Knock Count

koolbreeze said:

I checked the timing with another light this afternoon and the base timing is just about dead center 0 with the ESC wire unplugged.

But I noticed something I'm not sure about. I decided to check how much advance it has with the ESC wire plugged back in, so I turned the truck off, plugged wire back up, started the truck and check it again. With the wire plugged in, the timing never stablizes. It constantly fluctuates 6 to 8 degrees, making it difficult to know exactly how far advanced it is. It looks to be around 20-18 degrees, but as I said it fluctuates 6 to 8 degrees. If I give it a little throttle it will stablize for a second or 2, and then goes back to fluctuating. Is that normal?

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That sounds like a worn out timing chain, or distributor gear.

Possibly a worn chain or gear. But, the pcm does alter timing to stabilize idle. I forget how much is allowed on a stock tune, but I think it's around 5 to 6 degrees.

I don't fully understand how it all works yet. I've read some posts that suggest it is normal to see that fluctuation at idle and others that suggest it isn't. It's not dancing like a strobe light or anything or but it's moving enough that it never stops for over 1/2 a second. Constantly back and forth between the high point and low point of the fluctuation.

I've also read posts that suggests it could be the timing light, as it's the type with a dial advance. It's stable with the ESC wire unplugged though.

Here is a screenshot of the main spark v RPM v load table of a BDUY bin that I found. I can't say for sure that the bin on mine is exactly the same, as I haven't read it and didn't buy the truck new, but it is the same BCC number that is on the label on my truck's ECM. Anyway, if I'm reading this correctly (and I'm probably not) it appears to me it should be stable at 20.04 degrees as long as the RPM <= 1200 and MAP is <= 50 Kpa. When I checked the vacuum a few days ago at idle it was stable at 19" at idle. I will have to check it again.

koolbreeze said:

I checked the timing with another light this afternoon and the base timing is just about dead center 0 with the ESC wire unplugged.

But I noticed something I'm not sure about. I decided to check how much advance it has with the ESC wire plugged back in, so I turned the truck off, plugged wire back up, started the truck and check it again. With the wire plugged in, the timing never stablizes. It constantly fluctuates 6 to 8 degrees, making it difficult to know exactly how far advanced it is. It looks to be around 20-18 degrees, but as I said it fluctuates 6 to 8 degrees. If I give it a little throttle it will stablize for a second or 2, and then goes back to fluctuating. Is that normal?

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Is the engine hunting at idle? It will fluctuate back and forth as it is interpolating between multiple load/RPM cells but 6-8 degrees sounds like a large swing.

No, it idles pretty steady at about 650 RPM. I do get an occasional change in tone with maybe a 25 RPM shift and the timing changes with it maybe a degree or 2. But as it turns out, the timing fluctuating I was seeing is in the timing light not the truck! My bad on that.

I just connected TunerPro RT up to it so I could watch the data stream and timing together. If I turn the advance dial on the light to 0, the timing doesn't fluctuate even with the wire connected. It's steady about 1/4" above the top of the scale on the timing tab that's on the timing chain cover. If I use the dial to advance it (in an effort to tell how much advance there is) it starts dancing like a rabbit. That's with a new Innova 3555 light. I disconnected it and connected my old Harbour Freight light, which also has a advance dial, and it doesn't cause the timing to dance when the dial is moved off 0. So it's in the light.

On another note, I put some premium 93 octane gas and a bottle of Techron in it Saturday and have ran about half of that through it. One or the other, if not both, seems to have made a big difference in the way the truck runs and the knock counts. I took it for drive this afternoon and only got a couple knocks on the counter while cruising at 75 MPH on flat highway for 20 miles or so. Before I was seeing them every couple miles. I'm also not seeing anywhere near as many under heavy load, but still seeing some. So I don't know, it may just be the better fuel. I'll run a couple tanks of Techron and premium through it and see where it's at after that.

It's still running a little rich also, but that seems better too. I'm betting the exhaust leaks near the O2 sensor are contributing to that. Not looking forward to fixing that at all.

I got some time to tinker with the truck this weekend, so I pulled the plugs and checked the compression.

