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SM465 eternal rattle (spark knock) on highway

Got an appointment at my friends shop for next saturday for their best mechanic to check it out. Its a restoration shop that doesn't typically work on anything newer than 1985 unless its something special. The guy there wants to test it on the highway in 4wd to eliminate the tcase as a potential cause. While I trust him, I can't find any info about what the max recommended speed for a locked-in NP208 would be. Anyone know? I can usually get it to start making the rattle around 60mph. This would just be for a short stretch to see if it makes the noise.

Also, the trans started dripping from the input shaft yesterday, probably 8-10 drops worth on the ground since i parked it saturday night. This is with a fresh retainer gasket in place and the seal was replaced 3k miles ago. Was a little surprised by this... would it be an indicator of a sloppy input bearing?
 
As long as you are not actually in 4WD, in other words, hubs unlocked or slugs removed, most any speed should be fine. Of course it depends on the condition of your front driveshaft.
I would not want to run one of the homebuilt, square, unbalanced shafts at high speed.
Despite what some of the wild guys online here say, its not good to run on dry pavement in 4wd unless you have some way for the front and rear to run at different speeds.
 
I agree with Fordum..

The NP208 is a part time case,it is not adviseable to drive in 4wd on dry pavement with the hubs locked,I would not go over 25 mph tops on tar if you try that--you'll feel the wheels fighting you some because the front and rear axles are not "perfect matches"--they are very close but are slightly different ,like if you had 3:08's in the rear,the front axle may have 3:07 ratio due to the different ring gear sizes..any variation in tire sizes will also cause them to spin at slightly different speeds,that can lead to pulling or a sudden swerve situation..

I often have to leave my NP208 in 4wd in winter when I drive local snow covered streets,hubs locked in--then I'll come to a busier road that has bare tar and the truck will start pulling and jerking around corners and you can feel the extra resistance and strain put on the driveline..my t-case wont "unlock" when I put it in 2wd for some reason,it still sends power to the front shaft,and I just leave the hubs unlocked during warm weather...been told if a hub ever decided to lock in by itself at speed,it could lead to loss of control,or even flip it..(so far that has never happened)..

I would think the leak would indicate a loose input bearing or seal on the main shaft..ball bearings can "feel" ok as far as being tight ,but still be noisy..
 
The front seal shouldn't gonbad that quick unless there is a problem. And bad bearing or a worn input shaft could still be a problem. My front main bearing is noisy but so far hasn't damaged the seal. I've been needing to replace as well but I only put a few hundred miles in mine every year so it hasn't been replaced yet.
 
Ok thanks for the input. I was concerned about running it locked in that fast as well. The truck has the OEM autolockers (that work) so I think I'll just try pulling the front drive shaft and see if it makes the same noise in 4HI.
 
I forgot to mention that as well. 4 wheel
Drive doesn't like to work well on dry pavement and usually not good to drive above 25-30 mph in my opinion. Pulling the front ds should alleviate the transfer case locking up or binding. I had thought the transfer case was my problem as well. But when I pulled it everything there seemed okay. The rear had a little play so I replaced the rear output shaft and bearings. Still had the trans sound. Have you checked both rears as well. Gear howling can go through the drivetrain as well. It's frustrating as all hell trying to find that intermittent noise sometimes.
 
Yeah I know it wouldnt be "good" for it to do the 4x4 test... if i did it with everything locked it in was going to try to keep it perfectly straight and only run it long enough to get it to high speed and then back down again.

At this point if there's any issue with the trans im think im just gonna have the entire thing rebuilt. Really don't feel like pulling the trans a 4th time if i can help it.
 
<<<<At this point if there's any issue with the trans im think im just gonna have the entire thing rebuilt. Really don't feel like pulling the trans a 4th time if i can help it.>>>>

I have heard it gets easier after the sixth time.............
 
Fordum said:
<<<<At this point if there's any issue with the trans im think im just gonna have the entire thing rebuilt. Really don't feel like pulling the trans a 4th time if i can help it.>>>>

I have heard it gets easier after the sixth time.............
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Fact, because I'll have sold it by that point :D
 
Update (Short version) - The noise isn't the transmission, its spark knock

Update (Long version)- took it to my friends shop and the verdict was that this sound isn't coming from the clutch/trans/transfer case at all, rather it's actually spark knock. It always did sound like spark knock to me when the noise was mild, but i had no idea spark knock could get this loud and "metal on metal" sounding. Last year I had also tried 93 octane fuel and turning the timing back and it didn't seem to affect the noise.

