Ok, before commenting that this topic has been covered substantially 100 times, hear me out. I imagine I've read nearly every forum on pinging and have tried most things and am at wits end. The problem is that it will start up fine, but ping worse as I drive it (within 10-15 minutes). Idles smoothly (plus or minus 10-15 RPM), accelerates fine (in the beginning), will do 65 without hesitation or pinging, but the pings and shakes almost like its going to stall when I TAKE MY FOOT OFF THE GAS AND AM COASTING TO A STOP. If I shift into neutral it smooths back out like nothing is wrong. Revs fine and won't ping in neutral. Here are the details of the truck:

1979 F250 2WD 351M, (shop that did the trans told me they looked up the serial on the motor...was out of a Gran Marquis but forgot the year and to write down the number off the block...not too helpful)
C6 trans with the larger 14 inch flywheel (rebuilt by Jasper right before this became a problem)
4 barrel Edelbrock intake with Holley 4160 (4 barrel)
No EGR, no cats, no smog pump, no AC, no emissions anything
Vacuum at idle is 18psi
Current base timing is 4.5 btdc, mechanical advance arm is stamped 8, vac advance is set as low sensitivity as it will go (meaning as little vac advance as possible).
Vac advance is hooked to the timing port on the carb
Appears to hold good oil pressure (assuming the gauge and new sending unit aren't conspiring against me)

This truck lived on a farm I worked at for the better part of 15 years. It got little maintenance over that time other than adding oil as necessary (leaky valve gasket), but never had any problems (drove around town and occasionally on the highway). I ended up with the truck and of course all of the deferred maintenance caught up to it at precisely that instant. Drove it home and the transmission stopped shifting into drive and then out of first (the fluid was full but quite black). At the shops advice, I replaced it. This took them an eternity, and when they gave it back they said it needed an ignition module. Barely made it home, but ran pretty good once I slapped the new module in a few days later. Drove it a little more that day and then suddenly pulling away from a traffic light it sounded like somebody dumped a can of bbs into the motor. It was a 30 degree day in January, and the temp gauge was reading around the N in Normal, and I feel fairly confident it wasn't overheating (also new radiator). Went home, checked the initial timing (20 btdc), nudged it back a few degrees, little by little, still no improvement. Looked for vacuum leaks found a few and replaced all of the vacuum lines, the PCV valve, crankcase breather, and air filter (which is some oversized aftermarket one). Still pinging. Turned the base timing back a few degrees gradually until it almost wouldn't run, no change. Everything looked like it had been neglected, so figured it wouldn't hurt to do the cap, rotor, plugs and wires. Ran smoother, but still pinging. New plugs are still pretty darn clean looking, but maybe a tad tannish. Sprayed carb cleaner all over the intake manifold, carb base, and brake booster and no leaks. Took it to the shop and said it was the dizzy and replaced it (without asking or keeping the box with a part number). Didn't change a thing. Mechanical advance number stamped on the arm of the is 8. I also replaced the ignition coil, since it looked original and it runs even smoother with the new coil but still pings. Rebuilt the carb because it looked like it had at one point been leaking from the idle adjustment screws and the vacuum secondary diaphragm. Zero change in performance. I read that some later 351Ms had a factory base timing of 6 btdc. Mine is at 4.5 right now. I also checked took spark plug 1 out and carefully checked that TDC is where its supposed to be. It seems to be darn close if not perfect. And when I check the timing its with the vac advance disconnected and the line plugged. Tried running premium gas, and seafoam in the carb, both with no improvement. Like I said, it starts pinging when DECELERATING/COASTING TO A STOP after driving about 10-20 minutes, and then gradually pings worse and worse acceleration too (but is always the worst when I take my foot off the gas and am decelerating below 35 mph). Temp gauge reads O in Normal (although on hot days it creeps up a little more when idling). It almost feels like it wants to stall when its decelerating and it runs real rough at that point. Also I loosened my exhaust pipes/mufflers at the headers and tried to rattle them around in case something was somehow making noise in one of them. Of course, nothing.

I'm at a loss for what to do next. Could the issue be in the rebuilt trans? Faulty new ignition module? Lazy fuel pump (I've had this with an electronic fuel pump in a newer truck)? Wrong mechanical advance curve? Slop in the timing chain/gear? Head gasket failure between two cylinders? Thoughts?

