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Discussion Starter · #1 ·
I've got a 1999 cougar 2.5 MT with a stumble problem. If I give it around 50% throttle under 3k rpms, it tends to stumble pretty bad. I do know it's throwing codes for the oxygen sensor on the rear manifold, could that be causing it? Thanks!
 

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I've got a 1999 cougar 2.5 MT with a stumble problem. If I give it around 50% throttle under 3k rpms, it tends to stumble pretty bad. I do know it's throwing codes for the oxygen sensor on the rear manifold, could that be causing it? Thanks!
Replace your fuel filter, maybe 15 bucks at NAPA I actually just changed mine today and it took 20 minutes at most. Or you could need a new air filter, but it wouldn't run right at all if that was the issue. Those are just the easiest of fixes.
 

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I've got a 1999 cougar 2.5 MT with a stumble problem. If I give it around 50% throttle under 3k rpms, it tends to stumble pretty bad. I do know it's throwing codes for the oxygen sensor on the rear manifold, could that be causing it? Thanks!
How many miles? Injectors go out over time but just like the air filter it wouldn't run right the entire time. Most the time though a lag at 2000 to 3000 rpm will just be a simple dirty/clogged fuel filter.
 

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1999 3.0 SilFro
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This is pretty classic ignition problems. Probably needs spark plugs and plug wires.
 
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This is pretty classic ignition problems. Probably needs spark plugs and plug wires.
But the car is already running, how would it then be ignition? I can definitely see the spark plugs but wouldn't it fail constant and not just at 3k? I just did my fuel filter and it got rid of my lag/jumpiness at 2k around 60mph and my weird jumpiness when I go into reverse.
 

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Discussion Starter · #9 ·
Soooo,

It wasn't the filter, put a brand new one on today, didn't change a thing. I noticed while I was under the car that the o2 sensor was missing wires, (ripped out) so could that be causing the problem?
 

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Soooo,

It wasn't the filter, put a brand new one on today, didn't change a thing. I noticed while I was under the car that the o2 sensor was missing wires, (ripped out) so could that be causing the problem?
That's sad :( that would've have been the easiest fix. From what I know and have experienced you'll notice idling issues with a missing or bad 02sensor. Needless to say it's meant to be there so definitely fix that. 02 sensor just scans the amount of oxygen in the exhaust stream as it exits the engine but also makes adjustments to your fuel air ratio based on what's being put out. I'm just kind of thinking out loud here feel free to tell me I'm wrong. We are all here to learn :)
 

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Which O2 sensor? In the manifold prior to the catalysts ( monitors air/fuel ) or downstream of the catalysts ( monitors catalyst efficiency )?
 

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Discussion Starter · #12 ·
It's between the precats and the main cat, there's two of them, one on each manifold. There's no o2 sensors before that, and only the catalyst efficiency sensor after that.
 

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Discussion Starter · #13 ·
Two of the codes it's putting out is P0136 (o2 sensor behind cylinder bank 1 malfunctioning) and P1131 (lean fuel in cylinder bank 1), so I think it might be trying to over compensate for the bad o2 sensor.
 

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If its between the precats and the main cat thats your efficiency monitoring one. Its also the one that pops up the P0420 code. The one for fuel mixture is higher up the manifold closer to the engine. Pop the hood and look down where your fan shroud is you should see the front side O2 sensor sticking out between the fans.
Bank 1 is in roughly the same spot on the back side of the engine between the engine and firewall.
 

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Discussion Starter · #15 ·
Huh, thanks, I never realized there was one farther up on the manifold. So I'm guessing now it's either a fuel injector clogging or spark plugs/ wires. The spark plugs haven't been changed in a while, come to think of it. They didn't look terrible when I took them out, so I didn't bother replacing them.
 

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I put those super help you get better gas mileage by burning the fuel better E3 plugs in mine once. Drove it less then 10 miles and developed a missfire from the car not liking them. They still looked new but I pulled them out and put the motorcraft APP103 plugs in and it cleared it right up with used plugs that looked like they should probably be thrown away. The accel coil I had similar experience but made it about 2 weeks of normal driving. It still looked new. And holy crap lets not even go into wires.
Point is just because it looks good in these cars doesnt mean it is. These cars have the temperment of a high class hollywood housewife especially if something is a brand it doesn't like.
 

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But the car is already running, how would it then be ignition? I can definitely see the spark plugs but wouldn't it fail constant and not just at 3k? I just did my fuel filter and it got rid of my lag/jumpiness at 2k around 60mph and my weird jumpiness when I go into reverse.
Ignition refers to spark plugs igniting the fuel; it happens billions of times every time you drive it. It is a classic problem that has been covered dozens of times over the past 24 years on this forum - stumble/misfire under load, and only at lower RPMs, is almost always an ignition issue with plugs or plug wires, and sometimes also the ignition coil.

I pulled the plugs a few days ago, they looked pretty new though, so I didn't bother changing them. The plug wires are in great shape, too.
Plug wires can appear to be in great shape and still have problems under the insulation that you cannot see.

It is a very common problem with this engine; it is extremely sensitive to ignition issues.

ALSO - you cannot just put new plugs or new wires on it when you're having ignition-related problems; the misfires damage the wires and the plugs simultaneously, so it you don't replace both, the remaining damaged parts will destroy whatever new ones you install, and it starts all over again. You need to replace both plugs AND wires at the same time.
 
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