Surging at highway speeds

I’ve had my 86.5 for three and a half years and I have a developing problem. Two summers ago I noticed at idle the engine has a bit of a lope to it, almost like it has a big cam in it. I obviously don’t suspect it does, I don’t even think anyone makes a bigger cam for these. Anyway, last winter I swapped in the S300 chips from  liftbars.com  and installed a higher pressure FPR from Roger, new spark plugs, rebuilt MAF, accelerator cable, and I had replaced the O2 sensor the year before when I replaced the cats with an X pipe. The PO had done a top end refresh and I believe replaced the Cam Position Sensor before I got it.
So, after doing all that I took her on a 300 mile road trip to the Tail of the Dragon last summer. Fantastic trip by the way! On the way home I began to notice her surging while cruising at highway speeds. We would be sitting at 80 mph and she would all of a sudden lose power for a split second. I wondered if it might be the idle control valve when I first noticed the lope, but now I’m wondering if I’ve got a fuel pump going bad? Any suggestions would be greatly appreciated!

Power cutting out could also be a symptom of failing relays.  If the relays for the fuel pump (XX), EZK spark (XVI) and LH (XXV) have never been replaced, I suggest replacing them.  there is also a fuel pump fuse (#42) that if the connection is corroded could cause problems. 

Generally, a failing Porsche 928 fuel pump will be much noisier than the low hum sound of a good one.  

I suggest checking the fuel pressure and delivery.  Note, for the S300 chip installation use the fuel pressure for the 1987 and newer 928 S4. Low flow could be caused by a clogged fuel filter or missing screen at the in-tank fuel strainer as the fuel filter is after the fuel pump.

Testing the fuel pressure is done with the engine not running.  It requires removing the Fuel Pump Relay (XX) and placing a jumper wire between socket positions 30 and 87.  Listen for the pump noise before checking fuel pressure.

Your 1985 - 1986 Porsche 928 requires setting the base CO% and idle speed for the lambda air/fuel regulation and idle speed regulation to work properly.  See the factory workshop manual for the procedure.  A CO% exhaust test is done before the exhaust reaches the catalyzer (setting is 0.6% plus minus 0.2 and less than 300 ppm HC). The CO% adjustment is done by turning the adjuster at the MAF. 


 

Thank you for this detailed response! I’m definitely saving this for future reference. I was able to track the issue down to a vacuum leak on the passenger side. Thanks again!

Glad you found it! Vacuum leaks can definitely cause that kind of surging, especially at steady cruising speeds when the engine is trying to maintain consistent air/fuel ratios. Had to rectify it on mine in November