regulator too just to be sue I was getting flow. I don't however run a check valve on my fuel pump(bosch 044), but didn't think this would really matter.