Thump and knock
 

My friends 99 986 base has the codes p1128 and p1130. It seems to idle okay but under throttle, the power cuts in and out (we actually stalled driving) and there is this thud coming from I believe the left side. It starts fine every time. He described as thumping and choking? Car has just under 100k on it.
Any thoughts on easy first steps?
Thank you.

uh...replace the o2 sensos that are throwing the codes?

On the early (pre-2000) cars, these codes indicate an overly lean condition, usually an intake air leak or low fuel supply.

Thanks, should I start with spraying some starting fluid around idling cold motor and if no increase in revs, go to fuel filter? Would dirty MAF be a possible problem. When he 1st had this issue, the air filter was really dirty. We replaced but didn’t fix problem. Maybe that would do the opposite of running lean.
Thank you.

What you should be doing is running diagnostics, not randomly spraying stuff and changing out parts to see what happens.

Either get a vacuum gauge and check the idle vacuum on the car, or hook the car up to a Porsche specific scan tool and look at the fuel trims, and hook up a fuel pressure gauge and evaluate pressure and delivery rates.

Would also add to the list looking at the o2 sensor voltages. loads of videos on what to look for

did he have a k&n filter before?