After the test drive he spent about 20 minutes messing around with the timing gun, checking operation of various vacuum lines, vacuum advance, testing the egr valve, etc. After all that, his theory is that aftermarket EGR valve is not flowing to OEM specification during those highway conditions combined with the fact that the truck was originally designed to run ethanol free fuel. My carb is recently rebuilt (had the same spark knock before the rebuild) and the ignition system is all OEM spec and timed to OEM specifications. They spent all of monday trying to source me an OEM egr valve with no luck. In the meantime he suggested I test his theory by running a tank of ethanol free recreation fuel in it. I didn't do that, however I did find a gas station selling turbo blue (110 leaded) and mixed 1/4 tank of that with the 89 already in the tank... "only" a $30 test haha. With the turbo blue mixed in the noise on the highway was immediately about 80% quieter. So we are looking into other options now. He said he wanted more time to think about the best solution but we could be looking at some slight re-jetting of the carb and/or recurving the carb/distributor. I have a crane cams HEI recurve kit sitting on my shelf already if we need it. Unsure if we will try a different EGR valve or eliminate it at this point but looking back I never had this issue with the OEM EGR valve before it failed (almost a year ago now). The one currently on the truck is a BWD from OReily.

The good news is, I think I am done pulling transmissions for now :) The leak also seems to have cured itself, it hasn't left a drop since my last post.
 
Basic timing adjustments and fuel octane increases normally should cure the problem. If the detonation is so bad that they will not, then there is probably some internal problems with the engine...like the piston crowns are cracking.
 
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When the EGR valve failed last year I was 3 hours from home and had to make a block off plate out of a beer can to get it home. It made the same noise with the block off plate but if anything it was worse not better. Also, the spark knock I experience now is not loud all the time. It typically starts out mild enough that you barely notice it and the longer you go it eventually becomes obnoxious. I'm guessing because combustion chamber temps are gradually getting hotter or colder but I really have no idea. The engine seems healthy otherwise although it does use some oil. The power level seems appropriate for a 305. But I have been chasing this issue so long that there very well could be issues with the pistons. It only makes the noise on the highway... if you keep the truck at 55 or less you'd never know there was an issue.

If this turns out to be an excuse to order a crate 350... that would be really unfortunate :pimp:
 
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You may have something getting hot enough in the combustion chamber to pre-ignite the fuel/air charge before the spark plug fires--like carbon deposits,a spark plug electrode getting red hot ,or an exhaust valve getting hot enough to ignite things too far in advance..another thing often overlooked is a heat riser valve stuck closed or partly closed,(if you still have one & stock exhaust manifolds)--or some other restriction in the exhaust,which will tend to make the exhaust flow through the intake manifold heat passages and lean out the fuel mixture a lot..the back pressure adds more heat too..

Even my lowly 307 V8 of 1973 vintage said on the emissions sticker "designed for use with 91 octane or higher rating gasoline",and this was when lead was in the fuel,and long before any alcohol was added to gas...the engine only has like 8.5 to 1 compression too,but it'll ping with todays 87 octane unless I retard the timing several degrees--this reduced the power enough to justify putting "premium" gas in it..its only 115 hp stock to start with!..
 
diesel4me said:
You may have something getting hot enough in the combustion chamber to pre-ignite the fuel/air charge before the spark plug fires--like carbon deposits,a spark plug electrode getting red hot ,or an exhaust valve getting hot enough to ignite things too far in advance..another thing often overlooked is a heat riser valve stuck closed or partly closed,(if you still have one & stock exhaust manifolds)--or some other restriction in the exhaust,which will tend to make the exhaust flow through the intake manifold heat passages and lean out the fuel mixture a lot..the back pressure adds more heat too..

Even my lowly 307 V8 of 1973 vintage said on the emissions sticker "designed for use with 91 octane or higher rating gasoline",and this was when lead was in the fuel,and long before any alcohol was added to gas...the engine only has like 8.5 to 1 compression too,but it'll ping with todays 87 octane unless I retard the timing several degrees--this reduced the power enough to justify putting "premium" gas in it..its only 115 hp stock to start with!..
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Oh wow I thought these trucks were meant to run on 87, I have been putting 89 in it since I bought just for the "hell of it." The technician I talked to also suggested running some of this engine cleaner through it, but now I understand why he suggested that thank you. Come to think of it, one odd thing I noticed about this truck a long time ago is that the exhaust from one bank seems significantly hotter than the other. Evidence of some internal issues maybe? I noticed this just feeling the air out the tailpipes with my hand, and its further evidenced by the there are fact stainless clamps discolored on one side and not the other. Its a free flowing dual exhaust (the truck sounds really good in my opinion), and I've verified the riser valve is in proper working order.


