Getting rid of spark knock

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85lsxMCSS

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I recently bought a 94 2 door blazer and it has 107k on it .the guy I bought it from said it'd been sitting for 1.5 years since he got s newer Range Rover to drive around. He said the Blazer was borrowed from a friend who put in 87 octane and he said it started to have a knock .

So I bought it and we dropped the oil pan. All the bearings and caps were in great shape. We swapped the oil pump for a new one while down there. Oil pressure was always good.

I replaced the plugs and wires, all the fuel lines and a new fuel filter. We dropped the tank and drained the old gas out and put fresh 89 octane in it . It still has spark knock at certain tikes .most evident is when you are at a steady rpm while in park and do a quick rev. That is when it is the most frequent.

Also the trucks idle in park is around 900 but in gear (drive or reverse) it drops to like 300 rpm on the tach and feels very rough .

Any thoughts? No codes are being thrown .
 
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85lsxMCSS

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Well, seems to me that the timing is off as is the idle obviously. Check for vacuum leaks. Most of the GMT400 experts (not me) will show up tonight after work.
My guy did say he checked the timing and it was spot on. Thats the first thing he checked . He checked the EGR valve as well and it's working accordingly.
 

iamdub

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Correct firing order? I've seen people cross a couple wires and the engine still ran almost normally.

When quickly stomping the throttle from idle, does it knock any less when the engine is cold?
 

bigfootchiro

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OOOOO! I love knocking and ticking sounds!!!

Like swath and dub said, I would be checking timing, vacuum leaks, firing order. Double check the timing, because being on just a couple degrees can throw everything off. 900 at idle in Park seems a little high, but 300 doesn’t make sense. The engine would cutout and die. Check cap and rotor, as well. They may need to be replaced. Are plugs gapped properly? Any way for you to take a video of the sound or record it? You’d have to post it to YouTube, then post the link here. Trust me when I say, you may think that knock or tick is one thing, but could turn out to be something entirely different!
 
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85lsxMCSS

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Correct firing order? I've seen people cross a couple wires and the engine still ran almost normally.

When quickly stomping the throttle from idle, does it knock any less when the engine is cold?

I haven't tried to do it from a cold start. Usually have let it warmed up for s few minutes prior to trying it . It never made a noise at a idle .At all.

Wires wise we did one at a time. Not sure if the guy before mixed them up we went off what was on there. But all the wires seemed to be correct length per dpit they are currently in right now
 
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85lsxMCSS

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OOOOO! I love knocking and ticking sounds!!!

Like swath and dub said, I would be checking timing, vacuum leaks, firing order. Double check the timing, because being on just a couple degrees can throw everything off. 900 at idle in Park seems a little high, but 300 doesn’t make sense. The engine would cutout and die. Check cap and rotor, as well. They may need to be replaced. Are plugs gapped properly? Any way for you to take a video of the sound or record it? You’d have to post it to YouTube, then post the link here. Trust me when I say, you may think that knock or tick is one thing, but could turn out to be something entirely different!

He referred to his buddy who's a tech in GM about the timing and he said the ting is where it needs to be .that was the first thing we checked .

It's definitely not a tick .it sounds like a loose nut in a tin can type sound that isnt steady .

We did not do the cap or rotor just the plugs and wires .Plugs are at .40 gapped .They were pre gapped and I checked to make sure they were
 

bigfootchiro

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He referred to his buddy who's a tech in GM about the timing and he said the ting is where it needs to be .that was the first thing we checked .

It's definitely not a tick .it sounds like a loose nut in a tin can type sound that isnt steady .

We did not do the cap or rotor just the plugs and wires .Plugs are at .40 gapped .They were pre gapped and I checked to make sure they were

Like I said, there are far too many ticks/knocks that could potentially happen. Try and take a video and post it. Could even be a loose/broken flywheel.
 

iamdub

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Weak fuel supply (volume and/or pressure)? This could cause it to be really lean and knock when goosing the throttle. The idle could've been low from the weak fuel supply so maybe the previous owner cranked on the idle screw to keep it running but it bogs immediately under load, even a small one such as just being in gear. Just throwing out guesses.

*EDIT*

This is TBI, yeah? Run it with the filter off and spritz some gasoline from a spray bottle when revving or just idling in gear to see if it smooths out any. Try to give it a consistent amount.
 

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