Knocking¿¿¿¿¿

[junglestylz]

I just got a TMS chip and Cold Air Intake on my car (1994 325isa), and it was running beautifully. I them changed the oil (from whatever was in there, to 10W40 Syntec) and I developed a slight knocking noise at 3,000 rpm and above when I gun it. It's not constant, but a worry regardless. Any ideas what it could be¿ Thanks for any help before hand.

 

[andreyiv]

Maybe knocking from low grade gasoline? I heard it happends in the hotter climates and after you get a performance chip. Some people say the only way to avoid it is to use 93 octane fuel, if of course that is the same knocking.

 

[junglestylz]

Thanks for the input. I make sure to use 91 or above, but the thought had occured to me that it could have been the gas. It is also freakishly hot out here right now (115-120) and that could be a consideration. I will have to find a place that sells 93, or add some octane booster on my next fill up.

 

[andreyiv]

No problem man, hope it works out for you.

 

[Big Daddy]

I bet Andrey is correct, the heat combined with the new chip, you need 93+.

 

[junglestylz]

I am doing a mandatory search of all the gas stations in Vegas to try to find some 93. There is a lot of offroading out here, so I am sure someone has to have it. I found that the connector on my charcoal canister is busted to, but I have a new one ordered from Bavauto.

 

[Big Daddy]

We have a service station here that sells Trick Racing gas at the pumps, they have it in three octanes 99, 103 and 114. Can you imagine!

 

[Kirby]

I also think it is knock from pre-ignition, higher octane should fix it, or prove otherwise. Please let us know how it pans out, I'm quite curious about this one.

 

[junglestylz]

From my best guess, it had to have been the busted Carbon Canister, mixed with the unnatural heat. I threw a new one in today, and it completely stopped.

 

[Kirby]

Quite Interesting, but makes you go

 

[andreyiv]

Hey, who cares as long as it fixes the problem, right?

 

[junglestylz]

That's my theory. I am guessing that it had to do something with the fuel pressure.

 

[junglestylz]

Still haven't remedied it. I took out the chip, and it still does it when it gets hot. The temp gauge is reading as well within normal limits, and I put some 100 Octane in it, and it is still doing it with the OEM chip¿ Any ideas¿

 

[andreyiv]

Then I really haven't a clue. I know my exhaust rattles and sometimes you can mistaken it for a knocking sound but that is only at idle and goes away with even the smallest pressure on the gas pedal. I hope you get it resolved and keep us posted about it.

 

[Kirby]

A few ideas, no real answers though -

Timing Problem due to worn components? - If a timing chain is stretched, or gears are worn, the timing will not be consistent, and will actually change several degrees under load/no load/engine braking conditions. Perhaps enough to cause a pre-ignition knock under load. I know this from first hand experience on a GM 2.8L V6 (crap crap crap engine).

Piston Slap? - If an engine is reallyworn, a knock under load can be piston slap, rod or crank knock. It's because the piston/cylinder is worn enough that under load the piston shifts and slaps the cylinder wall as the crank rotates, or the rod or crank bearings are bad.

But your message seems to say that this only occurred after the oil change? I don't think even a thick oil or additive would mask piston slap. It might mask a crank bearing noise.

Again, these are just some ideas to think about. I know it's tough to describe sounds, but can you give a better idea? High pitch, low pitch, long, short, thunk, bang, pop, clunk. Sounds like a cereal commerical!

Any idea where it comes from - top end or bottom end? Have somebody ride in the engine compartment while you drive.

 

[knightpitt]

So, it happened after an oil change and there's hot weather. I'd say pull out that oil ASAP. Always think "simple". If you change one thing and something breaks, then, it's probably the thing that you just changed, not something exotic and different. Run some Mobil 1 Synthetic 15w50, and my bet is that it'll fix everything. I know, I know, BMW now says something different due to their contract with Castrol or something of the sort... but just run the Mobil 1 15w50 now, and see what happens.

Oil viscosity being the problem does not intuitively make any sense in the situation, except that it's what you just changed. Also, barring a better description, I'm going to blame low oil viscosity and an aging VANOS unit. It's a real stretch, but it's a cheap attempt, and at least it'd take it off the table.

Because you mentioned in your first post that it was intermittent and at 3k rpms, not 4k, and that it is "knocking", not "tapping", I don't really like my solution idea and I agree with people thinking that it's pre-ignition. I'm assuming you're throwing no CEL, though?

As for piston slap, or worn crank bearings, etc, I think that it'd be less intermittent (more constant), and pretty unlikely for a BMW engine with reasonable miles on it.

My next step after the oil change would be to look for a misfire, and probably just change the cheap ignition components, starting with plugs, cap, and rotor.

Or, maybe you just have a bad knock sensor. That's supposed to throw a CEL if that happens, I believe. Because, if the engine is actually pre-igniting, it's supposed to dump in more fuel to keep that from happening. Hmm...

I'm pretty late to the party here, so hopefully you already have it fixed. You'll have to let us know!