I own a 1998 Nissan Maxima, recently my car has been experiencing acceleration retardation. At times, it wont even go over 20mph. I recently took it to the shop and after diagnostic work was told I needed a new knock sensor. As you all may know, this repair is kind of pricey. I was wondering if spontaneous lack of acceleration was a sign of needing a new knock sensor and if so, how soon should I get it fixed before anything worse happens?

Also, before my car begins to lack acceleration, my breaks become stiff, as in a light tap is equivalent to me pressing down with force upon them, the car no longer drifts forwards or backwards if it's in drive, reverse or neutral, and sometimes does a high pitched squealing sound...

(my check engine light did not come on)

 

A knock sensor would not cause your car not to accelerate over 20 mph. A mass air flow sensor would. Go get a used OEM mass air flow sensor. Make sure its original other wise your car would not run right.

 

My money is on your MAF being bad. Bad knock sensor would still let you get up to speed, just sluggishly.

If i recall correctly not being able to go over 20 is a hallmark symptom of a bad MAF.

 

The brakes are what's causing your issue, not the MAF sensor.
You have a leak in the brake booster which is letting un-metered air into the engine. The failing booster is what's causing your stiff brake pedal.

Have the brakes fixed first, that will most likely resolve the acceleration issue.

The knock sensor error might go away as well, monitor that and if it doesn't, replace the sensor. It's really a 20 min. job at best.

 

Yeah, I'm with the other guys. A bad MAF will cause your car to have a 2500 RPM limit, causing your car not to go fast. Ignore your speedometer, what are your RPMs reading when this happens?

On top of that, you would definitely get a code for a bad knock sensor. You will probably end up getting one with a good knock sensor if you keep the car long enough Lol

Also, if you ever do have to replace the knock sensor, do that yourself. It's about a $20 fix, and it's not hard. A complete beginner can do it.

 

Same advice I was going to say as well. If your car is not going forward or backward by itself without tapping the gas on a flat surface, inspect the brake system. I would also check to see if you have seized calipers which may prevent this also.