Tried a search but no results. I know it's an old question to ask but...

What do YOU think the main reason for "pinging" valves under under heavy load requirements ?

Normally, the vehicle runs cleanly, quietly and smoothly.

I've tried retarding the timing and it DOES help but some say that's not the way to fix it.

I run 87 octane fuel.

Thanks,

Ken

 

Pinging has nothing to do with the valves. The usual cause of pinging or detonation is too much timing and retarding the timing is the usual fix. Hot spots from carbon build up on the pistons may cause ping/detonation too.

 

I'll add that pinging can also be caused by a lean fuel mix. All of these symptoms happen when the EGR valve goes out. Engines with EGR come with lean jets and a steeper timing curve to compensate for the change in combustion chemistry. When the EGR valve goes out, the motor is left to run lean and over-advanced. The EGR valve is the first thing to check.

 

I noticed this with my engine too. It happens during acceleration at 2200 rpm through every gear. It goes away above 2200 and does not happen when cruising at 2200. I am suspicious that

1- Higher than 2200 rpm creates enough vacuum to get a slightly leaky EGR valve to function...?

2- EGR valve is junk and the mechanical advance kicks in above 2200 to get rid of the "ping"...?

I have not yet looked into it, but I think this is where I'll start.

 

If the EGR valve leaks at all, it will never operate. Vacuum-operated devices are all or nothing; it takes a perfect seal to function. The reason your motor pings when you accelerate is because the vacuum advance on the distributor is advancing the timing. Pinging comes from advanced timing. Motors with EGR have a more aggressive timing curve than their pre-smog counterparts. If a motor that is tuned to run with EGR suddenly is given a fuel mixture that is free of inert exhaust gas (which happens when the EGR valve goes out), the timing curve will be too aggressive for the combustion chemistry, and the motor will ping. Pinging occurs when the piston is forced back down too early on the upstroke. That's where the rattling noise comes from. As Harte said, it's not valve-train related. The reason the pinging goes away after a certain RPM is because vacuum drops off as the throttle plates open. Once the throttle plates are open past a certain point, manifold vacuum is effectively equal to ported vacuum. Because vacuum is very low when the throttle plates are open, the vacuum advance has a weaker signal and no longer advances the timing (only the mechanical advance under the baseplate of the distributor plays a role by this point).

 

Excellent as always fmc400. Thanks.

 

I'm confused on one point. If the pinging is being caused by a leaner air/fuel charge in the cylinder and a timing advance too far forward caused by the vacuum advance, shouldn't the prime time for pinging be when he is cruising along at 2200 RPM? With the throttle plates barely tipped and the engine running at moderate speed the vacuum should be high, giving the highest vacuum advance, and the air/fuel charge should be at it's least dense. Lean and advanced, prime for detonation. Is it possible the pinging is going on all the time but it's only being heard and felt when a denser, more open throttled, air/fuel charge is being preignited? So the pinging should stop completely with the vacuum advance canister disconnected right? Even with a completely non-functioning EGR the mechanical advance alone would not cause preignition, correct?

 

What I was thinking is that the noise is caused by the higher engine stress under these conditions (more advance, lean mix).

That was my assumption, now that you bring up the question I am curious again.

 

Typically, pinging can occur both during acceleration and at cruise. However, there are many factors to take into consideration, and they don't always go hand-in-hand. For example, engines have a greater chance for leaning out during acceleration than at a constant speed because of the difference in weight per unit volume for fuel and air. For a given volume, fuel is heavier than air and takes longer to move. This is why carburetors come equipped with an accelerator pump, to provide an extra shot of fuel to compensate for the fact that fuel takes a lot longer to "catch up" to air when there is a sudden change in manifold pressure and the carburetor switches fuel circuits. I've also experienced the opposite, where the motor only pings at cruise and not during acceleration. I'd buy the notion that altitude has a role as well. It just goes to show that several factors can come into play. However, if the noise only happens during acceleration and not during cruise, it's also possible that the noise is instead piston slap and is mistakenly being identified as detonation (pinging). Pinging sounds like marbles inside a coffee can, whereas piston slap has a deeper tone.

To answer the other question, it's doubtful that mechanical (centrifugal) advance alone can cause detonation, although I'd be hesitant to say that it's impossible.

 

fmc & everyone else,

You are hittingMY problem square on. If I am cruising a highway at a steady 60 and I hit the gas to pass or climb a rise the "marble filled can" sound starts. As I ease off the gas it goes away and rides quitely until a repeat of the aforementioned activity.

After reading the posts (thanks for the debate guys) I'll try retarding the timing (some) again. It DID run a little better that way. I have it set on the factory mark.

Ken