Timing Issue

[srgould41]

Hello,

I am chasing an issue on my 1990 5.7, 1228746 ECU. Yesterday it was acting like it had a vacuum leak, idling about 1850. Today it started and idled normally except the RPM was bouncing from 550 to 950 or so. Vacuum is stable at about 19 in low idle, up to 21 at high idle. Injectors look fine and have an even spray pattern. Fuel pressure is steady at about 14 psi.

I decided to check the timing. Unplugged the plug for setting base timing and took a look. The timing is all over the place, bouncing 15 to 20 degrees total, moving both retarded to advanced.

I pulled the cap (cap and rotor look great), the rotor does rotate a little bit (maybe 1/16" to 1/8") as well as a little more when riding up the gear teeth. I don't know how much movement is considered normal. Anyone?

The ignition module is a Delco and has about 2,000 miles on it. The coil has about the same. I replaced both at the same time along with the cap and rotor. The spark control module is original.

Before I go do any further testing and troubleshooting I wanted to get some advice. Suggestions?

Thanks

[In-Tech]

IAC sticking is pretty common with age.

[srgould41]

IAC. That could definitely be an issue with the bouncing idle. I wonder if the timing would be affected that much with the 400 RPM swing? The IAC is 3 years old, but even new parts do go bad. I will keep this in mind. I was data logging when this happened yesterday so I'll look at the IAC steps more closely.

Edit: IAC was 3 years old. The IAC was what I heard and thought was a vacuum leak. When in gear (700R4) it's trying to command a 1050 ish RPM and opened up all the way. Replaced IAC just now, same issue. Put it in gear and the IAC opens up to command a high idle. Put it in neutral and the RPM jumps to 2k ish then nearly stalls as the IAC tries to bring it back down.

[PlayingWithTBI]

Maybe the teeth on the distributor gear are worn or you have slop in your timing chain? I'd pull the distributor and check the gear out.

[srgould41]

Rebuilt engine in 2017. I used the original distributor with new stuff in it. Could very well be the gear is worn. It did look good enough to reuse when I rebuilt the motor.

Maybe the teeth on the distributor gear are worn or you have slop in your timing chain? I'd pull the distributor and check the gear out.

[srgould41]

When replaying the last log I noticed the spark advance varies from 36 to 54 degrees. Is that right? Should it be that high at idle?

[PlayingWithTBI]

When replaying the last log I noticed the spark advance varies from 36 to 54 degrees. Is that right? Should it be that high at idle?

Not really, it s/b steady and closer to 20* or less.

[srgould41]

That's about what I thought.

Could the ESC be doing this?

Thinking on the worn distributor gear I can see how it could retard timing and make for unreliable numbers, but I would not think the timing would bounce around by 20ish degrees. I am not opposed to replacing it (already found one just in case) but I don't feel like that is the root cause.

I might have a spare ESC, but it would have been mated to a 7747 ECM (kept off my last engine). I am not sure it can work with the 8746. Does anyone know?

Not really, it s/b steady and closer to 20* or less.

[PlayingWithTBI]

I might have a spare ESC, but it would have been mated to a 7747 ECM (kept off my last engine). I am not sure it can work with the 8746. Does anyone know?

Look up the ESC, for each, on someplace like Rock Auto and see if they're the same (they should be if it's the same engine). On your logs, do you see an unexplained jump in timing anywhere? That could be an indication your ICM is bad. I know you just put in an AC Delco a while back but, they aren't as good a quality as they used to be. When you installed it did you use plenty of thermal paste on the underside of it? If your coil is bad, it can take out your ICM too. Spray some water on your coil, not the wires, while running and see if it stumbles.

[srgould41]

Thanks, some good advice. I will find my spares this morning and do some research. I have a spare coil and ignition module so I can test each part one at a time. I went through the logs again and two of them did show an error for the ESC, VSS and Knock Sensor. The errors weren't consistent across sessions so I ignored them.

Look up the ESC, for each, on someplace like Rock Auto and see if they're the same (they should be if it's the same engine). On your logs, do you see an unexplained jump in timing anywhere? That could be an indication your ICM is bad. I know you just put in an AC Delco a while back but, they aren't as good a quality as they used to be. When you installed it did you use plenty of thermal paste on the underside of it? If your coil is bad, it can take out your ICM too. Spray some water on your coil, not the wires, while running and see if it stumbles.

