Hello,

I have a 1990 Chevy K1500 5.7 350. A few months ago the truck started making a “knocking” noise ONLY UNDER LOAD. When I’m not giving it too much gas it won’t make a noise but when I’m getting up to highway speed it’ll constantly make the noise. If I’m cruising around 50-60 mph it will also knock constantly. It also runs rough at idle.

I have replaced the EGR valve twice, the EGR solenoid, the small hose between the valve and the solenoid, all the spark plugs, I have checked the time the timing is spot on, drained the oil (no glitter)/ replaced the oil, new o2 sensor, replaced a couple vacuum hoses that had small cracks and ran a tank or sea foam to try and clean anything out.

im currently at a loss and dont know what to try next. Any advise or anyone have this same problem with a fix?? Thanks in advance!

 

What you’re describing sound's like spark knock - if the noise has a kind of ping or bell type tone, that’s the most likely cause.

First make sure you’re using the correct octane rated fuel - 87 or 89 octane should work - if the fuel is outside the correct rating, the ECM may be unable to compensate for it.

The knock sensor is located down by the starter - some engines did have a 2nd sensor on the opposite side of the block.

Key on, engine off, unplug the sensor and check for 4.5-5.5 volt reference from the ECM. If it’s there, replace the sensor making sure not to over-tighten it. Note that some of these sensors doubled as block plugs for the coolant passages, so be prepared to catch some fluid.

 

well it's an old old truck. The cylinders might be full of deposits, or the chain is stretched. Have you looked at the spark plugs?