Since the year 2000, I have been searching for the source of an INTERMITTENT 'rat-a-tat-a-tat' noise. No apparent reason for the noise appearing and disappearing when it did. Even when it was not loud, it was still sort of there.
Thought it was the carb or T.P.S.. Repaired that. l Replaced the chip. Replaced the water pump. Replaced the alternator bearings. Belts. Dropped the 'Y' pipe and checked the heat-riser valve. Started adding Marvel Mystery Oil to gas and oil [never adding past 'full']. Disconnected all vacuum hoses, as I thought it was perhaps the heat stove on the air cleaner. Thought it was the 700R-4. Thank goodness I only replaced the filter and fluid. Replaced the fan clutch. Replaced the A.C. compressor [it was leaking anyway]. Replaced a leaking p.s. pump. Replaced my underwear [needed to see if you were still reading this post]. etc., etc.,etc..

Now to the point. Two weeks ago. There was the noise again. No particular reason. Drove all day to do shopping. Saw a 'nearly-empty' on the gas guage. Pulled into a station. Stalled right at the pump. Wouldn't restart. Filled up. Started up and no more stalling. But wait--the noise was still present.
Then I thought. Tank not quite empty, but stalled. Could it be WATER-IN-THE-GAS?

Bought a gallon of pure wood alcohol at Lowes. Added a cup or so to gas. After third tank-full, no more noise. I have been waiting for it, but to no avail. Seems to be gone for good. I feel that there must have been three gallons of water in the tank.
I still add MMO to the gas tank, no longer to the oil, though.
I use many different filling stations. Are they all so very bad?

Sorry for the rambling on, but I am thrilled over this minor revellation about the car.

Seth

Edit: That noise had a mind of its own, and just might re-appear after reading this post.

 

Congrats on the fix...
I didn't know about using the pure wood alcohol..
That beats dropping the tank...

 

Couple of problems with this theory:

1) Gasoline is less dense than water, and therefore floats on top of water. The pick-up is in the bottom of the tank - what's going to get picked up first? Right, the water.

2) Gasoline in the United States has 10% ethanol in it. Ethanol absorbs water, and it will be carried out with the gasoline. The typical "gas line antifreeze" is nothing more than methanol - it also absorbs water, and if you're adding it to your tank these days, you're wasting your money - the "antifreeze" is already in the gas.

Have we talked about the tank pressure relief valve in the past?

 

When the noise began way back when, I drained out much of the gas, but it persisted. Perhaps I needed to pump it dry.
So what if the gas is 10% corn or grain alcohol. If gas is watered down enough, the alcohol content would probably be moot.
I have no explanation for the results. Today, I am back down to 1/4 tank, and the noise appeared for about 10 seconds, but very low in volume.
When I first began using the alcohol 2 weeks ago, the noise only reappeared at 1/4 tank or less. Now seems to only happen at same.
You may have touched upon the tank pressure relief valve in the past, but I have no idea of what you speak. Did a search for it. Seems that if that thing were bad, the exhaust might become rich. The car gets crap gas milage, but has not run rich since fixing the vacuum leak that was confusing the O2 sensor.
Is my stock tank internally divided by panels? I used to notice that the noise would begin right after making my 1st left turn, just past Brooklyn College campus. For a time, I figured that it might be an engine mount, which it wasn't, of course.

So if you have a better explanation for this, then please offer it.

Better water in the gas, then something mechanical. But the stall with a nearly-empty tank seemed to offer the clearest clue.

Seth

 

Idk man coming from someone who drives through 5 foot deep streams/swamps in his other vehicle I dont think that your car would be doing very well if it was cycling that much water through the motor. We fear water. Why? because it doesnt compress and when you get enough of it in there somethings gotta give.

 

Wood alcohol is methanol or methyl hydrate. Same stuff I use for race fuel. As mentioned above, it's also the main ingredient in the gas line antifreeze. A cupful to a tank of fuel is all that's required. The most you could safely put into gasoline is 10%. A 10 gallon tank could have 9 gallons of gas and 1 gallon of methanol and still run properly.

Methanol is hygroscopic. It absorbs water.

Methanol burns slower than gasoline. Your "rat-a-tat-a-tat" noise could have been pinging for whatever reason and the slower burning methanol increased the octane rating just enough to stop the pinging.

I can't see a "water in the fuel" problem for 8 years. You would have gone through enough fuel to dilute any water in the tank. Since you use many gas stations, I'd rule out them as a constant source. Maybe someone has been pouring water into your fuel tank from time to time.

 

So, this leads us to the question of why it was apparently pinging [intermittently, and I had forgotten to mention, always from 800 to 1,500 rpm]. Hot idle is 600 rpm in gear.

A mechanical reason would be more constant. Hydraulic lifter would even be more constant. I do have an intermittent ticking [not loud] from a hydraulic lifter, that seemed to just very recently appear. When I change the oil this weekend, will do the 4 quarts oil + 1 quart Marvel Mystery Oil thing. It has been known to clean these things out. Kept the innards of my Triumph minty-fresh and squeaky-clean for an amazing 300,000 miles.

Seth