pinging

[manlion]

I have a 94 RM wagon. Whenever I accelerate I have a slight pinging. If I use premium fuel this seems to go away. Are there solutions other than using premium fuel?

 

[dcraig1000]

how long has this been happening?

 

[buickwagon]

Step 1 is to investigate the knock sensor. If possible, borrow a scan tool and see if the knock sensor is detecting the pinging. If it isn't, check the connector at the knock sensor, as they seem to be rather fragile and located down where they are potentially exposed to a lot of water etc. thrown up from the wheels.

The older vehicles had a separate Spark Control Module under the hood that interfaced between the knock sensor and the Engine Control Module to retard the timing when knocking was detected, however I believe that function was relocated to a daughter board in the PCM by 94. Failure of this system should set the Check Engine light and throw a code. In both cases, the system starts slowly retarding the ignition timing when a knock is detected, then dances around the point where the knock disappears. If the sensor is not as sensitive as it once was, then the system could allow a light knock without throwing a code.

Note that the sensor is located in the water jacket, so you WILL be boiled like a lobster if you attempt to change the sensor with the engine hot! Allow the engine to cool and drain the coolant before attempting to change a knock sensor!!!

If the knock sensor system IS working properly, then it's possible that there is something else going on in the engine causing a knock that is beyond the range of adjustment. Heavy carbon build-up may be causing hot spots that generate multiple flame fronts. Excessive oil consumption actually lowers the net octane rating of the fuel mixture.

 

[manlion]

That is all good information. If there was carbon buildup wouldn't this be a problem using premium fuel also?

 

[buickwagon]

Not necessarily.

An engine knocks when there is too much pressure for the octane rating of the fuel or too much heat that ignites multiple flame fronts which collide in the cylinder.

Retarding the ignition timing works by reducing the time for pressures to build between the moment the mixture ignites and starts to expand and TDC.


Carbon can cause knock two ways: first, it can create hot spots that act like glow-plugs to ignite fuel at lower pressures (ie: before the spark). When that happens, the burgeoning flame fronts are fighting both each other and the piston's rise to TDC.

Second, a layer of carbon takes up space, which raises the compression pressure in the same manner as shaving the heads would.

Premium has a higher octane rating, which means it is more resistant to detonation and burns slower so it doesn't self-ignite as readily and the flame fronts don't propagate as quickly if they should ignite. Therefore the pressures don't build until the piston has passed TDC. That's why it is necessary to use premium fuel in high-compression engines -- ignition can be early enough for thorough combustion, yet pressures still remain manageable before TDC.

Besides spark advance and octane rating, knock is affected by a few other things too:

The wrong heat range plug can cause knock in the same manner as a hot carbon deposit.

A hot engine is more likely to suffer knock. Besides low coolant, a defective fan clutch, thermostat, or water pump, lime and rust deposits in the head and block can reduce heat transfer so the head temperature may be higher than the temperature gauge indicates.

Air intake: Cold Air Intake systems are a popular performance mod not just because cooler air is denser -- it has the potential to reduce knock. Things like stuck pre-heat valves can contribute to knock.

Lean Fuel Mixture: Lean mixtures not only burn hotter, they require less heat to auto-ignite. The system is supposed to learn and self-adjust, but there are only a limited number of O2 sensors for all those cylinders and they adjust to the average value. Vacuum leaks or dirty/sticking fuel injectors can cause one or more cylinders to run leaner.

The EGR helps keep combustion temps down too. If the EGR valve is stuck or passages plugged, that may contribute to knock.

I already mentioned oil consumption in the previous reply -- excessive oil vapour lowers the mixture's net octane rating -- which reminds me of something that happened many years ago:

The cops busted an organized ring that was charging for disposal of industrial waste oils and disposing of it by diluting it into no-name tanker trucks of gasoline, which they then sold to small independent gas stations at a slight discount. While that possibility is rather remote, are you buying your gas from independents that have to undercut the big guys by a few cents per gallon?

 

[manlion]

Thanks for all the great info. One thing you mention is EGR. A couple of years ago I plugged the EGR vacuum line. I was having a slight hesitation when going from a stop to a slow take off. This was more noticable in reverse. Maybe my carbon build up is a plugged EGR valve opening or something.

 

[buickwagon]

If you plugged the vacuum line, then the EGR can't open. I don't know that would cause carbon build-up inside the engine, but since the EGR keeps combustion chamber temperatures cooler (by slowing the flame propagation) the lack of EGR might contribute to heating of carbon deposits that are there from other causes.

 

[manlion]

I'll plug it back in and live with the hesitation for a while to see if the problem is related.

 

[milkman]

Awhile back i posted about a "pinging" which to me is different then a knock.
A ping is a higher pitched ticking like sound
While a knock is that low grumble from within.

If yours is indeed the ping check your exhuast manifold's are securely tightened. thats what fixed me up.

 

[buickwagon]

Did your sound only occur under acceleration, or at all times?

 

[manlion]

The pinging is only on acceleration. It is slight. I notice it more when I am already moving, if I accelerate even slightly I hear it.

 

[manlion]

Milkman....how did your exhaust manifold loosen? I have never had mine apart. I suppose if you had it apart and there was a small leak it would make a slight ticking sound. That is how I would classify that sound. Mine is pinging when I accelerate. Thanks for your input. I think you had a different situation.

 

[milkman]

i took it to a mechanic(i know i know never again) He was the family mechanic. I assume one of his helpers are half retarded, they put the gasket on upside down, DIDNT USE A NEW ONE. After they shaved down the head to fix a oil or coolant leak i forgot it was years ago.

Then i started working on the vehicle myself quite a lot, and figured i would finally fix the pinging, new gasket and correct torque, no more pinging.

 

[paart]

GM usually doesn't install exhaust manifold gaskets. With new, flat manifold and head surfaces, they aren't needed. After removing the exhaust manifold for service, most mechanics install them to prevent leaks, which isn't a bad idea, since exhaust manifolds have a tendency to warp slightly over time. That is unless you have a Ford. Then they have a tendency to warp a lot! (A least they used to--I don't know if they fixed the problem or not.) The problem with exhaust manifold gaskets is that they compress over time, resulting in loosening of the manifold, and sometimes a leak. It's a good idea to check and re-tighten the manifold bolts a month or two after installation, then from time to time after that. The only gaskets that I've seen that don't compress are graphite gaskets, and they are hard to find, if they are even available at all.