Crackership

Ok, so I just finished doing a ton of work to my 951. as part of the work, I had the cam box and intake manifold off. now that it's back together, it has a peculiar and intermittent running problem. it occasionally hunts a bit at idle, and sometimes it will hesitate under acceleration, but only briefly. It will also occasionally backfire on throttle overrun. Thanks to my Zietronix data logger/gauge thingy, I can see that when it's hunting at idle, vacuum drops and Air/Fuel ratio goes rich. It also goes fairly rich on throttle overrun, which I assume is why it's loading up and backfiring. I'm not sure what happens when it hesitates under acceleration, cause it happens very quickly and I think staring at the gauge instead of the road ahead while accelerating might turn a minor problem into a major one.

I've spent a ton of time looking for vacuum leaks and haven't found any.... doesn't mean there isn't one, just haven't found one. But I would think that a vacuum leak would result in a lean condition. Correct me if I'm wrong.

For what it's worth, the car has a Vitesse MAF, 83# injectors, LR manual boost controller and Tial 38mm dualport wastegate, so the vacuum system is a bit modified.

anyway, I'm stumped. Any ideas?

Excelerate

hunting at idle and the hesitation sounds to me like the throttle position sensor. I see you have a MAF so we can eliminate a sticking AFM barn door, maybe check that the MAF is putting out good numbers?

Crackership

Thanks for the ideas. I just checked the TPS for adjustment and it's spot-on. Of course, that doesn't mean that it's working right, but it's not out of adjustment. I suppose I'll contact Vitesse and see if they have any troubleshooting suggestions for the MAF.

Tedro951

Did you happen to rebuild the throttle body? If so, check that the hard throttle stop isn't allowing the butterfly to close completely off, making the ISV jump in to help.

running_cold924

or if the seals are shot on the throttle body shaft and oily-boost is going into the TPS..

if and 86, then venturi delete?

Crackership

it is an 86... and I've been wondering for some time what this "venturi" everyone keeps talking about is and why it's good to delete it.

also, I think I got it... the vacuum line from the intake manifold to the ISV was loose. I tightened it back up and it seems to be better. No more poopy idle and the backfiring is gone. I've only driven it in the rain and in traffic since though, so I haven't seen whether or not the hesitation on WOT is still there.

running_cold924

the venturi delete just cleans up under the intake, less vac. lines, less chances for leaks