Does anyone know how much the ecm advances ignition? (Page 1/1)
2greenfrogs OCT 18, 09:14 PM
I have an 84 fiero with a moderately built 2.5 Duke. The engine has about 6-700 miles on it, and had to replace the cam, lifters, and gear because the fiber gear that was on it catastrophically failed... like the gear itself was in pieces, not just the teeth. Anyways, I got a different cam from comp cams than the last one ( it was only $115 on ebay) and the advance I was running on the old one doesn't work at all for this one. Anything under 20° initial advance and she sputters at takeoff until she gets above 2k rpm. The old cam ran fine at 12°. Long story short I'm curious if anyone knows how much the ecm advances the ignition timing so I can figure out the total advance I'm running. Comp cams told me it should run optimally between 30-36° total advance. The other thing is, it's at 20 initial right now, but is still on the sluggish side initially. I will probably keep moving it up till it goes away, or it starts to ping, but really would like to be able to figure out where I'm at.

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84 SE Iron Duke .30 over, flowbench optimized head, decked .040, ported intake/exhaust, and street cam

Patrick OCT 19, 05:31 PM

I don't know if this might be what you're looking for, but I was involved in a discussion here regarding ignition timing way back in 1999. Have a look at This thread and see if anything is relevant to your inquiry about your '84. Apparently the '87 duke (with dis ignition) had a total advance timing at idle of 38°.
fieroguru OCT 19, 06:45 PM
Generally speaking total advance depends on rpm and engine load.
At WOT 30-36 degree is normal for older cast iron heads, but at part throttle cruise and other light throttle conditions, the advance can be 45 to 50 degrees.

When you make the adjustment at the distributor, it changes the timing across the entire load and rpm range. It might be OK for idle, but not OK for WOT or cruise conditions.
theogre OCT 19, 07:16 PM
Initial Timing of All HEI is 100% mechanic at the distributor. 8° w/ ALDL A B Jumped. If auto trans in Drive.
ECM takes over Timing when the engine is above ~ 500 RPM and advance/retard from the Initial timing.

If adjust the Dist for whatever other then 8° then ECM still tries to set timing and often have problems as it looks at other sensors like O2 and adjust timing partly to see right numbers for them.
Extreme Initial Timing the engine won't start or run right even if doesn't set "codes."

Even if Everything is 100%, doubtful, that Cam can still cause problems.
As well as cutting the block and/or head because often changes nonadjustable valve train. That's why they made "Head Saver" shims to go w/ the head gasket for Dukes and others.

The Only way to see some ECM timing is use scan tools to read timing in the data stream. Even that often won't help you because only update that 1 or 2 times a second vs ECM can change timing each firing of a plug.
Plus ECM hates scanning and adjust timing just for you scanning. Is Why most OBD1 engine idle at 1000rpm while scanning. And why most scanner made by big companies say do not drive when scanning.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

2greenfrogs OCT 19, 08:37 PM

quote
Originally posted by Patrick:


I don't know if this might be what you're looking for, but I was involved in a discussion here regarding ignition timing way back in 1999. Have a look at This thread and see if anything is relevant to your inquiry about your '84. Apparently the '87 duke (with dis ignition) had a total advance timing at idle of 38°.



That helps a lot actually, thanks! That gives me a rough guesstimate at least on how far the advance is getting adjusted after my initial timing.

I jumped the pins, and adjusted the initial timing again today before my way home from work. The marks obviously don't read far enough, but using a caliper gauge and the stock timing marks as a guide, I'm at about 26° initial advance. For the first time she doesn't sputter when I touch the gas from a stop. That just seems crazy high to me. She ran fine on 87 gas the whole way home with moderate throttle. It's still a new cam so I'm not really wanting to go wot till I get at least a few hundred miles on the cam... but it seemed fine... no pinging. She also had a bit better acceleration not just to 2k, but past that as well. I do have a 180 Stat and a 180 fan switch so I'm not sure how much that will affect it all. I just don't want to go overboard on the timing with this thing, and am not sure why it's taking so much advance just to run right.

I do know that when I unplug the battery for a few hours, and the computer resets, the car will hardly start anywhere under 20° initial... and immediately will stall if you touch the gas. The head has had a lot of work done to it opening it up. Would that mean greater care should have been taken in pairing an appropriate cam?
pmbrunelle OCT 19, 10:12 PM

quote
Originally posted by 2greenfrogs:
I do know that when I unplug the battery for a few hours, and the computer resets, the car will hardly start anywhere under 20° initial... and immediately will stall if you touch the gas.



It might be that after driving a for while, the computer learns the airflow (and therefore fueling) difference between the stock engine and yours in the BLMs. When the computer is unplugged, the BLMs are reset to default values, nothing is remembered, and the engine doesn't run as well upon restart.


quote
Originally posted by 2greenfrogs:
The head has had a lot of work done to it opening it up. Would that mean greater care should have been taken in pairing an appropriate cam?



Sounds like you need to work on the tune in general... fuel also, not just spark. Either learn to do make adjustments yourself, or work with a tuner such as Ryan.

It's not necessarily a bad sign that your engine won't run well on a stock ECM/PROM. That just means you've modified the engine significantly enough (relative to stock) that the stock ECM/PROM won't run the (now different) engine smoothly anymore.

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If you want to try a lot of ignition advance down low without having too much advance in the upper RPMs, it might be an interesting test to run the car with the ALDL jumper, with the fixing fixed at some value such as 30°-35°. This information may turn out useful for you / the tuner.

It is probably not ideal to try to start the car like that (only rotate the distributor there when the engine is already running); too much ignition advance during cranking might cause the engine to kick back during cranking.

[This message has been edited by pmbrunelle (edited 10-19-2021).]

theogre OCT 19, 10:48 PM
ECM "forgets" by loosing standby power in Seconds.
All GM OBD1 has a single pin Weather Pack connector in the engine bay just for this job to wipe Codes etc.
84 and maybe 85/86 L4 Fiero that is on the right side of the head.

You will have problems until you drive > 35mph to reset the IAC and relearn other things.
2greenfrogs OCT 20, 03:30 PM

quote
Originally posted by pmbrunelle:


If you want to try a lot of ignition advance down low without having too much advance in the upper RPMs, it might be an interesting test to run the car with the ALDL jumper, with the fixing fixed at some value such as 30°-35°. This information may turn out useful for you / the tuner.





That's another good idea as well... I may try running it in diagnostic mode at some point. Like you said the info it yields would be helpful.

Ogre - I understand the ecm resets. I guess I should have worded a little better. I was trying to give more info about the situation to see what suggestions others might have. As was mentioned, I gather enough to realize the engine is modded to a point it will no longer run on stock values. When I don't want the computer reset but have work the requires removing the battery or alternator, I have a line I can clip in that I attach to a motorcycle battery... then disconnect the main cable. I had to rewire the chasis cables anyways because the manifold melted and shorted them out. That's something I never understood either... placing the chasis cables on the starter that sits 3" from the exhaust manifold.

Pmbrunelle- tuning would be nice... that's just more time consuming than I can afford at the moment. The engine I bought off a guy in Chicago for $200, still on the stand in his shop. He had it built for a sunbird (I think) that he no longer had or something. I was needing a new engine for the 84, and well that was a no brainer. He showed me and gave me a list of all the work done to it and he spent a chunk of change on it for sure. The cam that got wiped out on it I would've had to get custom ground as I couldn't find one with the same specs. It would definitely benefit with a solid tune, but that's for a different day.

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84 SE Iron Duke .30 over, flowbench optimized head, decked .040, ported intake/exhaust, and street cam