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SM465 eternal rattle (spark knock) on highway

If the engine uses enough oil to smoke some,then its likely enough oil is getting past the rings to carbon up the top end,and the deposits can glow and pre-ignite the fuel mixture..the top end cleaner may help in that case..also excess carbon build up will increase the compression by reducing the combustion chamber volume,which would encourage pinging on lower octane fuel...
 
That all makes a lot of sense. I'm going to check plugs and do a compression test. It seems to have two different issues with oil consumption, each on a different bank. A warm restart gives a nice puff out the passenger side (but clears right up), and a cold start gives a very light haze out the driver side for several minutes. I think the latter issue has developed since I have owned the truck, probably from all of this spark knocking. I think its in good enough health for now but I'm going to keep an eye on it. Wish I had known this was a spark knock issue a long time ago.
 
Could just be valve stem seals if it only puffs for awhile after being started--but eventually enough oil will build up and coat the piston tops,valves under their heads,and maybe the spark plug electrodes ,which can also turn red hot fairly easy and cause pre-ignition..sometimes a colder heat range plug helps reduce pinging,but they may foul faster in low speed driving..

I'd also make sure there are no vacuum leaks,unplugged or damaged hoses,the power brake booster is holding vacuum,and the transmission modulator too,if you have a TH350 or TH400....I've seen a defective PCV valve or the wrong one cause a lean mixture and encourage pinging too..

I've seen one engine that pinged a lot and also would develop a rough idle and a "flat spot" on acceleration after it warmed up good,an old 283 that ran decent otherwise and didn't use oil--nothing was found doing all the usual checks,then one day the owner decided to upgrade to a 4 barrel aluminum intake and carb,and when he removed the original cast iron 2 barrel intake,he found a long crack on its underside,which opened up when it got hot--it was the heat riser passages casted into the intake..the oil in the lifter galley was cooked to a carbon like substance from the exhaust gasses escaping from the intake..

After the mess was cleaned up and the new intake and carb were installed,all the problems vanished--it was evidently cracked on the "carb" side and letting exhaust mix in with the incoming air/fuel mixture...(EGR gone wild !)..there was a lot of sooty deposits under the original carb when it was removed..
 
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Great news. I got the AC Delco EGR valve last night and tried it out. I estimate it cleared up about 90% of the noise... MUCH better than it was. And surprisingly, it appeared to be made by the same manufacturer as the BWD from OReilys I removed. Same case, and same print on it, but the part number ended in 16P instead of 15Y or something like that. i will take pictures with the exact information later in case anyone else runs into this issue because what a freggin' headache this caused me... It would seem that Oreilys stocks the incorrect EGR valve for an H code 305.

The truck runs better now too, the throttle tip in is more responsive. I'm going to try some engine cleaner next to see if that takes care of the rest of the noise, if not I might still try that adjustable vacuum advance. The remaining spark knock is minor and I should at least be in the ballpart that some small adjustments clean it up for good. Or worst case I keep premium fuel in one tank for use on the highway. While I was running it down the highway yesterday I was switching back and forth between fuel tanks and the noise would disappear entirely when I switched to the tank with 89 / 110 octane mix.

Whats interesting to me though is Ive had a number of people suggest removing the EGR. I did that last year temporarily and that was the worst spark knock condition it ever had. Is my engine unusually sensitive to these things or is it normal to have to retune the distributor / carb to account for this?
 
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Engines made in the 80's were designed with emission controls ,they had cam profiles and spark advance curves tweaked to allow the lean mixtures and EGR,etc,to reduce emissions and yet not have too much driveability issues...

When you go deleting things like an air pump,EGR,and change the carb and or camshaft,things are thrown out of kilter,and the engine will need to be re-tuned to make the most of the altered state it has been modified to--many are dissapointed when they put all those aftermarket parts on and find it does not really run that well..the spark advance curve,fuel mixture,and initial timing all need to be tweaked for it to run well...some engines will have issues with spark knock due to the combustion chamber design and compression ratio..
 
My understanding is that EGR increases the amount of timing advance you can safely run by a few degrees. But deleting it seems to make a drastic difference on mine. Just seems odd that other people are able to do it without a problem.

I think i can eliminate the issue entirely by limiting some of the vacuum advance, because throttle tip in on the highway is when it knocks the worst.
 
