I recently had to change the motor in my 2004 Hyundai Sonata. I bought a used one from the junkyard, and it fired right up after I got it put in, and all the wires, hoses, etc. connected. The only thing that I don't have hooked up is the exhaust pipe. I've got the manifolds on the motor, and the Y pipe is lined up on both of the manifolds. The gaskets won't be here until the morning, so I just hand tightened the bolts. The car is obviously loud, and runs pretty smooth. I drove around the block, and it seems to want to "fall flat" when it gets to a shift point, and it won't shift. The first time around the block there was no check engine light. I came home and topped of the transmission and coolant, and ran around the block one last time. It seemed to not run as strong, and the check engine light came on. I am guessing that the O2 sensors, which only sit about 1" above the connection between the Y pipe and the manifolds are not operating properly with the exhaust open. Just curious if any other ideas are out there? Do you think that finishing up the exhaust will fix this issue, or do I need to dig into the issue a bit more? If I can get it to run and shift, I will take it into the auto parts store and have the codes checked. Thanks for any input.

1 Answer 1


With the check engine light on, the car is going to be running a fixed set of parameters from the Engine Control Module.

For instance Fuel Pressure will be held at max pressure and timing and spark adjustments (if done electronically) will not retard or advance.

One of the reasons this happens is because the Engine Control Module is not recieving all of the needed information from the required sensors in order for it to calculate things like fuel mixture, timing (spark/valve), etc. This will also in turn affect transmission points. Where as Transmission Pressures are usually set to max during a fault situation and if the fault is specific to the Transmission then it will force only 2nd and 4th gears (limp home mode).

All in all. Get that exhaust hung properly, clear all your codes, and repeat your test drive. As long as you didn't forget to plug something in somewhere and all the parts are good then, I think you will be just fine.

  • The engine will operate on its ECU map, based on crank or crank-and-cam sensors. Commented Sep 25, 2014 at 18:39

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