I am working on a 91 speed density TPI motor and have a question on the timing. I burn my own PROMS and have about 25 updates on this eprom as I fine tune the fuel and spark. When I watch my timing at idle with a timing light or on my scanner, I see the timing jumps around about 6 degrees or so. Looking at the spark table I do not see why this is happening..... My MAP is stable(35kPa), rpm is stable(650 rpm), and the cells around idle have the same timing values. I dont see why it should be changing...... Am I missing something about how the ecm operates??? Is this a normal phenomenon with these ECM's???? Thanks alot

 

Yelofvr,

I can't offer any explanation, but my car ('90 305 TPI) does the same thing. Closed throttle spark numbers and main spark table all have same timing numbers but at idle advance jumps around @ 6-8 degrees.

For awhile I suspected a loose timing chain or something as my motor has rather high mileage. But then I set minimum TPS% for idle to something really low like .4%. Instantly at that TPS% the timing becomes rock solid at whatever I have it set for, both on my scanner and with a timing light.

So, like I said, I can't help explain it but I certainly can verify what you're saying. I'd like to know more about it myself. Must be that mysterious cell #4.

Regards,
Gary Anderson

 

Gary when you say you set your TPS% at .4% your timing steadied. Are you talking about TPS% at idle. I am asking because I am on about my 50th revision of 7730 eprom for a Beck racing engine I put in my 77 corvette. I bumped this number up to 6% tonite thinking this would allow me to adjust the throttle blades without messing up the tps enrichment, but I also started to experience weird timing. With the timing bypas unhooked it steadied, but it took alot of advance to bring it to 12 degrees BTDC. Does anyone know if the ECM can still change timing with the black and tan wire unhooked.

 

Hey guys , Trax just answered that question in another thread.
Here it is.


https://www.thirdgen.org/techbb2/sho...hreadid=110734

 

Thanks guys for taking the time to respond.......
Guess it is normal. At idle I normally see 11 deg added, so that is a total of 17 deg BTDC. That seems normal, when it jumps, it can go as high as 24 deg BTDC total timing. Seems a little high, but nice to know its working OK. This way I can move on to my new found stumble I am getting on acceleration I put in new injectors, new ignition module, new ignition wires and new 7730 ECM(had spare) but no better. Fuel pressure is rock steady, put in new fuel filter, no trouble codes and still stumbles..... I am starting to suspect the memcal might be bad or intermittent since it has the EST circuitry on it and I have had it in and out several times reprogramming it

 

If the timing is jumping is this effecting the injector pulsewidth? If so this will not do. Possible we can intercept the signal at the pick up and use the new MSD timing computer and then refeed the injectors and ecm with a true rock steady timing. That is providing that the MSD unit has the ability to hold steady. Anyone used one of these ECMS yet on ANYTHING? If it does work I will buy one and wire it up. I would be happy to tell all how I did it. hopefully I get some feed back.

 

The timing and the injector pulsewidth are NOT tied together. I monitor the inj pulsewidth on my scan tool and it is rock steady at idle when the timing is moving around

 

Well, they are tied together, for when it's the injectors time to fire, and for counting, which pulse to fire the injectors on.

So while the injector firing pulses are jumping around, their length doesn't change, and that's what's important.

 

I fail to see the point, of adding a computer when you already have one. Once you buzz the engine up a little it's rock steady. There is so much slef EGR'ing going on with a cammed motor at idle the few degrees of timing bounce is a no-matter item.

 

Sounds like you need to read thur things a little more. There are lots of things going on at idle, and knowing all about them might be an aid.

FWIW, I generally run 26-30d BTDC at idle on cammed engines. That's true timing, figured out by reading the all the variables.

 

Thanks for your inputs as always Grumpy. The reason I asked about the timing moving is that in all that I have read....... which is ALOT, I have never seen mention of an algorithm in the ECM that uses timing adjustment to stabilize the idle speed. Now I know based on Traxions input that this occurs. I agree cammed up motors can run much more advance at idle, my engine however is a rather mild 383 with a 204/214deg @0.050" cam

If you have any suggested sources for additional information on this subject I would love to read them. Thanks again for responding!

 

I noticed that idle seem to be tied to the timing. I realize I have a long way to go as far as tuning is concerned, but I have always started my tuning by trying to find a liveable idle. Right now at start up the car idles a 1500 for approximately 30 secs then it settles into the commanded 700 rpm, but it will lope and stumble from time to time and ocassionaly rev back to 1500. I was hoping stabilizing the timing would solve this. I am wondering if my knock sensor is interfering. I am using a pre 87 block bored to 355ci and a hydraulic flat tappet cam. I have closed loop set up to come on at 140 degrees farienhiet, but I think it would take longer than 30 secs to reach this temp. The clt doesn't read 140 when the idle settles.