99 Blazer v6 vortec 4.3l. We had a valve job back in 08/09 and it ran fine for a while. We replaced the Crankshaft Position Sensor the first time... it went bad. Replaced the connector and the cps again... went bad. We are doing it a third time now (and no I am not paying for this again. I paid once).
My question is - When doing a valve job, did the mechanic have to move or remove the crankshaft or do anything that might cause the crankshaft to need adjusting? The problem seems so intermittent. As of right now, it isn't running at all. Starts up, runs for 5 or 10 minutes and cuts of like the key was turned off, just dies. Funny thing is, we had the fuel pump replaced and it ran great, for a day. The mech said the fuel pressure was too low. It was sitting at about 54 at idle. He said 'The Book' says it should be 85...? When it runs (as I said for 5 or 10 minutes), it runs great... but this is so damn frustrating.

 

Nothing that the mechanic did during a valve job should have impacted the CKP sensor. It is a non-contact sensor located in the front timing cover. Now, if he also replaced the timing chain/gear-set, then he would have had to remove the front timing cover. If he damaged the cover (it's plastic) and ended up replacing it, he may have replaced it with the wrong cover, either increasing or decreasing the air gap to the crank shaft reluctor ring.

As far as fuel pressure goes, we have a very good write up for that. Check the Tech Article (DIY) Index link in my signature.