96 Ford Taurus - Running Lean

[somethin_random]

Hey..

I got a Taurus last week and two codes came up, P0401 & P0430.

P0401 was a bad DPFE Sensor which I replaced and the code is gone now.
P0430 - I am 90% sure it is the catalytic converter.

I am yet to get the new catalytic converter as I need a little more money but also I wanted to be 100% sure before I get it. So I spent some time trying to figure out why assuming the converter is done went bad. I replaced the plugs & wires and looking at the plugs, I suspected a bit of a lean condition. I then checked the fuel pressure at the fuel rail and it was good but I went ahead and replaced the fuel filter anyway. Next I tested the MAF sensor and it was good.

Then I checked the vacuum pressure.
- The pressure held steady at the correct reading at 18
- When I held the RPMs higher, the needle started to drop initally but then went straight to 20 and stayed there as I held the RPMs steady

This is not supposed to happen when I add RPMs doing a vacuum pressure check, correct?

Then I hooked the scanner up to check fuel trims.
STFT all checked out good
LTFT was where I saw the problem

LTFT
Idle = Averaging +3
1500 RPM = +8
2500 RPM = +11 to +15

So at higher RPMs, the car runs lean.

Does anyone know what the next step would be in diagnosing this?

This is one reason why I don't want to replace the catalytic converter immediately, I read that if the car is running lean, then the catalytic converter would perform inefficiently. So I figured that maybe I should fix the lean condition and then check the converter again so I wouldn't be wasting money.

3.0 OHV

Thanks in advance

 

[DRIVEN]

The vacuum readings you have are normal. There is a finger sized vacuum coupler near the throttle body that is a common failure. Some will be lean at idle and suck themselves closed at higher rpm -- some will do the opposite and be fine at idle and suck open at higher rpm. Also, Fords seem to be more sensitive to dirty injectors than most other manufacturers. They ALWAYS benefit from an injection service like MotorVac, etc. Fixing those things will bring your fuel trims closer to where you want them but won't save your cat. Sorry to be the bearer of bad news but it looks like you'll need a new one.

 

[somethin_random]

Thanks for that, I will try to find that coupler tomorrow and check it out. I was suspecting clogged fuel injectors as well, I wonder if there is a place locally that professionally cleans fuel injectors. Have you ever used MotorVac? If so, where are they located and how much did they charge?

I'm not too upset about replacing the CAT, I figure if I could find someone to weld it, I could probably get it done for $200. It's $125 at the auto parts stores. I'm waiting on someone I know at Car-X to get back to me, he claims he might be able to get the whole 2 converter assembly for $100, that would be sweet because they cost $400-$600 plus they are bolt on. I'd be shocked if that holds true but if worst comes, he can weld the single replacement for me.

I'm more concerned trying to figure out what caused the converter to go back in the first place which is what led me to running all these tests. I can't find anything else wrong with the car other than this lean issue. That is when I googled "can a lean engine cause a CAT to fail", the consensus was no and here is a quote from aa1

A rough idle or stalling. A performance cam with lots of valve overlap can give an engine a lopping idle, but so can a vacuum leak. A really serious leak can lean the air/fuel mixture out to such an extent that an engine won't idle at all. An EGR valve that is stuck open at idle can have the same effect as a vacuum leak. So too can the wrong PCV valve (one that flows too much air for the application), or a loose PCV hose. The rough idle in these cases is caused by "lean misfire." The fuel mixture is too lean to ignite reliably so it often misfires and fails to ignite at all. Lean misfire will show up as elevated hydrocarbon (HC) readings in the exhaust, enough, in fact, to cause a vehicle to fail an emissions test.

If that was the case, then most likely BOTH converters would be reporting bad but they aren't and only one is. So I'm back to trying to figure why the one failed.

You said my vacuum readings are good? Isn't the reading supposed to drop to 0 when I rev the engine?

Thanks

 

[DRIVEN]

MotorVac is the trade name of the machine. You'll be most likely to find it at independent shops but possibly at dealerships too. We have one at the shop I work at and charge $120 for the service. Did one on a Mustang just yesterday. I think you might be over-thinking the cause of death of your cat. The car is 17 years old after all. They don't have an infinite lifespan. You've given it a proper tuneup and if there are no vacuum leaks there isn't really much more you can do. Your vacuum readings look good by your description. You will see high vacuum at idle then a momentary drop when you open the throttle then it will climb back up. High readings indicate that your engine is efficiently pumping air. That's a good thing.