Mazda Repair: Can you help me?, fuel pressure regulator, air flow meter

Question
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Followup To
Question -
I own a 4 cylinder '96 Mazda Protege, manual transmission.  Recently, the car has started shuddering violently no matter if it is in neutral or in gear.  It also is slow to respond whenever I hit the gas no matter what gear I'm in.  I've never had this problem before.  I was wondering if you could tell me what the problem sounds like and what it typically costs to get it fixed so I will be armed with this information when I go into a repair shop.
Answer -
hi jennifer,
The problem you are having could be caused by any number of situations and it is quite hard to make a diagnosis without me having seen the car. However, I can suggest some possible causes. If the problem has just built up over time, it could be something as simple as dirty fuel injectors or dirty throttle body. A good dose of fuel injector cleaner into the gas tank should solve either of these problems. It would also be wise to check your air cleaner element as it just may be dirty. If the problem has just appeared suddenly then you are probably looking at something like a faulty fuel pressure regulator, air flow meter or coolant temp. sensor. If the check engine light isn't on then a faulty fuel pressure regulator or a faulty or dirty Idle Speed Control Valve is the likely culpret. If you need any further assisstance or want any further information don't hesitate to write back with any more info you might have about the problem such as: has there been any noticeable jump in fuel consumption since the problem or is there any smell of petrol when the vehicle idles.
hope this helps

James,
First of all, thank you for your help!  The problem just happened recently and the check engine light IS on.  I talked to a friend about it, and I was told it was probably my spark plugs that needed changing; I changed them, and it didn't fix the problem.  There's been no noticable change in fuel consumption, nor do I smell any gas when then engine idles.  Is it possible the problem is being caused by a dirty fuel filter?  I have not changed it since I bought the car in 2000.

Answer
Jennifer,
The fact that the check engine light is on indicates that there is a problem with one or more components of the engine management system. As I said it could be any number of different sensors, particularly the coolant temp sensor or air flow meter. To find out the exact problem you will need to take it to a dealer and have them hook it up to a diagnostic computer which will tell them exactly which part of the engine management system is failing. A dirty fuel filter wouldn't cause a problem this severe nor would it cause the check engine light to illuminate.
hope this helps