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I am hoping that since the engine ran mostly quiet with some turbo blue, nothing is significantly damaged internally.
 
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Probably no damage yet,but it isn't good to let an engine ping for too long..not too many engines "carbon up" nowadays,gas has a lot of detergent in it now and ethanol acts as a cleaner somewhat also..but it wont hurt to try that stuff..the valves may have a build up of crust under their heads,which will lean the fuel mixture out a lot and reduce its ability to "breathe"..

I had a few 250 straight sixes with the "integral head" (aka as a"Camel humper " due to the valve cover shape--those were notorious for spark knock,they had either a one or two barrel carb depending on the year,that were leaned way out to reduce emissions--add in the fact these engines were overloaded ,being put in full sized cars and trucks ,that made them run very hot and they would ping ,even with high octane gas...I tried retarding the spark,that did not help,in fact they ran even hotter that way and pinged regardless,and lost a lot of HP..

I had one in a '79 C-10 Bonanza that eventually had the #6 piston lose the top ring lands suddenly...it only had 117,000 miles and I had adapted a Edelbrock 4 barrel to its 2 barrel intake after giving up on the E2SE Rochester original carb,that always surged,and made it ping badly...

I took the secondary linkage off the Edelbrock so it only ran on the primaries,and it ran sweet,till one day the piston decided it had enough on a highway trip!--barely made it home,and it ruined the cylinder,I decided to swap in a 305 V8 instead..

The 250 six in my '82 G-10 van also pinged,but not as bad as the '79 engine did,that had the original carb and I opened the main jets up a tad when I rebuilt it,that and the fact the van was not as heavy as a pickup seemed to let the engine work less hard and not knock or ping as much..

You may be able to change the advance weights in the distributor and or add an adjustable vacuum advance unit if your running an HEI distributor,enough to silence the spark knock without reducing power that much..
 
I didn't read the history of this engine, but be sure of the simple stuff here. Could 2 plug wires be swapped? Do you know for sure that your timing tab and mark are lined up with real #1 TDC? Have you measured the amount of advance your vacuum can is giving and how much the weights are giving? Distributor weights also like to stick. When's the last time you pulled each plug and looked at it? If something is running different on one bank, it should eventually show up on the plugs.
 
I have heard of cases where the damper gradually walks away from a true timing reading and 0 degrees isnt actually TDC. Right now i dont have any reason not to believe the reading we are getting but that is possible.

The tech measured the advance of each system while i was there, but i havent done it myself. I think the initial total advance he found was 42 degrees which he thought was high, and maybe upper 30s without the vacuum advance hooked up. We dialed it back 4 degrees because thats how much higher we were on the base idle from OEM spec. Haven't pulled a plug recently, I'll do that.

Basic history of the engine is it has 111,000 miles. I did a full tune up on it last year including ignition, carb, timing set, and cooling systems. I just ordered a new set of delco plug wires since thats one of the only things I haven't replaced on this engine (the ones on it looked new).
 
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Yes, the balancer can slip, but there has also been more than 1 timing tab setup for SBC over the years. If parts are mixed between engines you can be off. It's easy enough to figure out with a straw in the spark plug hole. Normally that would be far enough off to either belch smoke or be hard to crank. Retarding the timing will make the exhaust very hot, as the mix is still burning in the pipe. I suppose if two wires were swapped you could get 1 advanced to ping and 1 retarded to be hot.
 
Good points there. I'm at least confident this truck isnt a mixed bag of parts. Other than a couple small changes like dual exhaust and air filter, its pretty much an unmolested survivor.
 
I managed to track down a delco EGR valve available on Amazon and Rock Auto: https://www.amazon.com/gp/product/B001KSAOA2/ref=ox_sc_act_title_1?smid=A1BB0GDPEIAGXP&psc=1

Plan of attack is to install this first and see what happens. I also have a Crane Cams HEI recurve kit sitting on the shelf. If the truck is still spark knocking with the Delco EGR valve (I honestly believe it will cure it) then my next step will be to install the adjustable vacuum advance portion of the recurve kit and use that to pull some timing out of the vacuum advance, since the knock is heavily influenced by throttle tip in.

If that doesn't work the shop says they can make adjustments to the carb which will cost $200-500. However, i think if its still knocking after all that it would be evidence of internal issues (I already know the engine uses oil) so I'd probably leave the carb alone and start looking in the direction of a goodwrench 350.
 
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