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2003 Taurus DOHC with 98k miles, getting the famous 171 and 174 codes. Was getting 2 others that I don't remember. Could hear the vacuum leak so we replaced the elbow. Reset the CEL, 171 and 174 came back. Replaced all 4 O2's and manifold gaskets. Cleaned MAF. Plugs and wires changed about 10K miles ago. Any ideas? Fuel filter, possibly? Thanks
 

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Redden,
I have a 1999 DOHC that I fought those codes for a while and I will tell you how I found my problem. You will need a scanner that has the live data feature. Borrow or buy one. You will need to monitor your Short Term Fuel Trim (STFT) readings. They will problably be pegged out around 22. First you need to determine where the vacuum leak is by isolating the engine from other areas that use vacuum. There is a small plastic box on the firewall that has vacuum lines attached to it. I think it is called the vacuum source distribution box or something to that effect. It directs vacuum to your heating and A/C, your brake system, and the EVAPs system. Problems in any of these areas can cause those codes.While monitoring your STFT values, disconnect and plug the main vacuum line from the motor to this box. If your STFT levels go down then your problem is not in the engine itself but in one of the other areas mentioned. Do not drive the car with that vacuum line capped off, you will not have brakes. If your values do not change then you can concentrate on the engine. I had two vacuum problems on my car, the lower intake gasket and the EVAPs system. I used the propane method to find my lower intake leak. You can do a search and read up on that.

Ed
 

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Several hoses might be leaky. One pcv system elbows rots on each valve cover. Dorman 46017 is a good substitute for the factory elbow which cannot be bought separately from the "z" tube. And where it connects near the throttle body at the other end of the "z" tube. There is a pcv hose under the intake manifold. Connects to a plastic tube to the elbow you replaced and pcv. And a hose next to the pcv elbow. And several other hoses off the intake nearer the firewall.
 

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When I changed plugs on my 'tec I pulled the intake, replaced the gaskets, and I replaced all the vacuum hoses with high grease silicone hoses. I used plastic elbows where needed. Not quite as clean looking as preformed hose, but it works well. You can't see most of those hoses anyway.
 
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