catalytic converter

[Steve Nylin]

Hi Geo Folks,

I have information to pass on that happened to me, and it sounds crazy but true. I have a 92 Geo Metro Convertible w/auto transmission. It started to not shift properly, so I changed filter & fluid. No change. I unhooked one sensor wire to shift manual and that worked out good, but still not shifting on its own. The engine seemed to lack power also. Now the crazy part. I was under the hood and I ran the throttle by hand and the exhaust was blowing back from the manifold pipe hook-up past the donut. I went to the back end and felt for exhaust and there was nothing and the pipe was cold. The catalytic converter was plugged. I took a grinder and put a hole in the converter, and it started to run fine. I ran it out the road and it worked fine, so I thought I would try the transmission in auto again. I pulled over, rehooked the sensor and the thing works like it should. The problems with the converter being plugged caused the engine to run hot, lack of power and I'm guessing the computer malfunctioned because of that also. I know I had to pass this on to you folks, and maybe solve some of your problems too.

[RossT]

You did a good job in your diagnosis!

[91Caprice9c1]

Hey Steve, good job solving the root cause!

With the exhaust clogged as it was, your MAP sensor was seeing false high pressure indicating high load conditions to the PCM. This would certainly delay shift points.

Thanks for sharing, a clogged exhaust wouldn't have been the first thing i'd check for shift abnormalities. Now I can see how it's possible.

-Matt