2000 Oldsmobile Alero Random Multiple Misfire

RCHINOY

MEMBER
2000 OLDSMOBILE ALERO

Engine Performance problem
2000 Oldsmobile Alero 4 cyl Front Wheel Drive Automatic 105k miles

Engine is 2.4 L

My car was working fine, and I changed the spark plugs with NGK platimum bought at Autozone.
the problem started after I put everything back together. The engine starting running very roughly and stalls after a few miles.
just to make sure the new plugs were not bad, I put the old ones back, but there was absolutely no difference. Just to make sure none of the plug coils were bad, I replaced them as well, but it made absolutely no difference.
I'm thinking that I must have knocked something loose like some vaccum hose or something.
the funny thing is at random times, everything is OK. I mean in the middle of coughhing and sputtering, all of a sudden I'll get full power for a few minutes then it will go back to the misfirings.
At one point, I drove almost 12 miles back to the house all the time sputtering and stalling. After I shut the engine down, I restarted it after a while, and it was as if nothing was wrong. It ran absolutely smooth for approx 5-10 minutes before dying and then the same issue happened.
I believe the error code obtained by the engine analyzer at autozoone was PO300. It did not specify any cylinder.
I'm completely stumped!
Pls help

5 Replies

BMRFIXIT

MECHANIC
19,053 POSTS

Check if you had damage the wire boots and or the coil

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Tuesday, May 19th, 2009 AT 9:10 PM

RCHINOY

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3 POSTS

Hi Jack, Thanks for the prompt response.

I have already changed the plug boots.
If I'm not mistaken, the coils are inside the plastic housing which I have not even opened.I will attempt to test these, however, it remains a mystery that before the spark plug change all was well.

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Tuesday, May 19th, 2009 AT 9:48 PM

BMRFIXIT

MECHANIC
19,053 POSTS

That housing also serve as a cap used to
and in some cases can be bad
check that too
I would suspect that before the coils

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Wednesday, May 20th, 2009 AT 5:17 AM

RCHINOY

MEMBER
3 POSTS

OK, I checked the coils out (compared the resistance on a new one vs the existing one) and all looked OK.
Since my gut feeling was a either a loose connection or a grounding issue. I took the cover off the sparkplugs, and took the coils out.
I then cleaned all connection points with a fine sandpaper, then I cleaned the head where the bolts for the cover (plug housing) would go in.
When I put everything together again, the car amazingly started up fine. After about 1/2 a day the check engine light was gone, and the engine smooth.

Now here's where the issue is. Approx 24 hours later, the check engine light came back on with no visible symptons. The error code was
P0420
(Catalyst System Efficiency Below Threshold (Bank 1) )

Except for some slight rattling noise coming from underneath the car all seems well. So I reset the light, and after a couple of days it came back on.
I seriously doubt if my Cat converter is bad, since I had no prior issues with it.
One thing I am noticing that when I start my car after it has cooled don, the RPM will run at approx 1100, instead of the normal of around 600. After 5 minutes or so, it will come down to 600 and stay there. Till I let the car cool down for several hours.

Unfortunately, while I had the earlier error (P0300), I had driven the car almost 40-50 miles over several days all the time while it was misfiring severely. Could this have damaged the Cat converter?

Any help would be appreciated.

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Tuesday, May 26th, 2009 AT 11:09 PM

BMRFIXIT

MECHANIC
19,053 POSTS

Misfire is the main killer of all converters
tap lightly on the catalytic converter if its rattling on the inside
suspect a bad converter

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need manual CHECK IT @
https://www.2carpros.com/kpages/auto_repair_manuals.htm