I purchased my car from a public auction and then had my mechanic examine it. He told me that it was misfiring and should have a tune up. He changed the distributor cap, rotors, plugs, and plug wires. After those replacements, the car is still misfiring. Even though the check engine light is not on, he still hooked up the hand-held computer to the van but there was not any code. He had me drive the car for 30 minutes, he re-tested it, but no check engine light ever appeared. Now, he is speculating the it might be the fuel filter, fuel pump, intake manifold gasket, or something else. He also suggested that perhaps I should take my van to the muffler shop so that they can change the intake manifold gasket. He is a good mechanic and usually can spot the problem right off and fix it, but this one has him puzzled as to what the solution should be. I’m wondering if perhaps I should take the car to a dealership for a diagnostic. Before I spend more money on replacements, would you give me your assessment of the misfiring problem and how it can be fixed?
You may want to consider having another mechanic look at the vehicle to get a second opinion on this. This inspection would include testing of the fuel pressure to determine whether or not it is within the specifications set by the manufacturer. If the fuel pressure is low, the fuel pump is very likely having an issue.
If the fuel pressure is within specification, I would recommend having a vacuum test performed on your vehicle to determine whether or not the vacuum level is within specification. If the vacuum level is low, the intake manifold gasket may be the issue. Another way to confirm the intake gasket issue would be spraying some brake cleaner around the area where the intake manifold connects to the engine. If the engine revs up when the brake cleaner is sprayed in a certain spot, the intake gasket is most likely the problem. Once the problem has been verified, the mechanic will be able to give you a list of parts that may be needed in order to resolve the misfiring issue.
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