CAR IDLES AS IF MISFIRE AND CAT CONVERTER OVERHEATS

  • Tiny
  • laarcae
  • 2003 Mitsubishi Galant
  • 120,000 miles

Hello, I'm a mechanic and have been stuck on this one car for nearly 2 weeks now. Initially the car came in for a blown motor. We replaced the motor and everything was working smoothly, only the transmission wouldn't shift. So we replaced the trans after determining no other possibilities. Upon reinsertion of the motor and trans the vehicle now runs roughly, as if shorting out. All fuses are now good, though I did have fuse #9 in the fuse box burn out multiple times. After tracing the several wires, I found 2 damaged relays (overheated and melted).

But for the time being all fuses and relays are within proper operation. I tested all fuel injectors, replaced spark plug, checked for blocked muffler and cat, checked for bad mounts. But none of these are the problem.

I'm wondering if the wiring from an injector or perhaps the circuit path of fuse #9 might be causing this issue? Any and all insight is welcome.

(please ignore the yellow highlight, this was the only valid image I could find. It is fuse #9 that had the problem)

Monday, July 23rd, 2012 AT 5:16 PM

4 Answers

  • Tiny
  • saturntech9
  • Expert
  • 28,251 posts

So is the cat clogged or is it getting too much fuel from the engine?Thise are the two main reasons I have seen them overheat. Also have you done a back pressure test yet?Any codes in the computer?Have you checked the fuel pressure and let the fuel pressure sit with the engine off to see if it bleeds down?

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Monday, July 23rd, 2012 AT 5:23 PM
  • Tiny
  • laarcae
  • Member

Back pressure test shows no problems in the exhaust. There are no codes at the moment. I'm about to head out to retrieve my fuel pressure tester.

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Monday, July 23rd, 2012 AT 6:05 PM
  • Tiny
  • saturntech9
  • Expert
  • 28,251 posts

Sounds good let me know.

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Monday, July 23rd, 2012 AT 8:39 PM
  • Tiny
  • laarcae
  • Member

You know I wish I could come report that the car has been fixed, but I am not that lucky. I know I reported before that there was no backpressure abnormality, but apparently the entire exhaust pipe, both cat converters and the muffler were clogged up. We've removed the entire pipe and placed a hollowed cat/exhaust manifold. Now that we've got the engine running right, The transmission won't shift. We've purchased a replacement exhaust pipe. Question for you all, if I used the wrong ATF on this car, would it cause the engine problems I've been experiencing? I read somewhere (as I'm not a tranny specialist) that a bad torque converter can cause the Mitsu engine to overheat and/or run too rich. Also that the torque converter can make the transmission perform sluggish and will transfer the torque to. The transmission preventing the car from moving beyond a crawl.

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Tuesday, July 31st, 2012 AT 11:19 PM

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