Author Topic: 79 K10 Misses under load  (Read 3253 times)

Offline cymrych

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79 K10 Misses under load
« on: February 26, 2012, 10:29:17 AM »
I've scrounged this forum for a few weeks, but this is my 1st post here.

I have a 79 K10, 230k miles, 350 with 4sp (3 + granny low). All stock. Bought the truck from my uncle, who had it from new. He rebuilt the motor at 80k, due to soft factory cam wearing out.

My job takes me all over the country from project to project, and 90% of the 40 thousand miles I've put on the truck in the last 5 years have been on the interstate with a small (under 3000 lb) trailer on the hitch.

About 2 years ago, cutting across Wyoming in the dead of summer, I developed a slight irregular miss at highway speeds. It only popped once or twice, but I had to drop my speed to about 55 mph to keep it from happening. Got to the job, checked into the hotel, and promptly changed cap, rotor, wires, and plugs. It seemed to have fixed it, but I wasn't pulling the trailer for around-town driving, so I knew it wasn't simulating the load conditions exactly.

 Over the last year, this miss became *slightly* more apparent, but still, only when pulling a load up a grade at highway speeds, and only when it was pretty warm outside. If it was nightime driving, or from October through about April, it didn't seem to do it. Idle, 1st and 2nd run great, with no miss no matter where in the rpm range I push it, or for how long I have to keep the hammer down (definitely needed on some of those Colorado mountains). Sometimes, she won't do it at all no matter what the conditions. For example, drove all 2100 miles from Utah back home to Pennsy in October, she purred like a kitten the whole way without any issues.

Well, Thursday I got the call to return to work, so I left last night from Pennsy for Indiana, trailer on the hitch. About 150 miles into the drive, she started missing, much worse than anytime before. On flat ground, if I tried to push her past double nickel, she'd start sputtering and I'd have to back off. On a grade, about all I could do was let my speed bleed off to about 40-45, and she'd mostly be fine. From stopped to 50 mph, no issues at all. 1st gear and 2nd, full throttle range without any misfired. But try to give her more gas on a hill to get to 55? Forget it, she just spits and sputters and I can't build up any more momentum. 45-50 mph, seemed about all I could manage without constant sputtering, and that was only on the few flat areas. Of course, then I was getting buzzed by idiots in rice grinders on icy I-80 in hilly central PA ... not a good place to be.

Anyhow, only so much I could do on the side of road. I dumped a fuel cleaner/octane boast in the tank (93 oct is what always ran best), but no change. Before rattling the motor apart from the misfires, I opted to play it safe and nurse her back home where I could fix the issue. Took me twice as long, but 2 lane roads at 45 mph let me nurse her home, not misfire-free but drivable.

So, my intial thought is distributor putting out uneven/weak spark at high rpm and moderate to heavy loading. This would also explain why the truck just felt like it couldn't handle the hills as well as it used to, even when it didn't actively misfire, plus my ever-worsening fuel "economy" (I use that term very loosely, lol.)

My only concern is what if its not the distributor itself, but the cam bearings? I honestly wouldn't have even considered the cam, except for knowing that my uncle had a soft factory cam when he got this truck from Detroit, which started presenting problems under load and led to his rebuild of this motor at 80k. How can I test if it's the distributor itself causing my misfire, or if the weak distributor is only a symptom of a cam with worn bearings warping under high rpm and load? Or maybe it's not distributor at all, but valve float ... which would again point to a worn cam?

Any thoughts would be *greatly* appreciated. Been laid off all winter, it'd be nice to hit that highway again and starting making some cash again! Thanks

Offline zieg85

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Re: 79 K10 Misses under load
« Reply #1 on: February 26, 2012, 01:13:26 PM »
Welcome from NW Indiana.  Unless I missed it, by chance did you change your gas filter?  I know the timeline with the issue has stretched on for a while but my 85 C20 had the very same issue.  Sometimes it ran near perfect and other times it was a challenge to keep it in 4th.  My gas filter was almost plugged solid and I was surprised it ran at all.  I have also had my sock filter in the tank clog up.  I blew back through the fuel line and it cleared it...
« Last Edit: February 26, 2012, 01:35:00 PM by zieg85 »
1984 C10 Custom Deluxe 4.1L 3 on the tree
1985 C30 Custom Deluxe Ext. cab 5.7L TH400 4.56 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 Custom Deluxe 5.7L 3 on the tree 2.73

Offline bd

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Re: 79 K10 Misses under load
« Reply #2 on: February 26, 2012, 01:14:48 PM »
Hello cymrych!  Welcome to the site from Central California!

