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Why does my car balk on freeways?

Here is a description of the baffling trouble I have been having with my car - a 1995 Subaru Impreza.

In the past 2 years, shortly upon entering a freeway my car will sputter and lose power. I will pull over to the shoulder. Normally, I can restart the car and get back on the freeway.

Sometimes the Check Engine light comes on, sometimes it doesn?t.

Sometimes the power goes out completely, sometimes it doesn?t.

When it acts up, it is the first freeway trip of the day. It does not seem to act up on subsequent trips.

When the Check Engine light has come on, we have replaced the ignition coil (twice), cleaned the fuel injectors, and then replaced the fuel injectors.

The mechanic who has worked on my car all these years has done all sorts of tests ? smoke tests for vacuum leaks, scans. He has driven the car and has experienced the problem first-hand.

To me there is one symptom that seems to be related. My rpms vary quite a lot. In the same drive they can vary by 500-700 rpms. For example, I will be at a red light the rpms will say 500; once I speed up to 40-45 mph the rpms will say 2000. Then at the next red light my rpms will say 1200, and as I speed up they will go to 2500-2750. Then back again, or not. I can find no pattern to this variance, but it doesn?t seem right to me. The first time I noticed it was about 2 years ago shortly after my car acted up for the first time.

The Check Engine light has come on twice lately ? not on the freeway. I have replaced an O2 sensor, and a vacuum hose had come loose.

Has this ever happened to anyone else?

Has anyone checked the fuel line pressure?

My mechanic said the fuel line pressure was checking out okay.

Has he checked the exhaust system for obstructions?

I’m assuming from your post that he’s checked the usual things like the oxygen sensor, the MAF sensor, the MAP sensor, the vacuum readings, etc.

what are the exact codes that are coming up when this happens?

I don’t believe the mechanic has mentioned checking the exhaust system for obstructions. He has done a smoke test, if that checks the vacuum readings. And the O2 sensor has been changed lately - the Check Engine light came on when the car was not on the freeway. I cannot speak specifically to the MAF & MAP sensors, although I would assume they were checked.

My mechanic said the codes indicated the fuel injectors and ignition coil. These parts were replaced. I do not know the code numbers.