boost drop off

[schmidlapp]

Hello,
My 84 930 has an issue when boost comes on, and it comes on strong,
suddenly the boost drops off to nothing. Recently (last fall before winter storage)
it was in the shop for a major service, valve adjustment, valve cover gaskets,
plugs, rotor, cap, timing and a new intercooler o-ring. It ran fine when I got it back, boost like crazy and went like stink. Before I take it back to have checked by a "pro", I'm going to ground the overboost lead to see if thats the issue. I really am suspecting mabye my secondary fuel pump is not right. I do have an afr gauge and it seems to be running leaner than it has in the past. It normally runs rich most of the time,
now at highway cruise speed 2600 rpm, it is showing very lean...
Any ideas, am I on the right direction with the fuel pump?
Also is there a wiring diagram available for the fuel pump circuits?
Thanks in advance...

[mark houghton]

Listening check::: "your boost drops off to nothing". Do you mean that your boost gauge confirms that the boost presssure has dropped to zero, or are you just talking generally that the power seems to fall off (seat of the pants). If it's just lack of power, then yes your lean AFR's and perhaps the fuel pump may be a clue.

What does your AFR look like when on boost? Is it pig rich, perhaps indicating a blown o'ring or other large boost leak? Can I assume that you're able to reach full boost, but it quickly drops after that point?

The overboost sensor (lead) that you mentioned won't cause your boost to drop off. Instead, if it were faulty it will cause your car to shut down completely and violently. Face-in-dash, ya know? That overboost sensor simply shuts down the fuel pumps.

[RarlyL8]

A failing rear fuel pump can cause these symptoms. System pressure check or volume/time check will help diagnose.