165hp lifter like noise

[wolfhound]

Have a 1975 gm 165hp inline6 with a lifter like noise.
Sounds like it's from the front of the engine at the middle or lower by the crank. hard to tell
here's what I have tried: adjusted the valves cold(not running) per the service manual.
adjusted the valves engine running(normal temp).
nothing changed. So i tried some other clean used lifters in the first three cylinders one at a time in each(that was a PIA). again still the same noise.
my next trick : STP additive, octane booster, can a deep creep.
I was thinking of a valve job, new lifter and pushrods, but would rather not if I don't need it. Could it be the water pump, timing chain or maybe a rod knock?
Forgot to ask what plugs can be cross referenced for this engine, as the original AC type are no longer available.
thx

 

[Lyle29464]

Re: 165hp lifter like noise

I do not think anyone can talk you through this. You really need to get a mechanic. Tell him what you did. Maybe he can put new lifters in it and trouble shoot the knock and see if you have some flat cam lobes now.

 

[smokeonthewater]

Re: 165hp lifter like noise

fuel pump..... maybe

A sticky pump or one with a broken spring can cause a knock.... you could try removing it and either running with a gravity feed tank or just running on what is in the carb for a few seconds.... only cost ya a few minutes and a gasket to check it

 

[wolfhound]

Re: 165hp lifter like noise

I do not think anyone can talk you through this. You really need to get a mechanic. Tell him what you did. Maybe he can put new lifters in it and trouble shoot the knock and see if you have some flat cam lobes now.

I can install lifters, I've taken those in and out(and kept them in order) a bunch o' times. I did ask a boat mechanic he's stumped(or just doesn't want commit to a wrong answer maybe).
thanks though

 

[Silvertip]

Re: 165hp lifter like noise

Timing gears can cause that noise as can a broken or collapsed piston skirt. Start the engine and pull one plug wire at a time. If the noise goes away on the offending cylinder, you probably have something amiss with wrist pin, skirt, rod bearing, piston clearance, carbon build up, etc. All of those except carbon build up require engine disassembly. And just so you know, installing used lifters in an engine can cause improper operation as well. The wear pattern on the replacement lifter is not the same as the wear on the cam lobe in the new engine. Some of those inline sixes of the 60's and 70's did exhibit piston slap from collapsed skirts or wear. They would continue to run forever so unless oil pressure indications are low or the noise gets progressively worse, I would not tear it down at this point. There is more diagnosing to be done before doing that. And as was pointed out, I too know of engines that were rebuilt only to have the same noise present. The culprit was the return spring in the fuel pump.