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Discussion Starter · #1 · (Edited)
Hi!
I have a 1989 Mustang GT that's running lean. There's no check engine light while driving but after testing for codes I got a "41" and "181". It has a bit of a loss of power and tends to stall and stops. I've done lots of work to the car including new plugs, cap and rotor, TPS, O2 sensors, fuel filter, air filter, EGR, TFI, ignition coil, and cleaned the MAF. I've checked for vacuum leaks using this stethoscope my dad has and couldn't find any. I did a base idle reset with TPS, has about a .96. Vacuum was good until the idle dropped down after a few seconds. Still need to check the fuel pressure. The fuel pump was replaced not too long ago and I've been giving it treatments of fuel injector cleaner, "STP High Mileage".

If anyone knows anything that'd be awesome, thank you!
 

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Its only in one bank which would lead to possibly a clogged up injector(s) or an injector(s) that just isnt firing in that bank. I really hope you didnt replace all of those parts chasing this. As long as tps voltage is between .85 and 1.10v its fine. Having it set at a specific point is and has always been myth. You can swap injectors from bank to bank to see if the problem follows. If it does, its a problem with the injectors. Youll have to reset the computer after you make the change to see if it throws different codes
 
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Hi!
I have a 1989 Mustang GT that's running lean. There's no check engine light while driving but after testing for codes I got a "41" and "181". It has a bit of a loss of power and tends to stall and stops. I've done lots of work to the car including new plugs, cap and rotor, TPS, O2 sensors, fuel filter, air filter, EGR, TFI, ignition coil, and replaced the MAF. I've checked for vacuum leaks using this stethoscope my dad has and couldn't find any. I did a base idle reset with TPS, has about a .96. Vacuum was good until the idle dropped down after a few seconds. Still need to check the fuel pressure. The fuel pump was replaced not too long ago and I've been giving it treatments of fuel injector cleaner, "STP High Mileage".

If anyone knows anything that'd be awesome, thank you!

There are no three digit DTCs for that strategy. Are you positive you got 181 and not 91?
 

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Discussion Starter · #5 ·
Its only in one bank which would lead to possibly a clogged up injector(s) or an injector(s) that just isnt firing in that bank. I really hope you didnt replace all of those parts chasing this. As long as tps voltage is between .85 and 1.10v its fine. Having it set at a specific point is and has always been myth. You can swap injectors from bank to bank to see if the problem follows. If it does, its a problem with the injectors. Youll have to reset the computer after you make the change to see if it throws different codes
Thank you! I did have check engine lights in the past for a couple of things that I replaced there like the coil, EGR, and TPS. I wouldn't be surprised if it was the injectors because they're the originals. Are they hard to remove? I'm pretty new to working on cars
 

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Discussion Starter · #6 ·
There are no three digit DTCs for that strategy. Are you positive you got 181 and not 91?
I'm pretty certain, troublecodes.net showed it as a code for the right bank running lean. I've tested it a couple times and I think I also got a 91 on one test but that was system running lean and a malfunctioning transmission solenoid.
 

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I'm pretty certain, troublecodes.net showed it as a code for the right bank running lean. I've tested it a couple times and I think I also got a 91 on one test but that was system running lean and a malfunctioning transmission solenoid.
You might want to re check your source, there is no 181 on your strategy. Regardless, the codes are NOT lean codes, the codes are specific, 02s are reporting lean. That could mean anything from really being lean, or an open circuit and everything in between. You might want to verify them before assuming your engine is lean.
 

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Check the fuel lines from the tank to the engine bay. The nylon lines under the hood get kinked by accident and the kinks don't come out have to replace the lines with -4 aeroquip press on fuel line. And also the hard lines under the car get smashed and squished by jacks and jack stands and lifts when people put them in the wrong place while doing maintenance or changing a flat on the side of the highway. Visually go over them inch by inch and make sure they are all nice and round tubing.
 

