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Q: Car hesitation from stop to acceleration

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I put a new fuel pump and filter in my SUV after the old one went bad. It then started to give me a code 35 IAC. I replaced my IAC and idle was normal and not super high anymore. No more codes. I have put a new MAP sensor, throttle body sensor, spark plugs and wires (in correct firing order), vacuum lines, distributor cap, and rotor. However, it is still hesitating hard while accelerating and has miner surges like less than 50rpm and its noticeable while driving with small jerks like its trying to catch up. Any advice? Also, the spark plug gap says it should be .035 and I bought the iridium plugs that are pre gapped at .055 as advised by the auto shop.

My car has 193352 miles.
My car has an automatic transmission.

A: Hi, thanks for writing in. Prior to further...

Hi, thanks for writing in. Prior to further diagnostics you should install the spark plugs that are referred to in the factory service manual and/or obtain the plugs from a GM dealer. Installing alternative aftermarket plugs with different gaps, materials, heat ranges, and what have is OK if you are set up as laboratory to do testing but if not you have to go with OEM on plugs. GM has already agonized over the correct plugs for the vehicle and has spent millions of dollars on testing. Benefit from their experience. If an auto parts store tells you to use a part that is materially different that which is clearly documented in the Factory Service manual, 10 times out of ten the store is wrong and the Manual is right.

Given the age of the vehicle, I would consider the fuel injectors (remove and send to a lab for cleaning, for example Mr. Injector), vacuum leaks, and ignition wiring issues as a cause of the hesitation. But before checking those, install the plugs recommended by GM for the vehicle because that is the only way to eliminate a misspecification there as even a part cause of the hesitation. On a 20 plus year old vehicle, I would have the injectors professionally cleaned and tested no matter what symptoms you are having, but particularly if it is hesitation.

If you would like help, consider having an expert automotive technician from YourMechanic come to your home or office to inspect and diagnose this issue for you, and make or suggest any repairs as needed.

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