My idle speed with the truck in gear (automatic) and A/C on drops too low and the oil pressure lamp comes on. It did this a couple years ago and I had to bump up the idle speed adjustment. The adjustment has not slipped. As a matter of fact it seems "froze" and I can't turn thr jamb nut. What would make it lose idle speed? I put a new fuel filter on and it helped but there must still be a fuel restriction somewhere??

 

Have you replaced the pre-filter? Thats a possible cause.

 

Abbynormal....I've been having to bump up my idle every 6-12 months for over 2 years now. It would get so low with the a/c on and in gear the engine would stall. Have you checked your fuel pressure?
With my truck I'm starting to think that maybe my lift pump and or the overflow
valve may be getting weak. And that I've just been compensating by having to raising the idle. I'm in the process of replacing them. Just waiting on the new seals for the overflow valve to come in the mail.
And no...I haven't checked my fuel pressure. Buy the time I drill out the banjo fitting, buy and hook up the needle valve and buy a gauge I thought I may as well replace the pump and overflow valve and just see how it runs.

 

The first thing that you should do is check fuel pressure. As a real quick check, you can put it in drive with the A/C on and a friend standing on the brake and then pinch off your fuel return line and see if the idle speed increases.

All mechanical components do wear and change with time. To see a significant change in the idle speed is unusual but it doesn't necessarily mean that something is wrong.