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vacuum leak(s) are the #1 culprit with code 91 and 41

Oftentimes a traditional leak test won't find them either. Traditional meaning spraying carb cleaner around the vacuum sources, while listening to the engine change speed or tone. On EFI it usually doesn't work unless it's a BIG leak. A smoke machine will "usually" find them, though. One exception is any vacuum leaks under the dash (there are lines that run the heater box parts and the selector). Before the line that feeds all that (black one) gets through the firewall, there is a check valve. A smoke test puts a small amount of pressure in the intake tract of the engine, but the check valve in the a/c & heat system prevents the smoke from getting to the heater controls under the dash, making those leaks very hard to isolate. BTDT.

Those plastic vacuum lines under the hood are brittle by now. They crack, break off, etc. Small cracks are very very hard to find and that's all it takes to generate the code. Also, if you still have the smog stuff intact, look closely at the TAB/TAD solenoid (behind passenger strut tower), they often leak and those too are hard to isolate. Mine was found accidentally and it solved the code 41 issue for good. Tiny leak but it don't take much.

Intake manifold gasket leaks are also sources of vacuum leaks. Any hoses under the upper intake that may have a crack can cause issues. Fittings. PCV hose. Leaking brake booster. Speed control vacuum solenoid. The vacuum reservoirs and their hoses/fittings. All of that and more, can cause lean codes.

Injector sticking can be an issue there too but not likely if it's running decent. Normally if you have an injector sticking that cylinder's spark plug will show it.

HEGO harness issues can cause it

bad ground(s) can cause it

bad HEGO(s) can cause it.

41 and / or 91 are the most feared codes there are on an EEC-IV system! I fought it on my 93, 4 cylinder, for months--and all it was, was a simple loose clamp on the intercooler. Boy did I feel dumb.

if you have an exhaust leak at the manifold, it, too can cause the hego to report lean.
 

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Discussion Starter · #10 ·
Thank you so much for the help guys! I'll be sure to check all of that when I have some time on the weekend. Unfortunately I don't have a smoke machine but I heard you can use a cigar to test the lines through Scotty Kilmer.
 

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vacuum leak(s) are the #1 culprit with code 91 and 41

Oftentimes a traditional leak test won't find them either. Traditional meaning spraying carb cleaner around the vacuum sources, while listening to the engine change speed or tone. On EFI it usually doesn't work unless it's a BIG leak. A smoke machine will "usually" find them, though. One exception is any vacuum leaks under the dash (there are lines that run the heater box parts and the selector). Before the line that feeds all that (black one) gets through the firewall, there is a check valve. A smoke test puts a small amount of pressure in the intake tract of the engine, but the check valve in the a/c & heat system prevents the smoke from getting to the heater controls under the dash, making those leaks very hard to isolate. BTDT.

Those plastic vacuum lines under the hood are brittle by now. They crack, break off, etc. Small cracks are very very hard to find and that's all it takes to generate the code. Also, if you still have the smog stuff intact, look closely at the TAB/TAD solenoid (behind passenger strut tower), they often leak and those too are hard to isolate. Mine was found accidentally and it solved the code 41 issue for good. Tiny leak but it don't take much.

Intake manifold gasket leaks are also sources of vacuum leaks. Any hoses under the upper intake that may have a crack can cause issues. Fittings. PCV hose. Leaking brake booster. Speed control vacuum solenoid. The vacuum reservoirs and their hoses/fittings. All of that and more, can cause lean codes.

Injector sticking can be an issue there too but not likely if it's running decent. Normally if you have an injector sticking that cylinder's spark plug will show it.

HEGO harness issues can cause it

bad ground(s) can cause it

bad HEGO(s) can cause it.

41 and / or 91 are the most feared codes there are on an EEC-IV system! I fought it on my 93, 4 cylinder, for months--and all it was, was a simple loose clamp on the intercooler. Boy did I feel dumb.

if you have an exhaust leak at the manifold, it, too can cause the hego to report lean.
This this and this. Well done.

My money is leaning towards a vacuum leak. Oh, and pro tip for the OP - sometimes the vacuum leak will only occur when the motor gets hot enough to let the particular hose flex a bit and the crack to open up. Or vice versa. LOL.

Additional tidbit - an exhaust leak can also cause a similar issue. It can make the EEC do wacky stuff sometimes.

Good times.
 
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