What causes a motor to ping under load but only when hot?

Question

Once my 2000 Trans Am warms up, it is knocking or pinging under moderate to high load. It runs fine when the motor's cold. I almost always use Shell 93 octane gas, and I have always used 5W30 or 10W30 synthetic blend oil. The car has 115k miles on it.

For cold starts, it runs about 60 lbs of oil pressure, dropping to 40 lbs when hot. Would low oil viscosity cause this noise?

Or could it be running a lean air to fuel ratio at higher temperatures? With good fuel and no engine trouble codes, how could that be?

Is there some other cause?

The motor is completely stock, no chip.

I replaced the fuel filter at about 75k mi and the plugs with AC Delco platinums at about 95k mi.

I first noticed the pinging a few months ago only at full throttle and at high RPMs. It has only recently started happening more frequently, when driving around town.

Answer 1

The sound is detonation (also known as spark knock).

It's cause by a long list of things.
Carbon, too high of compression for the octane fuel being used, too low of octane fuel for the compression, bad fuel/water in fuel, engine running too hot, inlet air into engine too hot, timing too advanced, air/fuel ratio too lean, lugging the engine around (driving up hills in 5th gear), spark plug heat range too hot for the compression

The odd thing is, it shouldn't be happening on your car with out setting off a trouble code.

You have a knock sensor in the block that picks up knocks of any type and retards the timing untill knock is gone.

Sounds like the knock sensor aint doing it's job.

Now next thing is why do you have detonation in the 1st place on a stock engine...

Only things that come to mind is..
High amounts of carbon build up.. Try using a bottle of seafoam. Seafoam is about the best top end cleaner on the market.

Next would be, one of the many sensors on the engine aint working right or doing its job, letting the A/F run lean, or timing advance too much.

Answer 2

1

Answer 3

The boob who are telling him to advance his timing,...
his ECM/PCM controls timing and there are no changes that can be made.
Older a car gets the more carbon build up in the combustion area.
That carbon takes up space and actually raise COMPRESSION RATIO.
high compression ratio engines can't run on todays fuel because the
octane is not high enough. High octan fuel has more atoms of h2o
in it and that h2o keeps the high comp ratio motor cooled in the
combustion chambers. Some guys in the old days would buy
one plugs level cooler plugs.
Or you could by higher octane as an experiment and see if that has
any effect.
Also your heads may be getting hotter than they are supposed to be.
If you're knock sensor is checked and found ok then I'd go for a top
engine cleaner like seafoam or some such. (sometimes 2 treatments are needed)
If the knock still persists tell the guy at the part store your plug number
and ask for a cooler plug than you have now.
I have heard also that water temp sensors (not to be confused with your temp gauge sender)
can cause the ECM/PCM to make false adjustments to engine timing.
Your engine is behaving like a high compression eng would, when it isn't that type of eng.
If 93 octane makes the knock go away, then you know that your combustion area
is getting too hot.

Answer 4

In the old days it was quite common to do a decoke on an engine from time to time because with time carbon built up in the engine that can cause some pinging. A decoke means to remove the carbon.With modern fuels and engines the need for this is quite seldom but carbon can still build up. If the car uses some oil and it is mainly used in traffic carbon can build up.
An engine running too hot, ignition timing not 100%, air leaks, too low fuel pressure on fuel injection engines, bad EGR valve and low octane fuel can all cause pinging.
Better have it checked, pinging can cause severe damage to an engine.

Answer 5

The ping is called detonation, not a good thing
the oil pressure is not the problem
93 octane fuel shouldn't be a problem
Try backing the distributor timing back (retard) about 2 degrees . the knock sensor should be doing this .
map or maf sensor could be going bad.
not all the sensors will set a trouble code
What do the plugs look like ?
How is the fuel pressure ? filter could be stopped up or pump getting weak.

A fuel injector could be stoped up or going bad try Tech Ron Injector cleaner It is a bit pricey but one of the top best cleaners

Answer 6

I would suspect a worn fuel pump or plugged fuel filter on a GM product with that mileage. Preignition happens when the heat from compression ignites the air/fuel mixture. The EGR valve also helps cool combustion temp but not under full throttle. Another problem could be a dirty air flow meter but I hav not seen that happen on a Trans Am

Answer 7

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Answer 9

I had the same problem with one of my car, sounded fine but started knocking once it warmed up. I added half a qt of Marvel Mystery Oil to the oil and after a few more miles of driving it totally went away. Don't totally know why but it worked, it's worth a shot.

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