first off, the car is a 2001 Z28 M6 with 56k miles. The engine is bone stock except for a K&N filter and redline fluids throughout, and the only mods on the car are 17x9 wheels with 275 tires and a 35mm front sway bar.

i've got a problem at high rpms when accelerating. it seems like the engine is detonating. the sound is similar to a rattle can being shaken or maybe like a loose heat shield (checked and found no loose heat shield).

i have replaced the fuel filter and pcv valve.
i have used seafoam in the throttle body, in the intake through the brake booster, in the oil (and changed the oil) and in the gas. and i sprayed the aerosol can electrical cleaner on the maf wire.

i have tried bp gas and shell gas (93 octane only).

none of this has worked. it seems like the noise gets more pronounced with higher gears and the rpm's when the noise starts gets lower with higher gears, ie:
5500 rpm in 1st
5000 rpm in 2nd
4500 rpm in 3rd
4000 rpm in 4th

what should i be looking into in order to trace this problem down??
any opinions appreciated!!

thanks
tony

 

You've got alot of the basics but what about plugs/wires, I assume you have a clean air filter. I notice you have a K&N is it over oiled? The oil can get on the maf wires and mess the the air/fuel ratio. Hows the inside of your intake manifold look is it covered in oil?

 

Get rid of the K&N first, their useless.

See if you have oil on your spark plug threads, if you do, than most likely you have burnt oil building up on your pistons. When the engine gets hot that burnt oil creates much hotter spots on them and when you get into the throttle those hot spots will ignite your air/fuel misture prematurely (detonation).

If you do have oil on your plug threads, you need new valve seals in your heads.

Take the car out and do a WOT throttle run when the engine is still relatively cold. See if it DOES NOT detonate. If it detonates after the engine is at operating temp or hotter, than thats your problem.

Here's what it looked like on mine and I had your exact symptoms. After cleaning them my car can run way past the 210 mark if I let it, and no detonation no matter how hard I drive it:

Before:



After cleaning them before the newly rebuilt heads went back on:

 

the k&n was new when installed, and has not been re-oiled since it was installed. if it was the oil from the filter screwing up the maf sensor, how do i clean it now??

i have not checked the plugs yet, but i was planning on replacing them with ngk tr55s or tr6s. any opinions on which ones? and i was only going to replace the plug wires with msd's.


as far as cleaning the piston tops, is there any other way other than removing the heads?? is it worth it to rebuild the stock heads, or would be going with a set of ls6 heads and cam be easier at that point??

thanks guys!
tony

 

It's just my opinion about the filter, but it is a FACT that K&N filters allow more larger pieces of dirt to get into your engine and harm the piston rings. You can't beat the paper filter for engine protection, and the extra airflow that K&N "claims" is probably true, but its only gonna be worth .5 hp, if that. Not worth it to me.

If you're stock, TR55's are what you need. NGK are the best IMO. TR6's are colder and used for engines with nitrous.

Also, as far as wires go. Kind of like the K&N thing....you cannot beat the stock factory wires for a stock engine, no way, no how. I have a 436 stroker engine and I bought the Taylor 10mm Thunderbolts, they did absolutely NOTHING for performance. Than when I took them off to change my plugs, pulling them BY THE BOOT, 5 of them broke. So I put my factory stock ones back on until I could get new ones and the performance was identical, zero change. I STILL have my 9 1/2 year old stock wires on and my engine runs smooth and strong. Buying expensive plug wires on engines that make less than 600 hp are a complete, 100% waste of money.

Find out what your problem is first, I was just throwing that out there about the bad valve seals because your symptoms were exactly like mine. Also, when you cruise at say 55mph, and than stomp the gas to the floor, is there a puff of bluish-white smoke that comes out, than clears up? You can see it better at night when there's cars a few hundred feet behind you with their headlights on. You might not see it in the daytime. If yes, than thats another sign your valve seals could be bad. When you stomp the gas from a cruising speed, that instant drop in pressure in the cylinders sucks oil from under the valve covers through those bad valve seals, through the cylinders and out the exhaust. But than it levels out and that puff goes away as you continue to accelerate.


The ONLY way to clean them is by removing the heads. But if you need a new valve job they need to come off anyway. Its not that big of a job, but it makes you have to buy new head gaskets and have the heads checked and rebuilt. It'll cost about $600-$800. If you do have to rebuild them, PM, I have the best head guy in the country to recommend, and his prices are the best too.

But you need to find the problem. Detonation can be a few things, but what I mentioned about the hot spots because of burnt oil, is sooooo overlooked by many.

Did you do a WOT run when the engine was cold? I'm curious if it DOES NOT detonate.


.

 

Quickin, A couple things...

1. How could you tell the car was detonating? Was it audible, or did you use a computer knock logger?

2.I know you always have good advice and technical experitse, but what do you think of going WOT on oil that is not at operating temp(one time can't hurt?)?

Also, I found out my car was built 11/97, and your car is a 98 too, did you have the old style knock sensors with 4* of constant reatard, or did you have the new style sensors. And how many miles were on there?

I have a feeling my car might look like that I know that PCV sucks a lot of oil into my engine.

 

My car would only detonate after the engine got really hot, as in over the 210 mark. I would hear it, it was friggin loud. My friend driving next to me could hear it. After the engine was hot it would detonate leaving casually from a traffic light or if I got into it hard. It was really bad. But when it was cold it wouldn;t detonate AT ALL no matter how I drove it. My problem was the burnt oil on the pistons, when the engine got real hot so did that hard burnt oil and it was just like a spark plug. As the piston came up and compressed the air and it started to get hot from that compression, those hot spots would ignite the air/fuel the second the fuel started spray into the cylinder and it would explode before the spark plug fired.

I'm not talking start the engine and immediately in 3 seconds pull out and go WOT. Let the temp gauge rise to where its almost at the first tick mark (155* mark), than you're engine is warm enough where you won't cause any damage at all, its just fine to go WOT.

Man, I don't know that stuff. sorry. But my detonation issued were with my 436 stroker, never when I had the stock engine in there.

Also, the reason, back than, when I was getting so hot was because my condensor was so blocked up with crud that as soon as I turned the A/C on, my engine would overheat in literally 2 minutes. If I drove around with the A/C off it would never get hot enough to detonate from that burnt oil on the pistons.

 

Sounds like the same thing my car did, it was not detonation, but a loose 02 sensor. Before you go tearing you engine apart, check the 02 sensors, tighten them if you can.

 

ok guys, here the latest.

took the intake apart, pull the k&n and installed a paper filter.
sprayed the maf down with maf cleaner, and squirted some seafoam in the throttle body. also put some seafoam through the brake booster line.

put everything back together and let it sit for an hour or so.

went and drove the car, easy at first (lots of smoke as usual) and then hard once it was done smoking. NO DETONATION.

so i was happy as a calm. drove the car for about 2 or 3 days, and the detonation has come back now!!

i bought spark plugs yesterday, and will install them this weekend.

but i'm thinking maybe the start of a plugged up cat?? the driver side has already been replaced, about 2 years and 20k miles ago.

or maybe the beginings of knock sensors going bad??

or maybe just a really dirty engine internals and the seafoam just knocked off the top layer??

any thoughts??

i'll post info on the plugs when i change them.

tony