Misfire/Lean fuel o2 sensor?

petemakmt

New Member
Sep 26, 2020
8
0
1
20
Mississippi
Hi, so I recently purchased a 96 cobra with a cat delete already installed. Within a week I got a check engine light and pulled codes PO171/PO174 (system too lean bank 1 and 2) And PO300 (random/multiple misfire). I can definitely tell the car is misfiring but not too bad, so I bought some spark plugs. I read a little bit on my problem and It seems it could possibly be an o2 sensor?? I know theres 2 before the cats and im not sure if theres a mil eliminator installed. I just would like insight on what I should check/replace to fix those codes if it isnt spark plugs.
 
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08GT500

Active Member
Jul 12, 2018
896
139
53
50
Massachusetts
Hi, so I recently purchased a 96 cobra with a cat delete already installed. Within a week I got a check engine light and pulled codes PO171/PO174 (system too lean bank 1 and 2) And PO300 (random/multiple misfire). I can definitely tell the car is misfiring but not too bad, so I bought some spark plugs. I read a little bit on my problem and It seems it could possibly be an o2 sensor?? I know theres 2 before the cats and im not sure if theres a mil eliminator installed. I just would like insight on what I should check/replace to fix those codes if it isnt spark plugs.
Hi,
Sorry to hear.How built is thgry So it ran with no issue until it the DTC popped a week later Many things may cause this, need to start at the basics. How long was this fuel in the tak
First off, P0300 interpretation is a lack of fuel reaching the motor. That could mean, Pump, clogged filter, bad connection Ht
As you have 2 banks running lean, I’d start with:
1) Check fuel pressure at the rail.
2) Swap out your fuel filter.
3) Check your FP regulator vacuum
filter, Remember it’s pressure and Volume of fuel that are required, you may not own a fuel system test kit that tests both, but if your Scantool can freeze aww frame, watch y data(PID’s), Datalog, or can borrow one, excellent! Inexpensive tool you’ll use again on any OBD-2 car you have You’re Likely running MIL eliminators- sometimes they work, sometimes not. They are noticeable if you look.
As the EEC uses the upstream 02 for mixture, downstream 02’s only verify the Cat’s are reducing emissions as they should be.
Start with basics..
Have an adjustable fuel pressure regulator? What’s your fuel pressure at the rail,does your regulator’s vacuum line taste of fuel if you pull on it orally like a straw? Pressure should be 38lbs with line hooked up, 40 with vacuum line disconnected.
Fuel filter replaced recently?
An exhaust leak between the exhaust manifolds and upstream 02 sensor causes a vacuum as gases pass through, pulling in external air & diluting what the 02 sensor(s) read, more often it’s both sides, Do you have any exhaust leaks?
CEL‘s for it, MIL’s are likely installed.
Any unmetered air leaking in after the MAF is enough to cause upstream 02’s to tell the EEC there’s a lean condition, the EEC can increase injector pulse width to counter a lean condition, but only slightly.
This means going through your vacuum lines and looking for leaks. Check all connections on your CAI, and follow 02’s wire’s down as they melt on manifolds.
Hope this at least gets you started..
Best!
John
 

McFly2

New Member
Nov 21, 2020
18
1
3
63
Cincinnati
Hi, so I recently purchased a 96 cobra with a cat delete already installed. Within a week I got a check engine light and pulled codes PO171/PO174 (system too lean bank 1 and 2) And PO300 (random/multiple misfire). I can definitely tell the car is misfiring but not too bad, so I bought some spark plugs. I read a little bit on my problem and It seems it could possibly be an o2 sensor?? I know theres 2 before the cats and im not sure if theres a mil eliminator installed. I just would like insight on what I should check/replace to fix those codes if it isnt spark plugs.
Sounds like 2 seperate problems. Misfires and Lean bank 1 and 2. A live data scan would help a lot. If you have high short term fuel trims (+12 to 25%) at idle and the the fuel trims go normal ( + 5 or - 5%) at 2500rpm; you most likely have a vacuum leak. The scan also will give an indication of yor fuel pressure. Scan will also show you o2 sensor voltages and fuel trim% output at OS's. High dollar scanners (SnapOn Solus) will also show you injector voltage and frequency; and component :IAT, MAF, TPS and a multitude of other sensors and valve, solenoid tests and bidirectional controls of componest to try while monitoring live fuel trim data. If vacuum leak is diagnosed a shop with good smoke machine to detect hard to find vacuum leaks is valuable. I just paid Midas $89 to run a scan with the (SnapOn Solus) and run smoke machine through.