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Old 05-18-2017, 01:41 PM   #1
brown7373
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1972 Cheyenne 350 Engine problem

I am having a problem with my 350 Q-jet 4bbl. A few weeks ago, it seemed to start funny, almost made a pop sound. But it started, idled and seemed to run fine, until I tried a little harder acceleration. It was making some noise and lacked power. I have since decided it sounds like the engine is now pinging on any acceleration above gentle.

Water temp is okay and confirmed with infrared on the hoses, radiator, thermostat housing etc. which all read low 180, +-. Oil pressure also fine. It idles smooth, A/C runs fine and doesn't load up the engine. But even moderate acceleration, like to speed up a little from 50 mph and the engine pings. I have filled up several times from a couple different gas stations that I have regularly used. I have always used the 10% ethanol with no issues.

I just completed a 2 hour, 90 mile round trip, and other than pinging on acceleration, it was fine. Before this, in 14 years of ownership, it has NEVER pinged. 8:1 compression. I checked the timing when I got home, thinking maybe I jumped a tooth, but it was 6 degrees advanced, right where I have always kept it.

Any ideas what is wrong? 12K on plugs, Pertronix I. This did not happened gradually, it just suddenly started pinging.
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Old 05-18-2017, 01:48 PM   #2
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Re: 1972 Cheyenne 350 Engine problem

Did you ever check the timing?

Is the thermostatic air cleaner stuck that its only allowing hot air from the stove pipe from the manifold into the air cleaner?
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Old 05-18-2017, 01:51 PM   #3
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Re: 1972 Cheyenne 350 Engine problem

pinging will destroy your engine , check for a vacuum leak , get it fixed before you drive it that way
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Old 05-18-2017, 04:16 PM   #4
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Re: 1972 Cheyenne 350 Engine problem

Given your circumstances, I would guess you got a tank of bad gas recently. See if an octane booster will fix it up - or maybe fill up with 93. As dad used to say, do the easy and cheap stuff first!
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Old 05-18-2017, 05:59 PM   #5
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Re: 1972 Cheyenne 350 Engine problem

Timing was checked and at 6 btdc, I don't think my thermostatic air cleaner works...no vacuum hooked to it. Not sure of the gas, but you are right. A fill up of 93 is cheap and an easy fix if that is the problem. As far as vacuum leak, the idle is perfect. Wouldn't that ruin the idle and maybe cause stalls?
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Old 05-18-2017, 06:30 PM   #6
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Re: 1972 Cheyenne 350 Engine problem

Quote:
Originally Posted by brown7373 View Post
I don't think my thermostatic air cleaner works...no vacuum hooked to it.

Post picture of the engine compartment both sides.
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Old 05-18-2017, 10:29 PM   #7
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Re: 1972 Cheyenne 350 Engine problem

I'm pretty sure that the TAC needs a vacuum source to OPEN the valve. Vacuum is usually found coming via a thermostatic valve in the intake manifold near the thermostat housing.

Since you have good idle timing, maybe it's a weak mechanical advance spring that lets the weight fly once you get revs over idle?
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Old 05-19-2017, 12:02 PM   #8
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Re: 1972 Cheyenne 350 Engine problem

Could a weak coil do this? Mechanical and vacuum advance both work per specs.
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Old 05-23-2017, 05:54 PM   #9
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Re: 1972 Cheyenne 350 Engine problem

Put about 12 gallons of 93 octane in yesterday and drove about 10 miles. The light throttle ping is gone, but it still is sick if I try even medium acceleration.

Still smooth idle, normal starts, normal temperature and good oil pressure.

Is there a heat riser on a 72?