That is a very explicit and thorough writeup with a lot of good information.

So, my initial thought is distributor putting out uneven/weak spark at high rpm and moderate to heavy loading. This would also explain why the truck just felt like it couldn't handle the hills as well as it used to, even when it didn't actively misfire, plus my ever-worsening fuel "economy" (I use that term very loosely, lol.)

Temperature related failures can be difficult to diagnose, because they are hard to simulate under controlled conditions (like in a shop) w/o special equipment.  Thermal stress can cause intermittent module, pickup coil and ignition coil failures like you're describing.  Begin by inspecting for any indications of heat stress or carbon tracking inside the distributor cap and around the plastic casing of the ignition coil.  Make sure the steel ground strap between the frame of the ignition coil and the HEI harness plug is present.  Look inside both ends of your spark plug wires for evidence of black soot, indicating burn-back of the wires.  Check for discolored hard (baked) spots along the length of the plug wires.  If none of your tests or observations reveal definite faults, you may need to replace the module, pickup coil and/or ignition coil.

About 2 years ago, cutting across Wyoming in the dead of summer, I developed a slight irregular miss at highway speeds. It only popped once or twice, but I had to drop my speed to about 55 mph to keep it from happening. Got to the job, checked into the hotel, and promptly changed cap, rotor, wires, and plugs. It seemed to have fixed it, but I wasn't pulling the trailer for around-town driving, so I knew it wasn't simulating the load conditions exactly.

How many miles since your tuneup 2 years ago?  Maybe it's time for new plugs, again.  Replace your fuel filter and check your fuel pump vacuum (5-7+" Hg), fuel pump pressure (5-7 PSI), and fuel pump volume (1+ pint in 30 seconds) while cranking.

My only concern is what if its not the distributor itself, but the cam bearings? I honestly wouldn't have even considered the cam, except for knowing that my uncle had a soft factory cam when he got this truck from Detroit, which started presenting problems under load and led to his rebuild of this motor at 80k. How can I test if it's the distributor itself causing my misfire, or if the weak distributor is only a symptom of a cam with worn bearings warping under high rpm and load? Or maybe it's not distributor at all, but valve float ... which would again point to a worn cam?

In reference to "normal use" highway vehicles, cam failures generally result from one or more lobes wearing flat, decreasing valve lift.  As long as oil pressure is adequate, cam bearing journals are rarely, if ever, an issue.  When an exhaust lobe wears flat, the most common symptom is a backfire, "popping" back through the intake, especially with rapid full-throttle application.  Even with the transmission in neutral, rapid throttle opening may cause popping back through the intake.  A flat intake lobe can also cause backfiring, though not as pronounced, but is more likely to result in a simple misfire.

If you suspect a flat cam, connect a vacuum gauge to your intake manifold and check for a "bouncy" needle.  Or, remove the valve covers and check for any loose rocker arms and rocker arms with relatively decreased motion while cranking the engine.  If you're still not certain, measure the valve lift.  And/or, collect an oil sample in a clean plastic cup and have it analyzed by a local company who specializes in oil analysis for commercial fleets.  Check with local big rig truck shops for a resource and a sample cup.  The turnaround on oil analysis is typically about 2-3 weeks.

...Any thoughts would be *greatly* appreciated.

Remove the distributor cap and crank the engine until the rotor approaches #1 cylinder.  Rotate the engine in one direction by hand until the 0 degree timing marks are aligned.  While watching the rotor, rotate the engine in the opposite direction until the rotor just barely starts to move.  Measure the separation between the 0 timing marks (approximately every 0.1" relates to ~2 degrees).  This reflects the slop in your timing chain.  Conservatively, if slop is ~0.4" or more (8+ deg) the timing chain has reached its service life.

Of course, mine isn't the only opinion, but I hope it helps you.   :)
Rich
It's difficult to know just how much you don't know, until you know it....
In other words... if people learn by making mistakes, by now I should know just about everything!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline bake74

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Re: 79 K10 Misses under load
« Reply #3 on: February 26, 2012, 01:24:44 PM »
      Welcome cymrych from California.  I think the last 2 post will get you started and from there if more is needed help is here.
#1: The easiest and most obvious solution to any problem is 99% of the time correct.
#2: There is no such thing as impossible, it just takes longer.
  74 k10, 77k10    Tom

Offline cymrych

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Re: 79 K10 Misses under load
« Reply #4 on: February 26, 2012, 02:03:57 PM »
Thank you much for the quick replies.

