knock quesion.

[j894]

Could a bad spark plug or an incorrectly gapped plug
cause spark knock?

When I start cold there is a knock , until it warms up.
Then once it warms up, it only knocks sometimes if you leave it sitting at
idle in park. The ignition components are all new. New cap, rotor, coil, wires,
plugs. About the plugs they are the stock ac delco replacements(i dont
remember the number) about a year old. When we originally gapped them
we put them at .035 book was the wrong one, thena few months later found
they're supposed to be around 0.45.

I regapped them around .050 when they were last pulled. the thing is we
broke 2 plugs trying to get them back on #3 cylinder & had to put in replacements.
could that be somehow causing it to screw up. Or can the gap becausing a miss?
Pretty soon I'll be pulling the regular delcos for rapidfires, if the problem
doesnt stop.

Btw. Ive been running premiun fuel for months. I think the timing is advanced a
degree or 2. But it knocked even before I left it with an advance. Also I seafoamed
just a few weeks ago, and then a few months before that. So I dont think It would
have alot of carbon build up? or would it?

I just want the knock to stop. and get through the rest of the winter
thanks for any help

[bvr775]

yes an plug gaped wrong can cause knock. you need to gap them at 0.50. If you have broken plugs you need to replase them and quit arsing them down so tight.

myth: you need to tighten your plugs with an breaker bar and a cheater.
Not true. this is a commom mistake made by many weekend techs. your plugs merely need to be a bit more than snug but not super gorrila tight. If you touque them down to much your risk damageing you plug hole treads and haveing the plugs seize up makeing near impossible to remove them.

[j894]

It wasnt cause of overtightening them, if was cause of the extreme
angle goin in past the steering shaft, with the ujoint spark plug socket.
Since then ive used a different method for removing that one. All the
rest I just make them snug, maybe a little more.


I thought older plugs + a new one might affect something somehow.
But I did not try using broken plugs.
anyway. In the hanyes manual the recomended gap is like .045
i have 0.050 (maybe it was at 0.055 I cant even remember) I guess
that might cause it, like you said?

[OBI WAN]

What year, what motor?

[j894]

93, 4.3l Cpi ...like it says in the sig^

[biglouie_underpressure]

piston slap?

[j894]

I think its ok now. I checked it yesterday and the base timing was at 4*btdc
I must have bumped the distributer last time I was adjusting it, & bolted
it down without noticing. I set it to just about 1*dtc. The higher combustion
temperatures from the disabled egr didnt help it either. It doesnt seem to be
knocking now. DUH. Im putting in the mid grade stuff now, workin my way back
down to 87. Then see how much timing I get from it & work my way back to
91, 93, 94 again. Gas here is goin up again. Not good when you drive 300+
miles a week.

Im assuming it was knocking before, when the base timing wasnt advanced
because of the fautly knock sensor. Then was still knocking occationally till
now cause timing was readjusted.

I also just realized Ive been putting in 91 octane(sheetz premium),
for the past couple of weeks/months instead of 93 (sunoco). or even 94 which
I havnt run in months. I dont think that helped either

[LEADFOOT]

It wasnt cause of overtightening them, if was cause of the extreme
angle goin in past the steering shaft, with the ujoint spark plug socket.
Since then ive used a different method for removing that one.
How about letting us in on that method.

Also I thought the correct gap was .060.

[Matt 4.3 TBI]

The clearance is a bit tighter on 2nd gens from what I've seen. For the #3 plug I just put the socket on the plug, break it loose with a 3/4" wrench or a ratchet and unscrew it by hand.

I would go a bit smaller than 0.055 unless you're running an MSD box. My Chilton's says 0.035 for a CPI, but I would go with what the sticker on your fan shroud says.

[Matt 4.3 TBI]

Another question, have you checked for an exhaust leak? Around the exhaust ports, manifold flanges and EGR are places to check. They usually sound like a metallic tapping.

[j894]

How about letting us in on that method.
It has to do with using a 3/8 breaker bar. but theres more to it than that
I wouldnt be able to explain.

Yea accualy. I know theres a leak right off the passenger side manifold,
by the y pipe where i cant see. That does make the exhaust sound weird but
thats not what I was hearing.

[bvr775]

How about letting us in on that method.

Also I thought the correct gap was .060.


it is. for your truck.

the factory gap depends on engine size, plug type used for the engine, ignition voltage, compresion, fuel delivery, factory ingition timming, and allot of outher things. Usally the gap on older engine designs is tighter than newer engine designs.
There should be a tage on the rad housing telling timming, engine, plug gap and so on. Unfortunatly most of these tags long vanishied or due to a wreck are not incorrect. What to do? Call up a dealership and have them run your vin and allot of times with older trucks the parts stores will ask for a vin # and they have all the info on your engine in their comp.
your vin number is the surest way of getting the correct parts for you truck, but what if the engine is aftermarket or reman? this is where casting number come into play, but that's pretty involed so i'm not going to get into it.