 

You are running lean at idle and I can only imagine how lean at speed you are. Your base timing should be near 10 and total advance should be near 25. At idle on a stock engine you should be over 21 for the vacuum. You have a lean miss going on from what I can see, especially if your plugs are that white.

Also worth adding, the shop has no clue what they are talking about. There is no serial number on these engines and no part numbers either. There is only Ford engineering numbers.

 

Thanks for your reply. I think you probably already realized this, but I mistakenly wrote that the vacuum was 18 psi, when I meant to say 18in Hg. Chiltons says an average healthy engine should normally produce 17 - 22 inches, so I kind of took my 18 in Hg as being at least adequate. For what its worth, it holds a pretty steady 18in Hg at idle (maybe +/- .5 inch). If I recall correctly, it quickly dropped below 0 when I blipped the gas, went quite high (Chiltons says it should be 5 over but I'm pretty certain it was more than that), and then evened out quickly.

Also, a better description of the plugs is that they're more of a brownish tan on lower part of the insulator and really just look clean and new on the top part. Not trying to be contrary, but I didn't say they were white and they aren't really what I think of as white. I can take a picture to clarify tomorrow. Also my exhaust smells rich (and usually more-so on the driver-side), at least while idling in the driveway, making me unsure about the lean condition at idle (although lean would make for pinging). I've tried richening the mixture screws but then the vacuum goes down and the throttle responsiveness gets worse and it doesn't seem to be the issue.

Given this are you still thinking lean? And what would make it lean out after its already been warmed up and running ok for at least few minutes and doesn't appear to be running hot (fuel filter looks clear, for the record)?

 

Hot spots in the cylinder from a lean condition will cause the pinging. Adjust your timing up towards 10 BTDC you should pick up some vacuum and get rid of the exhaust smell. I would run the vacuum gauge into the cab and drive it and see where you are running at cruise. I suspect you do not have enough total advance.

 

I don't think the engine is pinging. The timing is too retarded.

And I don't think the timing is causing the shakes either.

The factory guideline is +/- idle 6* to 'all in' 36*.

Yours is 4.5* to 20.5*.

The shop had set your idle at 20*, and when added to 16* (8 x 2), gives the all in of 36.

As you stated, retarding it back from 20 made little difference.

Set your idle to 14*, which will give an all in of 30*. You should also see an idle vacuum reading increase.

Not ideal, but way better than it is now, and you should feel more power.

Dial in the timing curve one of these days.

The vacuum of 18'' with a pretty steady needle is spot on with timed (ported) vacuum advance, assuming your altitude is below 2,000 ft.

If you're curious, connect the vacuum advance to full manifold, adjust the carb down accordingly, and observe the +/-20'' reading.

I'm inclined to say that the C6 has an issue, not the ignition.

Maybe take the truck back to the shop and ask the builder to drive with you so that he can comment on the noises and the shakes.

 

 

It would be a good idea to verify the timing mark on the harmonic balancer is correct and that the outer ring of the balancer has not slipped. If it's off, no amount of trying to adjust timing will have any accurate meaning. You can remove a spark plug, install a piston stop (either store bought or make a stop from an old spark plug and a 1/2"-13 bolt), rotate the engine over by hand until the piston bumps up against the stop. Make a mark on the balancer, where the pointer lands, rotate the engine over in the opposite direction until it contacts the stop again. Make another mark. Halfway between these two marks will be the acutual zero timing mark.

 

I'm with FMJ, I don't think what you're experiencing is pinging...I've never heard of or seen an engine ping on deceleration...

 

Just thinking out loud... your pinging starts when you are decelerating, which is when your vacuum is highest, no? Could there be an issue with the advance pulling/sticking the point plate ? Besides the vacuum gauge in the cab, is there a way to check your timing under deceleration?

 

Good evening and thanks for all of your thoughtful replies.

