14cux. Engine rpm dips and coughs on throttle with cold engi

[jepper]

With cold engine idle is fine and steady but when I move the throttle just a little the revs go down and after some coughs goes up as it should. This occurs only with cold engine and dissappear when eninge becomes warm/hot.

This makes it a little hard to drive because after each gear change when giving some throttle it starts to dip and couhgs/hesitate a little and then starts to pull again as it should.

I also have removed my IAC-valve and exchanged it for a manually operated plunge (by the old choke wire) because with warm engine i had a idle around 1500rpm. The IAC-Valve remained open i think? Why I dont know. ECU operates the valve nicely with cold engine.
I bought a new IAC-valve, MAF sensor, lambdas, injectors, fuel pressure valve, temperature sender, fuel pump and filter, distributor including vacuum membrane, a known working used ECU and tried two other known good throttle position sensor.
All vacuum hoses are new (includes the little T-tube). Checked for vacuum leaks several times with start spray.

Same issue runnig closed or open loop. Tried both low and high fuel pressure and ignition. Tried base idle re-setting.
A strange thing is that the idle revs sometimes can cycle up and down when cold, despite that the IAV-Valve is removed!

If I open the IAC-Valve fast (with the choke cable) there is no dip in rpm but that's maybe because the TPS is telling ECU it should still be on idle management.

My thoughts goes to ECU map. The ECU senses a kind of big difference in engine characteristics it can't handle when going from idle to load management without doing big adjustments to the built in fuel map. Strange is that it's okay (maybe not completely) with warm engine.

Also thinking that the camshaft is too advanced, but then, why only when cold?

Enginge spec:
3.9 with 14CUX from RR 1990.
raised compression to 10:1 by 9.35 pistons and grinded heads. Mild porting of heads. Stainless custom exhaust - no cats but with lambdas.
Piper high torque cam from RPI with adjustable cam timing set to 4 degrees advanced for greater low rev torque. (This perhaps was a mistake as the camshaft already could be constructed with advanced timing)


Any thoughts and ideas?
Regards
John-Erik
Sweden

[Philip A]

Jepper, regardless of you checking, I would be pretty sure that you have a major air leak into the manifold somewhere that is causing your high rev idle.
If you do not have a leak it is impossible for it to idle at 1500RPM unless the throttle blade is way out of adjustment. Why not close down the air bleed bypass on top of the throttle butterfly and see what it does. Seeing you have the IAC valve disconnected, it cannot be that. Then maybe check the clearance on the throttle blade by seeing if you can pass a piece of paper through with it closed. In any case adjust it to closed making sure to remember how many turns.
The stumbling on cold idle is consistent with a large leak as a leak would lean the mixture seeing it is unmetered by the MAF. the surging is also consistent with leanness.
Maybe a wild thought. Disconnect all the vacuum hoses to brakes etc and plug the holes and see if there is a difference.
Regards Philip A

[jepper]

Thank you Philip!

I'm gooing to the bottom of this problem. I have not tested with brake servo hose plugged I think. That's a good point!
Maybe I could check for airleaks with dish soap and water if I connect around 0,5-1 Bar pressurized air to the inlet (plugged). I,ll report back any progress.

Regards John-Erik