Could a weak coil or failing Pertronix module cause this?
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Old 05-23-2017, 06:25 PM   #10
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Re: 1972 Cheyenne 350 Engine problem

Quote:
Originally Posted by brown7373 View Post
Is there a heat riser on a 72?
Assuming the passenger side exhaust valve flapper, yes, at least my '72 K20 had one. I removed it many years ago, no problem running without it.
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Old 05-23-2017, 07:37 PM   #11
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Re: 1972 Cheyenne 350 Engine problem

Are you certain it is pinging that you are hearing? Could it be a burnt plug wire arcing and making a clicking sound? Could it just be hesitation caused by a partially blocked fuel filter? I suggest checking your plug wires and perhaps changing your fuel filter and checking the sock in your tank for deterioration.
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Old 05-23-2017, 08:41 PM   #12
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Re: 1972 Cheyenne 350 Engine problem

It is definitely down on power, and the pinging, or whatever it is, sounds like it is from the left side. I'll check the exhaust manifold tomorrow. Plug wires are fairly new and all look fine and are tight.
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Old 05-24-2017, 08:44 AM   #13
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Re: 1972 Cheyenne 350 Engine problem

I don't see any mention of a heat riser in the exhaust on a 72. Does anyone know for sure if it was used in the left manifold on a 1972 Cheyenne 350 4bbl, THM 350?
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Old 05-24-2017, 12:16 PM   #14
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Re: 1972 Cheyenne 350 Engine problem

Are you sure the ping is a ping? Sounds like you might have a broken valve spring. Good idle, but when RPM increases the valve is slow to close. I would pull the valve covers and see what you see. I would also pull the spark plugs and see what's going on there, e.g. color, and any damage to electrodes.

BUT before I did any of that, I would check total timing/advance curve. You only checked static timing at idle. Find the timing chart for your engine and compare the spec with what yours is doing at various RPM. This will reveal any problems with fly weight springs in the distributor.
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Old 05-24-2017, 02:16 PM   #15
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Re: 1972 Cheyenne 350 Engine problem

No, I'm not sure it is a ping, but it is some kind of slight rattle and loss of power. f I press harder on the gas, it definitely shows loss of power, and sounds bad and forces me to immediately ease off the gas. I hadn't thought about broken valve spring, but that is a good thought. When it first happened, I got a funny sort of "pop" as I turned the key. It started and idled fine, but all the other symptoms were there.

I haven't done a thorough advance check, but I inspected the wires
and the cap. The vacuum advance works correctly and doesn't leak. I checked advance at idle and the distributor appears to advance correctly (up past the timing marks) when I blip the carb, and it immediately returns back to correct timing at idle.
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Old 05-24-2017, 04:31 PM   #16
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Re: 1972 Cheyenne 350 Engine problem

Okay. I popped the left side valve cover. Without turning the engine, none of the springs had obvious problems. I will rotate the engine at a later date to confirm the springs are not the problem.

I also got under the truck to make sure I had no heat riser on the exhaust manifold, and there is none. However, I think I found my problem. The number 1 & 3 plug wires were hanging down and very close to the exhaust pipe. One wire was split about 4 to 6 inches and the core exposed. It has been my perception that the noise I hear is coming from the left side, so I think the wires are the issue.

I am going to double check the valve springs while turning the engine, replace the valve cover gaskets, replace all the plug wires, distributor cap, rotor and plugs. It was nearing time for that anyway.

Thanks for all the ideas and suggestions, and I will repost when finished...hopefully with good news.
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Old 05-24-2017, 05:17 PM   #17
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Re: 1972 Cheyenne 350 Engine problem

FWIW, when I broke a valve spring in my BBC (69 El Camino, 396/375HP) it made a lot more noise than I would call a ping or a slight rattle. First time, I had several hundred miles to go on a cross-country trip and it was a weekend, nobody available to work on it and it was running OK so I kept driving. Second time, I recognized the sound and got it to a shop for repairs. Too many years and too many beers, don't recall if there was a third time but the problem was eventually eliminated with a set of Isky racing springs. Sorry for rambling but the point is I think a broken spring would probably be louder.
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Old 05-24-2017, 07:53 PM   #18
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Re: 1972 Cheyenne 350 Engine problem

Stocker, typically yes, but I've seen it where the main spring breaks and the secondary spring (if equipped) will operate the valve at low rpm. The noise won't appear until higher RPM.

I've also seen (don't recall if it was GM or not) , where a slow operating (lazy) valve can get kissed by the piston, causing a noise. Often diangnsitics are a matter of a process of elimination.

Certainly do the easier stuff first. Sounds like you're on the right track. Also the bare ignition wires could be cross firing, casing the timing to be off on the affected cylinder, causing the ping (pre ignition).

Interested to know what you find.

Last edited by Cman60; 05-24-2017 at 09:01 PM.
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