930 Power issues .... Argh!

[Murph329]

Ok, I have a 81 930, that won't go over 4k RPMs. Starts and idles fine, goes up through the RPM's to 4k then feel like it hits a wall, the power just stops and sometimes it feels as though I lose compression. Oil good, battery good, charging good, It feels as if the fuel is becoming starved when I reach that mark. Turbo shows boost but then its not a lot as at 4k RPMs it starts hesitating. Any help identifying this issue is greatly appreciated, just replaced cap, rotor, plugs and wires and still won't get past 4500... Frustrated and thanks for any help!

Chris

[James Brown]

try posting in the turbo forum also, what is the max boost pressure?

[spuggy]

Originally Posted by Murph329

goes up through the RPM's to 4k then feel like it hits a wall ... It feels as if the fuel is becoming starved when I reach that mark.

Sounds almost like classic "dash in the face" syndrome from excess boost and the overboost switch cutting the rear fuel pump. It's supposed to cut between 1.1 and 1.4 bar (you can remove and pressurize with a bicycle pump/check with a multimeter - be careful not to over-pressure it).

Does the wastegate open? (should be easy to hear/tell). If not, check the wastegate boost signal line is connected/not split. Should be connected to the side port on the IIRC - the top port is just vented to atmo (so the air above the diaphragm has somewhere to go).

The wastegate diaphragms can also split, because they're all pretty old now - and especially if you run more boost than stock - and then the wastegate doesn't operate (valve doesn't open) and excess boost will be developed.

Again, these can be checked for correct operation with compressed air; be careful not to over-pressurize.

Good luck getting rebuild parts for the stock wastegate; if that's the problem, many like Tial as a replacement - they do two 46mm wastegates with the correct flange pattern, one intended as a bolt-on 930 replacement with shaved fins to fit in the stock location; others report the "vanilla" one fits as well... Rebuild parts/customs springs available and it is supposed to respond faster too (no personal experience as both my factory wastegates work fine). Buy from a reputable source; there's enough pirate junk that's not worth the postage that Tial have a section on their website so you can tell if it's a genuine item or not...

If you're feeling confident that the wastegate is operating correctly, you could also run without the overboost switch connected - at least until you get a replacement (I don't run one at all, mostly because I don't have a rear fuel pump to cut).

It would be wise to monitor the boost gauge carefully - ideally a mech one, as the one in the tach isn't noted for being extremely accurate.

[Joe Bob]

Or a failing fuel pump.....

[spuggy]

Originally Posted by Joe Bob

Or a failing fuel pump.....

Good point! System delivery/flow and system (not control) pressure - should be verified before looking at anything else...

Stock 930/60 specs a minimum of 1170cc flow in 30 seconds (both pumps running) and system pressure of 6.0-6.7 bar.

Stock 930/66, 930/68 motor types are spec'd as 1500cc flow in 30 seconds and 6.7-7.4 bar of system pressure.

What motor type do you have Chris?

[totle]

As Spuggy mention.
Sounds like Overboost switch. Located on the bov housing on der the intercooler.
For test you can remove the wire on the switch and connect it to ground.
If all works well then, replace the switch