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1G Running very lean on idle only

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eclipsyco

10+ Year Contributor
582
16
Nov 19, 2011
Somewhere in, Central_America
Hi! been a while
So a got a 90 1g gsx last year it was running fine its just need a transmission car pretty much stock other than minor mods.
After the transmission swap car was running as usual afr was good, did a pull to feel out the trans all good until I came to a stop car was running weird didn't pay much attention since it still drives and didn't stall I noticed that the afr was a little leaner than normal high 15s almost 16s so i drove it easy back home then it sat for a few days since I didn't have the time to work on it.
This was a few months ago and now I'm fighting this issue where the car is very lean on idle only, high 17s same times 18s on the wideband and jerks (misfire) a lot if a try to rev it until a go past 3k rpm the afr goes back to normal.
I've owned a dsm for at least 10 years now I know my stuff around these cars plus I've been hording parts al these years.
Thinks I tried so far
BLT was number 1 thing to do it had a leak on the throttle body, seals were shot so I rebuild the throttle body new seals, gaskets, o-rings etc
Change spark plugs new bpr6es gap @ .28
Tried 3 different coil packs
Injectors 1g and 2g also a I have good fuel pressure (gauge on the line)
Compression test 152 across
Check timing mechanical and the CAS
MAF and o2 sensor
I did notice something weird tho, if I unplug any sensor while the car is running CEL will turn on except the o2 sensor, so this left me to believe it's probably a bad ECU.
if anyone have any input on this or any idea what else to check since this is stressing me out now and I don't have a spare 90s ecu
 
Nice 1g.
I might suspect clogged injectors. Did any spark plug look leaner or richer than others?
I might also suspect the idle position switch is not connected or working properly due to the misfires you mentioned while revving.
 
Yeah I did actually I though that was the problem at the beginning al 4 fire also tried some stock 2g injectors a had laying around
I'll try replacing the ISC I think I have another one, I did test the coils on this one and it tested good but at this point i'll just go ahead and replace it
 
Is the car on ECMLink?
 
I wouldn’t expect the ignition system to be the culprit of running lean. O2 and injectors were my first instinct. If you swapped both with “known” good ones, then the ECU could be next on the list to check.
I have a 90 gsx, and I sent my ecu to ECM tuning for a once-over while my car was down with other repairs. They checked it, did caps as precautionary, and sent it back all for less than $100. If you don’t have another ecu to swap, I’d try that next.
 
Hi! been a while
So a got a 90 1g gsx last year it was running fine its just need a transmission car pretty much stock other than minor mods.
After the transmission swap car was running as usual afr was good, did a pull to feel out the trans all good until I came to a stop car was running weird didn't pay much attention since it still drives and didn't stall I noticed that the afr was a little leaner than normal high 15s almost 16s so i drove it easy back home then it sat for a few days since I didn't have the time to work on it.
This was a few months ago and now I'm fighting this issue where the car is very lean on idle only, high 17s same times 18s on the wideband and jerks (misfire) a lot if a try to rev it until a go past 3k rpm the afr goes back to normal.
I've owned a dsm for at least 10 years now I know my stuff around these cars plus I've been hording parts al these years.
Thinks I tried so far
BLT was number 1 thing to do it had a leak on the throttle body, seals were shot so I rebuild the throttle body new seals, gaskets, o-rings etc
Change spark plugs new bpr6es gap @ .28
Tried 3 different coil packs
Injectors 1g and 2g also a I have good fuel pressure (gauge on the line)
Compression test 152 across
Check timing mechanical and the CAS
MAF and o2 sensor
I did notice something weird tho, if I unplug any sensor while the car is running CEL will turn on except the o2 sensor, so this left me to believe it's probably a bad ECU.
if anyone have any input on this or any idea what else to check since this is stressing me out now and I don't have a spare 90s ecu
I would suspect a bad alternator, bad voltage regulator and fuel pressure, too. Check alternator and battery to see if it has some voltage drop while you are having the issue.
 
I would suspect a bad alternator, bad voltage regulator and fuel pressure, too. Check alternator and battery to see if it has some voltage drop while you are having the issue.
You were absolutely right it was a voltage related issue more specific the voltage regulator on the alternator. Thank you I feel kind dumb, been working on this cars for years and not even one crossed my mind to check voltage anyway.. Thanks again
 
You were absolutely right it was a voltage related issue more specific the voltage regulator on the alternator. Thank you I feel kind dumb, been working on this cars for years and not even one crossed my mind to check voltage anyway.. Thanks again
Glad to hear that :thumb:
 
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