Suspension Better - Detonation NLG

[LVR]

Just got the X back this morning after 3 days at the dealer. Replaced two faulty sensors in the air suspension and the performance difference is amazing. Still not as good as the demo I back to backed but better than driving a battleship.

Detonation is still a problem..... all possible electronic fixes have been done..... preliminary inspection of the internals reveals oil up high in the intake manifold. I'm thinking we have carbon build up (which they say is impossible on this engine) and a compression problem..... we shall see.......

[vinuneuro]

Glad to hear it's handling better now.

-Oil in the manifold means you indeed have a compression problem, your rings aren't sealing properly (most likely) and/or your valve seals aren't.

-Let me make this clear though, you can't have detonation and a compression problem at the same time. To detonate, you need high compression or your ignition timing would have to be advanced a lot. Detonation with low compressin is virtually impossible, a statistical improbability to have both at the same time, and your ingition timing chain would have to be skipping all over.

-These engines really can't detonate. They have several O2 sensors in the exhaust always monitoring your air/fuel
ratio. If it senses knock (the result of detonation), it will pull timing aggressively to prevent detonation and protect
the engine. This is why your engine doesn't break if you happen to use a lower octane fuel sometime.

You can almost certainly rule out detonation...

**Get oil analysis done. It will tell you for sure if you have any kind of compression problem, whether it be at the rings or valve seals- this is such an easy way to diagnose the problem.** Also have them do a compression test and leak down test.

[LVR]

I know, the tech guy is scratching his head as the car is plainly detonating under light load or heavy load, but only when warm. The gearbox is catching it out because it is always in too low a gear for the load.

Detonation is caused by too rich fuel mixture causing carbon build up that pre ignites, or ignition timing that is too advanced. They say that the valve design eliminates the possibility of carbon build up, but to my experience it is exactly what it is. The timing has been eliminated, and now we are ready to start looking at the mechanicals.

He has already pulled the throttle body off and a few other bits to have a look. There is apparently oil up at this level. Whether that is getting there via blow by or via another method is what he will be looking for next. It will be going in to have the inlet manifold removed and examination of the internals.

He is talking valves by the way......

[vinuneuro]

Nooo! Detonation itself is caused by a lean condition. Fuel needs oxygen to burn; if you have too much oxygen and not enough fuel to 'cool' the combustion, temps are going to get high. For obv reason oxygen (air) is a lot more prone to igniting than fuel, hence the reason a lean condition would yield to detonation.

Has your tech pulled the spark plugs? What do they look like? On what basis do you know that it's detonating? If you are indeed detonating, it would have to be caused by some kind of electrical/electronic error, as the DME wouldn't allow it under normal circumstances. Substitute another DME and recheck. Keep in mind that prolonged detonation always means broken parts...expensive ones. Typically, the head will go first, but the piston(s) will follow suit as well. It may not be a bad idea to just go ahead and pull the head at this point, if the tech is positive about the detonation. He should also be able to get the knock sensor input values from the DME. I hope you get the oil analysis done as well...

SANguru just had a detonation problem in his M5. It was caused by a shorted-out DME. His cylinder heads did have to get replaced.

[LVR]

AAAhhh I LOVE technical discussions......

Detonation is actually the spontaneous ignition of unspent fuel air mixture AFTER the spark has been supplied, interrupting the flame front of the normal combustion burn. This is after top dead centre. The resulting very large pressure 'spike' sends a shock through the engine structure itself, which we all hear as pinging. This pressure wave exceeds the strength of the structure but as it occurs quickly the damage is limited to cracked plugs, rings, valves etc. The resonance of this pinging occurs at approx 6400 Hertz.

A knock sensor is simply a piezo microphone that 'hears' the sound and retards the timing.

Compression is not at all the issue, rather timing, mixture and the presence of a secondary ignition source. If you have a lower compression ratio you can actually increase the timing advance (minimum spark for best torque) for lower octane fuel.

Combustion of the fuel air mixture generates temps of around 1800 degrees, which in itself will melt aluminium pistons. A 'shielding' layer of a few molecules during mixture motion protects the piston head from direct heat transfer unless there is a shock wave in the form of detonation. This allows heat transfer and an engine detonating will want to overheat, increasing the problem.

An overly rich mixture for the timing allows a pocket of fuel to remain unburned and is then ignited by another source. That source can be the valves themselves, (which rely upon the cold fuel mixture flowing over them to cool them) such as a burr or sharp edge which retains heat or carbon build up. As my problem appears to be particularly bad when the engine is warm this is where I am leaning.

On the other hand, a lean mixture produces a 'lean misfire' and causes hesitation, rough idle and power loss. These I do not have.

Alternatively, we may simply have faulty valve operation.

Detonation, of itself will rarely result in major damage to an engine. The localised hot spots generated will then lead to pre-ignition, which kills the engine quickly through the ring lands, then blow by then the piston.

Oil up high in the engine may mean that the ring lands are already cracked. I have noticed that under heavy acceleration from stop I do produce smoke....... and I am certainly going to take your advice and get an oil test

[vinuneuro]

You win. My brain isn't working properly with 8 hours worth of sleep in the last two days. I just realized that I've been arguing pre-ignition detonation. The two really can lead to one-another, but that's a seperate case. Hey, I'm enjoying this thread as much as you ! I'm very interested in hearing the outcome of your quandry. Good luck and keep us updated .