2toys

So I have a 94 4runner with a rebuilt 3.slow after I got about 20k on the engine I started to notice a ping when I pulled a steep hill. Now I hear it anytime I romp on the engine. Here's the funny part when I go to the mountains and get up around 3500 feet it runs like a sewing machine. Anyone ever had this happen?

millball

Old carburettor equipped engines always ran richer at higher altitudes (because there is less oxygen per volume at the reduced atmospheric pressure), even my old chainsaws had to be adjusted to run leaner when I used them at altitude.
EFI engines should be able to compensate for altitude changes.
Your engine seems to be running lean under ordinary circumstances, then richening up to where it runs better when you gain altitude.
I'd check for stored codes and maybe change out the O2 sensor if its been in service for a long time. Let us know what you find.
Trying a mid-grade or premium fuel might help too.

JohnnyCanuck

The ecu is capable of adjusting for atlitude changes. Odd that it started happening after 20k on the engine, have you checked your timing?

RJR

Sounds more like the knock sensor to me. The ECU should detect the onset of pinging via the knock sensor, at any altitude, and adjust timing and mixture to keep it under control.

Generally engines ping less at higher altitudes because the absolute cylinder pressures are lower. That's why "regular" gas in Colorado is 85 octane, but regular is 87 octane in most of the rest of the country.

Couple of things:
- Severe pinging is hard on the engine and can lead to holes in pistons, broken valves and spark plugs, etc. Don't let this go on for very long.
- Running higher octane fuel may be a temporary fix until you can find the actual problem
- If the knock sensor is bad, the ECU should be setting a code.

2toys

There are no cel or stored dtc's 02 sensors are old so that could be it. knock sensor is new so I doubt thats the issue. And Premium fuel makes no difference. I have no idea how the truck ran before I got it cause it had a hole in the block the size of my fist.

snobdds

Have you checked your spark plugs...they will tell you what is wrong.

RJR

Normally, during combustion, the flame front proceeds in an orderly fashion from the spark outward through the fuel-air mixture, at a controlled rate which gives more of a steady "push" to the piston than an explosive blow. Pinging is evidence of unstable combustion which has spontaneous areas of flame occuring throughout the combustion chamber, going off like popcorn. When these areas meet, they create shock waves that greatly increase cylinder pressures and behave like hammer blows to the piston and chamber walls, creating the characteristic pinging sound.

Pinging is caused by a combination of
- too low octane fuel. Octane rating defines the stability of the combustion process.
- too lean a mixture (one cause could be a faulty O2 sensor). Lean mixtures ignite more readily than rich ones.
- excessive cylinder pressures (can be the result of wide open throttle at low rpm, or too high a compression ratio(perhaps caused by excessive carbon deposits in cylinder))
- excessively advanced ignition timing (causes combustion process to start early and develop while piston is still on compression stroke, raising cylinder pressures above normal)
- high cylinder temperatures (overheating, poor circulation of coolant around one cylinder, non-working EGR) cause fuel to ignite earlier than it should.
- preignition caused by cylinder hot spots (glowing carbon, valve edges ground too thin and thus glowing red, etc.)

Pick any of those and work on trying to figure out which one is the issue.

Does it ping only when the engine is warm, or immediately after startup? What rpm is worst?