[srgould41]

I went back through logs again. My old logs showed spark advance in the 30's and 40's with an occasional high of 51. My logs from a drive on the day this started showed spark advance between 60 and 62 degrees. Definitely narrowing the problem down. I lined up a spare coil and ICM. I'll try the ICM today.

I thought the ESC was controlling the timing advance and now read that it's only function is to signal the computer when the knock sensor picked up something. I'll check the wires to make sure they are intact and correct then ignore it.

@PlayingWithTBI, you mentioned if I was seeing jumps in timing. No, not in the drive logs. Pretty steady between 60 and 62. I did use the paste that came with the ICM.

[stew86MCSS396]

Post up your datalog so we can see what's going on.

[srgould41]

I have attached two logs. One from my drive then one from an idle only test.

Edit:
By the way, the ECM has a chip header installed so I can burn tunes. It's a 9C1 motor. I think I started with the ANLU bin.

Changes made (note I went from Stoich AFR 14.7 to 14.1 then decided to use 14.4):

Edit Log for 001-4.18.17-5.7.bin created by TunerPro.
************************************************** ************************
04/18/2017 10:19:31 Scalar: TCC - Lock (Low Gears) MPH threshold changed from 38 MPH (0x26) to 255 MPH (0xFF).
04/18/2017 10:19:39 Scalar: TCC - Lock (Hi Gear) MPH threshold changed from 32 MPH (0x20) to 255 MPH (0xFF).
04/18/2017 10:20:06 Scalar: TCC - Unlock (Low Gears) MPH threshold changed from 36 MPH (0x24) to 0 MPH (0x00).
04/18/2017 10:20:13 Scalar: TCC - Unlock (Hi Gear) MPH threshold changed from 30 MPH (0x1E) to 0 MPH (0x00).
04/18/2017 10:20:24 Scalar: TCC - Mandatory lockup MPH threshold changed from 80 MPH (0x50) to 255 MPH (0xFF).
04/18/2017 10:20:55 Scalar: TCC - Min Engine temp threshold changed from 102 a/d counts (0x66) to 151 a/d counts (0x97).
06/09/2017 13:38:44 Scalar: CldLp - Stoich AFR changed from 14.1 AFR (0x8D) to 14.4 AFR (0x90).

[brian617]

Idle speed is controlled two ways, IAC and Timing.

IAC will make slow corrections, timing will make fast corrections.

If the Idle is "hunting" your actual spark advance will also be "hunting" (idle overspeed and underspeed corrections) effect not cause.

If the problem is new I would still suspect a vacuum leak or lean condition causing the surge.

Vacuum leaks can leak into vacuum controller devices, brake booster, EGR, evap, etc.

What does your BLM look like at idle?

What is comanded idle?

What is TPS voltage?

Pay close attention to ECT sensor readings, very common TBI failure.

[srgould41]

The problem started last week. It did have some issues the week before that where when going through a mountain pass at high throttle position and the TCC locked the ECM went into limp home mode. I didn't have a laptop any more so could not look into it. The check engine light went off after restarts anyway.

The engine had been running OK up to that point. I suspected a vacuum leak at first. All ports blocked and no change. Of course the EGR could be partly stuck open, but I would expect a low idle or stall if that happens.

BLM mostly 120's when driving. Although when it was acting up in idle it was 108.
Command idle is what ever was stock in the ANLU bin. I might need to re-read the bin from the ECM though. What I have on my computer is a download from gearhead (ANLU5369 61.bin) and looks messed up.
TPS volts are .9 at idle.


Edit: I found the original chip from the computer and saved it to a bin. It's the same as the one on Gearhead EFI just looks goofy. I attached it here if anyone wants to look at it.

Idle speed is controlled two ways, IAC and Timing.

IAC will make slow corrections, timing will make fast corrections.

If the Idle is "hunting" your actual spark advance will also be "hunting" (idle overspeed and underspeed corrections) effect not cause.

If the problem is new I would still suspect a vacuum leak or lean condition causing the surge.

Vacuum leaks can leak into vacuum controller devices, brake booster, EGR, evap, etc.

What does your BLM look like at idle?

What is comanded idle?

What is TPS voltage?

Pay close attention to ECT sensor readings, very common TBI failure.