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Ms. Wrench
Ms. Wrench, ASE Certified Technician
Category: Car
Satisfied Customers: 554
Experience:  ASE Certified Technician-10 yrs., Service Writer-Foreign, Domestic & Fleet, Alternative Fuel Expert
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My car accelerates very slowly when I step on the gas.

Customer Question

My car accelerates very slowly when I step on the gas. People leave me behind in the freeway even when I step hard on the gas.   When I stop and go slowly on back roads, it slows down like the engine is gonna stop completely then it will abruptly pull. Two diagnostic tests showed nothing wrong with the car. My car is a 2002 Mitsubishi Montero LS. What could be the problem?
Submitted: 9 years ago.
Category: Car
Expert:  Ms. Wrench replied 9 years ago.

Hello,

If your RPMs stay low with the speed of the car then it sounds like you could have a severely restricted exhaust. If the engine revs but does not move like it should-then you may have a trans problem. Plugged catalytic converters will not always set a trouble code-certain makes and models will set catalyst efficiency codes quite readily but mitsubishi is not necessarily one of them. Anytime you step down all the way on the gas and the car doesnt go and the rpm's dont rev up-its a very good possibility that the exhaust is restricted.

You can feel the exhaust coming from the tailpipe when the engine is reved up-it should be strong and steady. From the information you gave it sounds like a classic case of exhaust restriction-if this does not make sense to you or if there are more details concerning this problem-please reply and I will give you a more precise answer. The more details the better. Let me know what you think.

Thanks for using JA

Ms Wrench

Customer: replied 9 years ago.
two diagnostic tests were done and no problem showed.
Expert:  Ms. Wrench replied 9 years ago.

Like I said it sounds like a restricted exhaust-classic symptoms. What type of diagnostic tests were done?

Ms Wrench

Customer: replied 9 years ago.
I just spoke with my mechanic and he said it's just general diagnostic test to check for error code. Is there a specific diagnostic test I should go for to narrow down the problem?
Expert:  Ms. Wrench replied 9 years ago.

Hello,

Yes to determine if there is a restricted exhaust issue-a manifold vacuum test needs to be done. Vacuum readings at the intake manifold at idle and under steady acceleration needs to be monitored. If the vacuum decreases under steady high rpm's then there is an exhaust restriction.

An exhaust backpressure test could also be done to determine excessive backpressure due to a restriction.

Temperature testing of the catalytic converter can also determine if that is the problem. The cat should be 100 degrees hotter at the outlet than at the inlet if it is operating properly. Any heat pocket at or before the inlet and a temperature differential that is cooler at the outlet side indicates restriction.

Thanks for using JA

Ms Wrench

Customer: replied 9 years ago.
I believe Catalytic converter test has already been done and it's not the problem. I will talk to my mechanic and get back to you. Thanks.
Expert:  Ms. Wrench replied 9 years ago.

Hello,

OK let me know. Have there been any other repairs done recently? Does it correspond to the symptoms I was describing as far as the low rpms go?

Ms Wrench

Expert:  Ms. Wrench replied 9 years ago.
One more question-is your montero an automatic?
Customer: replied 9 years ago.
rpm is okay. I'm at my mechanic's shop right now.
Customer: replied 9 years ago.
My montero is automatic.
Customer: replied 9 years ago.
Every possibility you addressed is not the problem as confirmed by my mechanic. I reiterate that he checked all of those prior to my communication with you. RPM is fine, catalytic converter is not defective, and diagnotic tests did not return results of bad engine or transmission. Anything other than you advised that might be causing the proble so I may take it up with my mechanic? By the way, I replied to your question yesterday saying that my car is automatic for which I did not get a reply from you.