hi
i have a 87 / 89 chev k5 blazer. it has 350 tbi motor and is a 4x4. trans is 700r4 and it is part time 4wd. my problem is that while driving at approx. 55 mph or up you start hering what sounds like a spark knock. if you back out of gas it goes away until you apply gas. it will then cut in and out. if you stay under 45 mph you dont notice it. then it goes away and will run fine. motor was rebuilt complete spring of 2009 and new crank,cam and brgs,bored to i believe .030 over,new hv oil pump, heads redone, new lifters and push rods, and so on. i've replaced every elec. part under hood including knock sensor and comp and dist. does anybody have any clue what to look at next. this problem does not happen every time.

 

Is it still the ECU controlled distributor or did you put in a manual HEI distributor?

Does the problem persist if you run a few tanks of higher octane fuel?

 

dist is ecu and problem started about 2 months ago -- replaced all elec parts and no luck. had mech. i trust put on diagnostics and the o2 was slow so replaced and thought dist bushings so replaced dist.-- still neg so replaced comp.-- could it be proms.

 

Is being bored to .030 normal or do you have to request it?

 

Not that this has anything to do with the topic of this thread at all, but unless the block needs a 0.030 overbore, I doubt a machine shop would do it as a matter of course. It requires new pistons & rings.

 

reason block bored .030 over was front two cyl. had wear and we decided to do right and had bored . also put in new pistons and rings. back to problem --- checked esc and that is working , tps is also in specs., and knock sensor is good. rechecked timing and thats good also. tbi is obout one year old -- got off ebay at good price and trk ran fine for 6 to eight months then started acting up.

 

Sounds like the fuel pump is going bad.

 

Like mentioned above, it could be a fuel pump. I had a problem like that with a brand new Walbro pump except I was not in it to hear if it knocked. Wife was driving down the interstate and said it just started to sputter and then it shut off. Got to the side of the road and let it sit for a few minutes and it started up and drove the last 10 miles home but it wouldn't make much power over 50mph.