codes 172 and 136 have me stumped

[musiccitysho]

This lean condition has me stumped. I keep getting 172/176 KOEO codes and 136/172 KOEO codes. The o2's and the maf (55mm) were replaced 20,000 miles ago. Ive tried cleaning the MAF and clearing the codes several times because of the K&N I was running, (I've switched back to paper) but the codes are still present. I've also checked for any unmetered air getting past the MAF around the TB, and all is secure.
The check engine light comes on after heavy acceleration, usually right after entering the highway and such, it used to go right off but now stays on much longer
I'm thinking its got to be a vaccum leak but I cant seem to find the source. I've checked the tubes coming from the egr, no cracks and secure. The secondaries function properly and I dont seem to have any loss of performance. It idles fine except I can occasionaly hear a howling sound at low idle but when I try to locate the source it stops before I can distinguish it above the chatter of the motor. It seems to be coming from the rear of the motor midway down but I cant pinpoint it cause of the engine noise and heat!
Does anyone have any thoughts or insights as to where the vacuum leak may be coming from or where or what else I should check.
A real big thanks in advance for any help

 

[AutoSHO]

Have you checked fuel pressure?

 

[musiccitysho]

I havent checked the fuel pressure, thanks,... I'm out the door for a gauge now! I'll run a search for what the pressures should be

 

[wood_e]

I've got the same **** going on... been that way for a while. I suggest fuel pressure is the culprit. As I had a post a while back and have tried nearly everything EXCEPT fuel pressure.

 

[musiccitysho]

It seems that I'll have to put it off until tomorrow since those gauges are high dollar (41.99 at O'reilly's) if you want to buy one. I'm going to call around to see if anyone will loan one out. Maybe Autozone? I'll post the results once I find a gauge
Thanks again.