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P0171 Rough idle

01-31-2010, 06:49 PM
I have a 2003 ZX2 with a P0171 code. I have had this code for a while now, and a farely regular very rough idle at stop lights that will cause the car to stall if you don't give it some gas.

I just replaced the entire engine for an unrelated issue and with it every sensor and hose connected to the intake or exhaust manifold was replaced as well, also the fuel injectors. So that pretty much rules out a lot of things.

The only things I have in mind that weren't replaced would be the MAF or the fuel filter. Are there any other things that I am missing?

02-02-2010, 07:13 PM
P0 171 indicates the fuel mixture is too lean. Many people with K&N air filters end up with this code because they over oiled the flter and it contaminated the MAF (Mass Air Flow) sensor. When the MAF gets dirty it will throw the 171 code.

1. Replace your air filter
2. Get a can of electrical contact cleaner at parts store or WalMart Automotive.
3. Disconnect the air hose from the air box to access the MAF.
4. Spray the MAF wire with the cleaner and don't touch it with anything.
5. Wipe out the interior of the air hose.
6. Reset your computer by disconnecting the negative battery terminal for 5 minutes. If the engine light comes on again it could be for the following:

1. Low fuel pressure due to a weak pump, pressure regulator or plugged filter.
2. Dirty injectors.
3. Vacuum leak at the intake or hose connections.
4. Leaky EGR valve
5. Leaky PCV valve or hoses.

You can get this code on many Fords because of the DPFE (Differential pressure sensor) The DPFE misreads the EGR flow as too low and the computer increases the amount of EGR flow causing a leaning affect on the fuel mixture.

I wish I could say it's this or that but PO171 can have many different causes.

02-02-2010, 08:17 PM
Consider a smoke test for unmetered air leaks.

Do a Google ''Youtube Auto smoke test''

02-02-2010, 08:21 PM
Thanks. I replaced the fuel filter and it didn't fix it. Most of the other things have been done as far as the fuel. Now it is clean the MAF, and blow out that hose because that was not replaced. Hopefully that will fix the problem.

02-03-2010, 05:48 AM
You might check fuel pressure as that is about the only thing that hasn't changed. It wouldn't be uncommon for it to have a bad fuel pump.

02-03-2010, 04:52 PM
Here is another question since you bring up fuel pressure. Since they don't make a book for the 2003 Escort, the best thing I have only goes up to 2000 and it says that you check the fuel pressure on the passenger side end of the fuel rail, but there is no connector there for this car. Any idea where there is a connector or what the best way to check the fuel pressure is in this. Also, what is the pressure supposed to be?

02-03-2010, 05:57 PM
Is there not a fitting that looks like a valve stem end? Anyway, if there is nothing easily accessible on the fuel rail you will probably have to T off of the fuel filter. I'm not sure of the exact spec but at idle it is usually 45-50 with the regulator and 50-60 with it off ( if it has a vacuum regulator to tinker with ).

02-10-2010, 12:02 AM
I would strongly suspect a vacuum leak rather than fuel problems. I would bet that the leak is right on the manifold.

04-28-2010, 08:06 PM
Yeah. It was a vacuum leak. It was the one that run the heater control valve. It must not have been all the way off, then for some reason it popped off all the way and I couldn't change the direction for my heater and popped the hood and could hear it hissing. I am guessing this wasn't the same problem as before I replaced the engine, whatever that was must have been fixed during the engine replacement.

Thanks for all the ideas.

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