running rich or timing?

[jim ross]

Hey there, after some recent mods -- larger throttle body / timing advance--ive noticed that when pushing the car hard, then dropping the revs by half for corner or blipping the throtttle. my exhaust is blowing out brownish smoke when the REVS are dropping...unburnt fuel? what is causing this? it never did it before the mods...timing off a little? or running too rich?? what should i look into? THNX! James

[MPDano]

Well, if you guys would listen to me and buy an Innovate Wideband setup, you would know this in realtime. You could watch your gauge and the numbers will tell you if your rich or not at any rpms. White Smoke (water in oil), Black (rich: I think), not sure what Brown is. Any weird noises in the engine?

[jim ross]

So its RICH for sure..blk'sh smoke. not on acceleration but on deceleration..I have a progamable signal calibrator. but no lap top. So i have to hook it up to my PC and guess at how to calibrate..i have an air fuel ratio and it shows perfect mixture...so im confused. I guess i need to tweak it a little? -J

[Normy]

My mother had a Cadillac that did the same thing- it would blow brownish orange smoke occasionally, but it did it when it was floored. We thought it had something to do with a plugged catalytic converter.

If you look around online, there is a procedure for testing via vacuum for a plugged cat. Can't remember where it is offhand.

N

[MPDano]

Originally Posted by Normy

My mother had a Cadillac that did the same thing- it would blow brownish orange smoke occasionally, but it did it when it was floored. We thought it had something to do with a plugged catalytic converter.

If you look around online, there is a procedure for testing via vacuum for a plugged cat. Can't remember where it is offhand.

N

That's a good idea. Cat's aren't that expensive and if it's OE, it might be time to change it out. I got my magnaflow cat on eBay.

Porsche 928 Forums dot com - View topic - Pics of my First 928 and Current Project

Porsche 928 Forums dot com - View topic - Pics of my First 928 and Current Project

[jim ross]

thanks guys.....ive got no cat though. I guess it must just be excess fuel. ill try re-calibrating the PSC1. its just unusaul as when car ran rich before-it would blow blk smoke on accleration, not deceleration. --J

[JhwShark]

Decel blowing could be caused by worn valve guides. However, the brownish color (odds are there is black in the mix) could indicate carbon blow from buildup/deposit. Orange may be rust from within the downpipes exhaust??

Something is odd?? Noting blowing during accel/load??

Just giving my thoughts.
Jon

[MPDano]

Maybe a bad Decel Valve? Does the 84 have one?

[jim ross]

thanks everyone, the problem was the PSC1 signal calibrator needed to be re-mapped. After all my mods the Maf signal calibration was off, i moved the peak of voltage curve to about 3 volts, from 2 volts. Its a tough process, without a lap top , i have to guess at the mapping.. download to car...drive it and return and adjust. No more smoke..it was running too rich at 2000 RPM to about 3500 RPM, causing the black smoke on deceleration. Yeh!-J