My engine knocks under heavy load at low RPMs.

The knock is intermittent, but occurs at 87 octane and 93. It occurs at any engine temp, including: just after startup (engine stone cold), when the engine is hot (180 regular temp), and when it is too hot (220). It occurs with the timing advanced 7 degrees, and advanced 0 degrees (stock timing on L03).

I know a lean mixture can cause it, but when I pulled my plugs they were all dark, as though the engine were running rich.

What else could do it?

 

excessive carbon build-up in the combution chamber can do that... or excessive wear in the cylinder bores... or incorrect timing...
need more info as far as type of sound... could be other mechanical failure...
zroc

 

timing wrong, not enough octane fuel, too much compression. hot spots in the chamber.

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-=ICON MOTORSPORTS=-
1st & 3rd

 

I've run Top Engine Cleaner from GM, which should have done a number on the carbon build-up. I've also run Techron fuel system cleaner (probably not as strong as the top engine stuff). Since knock was present at 7 degrees and 0 degrees I'm thinking it's not timing. Since it's present at 87 and 93 octane I doubt it's insufficient octane. Heads are stock so (other than carbon buildup) I doubt compression is too high.

How do I describe the sound, zroc? It's kind of a clanky sorta thing, not real loud, only present under high-load (low RPM) situations. Interestingly it is NOT present at high RPMs (where it would be more prevalent were it a timing issue)... I had a friend with me who noticed it and commented that he heard knocking, so he also identified it as knock rather than mechanical noise.

Excessive wear in the cylinder bores... What other symptoms would that have? This engine does have almost 150,000 miles on it, and the previous owner (125K worth) may not have taken good care of it (it's getting Mobil 1 now).

 

Before you place judgement on the "knock" being detonation, first realize that there are alot of things that will sound similar to it. By telling us (or at least myself) that a friend heard a "knock" sound coming from your engine really doesn't tell me anything as to what could be your problem. Could you be more descriptive?

Now, here are some things that will sound like detonation: exhaust manifold/header leak, poor/low oil condition, worn hydraulic lifters (both roller and flat tappet), timing chain slop, ... I could go on futher. I mention this because you said that if you retard/advance the timing it still knocks. Obviously not knock in my opinion. Does it knock when sitting on the driveway and "playing" with the throttle?

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George P. Lara
1994 Z28 LT1 T56
1984 Z28 High Output
Member: SCCA, SCFB, SC3GFB
Chevy Info --> MadMike's

 

Sound is very hard to describe in words :-). The english language doesn't address it very well.

It is not exhaust manifold leak, I've had those and know what they sound like. This is more metallic sounding than that. Oil LEVEL and PRESSURE are good (Mobil 1 5w30) although of course I have no way of knowing how well it is getting to everywhere in the engine (could be a blocked oil passage somewhere). Compression was good on all cylinders, I can get the numbers for you if you'd like (don't have them in front of me). Worn lifters are certainly a possibility (they are roller) -- what do they sound like and/or how do I identify them? Timing chain slop is a possibility, how do I identify it?

No it doesn't knock in the driveway no matter what I do. The only time it knocks is in 3rd or 4th gear with the TC locked accelerating hard (well, as hard as it will accelerate without downshifting or unlocking the TC) from a slow speed or a stop, in approximately the 1500-2000 RPM band (above the TC stall speed, but not very high RPMs). Basically the highest-load situation it will be in.

I have not gotten it to knock when in gear and revving the engine but holding it back with the brakes (to "simulate" high-load) but on the other hand, I can't rev it much above the TC stall speed in that case or I just wind up doing a brake stand.

The best way I can describe the sound is it has a "marbles in a coffee can" kind of quality. Metallic, rattley... It's hard to say if it knocks faster than the RPM speed because the RPMs are changing so much as I accelerate. But not too loud, I can hear it over the engine noise but only just.

 

How old is your Dist cap, rotor and wires?

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Cap and Rotor (and pickup coil, and ignition coil) are a year or a year and a half old. The cap and rotor are Accel, the pickup coil is a parts store brand (forget which), and the ignition coil is MSD (GM Blaster -- stock replacement). The wires are MSD 8.5 SuperConductors.