1998 Mercury Mystique


Ben Lee

May, 18, 2010 AT 11:17 AM

Computer problem
1998 Mercury Mystique 4 cyl Two Wheel Drive Automatic 145000 miles

The code P0171 on my reader says " system too lean bank 1". After failing smog, I realized I would have to fix the check engine code instead of just clearing it for the test.

I failed the test due to NO being too high at 15 mph. Max for my car is about 450 and mine was at 611. I just barely passed at the 25 mph speed.

The following is what I have already replaced within the last week or so, which was all done to both bring down emmisions and hopefully remove the check engine light. Cleared codes after many of these repairs to see if the CIL would come back, so it has been on and off many times.

-added hose clamp to both ends of the breather hose(one end had only a zip tie, the other end at the engine had no clamp at all)
-replaced BOTH 02 sensors (upper and lower, realize now that was a waste)
-replaced the mass airflow sensor
-checked the egr valve (this was done by a mechanic, who said it was fine)
-new fuel filter
-checked fuel pressure and pressure regulator, which were within normal psi
-changed engine oil and filter
-replaced spark plugs
-replaced air filter and breather filter
-added a 16oz bottle of Lucas fuel system cleaner

I attempted to use the " soapy water" method to check for any hose leaks, but I did not find any issues, and was not really sure what I was doing or sure where to locate all the hoses that I might need to check.


22 Answers



May, 18, 2010 AT 3:30 PM

First of all thank you so much for your donation. Sounds like you probably have a vaccuum leak thats about all you have left after replacing all the things you have.I would check for a vaccuum leak with some carb spray. What you do is let the car idle and spray the carb spray around all your vaccuum lines and intake manifold gasket also the base of your throttle body. If your idle goes up, down, stumbles, or dies you have a vaccuum leak there. Also what model Mystique do you have and what is your engine size?Let me know.


Ben Lee

May, 18, 2010 AT 3:43 PM

Thanks so much for your prompt reply!
It is a 1998, 4 cyl, 2.0 engine (let me know if you need more info)

I had not heard of the carb spray method, so I'll definitely try that.

I did hear once that the " propane" method (similar to what you described) when used on a car with electronically controlled idle would not work properly since the electronic control would not let the engine's idle change, even though you are introducing fluid/gas into the engine.
Is that not correct?

I was just reminded that my engine, when idling, from time to time will almost stall very briefly (rpm goes down to 4 or 500 rpm), but then it goes right back up and no problem. It only does this once or maybe twice each time I'm out driving, and it is only while idling.

One thing I forgot to provide was the freeze frame data from my code reader after the light went back on in the most recent instance (not sure if that will help you or not.)
Fuelsyst1: CL
Fuelsyst2: n/a
load_pcs % : 56.9
ETC (*F): 189
Shrtft1(%): 14.8
Longft1(%): 7.8
RPM (/min): 1630
VSS(mph): 40

I should also mention that I cannot find the EGR on my engine, and my Hayes book specifically says that the 1998 models are not equipped with them, but my mechanic not only said that all cars after the 1970's have them, but also that he checked mine and that it was fine. But again, I cannot locate it under the hood at all

Again, thanks very much!



May, 18, 2010 AT 8:27 PM



No its not true that on a computer controled idle you cant spray around looking for a vaccuum leak.So spray all around with the carb spray to see if you have a vaccuum leak.I also posted the trouble tree for your p0171 and posted a diagram for your egr valve.ley me know what you find.


Ben Lee

May, 19, 2010 AT 10:26 AM

Thanks for the diagram.

I sprayed all around the engine, and especially tried to focus on hose connection areas, but I did not notice any changes in the idle.

If I can't find a leak, then that would leave: -Restricted air cleaner (is this just referring to the air filter assembly?)
-Faulty PCV system (how do I check this system?)
-Incorrect cam timing (how do I check this? Or is that way beyond the scope of the DIY'er?)

I'm also wondering why the check engine light seems to be coming on more quickly after less driving than it used to. Before I started all these repairs, it would take between 30-40 miles of driving for the light to come back on after the battery had been disconected. And when I used my code reader to clear the code most recently, it took only 2 or 3 miles for the light to come back on. So either way, whether by disconnecting the battery or by clearing the code w/ the reader, it is coming on much more quickly than before all these repairs were done.



May, 19, 2010 AT 1:25 PM

For restricted air filter there talking mainly about a really dirty air filter. But if you had a kink or restriction in your air tube going to your throttle body or something like also.A faulty pcv you remove it and inspect it for a weak spring or all gummed up faulty hose etc. As far as checking timming you would have to remove the valve cover and line up all your timming marks lets wait on that one. We also have the excessive internal engine wear which you would do a compression check to check that. Also have you egr valve to check. Let me ask you this does the code always reset at cruise like 40 mph range you showed me on the snap shot?Let me know.


Ben Lee

May, 19, 2010 AT 1:43 PM

I found the PCV valve coming off the oil separator on the front of the engine, but did not get any idle changes when spraying the connections as best as I could (was difficult since the bottle does not spray at all angles). But where does the other end of the PCV valve tube lead to, opposite the oil separator?

I still can't find an EGR, and the one shown in my Haynes book looks just like the one in the diagram that you provided to me, but I still can't locate it. My mechanic did say that he checked the EGR (this was a few days ago) and that it was fine.

And to answer your question: I think the previous time I wrote down all the freezeframe data the MPH was about 60 (I've only copied down my freezeframe data twice at this point)



May, 19, 2010 AT 5:48 PM


I post the emmissions equipment your car should have you have to locate your vin letter or number for your car depending which letter you have you might not have a egr.When you find it refer to the chart i posted.As far as your pcv i couldnt find a picture or diagram for it.But the other end should go to a vaccuum source.Let me know what your find.


Service Writer

May, 20, 2010 AT 5:19 AM

Let me throw a few things out on this.

The intakes on these can leak after getting hot and without a smoke machine it can be hard to track. So do your testing with the carb cleaner while hot. Keep in mind carb cleaner is flamable so use good judgement.

Several years back we had a contour (sister car to the mystique) that had a lean condition that was tough to find. It did take the smoke machine to find it. There is a plastic tube that runs down the back of the engine compartment between the motor and the firewall that was cracked. It rubs against a main engine harness and creates a vac leak in this plastic hose. We had to have the car in the air on the lift to find it. It is a dealer part. I'll see if I can get more info later on it.

This car should not have an egr valve.

One word of caution, Mass air flow sensors were a problem with many ford products. Because you replaced it, does not mean it is good. Primarily if this is a reman one.


Service Writer

May, 20, 2010 AT 7:10 AM

The part number of that tube I referred to is: 5rz2420ba
That is a ford # and must come from them.
It runs from the brake booster I believe.
Assuming it is leaking of course.


Ben Lee

May, 20, 2010 AT 9:47 AM

How would I confirm whether the MAF sensor is now working properly or not?

I have sprayed everywhere that I can reasonably get to with the carb spray, but no changes in idle. I'll have to see if I can inspect that tube somehow that you mentioned (behind the firewall).

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