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Highway speed 50-70 mph. TPS rock solid according to the log, Speed steady according to the log. RPM oscillates up and down 25 rpm. Can be felt driving.
Highway speed 50-70 mph. TPS rock solid according to the log, Speed steady according to the log. RPM oscillates up and down 25 rpm. Can be felt driving.
Potential Causes?
Don
Williamsburg VA
Something going on with the IAC? What are the the IAC counts doing?
I am not sure who is on first. There is more to this story. On Tune 151, I had not issues. Not one single knock retard. Open loop idle Closed loop above 1k rpm
Tune 158, all sort of knock retards, cruising surging, RPM jumping around on the dash (not so much on Tunerpro, but think that is commanded RPM not actual)
So, Loaded BIN 151 back. No changes to the tune.
Drove it and had tons of KR, surging at highway, dash rpm jumping around.
Same tune, vastly different results.
Mechanical differences.
1. Tune 151 had the OEM distributor. Latest run used an MSD that looks to me to have serious phasing issues (but not according to the MSD tech)
2. Different Plug wires. Header clearance issues burnt the plug wires on 151. New wires are 10.4 mm Taylors
3. Same ignition coil
It seems to be worse when engine is hot. Strongly suspicious of a bad coil or bad coil ground.
Was this ever solved? Curious to find out what happened there.
HWY mode could lean you out.
Sounds like the most likely culprit is the "new distributor installation". Initial timing set properly? SA latency table match the new ICM? RPM jumping around on your data log also? The coil has to match the new distributor I believe (search TGO on that one).
Always a good idea to verify "real" timing at balancer vs commended SA across RPM band. You would be surprised how it can be off. To do so, you need to program a test tune where SA is constant when not in closed TPS. So you can read SA with timing light while operating the throttle manually. Can be a scary experience I give you that.
Last edited by SbFormula; Dec 30, 2022 at 09:49 AM.
I cannot say that I have resolved this issue. Candidly, I am not sure what to expect from a "stable" RPM. I have attached excerpts out of the last two logs. One at 73 mph and the other one at 46 mph. In both logs, the TPS is constant. But you can see some variation in MAP, BPW, RPM and VE from table. The VE from table is probably driven by the change in MAP, but who knows. I have Highway mode bypassed in both logs until I get the BLMs to tidy up. The distributor is the GM small Electronic with a new gear and a new module. New AC Delco coil. I have not done anything with spark latency relative to the ICM. Not sure where to get that information.
I had a heck of a time with bogus Knocks. Finally, removed the knock sensor and wrapped about six or seven wraps of teflon tape on the sensor and screwed back in until just "snug". Ran a ground wire from the engine block to the sensor and attached it to the outside of the sensor with a stainless hose clamp. No knocks. Moved the timing up 2 degrees across the board (above idle) and still no knocks. Will stay there for the time being. Both these excerpts from logs are with the advanced timing.
Knock Sensor: I was getting KRs off idle, KRs when encountering Decel (in and out), at low MAP, etc. This is a big HP engine stuffed into a 1938 body and chassis (modified some). Lot's of opportunity for vibrations and such. Dropped SA by 3 degrees across the board. Still had some KRs. So, did the wrap thing. I figured the extra teflon would prevent a good ground, hence the separate ground wire. See Photo.
Distributor: I was never comfortable with the MSD on the phasing of the rotor. When you aligned the magnetic pick up on the rotor and compared that to the location of the pin on the cap for No 1, it appeared to me that the rotor was well past number one when the magnetic pickup would fire. I never cut the cap to see what was really happening, but I did compare the alignment with the OEM distributor. I called MSD and they said it was supposed to be that way. That didn't jive with me. Seemed to me that even clocking the distributor body enough to get 10 base timing would move the phasing even further away. I guess I could be all wet here, but I had the OEM and decided to eliminate the MSD as a possible contributor. See photos. Red Rotor is the MSD.
Tell me about SA Latency versus the ICM. Tunerpro says don't mess with it.
Nothing wrong with your logs! You are trying to be accurate with antiquated EFI system. All the variations are normal. The extrapolation from tables are not accurate either. BLM cell# is also inaccurate at times, specially when engine parameters are near the BLM boundaries. RPM starts bouncing pass 4000rpm and gets worst pass 5000 rpm. Etc..
BTW, that knock sensor was meant to work with a 350cid (4" bore) or maybe a 305cid (3.73" bore) depending which motor the sensor came from. It was certainly not design to work with a ZZ502 (4.47" bore). IMO, I would not bother with it.
I had tried a few other KSensors with different parameters (like for a 454) and they didn't work at all. Engine wouldn't run. De-sensitizing it seems the way to go to get rid of all the KRs.
Tell me about SA Latency versus the ICM. Tunerpro says don't mess with it. Don
That's not quite correct if you're running an aftermarket module. The link below starts where I learned that fact. I found the stock tables to be off by as much as 5 deg over the aftermarket module. https://www.thirdgen.org/forums/diy-...ml#post6403022
That could also explain some performance differences between the OEM small top distributor that I am running now versus the MSD. The ICM I purchased is AC Delco 19179578. I think it is the same as the ICM that was in the distributor when I got it in 1995. Is there any place where the results of various member's latency tests have been accumulated so one can avoid having to run their own dial back tests?
That could also explain some performance differences between the OEM small top distributor that I am running now versus the MSD. The ICM I purchased is AC Delco 19179578. I think it is the same as the ICM that was in the distributor when I got it in 1995. Is there any place where the results of various member's latency tests have been accumulated so one can avoid having to run their own dial back tests?
USE at your own risk. I personally tested this on my own cars. I still recommend you do an SA latency test to confirm everything.
The OEM SA latency tune is not accurate. That's why you'll notice total SA at WOT has a weird convex curve to compensate for it. The curve got worse with years. Maybe GM tried to correct it that way. So if you recalibrate your SA Latency table, it will pull timing at certain RPM. On my OEM $8D, it was off by up to +6*!!!!!!!
This table is for 1989 OEM HEI distributor with remote coil (ICM 369) operating on $6E, 165 ecm
This table is for 1991 JEGS #555-40006 distributor with remote coil operating on $8D, S_AUJP, 7730 ecm
Last edited by SbFormula; Dec 30, 2022 at 05:46 PM.
That's not quite correct if you're running an aftermarket module. The link below starts where I learned that fact. I found the stock tables to be off by as much as 5 deg over the aftermarket module. https://www.thirdgen.org/forums/diy-...ml#post6403022