Forgot to mention that fuel filter was changed at same time as cap, rotor, wires and plugs 2 years ago. Looking at my maintenance log, that was at 215k, so 15k miles and a bit over 2 years ago. I know the fuel filter could stand to be changed, but all the rest should still be ok. (Based on mileage alone, anyway.)

I really doubt it's fuel filter related, since anytime I've ever had a fuel filter cause issues it affected much more of the operating range (for example, *any* time you reach a certain rpm, not just that rpm in a specific gear), but will change it as a matter of course just to rule it out all the same. Now fuel pump on the other hand, different story. I agree, it's entirely possible it just isn't putting out enough fuel for the available spark at high loading and rpm. Hadn't thought of that. Will have to find my pressure gage and test it ....

Haven't noticed any obvious external wear or blackening of the elctrical components, but I haven't started pulling them off just yet. I dropped the trailer, and tried to see if I could replicate the miss closer to home. Still have all my work clothes, tools, cookware tote, etc. for the job in the bed, so probably 4-500 lbs on weight. At lot less than total mass with the trailer being moved by the motor, in other words. Turns out, if I hit a nearby hill at about 55, and try to maintain that speed by hammering down (any county mounties reading this please disregard), I can get it to misfire.... this being with much less weight being hauled. Less than around 50 mph, and it won't misfire, but just chug merrily along. Just to compare, if I slow down to 20 and drop into 2nd, then floor it up the hill, she'll go nice and smoothly right up to 35 mph, lots of nice torque.

Thank you VERY much for that info on cam bearings. I haven't observed any of those symptoms, so I'm going to work under the assumption for the moment that the cam and/or bearings aren't the root cause. Which would be good news indeed. I wasn't really looking forward to yanking the motor to rip it apart to change out those bearings. At that point, I'd pretty much be looking at a full rebuild, since it'd be dumb not to while the motor's out and apart. Even a cam would have been a big pain in the keyster, but at least the block can stay in the frame for that.

And I'll check for chain slop too. That also didn't occur to me, but would make sense. High rpm at loading would put maximum tension on the chain, so if it's worn and stretched it'd effectively knock it out of timing while that operating condition exists. Once the speed and rpm drops a bit, the tension relaxes some and the valves don't fall so far behind their optimum.

Thanks guys, this gives me a bunch of things I can check for easily enough. I'll start with the easiest/cheapest things first (of course!) with the electrics and proceed from there. Will let you know what I find. If anyone else has a suggestion/recommendation, please let me know.

Offline zieg85

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Re: 79 K10 Misses under load
« Reply #5 on: February 26, 2012, 08:57:18 PM »
Sounds exactly how a set of Bosch plugs ran in a 72 C-10 I ran.  I replaced them with AC Delco and the problem went away.  I ran it on a Sun Machine brake torquing it and they were braking down.  They plugs didn't have 200 miles on them and only started acting up pulling my car trailer.
1984 C10 Custom Deluxe 4.1L 3 on the tree
1985 C30 Custom Deluxe Ext. cab 5.7L TH400 4.56 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 Custom Deluxe 5.7L 3 on the tree 2.73

Offline cymrych

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Re: 79 K10 Misses under load - UPDATE
« Reply #6 on: March 05, 2012, 12:45:47 PM »
UPDATE

Sorry, my computer is still all packed away for the job I was headed for last weekend, so I haven't been online much.

So, after checking the first order of business bits (cap, rotor, wires, fuel delivery), I checked my timing chain tension. I thin someone mentioned 4-8 degrees of slack is OK, anything more is bad. Well ... I had some slack. Guesstimate, 20-25 degrees, maybe more.

Obviously, even if this wasn't the primary cause of my problem, it was something requiring attention pronto. Needless to say, my slight misfire at highway speeds would be of minor concern if my timing chain should jump a tooth, or worse, ride up on a tooth and snap it off.

Tore down the front of the motor, and indeed, I had all kinds of loose slack on the chain. After a few ... lets just call them "miscommunications" to be civil, with the brainiacs at the local national auto parts store, the correct replacement parts all eventually showed up. Got it all cleaned up and slapped back together yesterday, got it running and set the timing for 4 degrees (what my chilton's recommends.) So far so good, just need to road test it.

