1995 J30 w/69K miles.

Under slight acceleration I'm sometimes hearing a slight ping/knock. I've been using premium fuel and fuel injector cleaner. This started about a week ago. I thought maybe it was bad gas but I've since refueled with no effect.

From a dead stop, I can't reproduce the ping whether I take off slowly or get on the gas hard. While driving, I can't reproduce it by flooring it either.

I can however, reproduce it by lightly pressing the gas pedal while cruising. Kinda strange.

Can you guys suggest anything? The car was serviced at 60K - throttle body was cleaned then.

Just keep in mind that you'll hear detonation more when it's hot outside. I'd just make sure everything is in tip-top condition, especially the cooling system, knock sensor circuit, and EGR system. It may be normal in some instances, but you shouldn't hear it too much. As a last resort the timing advance can be retarded a degree or two to eliminate it - but I'd avoid that until you've exhausted all other options.

Heath

In order to maximize fuel efficiency at cruise the ignition advance goes to around 40 degrees BTDC vs 20 degrees idle in gear or 26 degrees WOT at top rpm.

The ecu has a tip in program [for heavy and slight acceleration] where the timing is retarded fast and slowly added back in, but it assumes average conditions not summer heat with ac on.The knock sensors are supposed to take car of it pretty quickly before any damage is done but who knows their sensitivity after aging.Check your coolant temperature at cruise it should be under 190F on the highway, as even 5 degrees above will start retarding timing in 1 degree increments. A sure indication that the combustion is to hot!Check to see if the preignition occurs with the AC OFF under similiar conditions [turn it off and wait 2 minutes does it still occur?].Generally the EGR tubes are dirty and not enough EGR is flowing at cruise so the combustion chambers get too hot then a sudden acceleration and the detonation occurs and the 5 HP load of the AC compressor makes it worse.Same with the intake valves and deposits they absorb the sudden burst of extra fuel and it takes a few rpm for the richness to occur by then detonation has already started.

A good check is to see if it occurs on a very humid day [just after a rain storm] as the extra moisture will slow the flame speed down reducing detonation [preignition].

As previous posters point out assumming everything is spotless inside a 2 degree reduction in initial timing advance will usually solve the appearance of the problem in Summer.

Assuming the fuel is anywhere close to 92 blended octane.Never attempt regular or midgrade use in Summer.

Q45tech wrote:In order to maximize fuel efficiency at cruise the ignition advance goes to around 40 degrees BTDC vs 20 degrees idle in gear or 26 degrees WOT at top rpm.....

Thanks Q45Tech.

The problem appears to go away if I turn the AC off. It must be the heat because this morning on the way to work, I could not reproduce it even with the AC on.

At lunch time, it was about 92 degrees outside. I was able to reproduce it once.

After work, though, it was raining hard and the temperature was about 80 and I could not reproduce it either.

I'm not going to worry about it too much unless I hear it often.

Dennis,

Best octane I can find is 91. Should I use octane boost in the summer heat to reduce pinging?

I never notice pinging in the winter/cooler months.

You can try an octane boost but they do little when added to modern premium gasoline as most are already boosted with MTBE to get the 91-93.Check your cooling system for deterioration!

Fellas:

This thread is technical and infiniti car specifc enough, I moved it to the "Infiniti Online Mechanic board"

Fred...

Dennis,

Brand new radiator, thermostat and hoses. All OEM. Water pump has somewhere around 60k on it. Fan clutch seems ok, but is probably questionable. Fan has some small cracks in it, too.

I just don't go heavy in to the throttle when it is real hot out.

Other than that, the car ran a 7.9 0-60 in the summer evening heat (100) at 2000 feet. I had a little wheelspin, too.