I went to work on getting the car dialed in today, because its been running like total a$$ lately. Ever since I was able to get the car to idle on its own with the A/C on, its been idling a bit higher, around 1000 or 1100 rpm. I brought the idle back down to about 750 rpm or so, and was datalogging it at the time.

It starts to make a very loud rattling sound. I thougt something was loose and making that noise, but didn't find anything. I held a wrench against the passenger side header, and felt the vibration on the header. Went inside, all the gauges look fine, and no knock counts. I took a look at the timing, and I couldn't believe it... 40 degrees. If I gave it a bit of gas to raise the rpms, it would drop down to the high to mid 30s, and the rattling would stop. Below 750, it wouldn't rattle either.

I turned it off right away, and went inside to look at the spark tables, thinking I must have screwed up and stuck a 40 in the main spark advance somewhere. Well, at 700-800 rpm, there is no 40. In the closed throttle spark advance table, there is no 40. I spent about an hour looking through my bin, and couldn't find where this value originates from.

I decided to set the initial timing on the chip from 6 to 16, so the ecm would remove 10 degrees. It helped a little, but didn't stop.

I disconnected the EST wire as if I was going to set the timing, and fired the car up. It was dead on 6 degrees base. Timing light is a Snap on which is known to be good. The rattling was still there.

I didn't get any knock counts throughout this whole time. I'm running about 9.8 - 9.9 to 1 compression, and 9.3 octane.

I am pretty sure that its pinging. I can put my head next to he exhaust pipe, and hear the rattling sound very distinctly.

Anyone have any ideas? I wanted to post by current bin so hopefully someone can spot the error in it, but I'm not too sure how to do that.

 

have you tried with a "virgin" bin?

/N.

 

No, actually I haven't. The last time I had tried a stock AUJP in the car, it would barely run. It was surging in gear at low rpm, and popping on deceleration. I also had to raise the idle screw a turn and a half before it would idle on its own.

I'll give it another try though. Maybe the problem goes away.

 

If your rattling is still there with 6 degrees of timing its doubtful it is timing related as the ECM can't add more timing with the EST disconnected. In regards to looking for 40 degrees in your timing table, remember the 40 degrees will be a total number. Check your spark adder tables. Your base timing plus your main table plus any applicable SA table will give you the total timing. This is only a basic example but if your base was 6 degrees and you had 30 degrees in the main table and a spark adder table was being used at maybe 4 degrees then your total timing would be 40 degrees. Also do a search on this in the DIY prom boards. There is some great reading on how total timing is shown and what affects it, ect. Sorry I can't be more exact but I am not a speed density guy.

 

NO knock counts? Is the engine warm? I would think there should at least be a count or two from the "test tickle" of the KS during startup.

 

Well, you know how they say that the simplest explanation is almost always it? Guess what the problem was? My collector bolts on the passenger side vibrated loose. Oddly enough, it didn't sound as much as I would have expected (I knew I had an exhaust leak, but it sounded like a small one). The rattling sound was from my aluminum collector gasket. It was barely loose enough to vibrate, and was making the sound only at that rpm. I drained the oil on the car to inspect it, and decided to tighten down my exhaust a bit while I was down there, and found all three bolts loose enough to turn by hand.

The car really was running at 40 degrees at idle though. I verified that with a timing light (the type with the advance ****). It was just as razor said. The 40 degrees was coming from the combination of several tables. However, I decided to start from scratch with an AXCN bin instead of AUJP (like this last time), since I wasn't happy with how the car was running so far.

Vader, I had been wondering about that as well. Usually when I fire the car up, it reports one or two knock counts. From what I've read, those are forced knocks conducted by the ecm to make sure the KS is working. Every once in a while, it won't show any knock counts. Shouldn't the ecm drop into limp home mode when it can't verify that the knock sensor is working? I have the correct knock sensor for the application. IIRC, its the 3.9 kohm KS. That sound correct?

 

If its not knocking and your reading 40 degrees at idle your probably not at 40 degrees.

I bet the hub on the harmonic balancer has slipped and the timing mark has moved, pull the #1 plug out, stick a small pick or something in there and slowly rotate the motor by hand until you can feel the piston come to top dead center, then check to see if your mark is near the TDC mark on the timing tab. This method should get you within 2-3 degrees.

Let us know what you find!

 

When I assembled the engine, I had lined up the timing marks on the timing set, made sure #1 was at top dead center, and then checked the balancer in relation to the timing tab. It was dead on 0 degrees. The reason I had done this is because I was using a stock balancer, and had been told to watch out with the outer ring has moved. Not sure, but I think when the outer portion of the balancer slips its a gradual thing correct? I don't think that kind of thing happens in 300 miles. Wouldn't hurt to check again though.

I started from scratch with an AXCN bin (Vette w/ manual tranny), and took some timing out of the peaks to start. I have iron heads, whereas the bin was setup for the aluminum headed L98.

I've pulled some fuel out of the VE tables gradually, and the car has improved dramatically. I'm still not happy with it, but I'm making progress.

If I drive it for 30 minutes or so, I'll get 3 or 4 knock counts usually. That leads me to believe that my KS is working.