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1999 Sedan Deville 50th Anniversary Edition.
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24 Posts
Discussion Starter #1
I have had this problem from last year but the car has been off the road since November here in UK. At start up the engine runs fine for a short while, then begins to misfire, driving is ok, the car recovers but with lack of power for while, then runs fine. Idles uneven miss? Parking vehicle engine runs uneven, lack of power, and when restarted later runs uneven whilst manoeuvring, then lacks power for a while. The engine then settles down full power. I have to operate accelerator gently otherwise engine stumbles with lack of power. At idle engine is uneven, but rev engine and it has good power after a couple of splutters. I changed plugs last year to Bosch, these are fitted now. These are correct, checked with Rock Auto. I did try AC Delco plugs from Rock Auto, but these made the situation much worse with misfires and backfires on the road. I have no engine service lights and no current codes. Many thanks
 

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98 DeVille, 97 DeVille d'Elegance
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Have you checked codes? Even history codes can be relavent.
These only like AC plugs, it will not like others. Have you cleaned the TB? How old are plug wires?
I bet you have a simple issue like TB dirty, bad wire or coil going out.
If you have access to a handheld obd2 scanner with real time you can watch for misfires and it will show how many.
Another possibility is a vacuum leak, check the pcv hoses and grommets on vac hoses around the TB, the over pressure valve can stick or go bad causing vac leaks and bad running but it would be down the list.
 

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1999 Sedan Deville 50th Anniversary Edition.
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24 Posts
Discussion Starter #3
Re: 1999 Deville 50th Anniversary Edition Engine misfire no codes

Many thanks for your help and suggestions. I think I just need to work down the list, once the car is running on the open road it's great. I will run down the list. Many thanks again.
 

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1999 Sedan Deville 50th Anniversary Edition.
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24 Posts
Discussion Starter #4
Re: 1999 Deville 50th Anniversary Edition Engine misfire no codes

Ok, I've found two problems. The narrow vacuum hoses on passenger side crumbled when I flexed them. Replaced with 4mm vacuum hose. The 10mm rubber connector to the PCV had a split, so I have just replaced that today. So it runs better, but still idles uneven, in particular in gear. I have checked the exhaust backfire valve and there is movement when the engine is not running. I can rotate it clockwise by gentle twist at the vacuum connection. When the engine runs I cannot move it. I wil keep on with my list but any help would be appreciated.
 

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98 DeVille, 97 DeVille d'Elegance
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7,345 Posts
Tb cleaning solves alot of idle and drivability issues. The flapper shouldn't rotate it pivots at top and ahould snap closed nice and quick. If still idling bad and can't find another leak time for scanner... My 98 had coil getting wonky and missed off and on for a couple weeks before went out.
 

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1999 Sedan Deville 50th Anniversary Edition.
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24 Posts
Discussion Starter #6
Re: 1999 Deville 50th Anniversary Edition Engine misfire no codes

When I say it rotates I mean the hole unit. It's as if it rotates clockwise in order to remove the entire assembly for replacement. When the engine is off it is loose in its fitting. When the engine is running will not rotate and appears that the unit is solid and not loose. I have cleaned the throttle intake but not having a gasket yet am not able to proceed to a full clean and to examine the IAC. I take your point about a coil though. There is one that is light blue in colour and clearly aftermarket as the AC Delco units are black and numbered.
 

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1999 Sedan Deville 50th Anniversary Edition.
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24 Posts
Discussion Starter #7
Re: 1999 Deville 50th Anniversary Edition Engine misfire no codes

Just tried doing a relearn idle session, as suggested on another post. I now have code 300 engine misfire. Engine revs are showing 675 to 750 rpm.

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Just tried doing a relearn idle session, as suggested on another post. I now have code 300 engine misfire. Engine revs are showing 675 to 750 rpm. Also, only history codes are for airbags
 

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1999 Sedan Deville 50th Anniversary Edition.
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24 Posts
Discussion Starter #8
Re: 1999 Deville 50th Anniversary Edition Engine misfire no codes

Many thanks to rodnok01 for your suggestions. Replaced several vacuum pipes including those from intake manifold to pcv valve etc. Removed TB and cleaned thoroughly, new gaskets and seal, new TPS and IAC whilst I was there. Also got new plug wires and coils, not yet fitted. Good result so far, runs much better, sounds like an engine now with much more power, no misfires, steady rpm.
 
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