1. I did my homework and advanced my timing to 10*BTDC (yes with the vacuum advance disconnected). I've checked the harmonic balancer position in accordance with cylinder 1's position 3 times (most recently yesterday) and it is correct. I also rechecked the idle mixture, maintaining 800RPMs, made minor adjustments, and then drove it around and made some vacuum measurements. At idle in park its at a steady 20 in Hg (+/- no more than .25 in Hg). If I snap the throttle to 2500 as described in the Hayne's manual and let off the vacuum will go to zero (or maybe even past zero?) and then up to 25 before going back down to 20 a little more slowly than it moved in the other parts of this test (still probably taking less than a second to get back to 20). When in gear it idles at 17 mm Hg. I didn't cruise very fast, but at about 40 mph (I recorded about 15 mm Hg). The pinging seems a little less, but its actually been rumbling worse and worse as I take off at low speeds and when I decelerate. I would say its idling more smoothly and has a bit more power again, although I snapped the throttle a few times to repeat my readings and it popped a little through the carb on occasion. Still pinging a little when driving.

2. As I mentioned, the harmonic balancer hasn't slipped.

3. I'm kind of banking on something with the transmission myself. The rebuilt Jasper came with a warranty (3 years I think..gotta look at the paper), I just don't really want to take it back to the shop. Also, not sure if it has anything to do with it, but the more warmed up the truck is the harder and louder the transmission clanks into gear from neutral. I read that this is normal with C6s. Its definitely the transmission and not a u-joint clanking. This clank sounds like its coming from more where the bell housing is, whereas the pining sound seems to be more toward the top of the motor.

4. I actually ordered a spare ignition module to add to my collection of spare parts under the seat. Its raining now, otherwise I would have swapped it out tonight.

5. Nobody asked, but I'm running copper core Champion 25 spark plugs.

What do you think of my vacuum readings? The seem pretty normal according to my readings in Haynes. What do you recommend I try next?

 

That's an interesting idea. I guess I'll have to drive around and think about that. Its probably unlikely, because it did this with both the old and new distributor.

On the subject of looking at timing while driving, I this thought had actually already crossed my mind a few months ago. I bet you could rig up a timing light with a piece of steel strapping somehow and attach one of those little Mobius cameras to record your timing. If you had a second camera, you could set it to watch and record your interior gauges. If you "synchronized" them, say by sticking an atomic clock in front of each before you set them up, then you could synchronize both videos later on and watch how your timing changes while you drive.

 

As I mentioned, I did check this, however, I forgot to mention that my balancer has an extra long mark at 8* that seems to be from the factory. Anybody else seen that? It seems like it would be kinda low, but it looks, too perfect for somebody else to have made it.

 

Just a wild thought!!!! How's your oil level? Low engine oil will cause tapping on acceleration as well as deceleration. Could your pinging sound be valve tapping from low oil?

 

That's not a wild a thought, but I do check the oil pretty frequently since it has a few leaks and keep it leveled off.

 

I find that 'wild thoughts' and 'thinking out loud' tend to assist in finding a solution.

So, thinking out loud, pinging is generally from too much timing,I.E. too advanced.

But !

You just advanced the motor by 4 degrees and you observed a little less pinging, which contradicts the logic.

You also observed a bit more power, and that makes sense.

I'd expect to see an increase in vacuum with extra timing, but not a huge jump from 18'' to 20''.

Did you make any other changes ?

To satisfy my own curiosity, I know you mentioned the timed port, but is your vacuum advance connected to 'ported' or 'full manifold' ?

Pinging usually occurs when an engine is under load, so I wouldn't expect pinging in neutral or decelerating if it's not noticeable under acceleration.

Mind you, an engine can be unpredictable just like my girlfriend ! LOL

In addition to the sticking advance plate mentioned, maybe the springs are too light and therefore advancing too quickly.

In order to rule out timing as the pinging issue, and to fine tune the engine, I'd check and set the 'timing curve'. (A brand new dizzy doesn't necessarily work as it should. )

The timing is at 10* with no vacuum advance connected.
What is it with vacuum advance connected ?

The steady idle vacuum reading reflects a healthy engine.

The throttle snapping test simply measures piston ring health, and your test passed.

Popping through the carb on occasion implies fine tuning required with the fuelling.

Regardless of what you have read, a brand new tranny should not clank into gear. It should be as smooth as silk.

If you're lucky, it's something simple like a questionable vacuum modulator.

I'm not sure what the significance is of a 'larger 14'' flywheel' is, but a flywheel hitting against the bell housing might possibly sound like pinging.

My only recommendation is that you take the truck back to the shop, take the builder for a drive, and let him/her observe, and comment on, the issues.