LS6 runs rough when warm

[pormgb]

I have been battling an issue with my LS6 swapped Porsche 944. When the car is started from the cold, it runs fantastic with lots of power. When the car is shut off and sits for a while and restarted, it funs rough with noticeably less power. There is no issue transitioning from open-loop to cold loop, and the problem seems to occur when the car is restarted from a drive. An example is when I drive the car to the weekly car show, and I park the car for a couple of hours. When I'm ready to leave, the car runs rough. Going to the show, the car runs fine. Another observation, is car struggles at low rpm when pulling into my driveway, or driving slowly up a grade. Anyone has seen similar issues?

LS6
226, 229, 575, 585 Cam
85mm TB
40lb SVO Injectors

24005

24006

[joecar]

I can't view your files at the moment since I'm away from home.

Is intake air temp (IAT) getting hotter than ambient...?

Does fuel line get hot...?

Are you using return style FPR (has MAP reference hose)...? If so, is FPR leaking thru...?

Is there an internal coolant leak...?

Is there an air leak at intake manifold that shows up when hot...?

Is there oil pool on floor of intake manifold...?

[pormgb]

I can't view your files at the moment since I'm away from home. No problem, see my response inline.

Is intake air temp (IAT) getting hotter than ambient...? IATs average around 67F, very close to ambient. I have relocated the IAT to beside the air filter.

Does fuel line get hot...? I need to check

Are you using return style FPR (has MAP reference hose)...? If so, is FPR leaking thru...? I'm using stock Corvette C5 regulator with return, no reference

Is there an internal coolant leak...? I will pressure test the system, but I'm not losing coolant. I do smell coolant when the car starts, but my coolant level stays constant

Is there an air leak at intake manifold that shows up when hot...? I need to check

Is there oil pool on floor of intake manifold...? I run catch cans, but I will have to remove the throttle body to check for oil

[statesman]

Another observation, is car struggles at low rpm when pulling into my driveway, or driving slowly up a grade. Anyone has seen similar issues?

LS6
226, 229, 575, 585 Cam
85mm TB
40lb SVO Injectors


Yep, it's your cam.... it's too big for low rpm driving. The bigger the cam, the more torque you sacrifice at low rpm. For a good all-rounder, try something like a 218/224 on a 116 or 117LSA. It'll give you more power in the rpm range you use most often. Bigger isn't always better.

[pormgb]

Yep, it's your cam.... it's too big for low rpm driving. The bigger the cam, the more torque you sacrifice at low rpm. For a good all-rounder, try something like a 218/224 on a 116 or 117LSA. It'll give you more power in the rpm range you use most often. Bigger isn't always better.

Thanks, and I didn't think my cam was that aggressive.

That cam screams in the 3100lb 944, and I'm ok with the choppy idle and slight surging if that's due to the cam profile. The car has a 3.778 final drive, so I spend most of my time above 3000 rpm, where the motor best performs.

[statesman]

I never said your cam was too aggressive. I just said that it's not going to have much torque at low rpm. If you can keep the revs over 3000 as you're pulling up your driveway, the engine shouldn't struggle at all.

[pormgb]

I never said your cam was too aggressive. I just said that it's not going to have much torque at low rpm. If you can keep the revs over 3000 as you're pulling up your driveway, the engine shouldn't struggle at all.

3000rpm up my driveway, and my neighbors will love me:laugh:

It just takes a little more gas to get up steeper grades, and I also think my warm idle tuning may not be as good as it can be.