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Discussion Starter · #1 ·
I have a 2001 Mercury Sable LS. A problem started back about 5 months ago. I would start the car with no problems. The car would go thru about a 2 min cycle on a higher idle and then when it was over the rpms would drop down to 500. It would act like there was no gas and jerk or lunge causing lights to dim and brighten because the engine was surging on and off. Finally it seemed like the computer figured out a problem and would increase gas and the rpms would go back up. The check engine light didn't appear for a couple of weeks. When it did it showed the infamous P0171 & P0174 bank 1 and 2 lean. I replaced the Mass Air Flow sensor and it did nothing. Then I saw a posting on another site about the PCV valve elbow where it goes into the intake manifold. The elbow is smooth rubber and collapses causing vacuum problems. Sure enough mine was totally collapsed. The part was only 25.00 at the Ford Dealership and I had it put on. It actually seemed better for a while. I thought that would be the end of it. But about 1 week after the codes reappeared and the same symptoms as well. All this time since it started back 5 months ago I've had horrible gas mileage. I wish there was a way to check the whole vacuum system for problems because that is where seems to still be after reading tons of postings. I'm not sure if it is in the intake manifold gasket itself or some vacuum line. I'm frustrated and very leery of replacing 02 sensors because it seems unlikely that they are both bad at the same time. I also notice that when the car a going down the road I can feel it speed up a little and come back down. It sure seems to be an air/fuel mixture problem. Anybody have and feedback. thanks in advance
 

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Might check for a vacuum routing decal sticker underhood. 171-174 is most assuredly indicating a vac. leak, not faulty o2 sensors.
 

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P.S.: I hope you tried cleaning the maf sensor with crc maf cleaner spray prior replacing it, as they are quite pricey as you found out.
 

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If you have a scan tool that shows fuel trims, it is easy to eliminate most, if not all of the vac lines / fittings as the problem. Remove large vac from line from intake to vac tree and plug line or nipple on intake. Start engine and watch fuel trims to see if they head from high positive toward zero. If they do, leak is in vac lines / fittings. If they stay high positive, likely intake gasket leak, throttle body gasket leak, etc (something on the engine). DO NOT drive the car with no vac to vac tree, as you will not have power brakes!!!!!
 

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Discussion Starter · #5 ·
Thanks Jeff and Shelia. Do you guys know if a general mechanic can do such testing of the vacuum system or is it a specialized field ?
 

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Discussion Starter · #7 ·
Thanks guys I'm going to check around for a shop that has the right tools (smoke machine) to diagnose. It seems that since I've fixed the pcv valve elbow that goes into the intake manifold the problem is actually worse. It would only have the problem when I first started it after the initial 2 min idle cycle. Now it happens immediately and goes until the car is really warmed up good. It was 1 degree this morning. Definitely something to do air/fuel mixture or fuel itself. The injectors seem fine. No black smoke etc. If fact when the car stops lunging and surging the engine sounds great it just gets horrible mileage.
 

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Get your car scanned with a decent scanner that can read the PCM's "Freeze Frame" Data.
Freeze Frame Data will indicate the engine and ambient conditions at the exact moment the DTC was set. This is important because vacuum leaks usually occur within specific temperature ranges and must be duplicated to detect the leak.
It pretty much takes a picture of everything that was going on when your PCM set the code.

If you smoke test it, some things will leak the smoke out and fool you into thinking thats the cause. Idle Air Control Valve and EGR Diaphram will leak a little smoke every time.
 

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Discussion Starter · #10 ·
Thanks to all that helped me through this problem. I ended up taking it to a local repair garage. He hooked my car up to a smoke machine and smoke came out of intake manifold. They replaced the intake manifold gasket which also meant they had to put in new anti-freeze so I also had them do an oil change as to not get any anti-freeze into the oil. All in all the total repair with the oil change was $350.00. It runs great now. I would advise anybody that get's the P0171 and P0174 together to do two things. #1 look to replace the pcv elbow and vacuum line because mine was collapsed and #2 get your car hooked up to the smoke machine to find easily the vacuum problem. My car has 146,000 miles on it so the rubber is old. If it is a intake gasket problem like mine it is a good idea to just replace the other vacuum hoses while everything is apart. No codes now for two weeks and runs great even in this below zero weather that we have been having this week.
 

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If you have a scan tool that shows fuel trims, it is easy to eliminate most, if not all of the vac lines / fittings as the problem. Remove large vac from line from intake to vac tree and plug line or nipple on intake. Start engine and watch fuel trims to see if they head from high positive toward zero. If they do, leak is in vac lines / fittings. If they stay high positive, likely intake gasket leak, throttle body gasket leak, etc (something on the engine). DO NOT drive the car with no vac to vac tree, as you will not have power brakes!!!!!
Thanks for the tip. A very useful one.
 
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