Hot idle issue with 7A EX cam

[S2central.net]

Anybody btdt with erratic idle when hot with 7A EX cam

Got two cars with 440cc injectors. One with ABY cams is happy at hot idle. The other has same injector latency etc starts fine on cold, warms up fine and pulls strongly but the transition from liftoff to idle results in a stall when engine is up to temp. Very annoying.

ISV seems to be fine
MAF is good and no air leaks
TPS is behaving correctly

I even tried adjustment of idle speed and timing in the maps which helps a bit but something is off. Never done 7A ex camshaft before and wondering if that needs a particular tweak - mechanically or in the ECU.

[AndyP]

What turbo?

[S2central.net]

Funky AET hybrid but that is irrelevant for an idle niggle - or is it ????

[AndyP]

Stalling with the Garret BB turbo, under the conditions you describe, has been done before. Ross had it when Mihnea mapped his old coupe on a GT3071. Moving the MAF to the pressure side seemed to cure it. I don't have any issues with mine but I'm running std 3B cams, I'm sure Ross has 7A cams too.

[Ross]

I have a 7A exhaust cam on the RS2. Funny enough that had the same idle problem but only when the car was cold... very sticky oily throttle body and ISV probably to blame... it was always fine until very recently before I bought it aparently.

Back to my Coupe - that always ran fine with a 7A exhaust cam, until the change to the GT3071R. Then drops from mid-high RPM to idle would cause a stall. The turbo was the only change. After mapping, the issue was still there.

That one never got figured out, but others with similar idle issues found success in the old 'MAF to pressure side' trick.

It's an interesting thought that perhaps the 7A cam has an issue, maybe it's a 7A + big turbo combined issue

[S2central.net]

That sounds like ballache. Turbo has KKK centre section a K26 hotdogs of some sort and looks like turbonetics T04e coldside.

Its just odd that it's only at high temp. Water sensor into ecu is good too - gives sane readings as it warms up and when radfans cut in.

Fuel pressure seems good too with nominal 4bar from AAN regulator.

Car has a nice looking crank breather system which seems to vent lil bit on idle - there is no pressure regulating valve from the crank connection - wondering if very low crankcase pressure combined with 7A EX timing is causing some badness ????

[A80Avant]

can you modify idle settings on the ISV valve through the motronic? to make the valve become more Open by altering DC? (sorry, thinking in VEMS terms)

[S2central.net]

There might be limits on that somewhere in the bowels of motronic - not found them though. Generally you just dial in the desired idle speed and the ISV does what it needs to do.

[A80Avant]

ah, i see.

when issues like this arise in vems, we play around with PID and overrun fuel cut. That usually fixes it

[S2central.net]

Intuitively I feel it's the vent system. Will try a pressure control valve on the crankcase next.

[Thuppu]

Anybody btdt with erratic idle when hot with 7A EX cam

Got two cars with 440cc injectors. One with ABY cams is happy at hot idle. The other has same injector latency etc starts fine on cold, warms up fine and pulls strongly but the transition from liftoff to idle results in a stall when engine is up to temp. Very annoying.

Exactly the same issue on my UrS4 also. 440 injectors, 7A cam, CT BB turbo. On throttle lift off it stalls almost every time when it's warm. If you give it a small throttle bump before the rpm's reach idle it wont stall so easy. The idle is also a little unstable, humps when warm. When cold the idle is perfect smooth. I had this issue before the rebuild and 7A cams, so it's not a cam issue. My old breather system was almost stock before the build, under boost it breathed to atmosphere, but under idle it was stock. Now it's breathing to atmosphere all time.

Does the car have the stock airbox and airintake? I have the feeling it would be a problem with the air turbulence in the MAF if you have a cone filter plugged directly on it. I did a extension on the air intake before the MAF but it seemd not to help the problem. It could be to short though..?

[Ross]

Interesting point. My coupe had a cone filter on it as well.

[Thuppu]

Ones when a turbo hose popped off and I unplugged the MAF-connector so I could drive the car, the idle was perfect. There have been discussions on our finnish forum about the MAF airflow sensitivity, if the airintake before and after the MAF is to short it can cause idle problems...

[S2central.net]

Mmm interesting... This machine does have a short intake to the MAF. Dunno if the MAF screens are still fitted or if that makes a difference.

I was also wondering if it was sideeffect of bad/old lambda probe on this car seeing as it only occurs when hot.

[Thuppu]

On this also only when hot. I have a new lambda. Maybe 5-7000 km's driven.