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With carbed setups the primary rods (since EGR is used only at part/light throttle, such as cruise) and carb tune are intentionally "lean" where the EGR should be in operation...EGR being inert, they had to cut fuel somehow since there was less oxygen available to burn. If you run EGR but don't cut fuel, the fuel won't burn optimally.

If you remove EGR but don't adjust/tune the carb, it then ends up running lean. Did this with my '83 305.
 
I think it is running a little too lean even with everything configured to the factory spec, and this might be because of the 10-15% ethanol mixture in today's fuel. I haven't looked into whats involved in richening up a qjet at part throttle. I believe the screws at the base only affect idle?
 
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JoeliusZ28 said:
I think it is running a little too lean even with everything configured to the factory spec, and this might be because of the 10-15% ethanol mixture in today's fuel. I haven't looked into whats involved in richening up a qjet at part throttle. I believe the screws at the base only affect idle?
Click to expand...

That's correct. I stopped really digging into the Quadrajets years back, but as I recall things such as APT, rods/jets, and power piston spring are all adjustments tied into part throttle fueling.

I suspect if made, you could go with primary rods slightly richer JUST where part throttle falls. All I did before I knew better, was put in slightly richer rods/jets on the primary side. Didn't notice any downside to doing so, seemed to run ok, but the part throttle lean surge disappeared once I did. Didn't notice any knocking, but could definitely feel the lean surge once EGR was removed.
 
I'll look into whats involved in that. Going to try engine cleaner first per what diesel4me said, and from there i can either pull some timing out of it or richen the carb. Either way, I think im pretty close to nailing the coffin shut on this one
:burnout:
 
If your vacuum advance is hooked to manifold vacuum it will have full spark advance at idle,and drop off as the throttle opens more,zero at wide open throttle...most emission controlled engines are hooked up that way..
Back in the old pre-smog days they used ported vacuum,that would only let the advance kick in AS you opened the throttle,no spark advance at idle...you may try hooking the hose up either way,and see which way it spark knocks the least..

Most carbs made after 1975 have an APT (adjustible part throttle) screw that is hidden under a little cup plug somewhere on the carb casting--every book and carb kit info sheet will say "never attempt to re-adjust this screw as it is factory set--IF they even show it or mention it at all...

I have messed with a few of them ,one on a '75 one barrel rochester on a 250 straight six I had,pinged like crazy at "cruise" and would surge at 35 mph or so,until I opened the screw out another full turn or two,had to keep driving it to see if improved any--I was able to get most of the spark knock to go away after fattening up the fuel mixture and hooking the vacuum advance to manifold vacuum..

Then I had a hard time trying to plug up the "hole" in the casting where the little cup plug was,I tried a few methods,like a lead fishing sinker tamped into the hole with a punch,that leaked pretty bad,then I tried tapping the hole for a brass fitting and J-B welded it-- and the best I could get it was no leak at first,then a slow drip leak,which always bothered me ,I was always fearing it would really start leaking worse and start a fire..
I think on a Q-jet that APT screw might be located where fuel cant ooze out,but I haven't had one to look at in like 10+ years..you can find info on google about the APT screw..
 
For cruising down the road, you're looking at changing primary rods/jets.

Refresh my memory, did you try turning the distributor back a bit to try it, regardless of what the timing light said?
 
diesel4me said:
If your vacuum advance is hooked to manifold vacuum it will have full spark advance at idle,and drop off as the throttle opens more,zero at wide open throttle...most emission controlled engines are hooked up that way..
Back in the old pre-smog days they used ported vacuum,that would only let the advance kick in AS you opened the throttle,no spark advance at idle...you may try hooking the hose up either way,and see which way it spark knocks the least..
Click to expand...

Ported vacuum is an emissions strategy. I'm not sure if all Q-jets, even early ones were setup this way or not, but all the later ones have a slot in the primary venturi for "timed" (ported) advance. If the throttle blade was above or below that, vacuum wasn't getting to the distributor, or minimally. There is little vacuum when you are actually on the throttle. Doesn't matter where you check, vacuum does not increase as throttle opens (except some very specific conditions that aren't relevant here). Hook a vacuum gauge up to your intake and drive around. That's why the old school fuel economy gauges ran off vacuum. Opening the throttle decreases vacuum, and thus vacuum advance if it's hooked to manifold or ported vacuum 9again, assuming the ported slot is "open" to vacuum).