Got lots of new bits hanging on the front of my motor. I'm the type of guy who would rather replace a component that has at least 150k and probably up to 230k miles while I had it off anyway. So, new water pump, fuel pump, p/s pump, upper and lower hoses, and had my rad cleaned and pressure tested as well. (Well, attempted to test it. It was original and getting pretty brittle, so the fins just started flying all over when the mech hit it with air to clean it up. So, that's new too.) Yeah, all these were working just fine, but who knows for how much longer? Or even how many miles/years they each had on them? Nope, was simpler to me in the end to just replace what I could with new/rebuilt parts as I went. Yeah, it cost me more right now, but I'm willing to bet the frame will rust out of the truck before I have to worry about them failing from age!

Anyhow, the reason a road test hasn't been done yet is because I'm still trying to find a reman power steering pump which fits the stock bracket. So far, tried 2 and both are too large. The mounting holes and lines are spot on, but the reservoir itself is about 3/4 inch wider than the original, and the bracket won't slip onto it. I could mod my original bracket and weld it up to work, but I'd prefer to see if I can't find a pump to fit first. Got another one on order, should be at the store this afternoon. If it doesn't come in, or is still too large, I'll throw my original back on the truck just to road test the timing chain swap and see if it clears that misfire under load at highway speed. Then it'll be off to the junkyard to find a spare bracket to weld up to fit these new larger p/s reservoirs.

So, Final Update to follow....

« Last Edit: March 05, 2012, 12:51:48 PM by cymrych »

Offline bake74

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Re: 79 K10 Misses under load
« Reply #7 on: March 06, 2012, 05:49:44 AM »
     Glad to hear you found at least the problem with the timing chain before it created a bigger problem.  I think it is a good idea to replace worn items at least when you have to take them off also.  If you know they have that many miles on them, it only makes sense to replace them while they are off.
#1: The easiest and most obvious solution to any problem is 99% of the time correct.
#2: There is no such thing as impossible, it just takes longer.
  74 k10, 77k10    Tom

Offline cymrych

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Re: 79 K10 Misses under load FINAL UPDATE
« Reply #8 on: March 07, 2012, 11:37:56 AM »
FINAL UPDATE

Yep, never saw the point in putting important components with 150k+ miles back onto a vehicle. Besides, the parts for these older 350s are ridiculously inexpensive and generally easy to work with/install. My parts costs came to just under $600, more than half of that being the new radiator (I splurged on the brass and copper one, versus the one with plastic tanks). So for about $250, I installed new timing chain, water pump, fuel pump, p/s pump, p/s lines, water pump bolts, timing cover bolts, upper and lower hoses, new belts, and necessary gasket kits. Plus, I can do all the work easily enough, unlike with most of the newer cars out there today. If I tried to take the family Subaru wagon in to the shop for new timing belts, my total would be more than $300, and that would be for new belts and gasket only, reusing everything else. So I have no complaints at all!

At any rate, road test went great. In 3rd under heavy load such as going up a good grade, I don't have that misfire anymore. Even better, my power response on those hills is improved noticeably over how it was running before. I still lose speed, of course, but significantly less than before, with slightly less pedal needed as well. Haven't run enough at speed to see if my fuel "economy" (and I use that term loosely!  :) ) has improved much, but I suspect it will go up a little bit.

All in all, amazing what a difference a motor with proper timing and spec components makes!

Thank you all for your help! I greatly appreciated all the advice, especially as the solution ended up being something I hadn't considered!

Cheers
Jason

Offline bd

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Re: 79 K10 Misses under load
« Reply #9 on: March 07, 2012, 02:03:44 PM »
Well done!  :D   It's always gratifying when a motor purrs after a repair.   8)
Rich
It's difficult to know just how much you don't know, until you know it....
In other words... if people learn by making mistakes, by now I should know just about everything!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Grim 82

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Re: 79 K10 Misses under load
« Reply #10 on: March 07, 2012, 02:08:11 PM »
Bump your timing up to 8-12 BTDC and see if some of that mileage comes back. Whenever I have to spend some time under the hood I like to run all new vacuum lines as well.
Give a man a gun, and he might rob a bank. Give a man a bank, and he might rob the world.

 


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