Contour Enthusiasts Group Archives Forums General Troubleshooting Check engine light on over speeds of 70mph...

I just need some general council on this matter.

I've had the check engine light go off now three times, and have taken it to a local mechanic for the frist two (the third time that the light has come on being today).

He said that the code coming off the computer told him that "the engine was running too lean" and made three suggestions for it. Upgrade to 87 octane, put in a can of cleaner and octane boost, and if that didn't work and the light came back on replace a faulty O2 sensor. I've done all of this. After the replacement of the O2 sensor he said that should the light come on again, to let tank of gas go through and then the only other thing to do was to retrain the computer.

Today when it did it again, a little light went off in my head that said "hey each time that light has come on are days that I've taken the car up past 70+mph on the highway".

So given that he says the engine code is telling him that the car is running to lean, and I'm betting that it goes off after I reach speeds over 70mph, what do all of you think is happening?

Is it something more serious than just simply reprogramming the computer can make right? What does it mean if the light is coming on after I get past 70mph?

Like I said I'm just looking for some general guidance before I call this poor man to deal with the light for the thrid time in a week.

Thanks,
Ink

First off, identify your car by year, engine, and transmission. There are differences and without that information we can only guess.

Second, post the exact code number. Better yet, perform a SEARCH using the code as your search keyword to see if it's one that had been frequently brought up (sounds like it).

If you stiil need additional advice (like finding a new mechanic that understands what codes mean and how they relate to the CEL and the rest of your engine), don't hesitate to ask. Sounds like the poor SOB is batting 0 for 3 so far.

The light comes on then because the computer has decided that the operating conditions are met to perform a specific system monitor test. Certain of the monitor tests require steady-state highway driving (and a few other things) in order to execute.

Steve

You need to find another mechanic.

He found a code that said that it was running lean. He made some slightly off the wall suggestions to try first and then he wanted to replace the oxygen sensor.

Most of the time, oxygen sensor codes for running lean are not caused by the oxygen sensor. Someone needs to find out why it is running lean, then you can figure out how to fix it.

Since it comes on only after getting the car up to 70 MPH, it isn't likely to be a vacuum leak. It is very likely a fuel delivery problem. A fuel pressure test while driving is in order. It would not hurt to change the fuel filter first to see if it helps if the filter has not been changed in a while.

By the way, it would help if you told us what year the car is, which engine you have, roughly how many miles it has.

PO171 system to lean bank 1 I'm guessing.

Yeah I have that code to for a long time like February, comes and goes. I have cleaned the MAF many times, replaced the upstream O2 sensor twice, downstream once, replaced several vacuum lines, replaced the ECT done a whole bunch of [censored].
Anyways my reason is because my injector O ring seals are all leaking air when the engine goes for the intake stroke.

I'm fixing it tomorrow.

Its odd you should say it comes on above 70, my comes on while accelerating on the highway ro while on the highway, once going to Chicago, and a few other times while on the freeway.

So yeah fuel delivery problem.

I would take a vacuum leak detector, does it go beep, pause beep, while you have it above the injectors, if so your O rins are leaking.

Only $12 for new ones so dont be to worried.

I agree with jim - fuel delivery is probably what's up. But I have also seem a maf become biased and read properly at lower air flows and wrong at higher. There is a PID called BARO that reads a frequency. Depending on the altitude at your area, it should be 150 to 160 Hz. BARO is read at 2/3 throttle or more and is based on the volumetric efficiency of the engine. The computer knows how much air should be pulled in at high throttle settings, and uses the maf to measure it. A comparison between actual flow and expected flow yields the barometric pressure. Then baro is used as a compensator in the fuel trim calculation. baro is a good indicator of a maf sensor's health. The MAF pid should also track with the tp voltage - within about 1/2 volt or so, depending on how fast you're moving the throttle.

Jim is also right about vacuum leaks not causing problems at higher speeds. Once you get above idle, and certainly by the time you're holding highway speed, the percentage of unmetered air becomes small enough that the normal long and short term fuel trim compensators will hide it. At idle, so little metered air is there that the unmetered can be up to about half the air entering the engine.... Anyway, more than you wanted to know, huh?

Remember to clean the filter on the end of the fuel tank pick up

First of all I want to say a huge thank you to everyone that replied.

Second the car has been properly maintained according to the matience guide. It use to be my grandmothers car, and she assures me that she's taken it in. ;> (I'll check on the fuel filter)

Third the car is a 1998 Mercury Mystique GS model 2L with only 21,000 miles on it. I'm sorry for not posting this information in the first place, this problem is getting irratating, and it was the last thing I was thinking about.

Fourth, I trust this mechanic and yes he is certified. I have three other people in the family that take their cars to him, and he comes on high recommendation from a family friend. (I trust him far more than the dealership that I was taking it to for warranty work.) His work on all other occasions has been exemplary.

HITMANinMI thank you for the information you gave, it's a big step in answering this. My problem sounds exactly like yours. It's not so much that the light come on while/during the 70+mph it's a more of a common thread of the days that the light has come on and what I've been doing behind the wheel. The days it has come on are just after I have been excellerating to those speeds, days that I have not (staying around 50 or so) the light stays off.

And to GregH yes that is more than I wanted to know, but too much knowledge is never a bad thing when it comes to dealing with mechanics, or anything else.

Thanks,
Ink

So did you get it fixed?