You will have better quality idle, guaranteed, if you bump it up to 20* or so. It's just hard to do that, meet emissions, and have proper timing elsewhere with mechanical/vacuum advance, thus GM got "creative" with timed/ported advance, temperature switches, 10 miles of vacuum hose, etc.
 
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Well i started tinkering around with it a little last night. Its for sure hooked to ported vacuum (which is working), and that is aligned with the diagram on the sticker on the core support. So thats the way it came new.

I changed the vacuum advance mechanism out to a crane adjustable last night and took it for a ride. In first two initial settings I tried (quickest advance setting), the problem was the same if not worse than it was yesterday.

To get a better understanding of how everything worked i took the cap back off and watched the behavior of it with a range of adjustments, it seems like backing it way out will limit the advance substantially so im going to give that a try. I also have the advance lockout key, and a selection of different centrifugal advance springs at my disposal.

Right now the base timing is set at the factory specification, which is already as low as Id like to go with it since throttle response was way better with it advanced a couple degrees. I don't want to run it any lower than the factory spec, because that makes it feel really sluggish, especially off idle.

I think I am going to be tinkering with this thing for a while before I get it right. Switching to the 89/110 mix tank still eliminates the noise almost entirely, so i have to think its possible to get it to run right on 89 or even 87. I could probably just run 93 all the time but i dont think that should be necessary on this beasty 305.
 
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It must have had the ESC system stock. Is that still in place? The 305s had higher compression and liked to knock more than the 350s, so they all seemed to come equipped with it in mid-80's.

How have you verified where #1TDC really is? To get the cruise timing down, you may have to reduce the vacuum advance or go to stiffer springs. The easiest solution may be to turn the whole dizzy back to 0 and change the vacuum advance to direct (not-ported). Part of the laziness off-idle is that those factory settings have you under 10* when you crack the throttle.

You really shouldn't be able to hear spark knock ever. You can drown it out with fuel, but that's more appropriate for high load conditions. At cruise, if you're lean enough to knock, it should be running weak. Higher octane fixing it is proof it's real knock. It could be something is damaged in there from running with knock for so long.
 
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Youre right I forgot about that. This truck did have the ESC system in place and functioning when I bought it. I bypassed it after it was causing a no spark condition that took me forever to figure out and originally I never noticed any spark knock that resulted from that change. The entire issue began last year at the sand dunes when the factory EGR failed and I had to drive it 3 hours home with a beer can blockoff plate. After I replaced the EGR valve (with the wrong one) it "healed" the noise temporarily and then it came right back after 2-3 drives so that was when i started to suspect the transmission being the issue and not the engine since I had just had the trans out for another issue.

Have not verified TDC yet, but I am considering changing over to manifold vacuum. I wasn't sure how safe that was to do if ported distributors had different advance curves than manifold ones.

It would not surprise me if something is damaged from knocking so long, and if so it most likely happened on that three hour drive home last year when the knocking was getting louder over the course of that drive despite turning my timing back and running premium in it. Ever since then ive just done short highway trips trying to diagnose the noise, and it never spark knocks at anything under 60 mph so ive put plenty of miles on it keeping it within those speeds.
 
The valves and piston tops would be the first things damaged by long term spark knocking most likely,followed by the rod bearings..

I think I may have an EST distributor from a mid 80's 305 I got out of a G10 van ,I put a "regular" HEI in it when I swapped that engine into a '79 C-10 I had...(which never spark knocked with either distributor,but it still had a functioning EGR and Edelbrock 4 bbl)..
 
Yes, GM obviously knew the 305 was on the ragged edge of it's limitations when they did the whole ESC thing. Too much compression with crappy heads in a heavy vehicle. ESC was/is there to make sure that if something isn't just right, it doesn't detonate itself to death.

If there is any damage or carbon in the cylinder, any raised portion (as previously mentioned) can be enough to ignite the charge. That you can't fix it with premium and dialing timing back is a bit odd however.

The solution is obvious, 350. :) Or, if you want to be really part of the in crowd, you go LS.
 
Well, turning the dist back and premium fuel didn't fix it when I was trying to get it home with EGR blocked off. I think premium fuel would fix the knock today. 89 isnt enough. The new EGR valve probably fixed 80-90% of the knock ive been experiencing. Some might be happy with that, im not.

If some minor tweaks dont silence it, I will be looking in the direction of a 260hp crate 350. No LS, keeping this truck as original as possible.
 
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