I found out tonight that I can cause the M3 to ping under certain circumstances. This is on a 95 M3 with an AA S/C.

I was racing a Mustang and winning by quite a margin. I decided to let off the throttle and wait for the Mustang to catch up. The revs were steady and coasting at 4k rpms. When the other car caught up, I WOT'ed it and when it reached 5k the car started to ping. I immediately let off the gas to investigate.

If I start from a standstill and go WOT, I can go the entire revband without pinging (mostly) in all the gears. If I let off the gas and get back on, the car will ping like mad. Lean condition? Help! Happened to any of you FI folks too?

Any ideas? I can't figure it out. I guess I won't be able to wait for them to catch up anymore... :evil2

Sounds like a heat issue. Coasting at 4k rpms will increase intake temps. Depending on how long, it may have been enough to start detonation when the car went WOT.

The following can be done to help minimise the risk of detonation.

1. Retarding timing as intake temps rise
2. coast below 3k rpms and downshift when taking off
3. look at other ways of reducing heat, like finding a more efficient intercooler
4. Use better fuels

Good luck :)

It could also be due to a slow O2 sensor transition from full lean mode, as it coasting, to quick WOT. I see its a '95.. How many miles on those sensors? But first, you need to beg borrow or steal a Wideband sensor, to look at your afrs at that point of onset of detonation.

sounds like you ping when you put a load on the engine when it's near torque peak.

you haven't told us if the pinging goes away with additional octane.

sounds like you ping when you put a load on the engine when it's near torque peak.

you haven't told us if the pinging goes away with additional octane.


I'm limited to my octane choices so I think I will try other avenues (such as colder plugs) first.

do you have an FMU? if you do thats the problem.

imagine the situation... your at WOT, the fmu has been under boost so it has increaced the fuel pressure, you lift throttle, the fmu lets the fuel pressure drop, then you hit wot again.

in that split second, your supercharger makes boost, and pressureizes the fmu. before the fmu and fuel pump can increase the fuel pressure to the injectors, your already compressing air into your engine. after another split second, the fuel will catch up to the air and youll be fine.

i would suspect fuel delivery. low fuyel pressure, elderly pump, weak fmu, etc.

high air intake tempatures DO NOT cause detonation. it can contribute to an existing problem. the hot air is thinner than intercooled cold air, so in most cases the thinner air will prevent detonation. but the tempature of the air if to hot can cuase pre ignition and detonation.

like the others have said, get some colder plugs, maybe get a fuel pressure gauge and make shure that the fmu is working properly. (if you have one)

How can you tell that your motor is pinging?

The reason I ask is because, normally, pinging/knock from engine detonation is not very audible, especially at WOT with the sound of the exhaust and motor.

A knock sensor (which just happens to be a sensitive microphone that only listens to certain frequencies) even has trouble "hearing" knock somtimes. Regardless, you need to have your car tuned with a wideband oxygen sensor so you're not just guessing anymore. You need to know, for a fact, whether you're running lean/rich/stoich.

Chris

high air intake tempatures DO NOT cause detonation. it can contribute to an existing problem. the hot air is thinner than intercooled cold air, so in most cases the thinner air will prevent detonation. but the tempature of the air if to hot can cuase pre ignition and detonation.

:confused:

Statement #1: "high air intake tempatures DO NOT cause detonation"
Statement #2: "the hot air is thinner than intercooled cold air, so in most cases the thinner air will prevent detonation."
Statement #3: "the tempature of the air if to hot can cuase pre ignition and detonation."

Statement #3 just happens to be a direct contradiction of statement #1. High intake temperatures, especially in a motor with forced induction, can very well cause detonation. Granted, there are many other factors at work, but high intake temperatures definitely promote detonation. They do not, on the other hand, promote preignition, which is a slightly different beast.

Chris

imagine the situation... your at WOT, the fmu has been under boost so it has increaced the fuel pressure, you lift throttle, the fmu lets the fuel pressure drop, then you hit wot again.

in that split second, your supercharger makes boost, and pressureizes the fmu. before the fmu and fuel pump can increase the fuel pressure to the injectors, your already compressing air into your engine. after another split second, the fuel will catch up to the air and youll be fine.

This does not make sense to me. I cannot understand why this condition would occur only after running at WOT, then letting off, then going WOT once more. This should have no effect on the air/fuel mixture.

You said it yourself: when the throttle is lifted under boost, the FMU lets the fuel pressure drop. Now the ECU should be running in closed loop mode, just as if the gas had never been floored in the first place (it doesn't remember). At this point, going WOT again should be no different than if the driver had gone WOT from a standstill. In both cases, the fuel delivery would have to "catch up" to the airflow as a result in the delayed response from the FMU.

For this reason, I do not think the FMU is the problem. It would be a very commonly heard gripe amongst the FMU crowd, I'm sure, if a basic functional defect caused detonation.

Chris

GQ,

Is your car's knock sensor functional?
What octane gasoline are you using?

Go driving on a cold night and see if you can reproduce your problem. I'm guessing that you will be able to. If this is the case, I'd highly recommend retarding your ignition timing a few degrees until you can have your car tuned on a wideband to see if you're getting real detonation, "phantom knock" (caused by mechanical noise and falsely received by the knock sensor), or something entirely different. I'm unaware of the common BMW-method of retarding the ignition timing, but often times it's as easy as offsetting the cam angle sensor (on distributorless ignitions) or the distributor gear.

Chris

:confused:

Statement #1: "high air intake tempatures DO NOT cause detonation"
Statement #2: "the hot air is thinner than intercooled cold air, so in most cases the thinner air will prevent detonation."
Statement #3: "the tempature of the air if to hot can cuase pre ignition and detonation."

Statement #3 just happens to be a direct contradiction of statement #1. High intake temperatures, especially in a motor with forced induction, can very well cause detonation. Granted, there are many other factors at work, but high intake temperatures definitely promote detonation. They do not, on the other hand, promote preignition, which is a slightly different beast.

Chris

I said- "high air intake tempatures DO NOT cause detonation. it can contribute to an existing problem."

theres a point in witch hot air is either a help or a hinderence. but there are far to many factors to calculate to determine at what point it helps or hurts.

i guess it depends on your definition of "high" intake tempatures. he may see tempature ov over 220 degrees if un intercooled, where as i dont see anything above 145 degrees.

This does not make sense to me. I cannot understand why this condition would occur only after running at WOT, then letting off, then going WOT once more. This should have no effect on the air/fuel mixture.

You said it yourself: when the throttle is lifted under boost, the FMU lets the fuel pressure drop. Now the ECU should be running in closed loop mode, just as if the gas had never been floored in the first place (it doesn't remember). At this point, going WOT again should be no different than if the driver had gone WOT from a standstill. In both cases, the fuel delivery would have to "catch up" to the airflow as a result in the delayed response from the FMU.

For this reason, I do not think the FMU is the problem. It would be a very commonly heard gripe amongst the FMU crowd, I'm sure, if a basic functional defect caused detonation.

Chris

datalog your fuel pressure, boost/vacume, and tps voltage while driving. you will see what im talking about.

going wot from a standstill is slightly different. there is no load on the engine at idle, this is why you can rev it up all day and not make a pound of boost.

so while starting off from a stop, the engine builds boost while under load, and it does it relitively slowly. so the fmu has a chance to match the boost. but while driving, and at 4000 rpm, lifting throttle, and then suddenly going WOT there is boost right away due to the load the engine is under.

as far as i have known, it is a common gripe. i got rid of my fmu because of that reason.