Hey guys,i've got an 87 iroc with a 350 pretty much all stock.[no air filter baffles,no screens in maf,ported plenum,afpr,tbaf,ads chip]I recently developed a pre ignition knock.Initial timing is set at 6 degrees btc,fuel pressure@46.I've tried backing off the initial timing to 5,4,3 degrees and still get the knock.i've also ran a couple tanks of gas through it [91 octane] and changed plugs,wires,cap and rotor.could the coil be getting weak or mabee the ignition module going bad?Can anyone think of anything else that would cause this graduale onslaught of pre ignition knock?

 

GM,

Welcome Aboard!

A gradually increasing detonation problem could be the result of carbon formation in the chambers or on valves. An inoperative EGR system can contribute to this as well.

If you have been running a rich mixture from either adjusted fuel pressure or a poorly-written ADS "Super Chip", the carbon formation could be heavy enough to create hot spots in the chambers and cause the preignition regardless of how the initial timing is set or what the ECM/EST tries to do to retard timing.

Have you monitored timing after adjusting to make sure the EST is working?

 

I don't think it was graduale enough to be carbon build up,it was within 100 miles or so but has gotten worse.I replaced the egr valve,egr solenoid,and that temp sender on the egr base when I bought the car [was getting code 32]and I think that cured it,no more codes.Plus the plugs looked pretty clean when I replaced them.I have not monitered timing after adjusting but will do so right now.If the EST was not working would I get a code?

 

Alright I checked it with EST hooked up and it is working,I don't know if it is working right or not though.Any more ideas out there?

 

First, these TPI engines are EXTREMELY prone to knock and cannot tolerate much spark advance - sad to say. I have done plenty of "tuning" on "basically stock engines" and they all behave the same - anything more than 26-30* of total timing and you will get knock every time (this is with the Knock Sensor neutralized).

It may be that your KS is not doing it's job as effectively as it should. It may not be "bad" but it may be giving you erratic results and going "deaf" as you are in WOT. (A common problem with the Knock Sensor - going deaf just prior to actual audible knock occurring and then allowing "full timing" to engage and causing ping).

Also, do you have the original 87 Memcal? You should try putting that back in. Chances are, your ADS chip is trying to give you more timing but your engine can't really handle (few TPI really can FYI). Try putting the original back in.

FYI, those "off-the-shelf" eproms don't really do much and often cause more problems than they solve. Typically they add a few more degrees of spark advance, add a bit more fuel and lock the TCC a few mphs higher. That's about it.

And, of ALL the "off-the-shelf" eproms, ADS is the worst. I say this because I had one and when I got all my eprom burning equipment the first thing I did was compare my ADS (plus a variety of other "off-the-shelf" eproms) to the stock GM Eprom and I was aghast on how poorly the spark curve (and fuel curve) was. It was crap.

The only thing good for an ADS eprom is to "pop" the eprom, install an AT29C256 Flash Prom and reburn your own BIN on top of it. On the ADS, the eprom is easily "pulled" from the sockets and you don't have to "unsolder/resolder" it like a GM memcal. But that's the ONLY good thing I have to say about the ADS (and most "off-the-shelf" eproms).

 

Oh and I forgot to mention this.The engine is a supposed fresh rebuild,I was a little leary about that untill I found the manual from some crapper engine remanufacturer place in the car and discovered distributer was 180 out.

 

thanks 91L98GTA,I'll try that but I don't know if it is the problem.It ran fine for a few thousand miles after installing the chip.I have heard these chips are junk unfortunatly it was after the 30 day trial ran out.Live and learn I guess.

 

took chip out still getting pre-ignition knock

 

Then I suspect you Knock Sensor is not doing it's job or you have excessive carbon buildup or oil leaking into the combustion chamber.

If the KS has never been changed, I suggest you do that next. I have done some scans on my "old KS" and found that after 100,000 miles that it became "erratic". Sometimes it was "overly sensitive" and other times, it basically went "deaf".

 

I don't think it's carbon build up or oil leaking into the cylinders.It doesn't smoke or use any oil.The spark plugs were clean when I changed them.Even if the knock sensor was bad/going deaf I don,'t think I should get the knock with the stock timing curve,initial timing at 6 degrees and running 91 octane.I will try anything though so i'll do that next.I changed the ignition control module and that did'nt do it so I ordered a msd coil just in case.:lala:

 

You should understand that these engines do not tolerate much spark advance. When I've disabled/neutralized the Knock Sensor, few L98s can tolerate more than 26-30* of timing.

On a completely stock engine, most L98s can run on 89 Octane with no problems (audible ping). But if you look at a scan tool, you will see a TON of knock retard being pulled out to cause the effetive total spark retard to fall between 20-25*. Very common.

What I think is happening is the KS is going deaf (very common when actual audible knock retard occurs) at which point it no longer pulls any timing and BINGO their's your actual knock.

The KS is funny, it hears what the human ear does not but can't hear what we do. The KS is a "preventative" sensor and usually pulls out the timing prior to audible knock actually occurring. But, if the KS/eprom does not pull ENOUGH timing out the KS becomes "saturated with noise" and basically stops working.

As the KS becomes older (or if some fool wrapped teflon tape around the threads to "desensitize it") it becomes "saturated" much easier and ceases to function much quicker.

The real problem is you need to pull those heads, clean off the carbon and "clean up" the combustion chamber for "sharp metal edges and casting flash". This is the primary reason stock heads are so prone to "inducing knock".

Playing with your ignition (provided you have the timing properly set) is just going to throw a lot of money and solve nothing. I have done a lot of research on the reason "knock retard occurs" and how to combat it. I have reprogrammed my eprom in a multitude of ways and I am even looking at trying some different Knock Sensors. The problem is, the Knock Sensor is a "listening device set to a certain frequency". This frequency happens to be something the GM engineers have determined to be the "first signs of knock occurring, BUT NOT ACTUAL KNOCK".

The frequency where actual knock occurs can't be use because it also happens to be the same frequency where many "mechanical noise" is made by the engine and this would just cause "interference". Think of the KS as a device that "tunes a radio" just before you actually tune the radio station but NOT exactly the RADIO STATION because there is another station using that frequency.

This is why if the KS "does go a little off" you may find you are experiencing actual knock incidences because it is going "deaf" too soon from saturated noise. Many a person who has "neutralized" their KS hoping to get more spark advance have found "audible knock" once they get past 26-27* of total spark timing.

Also, if you HAVEN'T TRIED THIS, put some octane boost and use a different brand of gasoline.

Also, if you want to COMPLETELY control your spark advance and set you timing EXACTLY to the spot JUST PRIOR to knock occurring, you may want to learn how to burn your own eprom. This is what I do and the results are great. I get the max timing JUST PRIOR to having audible knock occur. I have mine tuned such that an increase of .4* of the spark advance within the eprom will cause knock. Also, you quickly learn that ELEVATION and AIR TEMP also affect the max amount of spark advance you can run without inducing audible knock.

Now, I only use the Knock Sensor for "part throttle driving" so I can go up hills without causing knock. But @ WOT, I "neutralize the KS", so I have COMPLETE CONTROL of my spark curve. A properly controlled spark curve can often give you a few more degrees of spark advance. And trust me, on these engines, a few degrees of spark advance can be the difference between going fast, going slow or having PING occur.

Good luck, gotta go.