Weird issue after fixing exhaust (PA 92)

GORIRNICHevo1

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Park Avenue
I had the check engine light on because of a hole in the rear outlet of the catalytic pipe. So after a “Holst” kit repair (without changing the part) the light went out. But following that another trouble came. The car starts fine when engine is cold but after a mile or two, if you turn off the engine it won't restart. And when it finally starts up again, the idle is at about 1500 rpm and the car moves forward on its own at constant speed (a bit like as if the cruise control on). I'm not sure (may have to check it out) but the check engine light takes some time to turn off and it is when it does that the problems I describe above would occur.
Any idea guys?
 

Ziggy

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Buick Park Avenue 91 base
If the check engine light comes on, check for codes first.
 

HotZ28

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This sounds like an ECM issue, that would require a replacement. Old ECM's can get like that and be funky. Did you disconnect the battery during the events mentioned? If so, was the negative cable removed first, then the positive?
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GORIRNICHevo1

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Park Avenue
I disconnect the battery each time I leave the car for more than 2 days without using. Well I only unplug the positive for reason of easiness. I know that It's better to unplug the negative for reasons of security, but I do that for years on various cars without problems if you care on what's around your battery. By the way, what do call an old ecm ? Is it a question of time, of mileage or "natural" deterioration?
 
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2007LucerneCXL

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Zapping the system using the positive may not be the best method, but to each their own. As it's unclear how many times it's being done a battery switch on the negative side would be a better solution.
71ZcFQECp1L._AC_SL1500_.jpg

Depending on what the code was that it was decided to be a exhaust leak past the catalytic converter is something that may be the unknown. The sensor location is upstream, while it could be related it's a OBD1 that is not that clever at times.
9507214C03-005.png
 

GORIRNICHevo1

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Park Avenue
By the way guys, let me relate you the exact thread of what is going on.
First start and 2 or 3 miles the Buick start and run as a charm - but with the check engine light on :s, then the light shut off and troubles appears, rpm needle goes to about 1400/1500, the car no longer slows down and keeps the initial speed until I push on the brakes firmly and finally went I switch off the engine, it no longer restart but after half an hour may be and in the meantime, impossible to move the gear box from P to N for example. This is where I am...
 
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