> I need some help. I have a 91 chev truck with a 350 in it. 108,000
> > miles. After it warms up and is at operating temp, it will knock when
> > it is under load, below 40 mph. The result of which makes the service
> > engine soon light come on. I did a code check and it sends out a
> > 43(electronic spark control). I started trouble shooting it. The
> > knock sensor is working, replaced plugs (checked gap), new cap/roter
> > and new plug wires and checked the timming. Tested the EGR valve by
> > plugging the vacume line(still get a knock). The EGR valve seems to
> > be working. Where do I go from here. Anyone know what to do next to
> > get rid of this knock. The knock seemed to subside after putting 93
> > octane fuel in it, however, the service engine soon light will still
> > come on. It was funny that when I put lower octane fuel in it, the
> > light would stay on anywhere from five to fifty miles or until I shut
> > the truck off. After putting in the higher octane fuel, the light
> > will come on and then turn off in about five miles of driving, or
> > again, until I shut the truck off. Any ideas. I have done nearly all
> > I can do without the proper testing equipment that I can't afford. >
> "Tested the EGR valve by
> > plugging the vacume line(still get a knock). The EGR valve seems to
> > be working. Where do I go from here."
>
> You have this test backwards. It's the EGR that PREVENTS knock.
> Bolcking it only makes it worse. (You block it to test whether it's
> opening prematurely.)
I'm back with this same problem. Here's what i did. I replaced the EGR valve. I went on line and found a GM parts place that was selling them at wholesale price. Called up the local Chevy dealer and they matched the price. Put the new EGR valve on, same problem. I advanced the timming roughly two degrees, still the same problem. I used ce-foam (spelling) in the engine to hopefully clean out any carbon that may have been built up, same problem. Outside of bringing the thing in to have someone hook up a scan tool to it, what else can I do. I could live with the Service Engine Soon light comming on, but it's aggrevating that I can't get this fixed. Any other ideas, thanks.