To start with, recently replaced the limg, thanks to everyone for the help.

I've had a hissing sound coming from the engine since before the limg were replaced, sounds like a vacuum leak. Even after replacing limg, still makes the noise. Only does it when accelerating, and I know it isn't the fan, since it's more of a hiss than a roaring sound. It makes the noise whether accelerating from a stop or if I'm trying to pass. Noise is gone when I let off the gas, idling or coasting.

Also recently replaced were the dizzy cap and rotor (AC Delco, of course), plugs (Delco double platinum) and wires.

Possibly connected is the poor mpg and surging while accelerating. I want to say that's the MAF, since I've had the P0101 code a couple of times over the last few weeks. I'll be replacing that this weekend anyway.

Oil pressure is normal, up to 60 psi on cold start and accelerating before fully warmed up, 40-45 psi with normal driving, 20-25 psi idling while in gear after warmed up.

Any ideas?

1997 Olds Bravada, 139,000 miles.

 

check all vacuum hoses especially in areas you were working

 

Think the MAF is on its way out?

 

I'd 1st follow newguy's sound advice. Rotting and/or split vacuum hoses are very common on older vehicles. These hoses are connected to either non ported or ported vacuum sources (ported hose leaks are often heard during acceleration/throttle opening).

Also consider that the MAF code could, of course, be related to the both the surging, poor mpg, and 'hissing' noise, so you might try cleaning it with a MAF spray cleaner. You might also check the rubber Air Intake Boot for proper fit, loose clamps, and cracks or tears (especially where the Air Intake Charge senor plugs into the boot).

Other things to check would be:
The PCV system hoses and grommets (both the PCV side and Breather side) as well as the valve itself. The PCV Breather hose that goes from the driver's side valve cover to the Air Intake Boot on my '95 Jimmy is very easy to push out of place without realizing it (that is, until I start it).

The exhaust system (especially upstream from the O2 sensors). This, of course, includes were the exhaust manifolds bolt to the heads (also look for a cracked manifold), the manifold to exhaust pipe joints, and so-forth down the line.

 

Had it running with the hood open, and I think I've found the issue. A couple of issues. The vacuum distributor (I guess that's what it's called) where one vac line runs from that to the vacuum reservoir ball, then over to the firewall, is letting out a faint hissing sound when idling.

Has a cracked 90° boot on top (to the PCV valve) and maybe the o-ring didn't set right. I might just go ahead and replace all of those vac lines and the boots. Already replaced the valve.

 

Getting the damn P1860 (TCC PWM) code. Looked at several videos, it appears I can replace it by dropping the tranny pan, removing the filter, turning it, pulling the clip, disconnecting the connector and pulling it out. I also found this thread about replacing the TCC valve with one better than stock. Think this might fix it?

Btw, I replaced the vacuum hose from the vac dist to the vac ball, that and the PCV boots. The hiss isn't anywhere near as noticeable now.

 

The weirdest thing happened this morning.

After leaving work, I stopped to get gas. After that, I hit the interstate to get home. On the way down the onramp, I started giving it more gas to try and get some momentum so it would get to a reasonable speed. Out of nowhere, it came back to life.

Acceleration became solid again, gas mileage came back up, torque converter lockup started working again, temps remained stable and I was able to get home with no issues.

That hissing noise also stopped. I'm getting a P0305 now, but I see I've repeated an earlier mistake and left a spark plug wire where it could get close enough to an exhaust manifold. Gives me an excuse to hit up Autozone, though.

Any idea why it would just come back like that? I'm certainly not complaining, I'm just wondering.