KenMAthisHD
Newbie
I’m not entirely sure how to correctly explain what I’m hearing, but I’m gonna give it my best shot as the problem is kinda driving me nuts and I can’t find much on it.
This is in my 92 K1500 with the 5.7 and 700R4. At speed, say about 55mph-70ish mph, the pitch and pattern of the motor hum starts to change. The way I think of it is think of a straight line as a constant sound pitch. Then take that line and add a bunch of waves to it to signify a change in pitch, but starting with some being spaced out, and as the line progresses the waves get closer together, and then further away, and it repeats. At the peak of each wave, the hum is a slightly higher pitch - at the bottom most point of each wave, the hum is a slightly lower pitch. It takes about 3-5 seconds for the frequency of pitch variation to increase, about 1-2 seconds where the frequency is much faster, and then about 3-5 seconds for it to decrease, then the cycle restarts.
It is never a suddenly drastic change, not like you’re dropping down a gear or you jammed on the throttle, it kind of creeps in as you get up to that speed and then varies the pitch of the motor. It doesn’t appear to affect the motor RPM noticeably, and it doesn’t backfire either. I’m trying to get a good sound clip of it but haven’t really caught it on my phone. I haven’t really found anything in trying to search for the issue online either.
I think it has something to do with the motor timing. It used to appear at lower speeds, but when I finally decided to time it with a light instead of by ear and feel, I brought it to the 0° timing mark (I did unplug the little plug thing to time it as directed in the block behind the accumulator/drier in the engine bay, and it ran kinda cruddy when it was unplugged while I timed it, but was fine when plugged back in) and now the pattern has moved from appearing at 35mph-65ish mph to appearing at 55mph-70ish mph. Outside of that speed range the hum does not appear. After getting it timed, I noticed that when cold starting the truck it fires up, but then has a half second where it doesn’t fire, and then fires again and continues running as normal. Once it fires the first time, I let go of the key, so it’s not cranking still when it doesn’t do anything after the first fire up, and I don’t have to crank it again to get it to fire up. It’s key turn, normal crank, fires, let go of key, nothing for a half second, then fires again and running. It’s not entirely consistent, sometimes it’s a quarter second or less, but it does it about 90% of cold starts and did not do the same thing before I timed it, so I’m assuming it’s related.
My plugs and wires probably have about 10k miles and 2 years on them at the most, and the cap and rotor are anywhere between 12k-15k miles and about 2.5-3 years. Still the stock distributor. The truck seems to idle fine, aside from what feels like a slight miss about every 7-10 seconds. However, it’s done that for the last 6 years and everything I’ve found about it feeling like a miss just seems to say “It’s the nature of the beast.” I know it’s not tire noise, the Cooper MTP’s make that typical mud tire on asphalt whir and that remains constant. I’d say it could be the exhaust, it’s got a 3 inch high flow cat with no muffler and is dumped at a 45° angle under the bench, but that’s been that way for the last 3 years and the noise is at most 3-4 months old (it doesn’t get driven very much).
Perhaps it’s the timing chain? There’s probably about 100k miles and 10 years on it. Or maybe the 0° mark was incorrect and the truck isn’t properly timed?
This is in my 92 K1500 with the 5.7 and 700R4. At speed, say about 55mph-70ish mph, the pitch and pattern of the motor hum starts to change. The way I think of it is think of a straight line as a constant sound pitch. Then take that line and add a bunch of waves to it to signify a change in pitch, but starting with some being spaced out, and as the line progresses the waves get closer together, and then further away, and it repeats. At the peak of each wave, the hum is a slightly higher pitch - at the bottom most point of each wave, the hum is a slightly lower pitch. It takes about 3-5 seconds for the frequency of pitch variation to increase, about 1-2 seconds where the frequency is much faster, and then about 3-5 seconds for it to decrease, then the cycle restarts.
It is never a suddenly drastic change, not like you’re dropping down a gear or you jammed on the throttle, it kind of creeps in as you get up to that speed and then varies the pitch of the motor. It doesn’t appear to affect the motor RPM noticeably, and it doesn’t backfire either. I’m trying to get a good sound clip of it but haven’t really caught it on my phone. I haven’t really found anything in trying to search for the issue online either.
I think it has something to do with the motor timing. It used to appear at lower speeds, but when I finally decided to time it with a light instead of by ear and feel, I brought it to the 0° timing mark (I did unplug the little plug thing to time it as directed in the block behind the accumulator/drier in the engine bay, and it ran kinda cruddy when it was unplugged while I timed it, but was fine when plugged back in) and now the pattern has moved from appearing at 35mph-65ish mph to appearing at 55mph-70ish mph. Outside of that speed range the hum does not appear. After getting it timed, I noticed that when cold starting the truck it fires up, but then has a half second where it doesn’t fire, and then fires again and continues running as normal. Once it fires the first time, I let go of the key, so it’s not cranking still when it doesn’t do anything after the first fire up, and I don’t have to crank it again to get it to fire up. It’s key turn, normal crank, fires, let go of key, nothing for a half second, then fires again and running. It’s not entirely consistent, sometimes it’s a quarter second or less, but it does it about 90% of cold starts and did not do the same thing before I timed it, so I’m assuming it’s related.
My plugs and wires probably have about 10k miles and 2 years on them at the most, and the cap and rotor are anywhere between 12k-15k miles and about 2.5-3 years. Still the stock distributor. The truck seems to idle fine, aside from what feels like a slight miss about every 7-10 seconds. However, it’s done that for the last 6 years and everything I’ve found about it feeling like a miss just seems to say “It’s the nature of the beast.” I know it’s not tire noise, the Cooper MTP’s make that typical mud tire on asphalt whir and that remains constant. I’d say it could be the exhaust, it’s got a 3 inch high flow cat with no muffler and is dumped at a 45° angle under the bench, but that’s been that way for the last 3 years and the noise is at most 3-4 months old (it doesn’t get driven very much).
Perhaps it’s the timing chain? There’s probably about 100k miles and 10 years on it. Or maybe the 0° mark was incorrect and the truck isn’t properly timed?
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