chickenfried209

I have a 91 silverado tbi 350 4x4 with 128xxx miles, its all original just my work truck. My problem is that when i start the truck its fine no noise, engine runs healthy but once i put it under any load(drive) its starts knocking. it sounds like its coming from under the truck towards the tranny/motor or lower part of the motor. What im thinking is that maybe its not getting enough fuel when it has load so i was going to replace the injectors and fuel filter. im not trying to replace part after part and to get no where so if anyone can help or had the same situation some info/advice would be great, thanks.

Gundog

I would try setting the timing. These older trucks get loose in the timing gear chain. Try a few degrees more and see if that fixes the issue. Are ou running 85 octane?

Allan In NE

Fuel has't anything to do with it. If it's knockin', it's knockin.

Pull that converter cover and see if the converter bolts have worked loose.

How is the oil pressure?

Allan

Gundog

Low octane gas can cause knock and so can timing. Just trying to give some ideas as to what to check.

Allan In NE

Dunno, there's a huge difference between knocking and pinging.

Allan

Gundog

Allen has a point, but as it was not knocking when idling, I went with low octane fuel and timing.

justimagination

How's the oil pressure cold & warm ?
david

chickenfried209

thanks guys, tomorrow my buddy and i will check the timing, oil and each cyclinder compression test, also its more of a "knack" then a knock if that makes sense.

a guy at autozone said its a rod, ummm... im not best mechanic by far but wouldnt it also "knack" at idle?

Chevy-Lover

I just had the same prob with my 01 suburban (5.3 vortec).... it was the oilpump in my case. Had the sound only when driving at higher speed like on highways. But you could also notice it at the oil pressure. The different you could notice when you shift from D to N with a warmed engine at the gauge. It felt down to 1/4. Shifting back to D it was almost back to 1/2. That was still enough pressure to drive at usual city speed, but was to low at higher rpm like you got on highways... Had this problem with 148 miles.
Hope you'll find your problem!

chickenfried209

ok so it ending up being the timing, it was advance past 8. how it got way up there i dont know. compression for all cyclinders but #3 and #5 were at 145 -150psi but #3 was at 70psi and #5 was at 80.