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Tech501
Tech501, Auto Service Technician
Category: Car
Satisfied Customers: 747
Experience:  38 years exp, ASE Mastertech with L1 Advanced Engine performance
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RPM GOES..back fire..the car in drive, the car drops down to 500 rpms

Customer Question

I PRESS ON TH GAS AND THE CAR'S RPM GOES TO NO HIGHER THAN 3000. The car sounds like it wants to back fire. when I put the car in drive, the car drops down to 500 rpms.
Submitted: 11 years ago.
Category: Car
Expert:  Tech501 replied 11 years ago.
 Why don't you just take it back tio the people that tuned it up. It's their problem.
Customer: replied 11 years ago.
Reply to Tech501's Post: Unfortunately this problem was happening before i took it to them. I thought I need a tune up that is why I took it to them.
Expert:  Tech501 replied 11 years ago.

 Aaaah, I see. That's why you let them do the diagnosing. The diagnostic charge is probably a lot less that all the unnecessary parts replaced.


First we need make, model, year and engine size.


Is the check engine light coming on at all?

Customer: replied 11 years ago.
Reply to Tech501's Post: 91 cutlass supreme 3.1 V6

only whne the engine shuts off
Expert:  Tech501 replied 11 years ago.
 Your really going to need access to a scan tool to read the sensor data to determine if everything is where it should be. One thing you could try is dropping the front exhaust pipe to see if the catalytic converter is restricted. I kinda doubt it but you might as well eliminate the possibility..
Customer: replied 11 years ago.
Thanks for the info
Expert:  Steve replied 11 years ago.

    If the check engine light is not coming on, there are probably no service codes in computer memory to read.


   You can test for a failed catalytic converter without dropping the exhaust pipe; that is rather difficult on many vehicles.


   If you have a vacuum gauge, attach it to the intake manifold and watch teh vacuum reading as you accelerate the engine; a restricted exhaust system will result in the vacuum dropping to or alose to zero, at which point the engine may stall.   If the engine cannot pump exhaust out, it cannot draw enough air in and will be very low on power under load although it will usually idle OK.    Another quick easy test is to temporarily remove the oxygen sensor that is screwed into the exhaust pipe ahead of the catalytic converter.   ALthough the check engine light will come on and the engine may not run terribly well due to the lost oxygen sensor signal, if there is an exhaust restriction it will be partially relieved by the opening created where the sensor was, and will result in increased power and RPM range.    If this is the case, you do need to remove the CAT to take a look inside.   


  Also, if you shake the exhaust system and can hear any chunks of debris rattling around inside, it is probably pieces of broken ceramic from a failed CAT.


   There are other possibilities for what you describe though: the one that comes to mind first would be a jumped timing belt.   If the cam and crank are slightly out of time due to a belt that has jumped one tooth due to belt stretch or worn tensioner components, it can result in symptoms similiar to what you describe.    Checking cam timing is fairly easy on most engines; you line up the timing marks on the crankshaft and remove the upper belt cover(s) to make sure that the cam marks are also aligned.


   If you need any further assistance, please do not hesitate to ask;


  If you think my answer may be helpful, please click on my "accept" button.


thanks!

Expert:  Steve replied 11 years ago.
   Note: I did miss the line in teh subsequent posting where you identified this as a 3.1 engine; this one has a timing chain, not a timing belt.   A failed timing chain or sprocket could give similiar symptoms, but is not very common on these engines.
Customer: replied 11 years ago.
I have one more question, even though it seems it might be my cat convertor. I tried to start my car and first it dod not today. Then it did, when i pulled off it seems there was no gas getting to the engine. I am assuming that your previous response nails it on the head.
Expert:  Tech501 replied 11 years ago.
 It was only one possibility and only a guess. That's why I didn't charge you for that answer. If it doesn't work, come back with a new question annd we'll try again..

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