What would cause engine surging/poor idle other than vacuum leak?

[kumasan616]

I'm trying to figure out an issue... I have an RB20DET that after changing fuel filters, idles VERY poorly, surges to 5K, then drops to nearly shutting off. Prior to changing the filter, the engine ran poorly when cold but when it heated up idled smooth and revved freely (no hesitation). What would cause this problem? FYI the following items were serviced (and engine ran fine after these changes) prior to this problem:

Cleaned/replaced AAC valvereplaced 2 cracked breather linesfresh plugscleaned contactsreplaced timing belt, water pump, etc.Tested fuel pressure (40+PSI, with no drops in pressure when load is applied)

What would cause this problem to happen?

[rotorolla]

pull the timing cover back off and check cam timing.It could have jumped if you didnt move the slack from the crank side to the tensioner side before you tightened the belt tensioner.I always spin a motor over a few times with the fuel pump fuse off and then recheck it to ensure I have all the slack out and ensure it didnt jump