Low rpm backfire thru intake; lt1
05-20-2013, 04:50 PM
#1
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Low rpm backfire thru intake; lt1 I've had this problem for the whole time I owned the car. I bought it knowing it was present but figured it would be something easy.... I was wrong.
Car is stock motor, 94 formula. Recent a4 to m6 swap.
Work done in the 6 months I have owned it
Plugs
Wires
Cap and rotor
Headers
Gutted cat
New Bosch o2s. Lots of debate on these....
Intake gasket
Fuel filter
52 mm throttle body. Stock had bad bearings/ vac leak on shaft
Map sensor
I have taken The opti a part twice, looks newish. I haven't Ohmed out the coil yet. Tested fuel pressure, looks normal, maybe a little low when blipping 30- 33.
Symptoms: car runs perfect if it sits for a day and I pull the PCM batt fuse. Gets worse the longer the PCM has to learn. Standard lt1 low rpm symptoms, backfire thru intake occasionally. Gets worse when it's hot, stop and go traffic.
Evap system is messed up, I can't figure it out. Won't hold vacume, tank always has positive pressure. Not sure if this matters.... I did tune out the ccp system and blocked that vacume line, no change.
Smoke machine shows a small leak somewhere I can't find.
Ideas?
Oh, I refuse to replace the opti because I have low tire pressure!!
Car is stock motor, 94 formula. Recent a4 to m6 swap.
Work done in the 6 months I have owned it
Plugs
Wires
Cap and rotor
Headers
Gutted cat
New Bosch o2s. Lots of debate on these....
Intake gasket
Fuel filter
52 mm throttle body. Stock had bad bearings/ vac leak on shaft
Map sensor
I have taken The opti a part twice, looks newish. I haven't Ohmed out the coil yet. Tested fuel pressure, looks normal, maybe a little low when blipping 30- 33.
Symptoms: car runs perfect if it sits for a day and I pull the PCM batt fuse. Gets worse the longer the PCM has to learn. Standard lt1 low rpm symptoms, backfire thru intake occasionally. Gets worse when it's hot, stop and go traffic.
Evap system is messed up, I can't figure it out. Won't hold vacume, tank always has positive pressure. Not sure if this matters.... I did tune out the ccp system and blocked that vacume line, no change.
Smoke machine shows a small leak somewhere I can't find.
Ideas?
Oh, I refuse to replace the opti because I have low tire pressure!!
05-21-2013, 02:30 PM
#3
You're right, the Opti is a joke. But don't condemn it just yet.
It sounds very simple and is often overlooked by even experienced people... but have you checked for crossed plug wires? Especially 3 and 5? Crossing those 2 in particular will cause a backfire at low RPM tip-in yet still run well at other RPMs and loads.
It sounds very simple and is often overlooked by even experienced people... but have you checked for crossed plug wires? Especially 3 and 5? Crossing those 2 in particular will cause a backfire at low RPM tip-in yet still run well at other RPMs and loads.
05-24-2013, 10:49 AM
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Update: while testing the coil I noticed the ground on the drivers side head was wet with oil. I pulled the wires, cleaned them with carb cleaner and a wire brush. I'm letting the car get hot, hoping for the best.
I think I've narrowed down my vac leak; at wot, my a/c switches from dash vents to floor vents. I'm thinking that would mean the leak is after the check valve. Anyone know of some common places to look? I originally found a leak on the hose going to to reservoir, but that's fixed.
I think I've narrowed down my vac leak; at wot, my a/c switches from dash vents to floor vents. I'm thinking that would mean the leak is after the check valve. Anyone know of some common places to look? I originally found a leak on the hose going to to reservoir, but that's fixed.
05-28-2013, 06:45 AM
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any error codes? Double check and make sure your fuel lines aren't on backward going into the fuel rails. Also maybe pop the injectors up and check make sure none are leaking and all are firing like they should. Scanning for codes will definitely help get you going in the right direction. Also check plugs and wires just to make sure none got cooked on the headers. Go over all your grounds make sure good and tight and clean.