#1 = 173 psi
#3 = 170 psi
#5 = 174 psi
#7 = 177 psi
#2 = 170 psi
#4 = 169 psi
#6 = 165 psi
#8 = 166 psi

Are those results normal for a TBI 350 or a little high?

A few of the plugs (#1, #5, #7, #4) were loose and the others were not very tight. #5, #7, & #4 had some guey, sticky leakage out of the spark plug hole. I’m not sure what it is, it doesn’t really smell like oil. I’m guessing it escaped the chamber due to the loose plugs. I ran a couple tanks of premium gas and Techron through the truck just prior to removing the plugs, if that matters.

All of the plugs had a small area on the porcelain that was burned with maybe a little glaze on some. Some also had some of the substance that ozzed out the holes. It could be oil but the truck doesn’t smoke or use any noticeable amount of oil between changes. There were a couple that had a few black specks, which I’ve read is detonation, but they weren’t bad.

I videoed each cylinder with a borescope I picked up. You can see the substance discussed above, as well as the carbon build up in the chambers. The worst was on cylinder #5. I’m not sure what a normal amount of carbon is for an engine with nearly 180,000 miles on it but I’m going to try and find some GM Top Engine Cleaner to run through it. I’ve read it works pretty well.

I don’t know much about reading plugs, so if any of you guys can take a look at them and the video, I’d appreciate it.
https://imgur.com/a/DeDQ3Gp

I also used the borescope to verify that the timing mark on the harmonic balancer is accurate. It’s dead on 0 degrees, so I don’t think the timing is off.

I also checked for slack in the timing chain/distributor. I measured it by turning the crankshaft in one direction until the rotor started moving, marking the spot and the turning the crankshaft in the other direction until the rotor just moved. I get about ½” as measured on the balancer. I think that’s about 7 degrees. I’ve read normal is 5 degrees or less, but I’m not sure.

koolbreeze said:

I pulled the plugs and checked the compression.

#1 = 173 psi
#3 = 170 psi
#5 = 174 psi
#7 = 177 psi
#2 = 170 psi
#4 = 169 psi
#6 = 165 psi
#8 = 166 psi

Are those results normal for a TBI 350 or a little high?

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I'd say that's a little high--assuming your gauge is accurate.

Altitude, cranking RPM and many other factors including combustion chamber deposits will affect the compression readings.

koolbreeze said:

I’m going to try and find some GM Top Engine Cleaner to run through it. I’ve read it works pretty well.

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Schurkey said:

You use any petroleum product to clean the combustion chambers, you'll create a huge cloud of smoke that the neighbors will despise. They might even call the fire department.

Drizzle a urine-stream of water back-and-forth down the carb primaries, or down the TBI bores, or into the throttle body at 2000 rpm. Water will steam-clean the combustion chambers so they look like new--and without the huge cloud of smoke.

Start buying "Top Tier" fuel. The fuel will be labeled "Top Tier" on the pump. Extra detergent to keep the fuel system clean.

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koolbreeze said:

I also checked for slack in the timing chain/distributor. I measured it by turning the crankshaft in one direction until the rotor started moving, marking the spot and the turning the crankshaft in the other direction until the rotor just moved. I get about ½” as measured on the balancer. I think that’s about 7 degrees. I’ve read normal is 5 degrees or less, but I’m not sure.

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I like to change the timing chain at 10 degrees or less. I don't like 7 degrees of slop...but I have to admit that I put a Genuine GM "Gen 6" timing chain in my boat's 454 and even with the new chain it had 5 degrees of slop. That was the loosest "new" timing chain I've ever installed.

The compression tester is a brand new OTC model 5606, of course that doesn't mean it's accurate. I can check it up to about 120 psi with my compressor, I should probably do that.

I'm not concerned about the smoke from cleaning the carbon, I live out in the country. I just want to use whatever works the best to clean it up. I'll look into the water thing though, thanks for the suggestion.

I almost always buy name brand fuel; Shell, Exxon or Chevron mostly; but I'm not sure if they are top tier or not. I'll look for that on the pump.

I don't mind putting in a new timing set, if the engine is worth the effort at this point.

I've read the spots on the plugs is likely valve seals, but the truck does not smoke at all, so I'm not sure about that. Those are easy enough also though.