Hello y'all, I have a '97 4-door 2.0 5-speed with about 120K miles. With regular maintenance it's been a good little commuter car for me for about 5 years now, but it recently developed a problem that's proving difficult to diagnose.

About 3 weeks ago while I was out of town the "Service Engine Soon" light came on. I took it into a Midas shop to see if they could give me an idea of what was wrong. The fella told me OK, but there were some issues he could not fix, only a dealer could.

He plugged in his OBD reader & got the code. Unfortunately I did not have him write it down, but he told me that the code translated into a "Problem with the upstream air sensor" and that the car was running lean.

I had been experiencing some minor idling problems (rough idle after warming up for 12-15 minutes), but nothing major, no stalls. The Midas man told me this was a problem for the dealer.

While I took his word, I wanted to check things with my local independent mechanic. While not a Ford specialist, he does come recommended by a friend. He plugged in his own OBD reader, got the same message that the car was running lean. However, he was not able to localize the problem. He instead felt there was a leak in the intake system somewhere, and proceeded to "smoke" the car. Apparently this is an old-school method of traking leaks whereby a smoke generator is hooked up to the intake & the engine is observed for any leaking smoke.

So my local guy couldn't find anything. He reset the light and sent me on my way.

Meanwhile, I have been thinking of all the things I might be able to replace in effort of fixing the problem. I own motorcycles and bicycles, and do quite a bit of my own labor (from changing oil and brake pads all the way to rebuilding clutches and gearboxes on old Triumphs).

I found an Upstream air sensor on rockauto.com, and I'm wondering how difficult it is to replace. My Haynes manual does not list it, but I know it's somewhere on the intake/exhaust.

Anyone have any tips? Thanks in advance.

 

I dont know how well the smoke method works, but I always found vacuum leaks by spraying starting fluid or carb cleaner around the intake and any vacuum hoses. Spray an area, wait a few seconds, and if the idle increases, you've found your leak. Good luck!

 

Mass air flow sensor maybe which is usally located in the air intake duct before the throttle plate.
Oxygen sensor is located in the exhaust pipes .



Good Luck