1999 Century 3.1 Rough Idle Full Throttle ok - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 01-10-2015, 04:04 PM   #1
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Default 1999 Century 3.1 Rough Idle Full Throttle ok

On start up cold or warm the engine idles and runs good for a minute or two. At full throttle - i.e. passing gear there is no missing or hesitation.

Regular driving idle is rough and sluggish. SES light Flashing a lot of the time.

Has a P305 Cyl # 5 Misfire. No other codes

The 2 / 5 coil pack has been replaced. # 5 Spark plug replaced. Tested for vacuum leaks using ultrasound equipment.

Can the #2 and #5 wires from the spark plugs be swapped at the coil pack to see if that creates a Cyl #2 Misfire? i.e. showing a bad wire?

Thank you for any suggestions.
John
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Old 01-11-2015, 09:57 AM   #2
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yes thats what should be done next
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Old 01-11-2015, 11:25 PM   #3
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Thank you jwfirebird

Unfortunately it didn't help solve the problem(*).

Same symptoms and only the P305.

I wasn't optimistic but worth a try.

John
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Old 01-12-2015, 08:20 AM   #4
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unfortunately intake or head gasket leak would be the next most common thing, you should check the compression. have been loosing any coolant?
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Old 01-12-2015, 04:52 PM   #5
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jwfirebird - again thanks for your time.

unfortunately (lots of those ) I am past the point of being able to do all but easy to reach stuff.

A little history I bought in 2009 with 68,000 miles on it. Now 85,000 - not high use

The P305 had been up for several months showing little or no effect. A few weeks ago under load the SES would flash a couple of times and missing could be felt.

Coolant leak: Not a yes/no answer. No sign of mikly at any time, recovery bottle never empty, no overheat. I check it often - I have a dip stick for the recovery bottle.

A couple of years ago there was an external leak that appeared to be coming from the area around the top left of the front cover. I put some stuff in the radiator which stopped that. The car only logs about 200 miles a month, all city driving and uses 1-2 cups of coolant a month. There has been seepage at a couple of hoses which I think I have recently stopped. The last week or so I have been pushing the engine a lot harder than normal to try to get a handle on the problem and the coolant level hasn't gone down.

Compression: I took it into a shop (a difficult thing for me to do) they replaced the 2-5 coil pack (weak spark?) and #5 spark plug - no help. I asked if compression checked and was told stuck finger/thumb in the hole while cranking.

They recommend taking the top off to check the #5 injector and while at it say might as well do the intake manifold gaskets.

I'm thinking to ask them to check #5 compression, if is about 150 ok, but if say 120 to pull all plugs for a better check. I read it is not a good idea to pull plugs until engine has completely cooled down - do you agree?

The thing I can't get my head around is that at start up it runs smooth for the first minute of so and on the road with pedal to the metal. I'm guessing their is a default fuel setting used at start up and at wide open throttle we don't care about emissions or fuel economy.

Aside - A few months ago I replaced the alternator because the voltage was varying quite a bit, 15.5 or more with high ripple - shorted diode(*). I wondered if that could have messed with the electronics.
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Old 01-12-2015, 07:12 PM   #6
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the cylinder 5 miss pretty much narrows it down to a mechanical problem with that cylinder or head gasket. you have to use a compression gauge. then put some oil in the cylinder to rule out rings. but given the 98 to 04ish 3100 is notorious for head head gaskets i would imagine thats what you are going to find. if your compression is good its probably just intake gaskets.

fyi the guy doesnt seem that knowledgeable. at least about gms, you dont use your finger and i have had many cars with your same engine and at least 200-300k on each before trading them in. i have never replaced an injector on any mpfi gm.

also should NEVER use sealers you should flush that out as soon as possible. they seal all the engine cooling passages, which is anything but good.

as far as the plugs you want to be as careful as possible with aluminum heads.

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Old 01-12-2015, 10:35 PM   #7
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Check compression.....

Check coil output with a spark test on on 2 and 5 wire......

Do an ohm check of the injectors(all around 12 ohms).......you need expensive equipment to check flow....have seen mice get under there and eat insulation of wires....if the injector is suspect, you can try swapping it.....it'* a little involved to do this......if misfire moves, you know it'* the injector....
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Old 01-12-2015, 11:22 PM   #8
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^^^^^^ What he said!^^^^^^^^
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Old 01-13-2015, 03:08 AM   #9
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The coil has been replaced with no change. Maybe even a little worse unless the real problem has got worse.

Yes compression test next on the list. I have had a compression gauge for many, many years but I am not as agile as I once was so much to my dislike have to rely on others. This might have to wait a week or so.

Testing injectors resistance with an ohmmeter would be nice if had an extra wire harness to butcher so could do without taking the plenum off. However, jwfirebird thinks it highly unlikely it is faulty.

Thanks

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Old 01-13-2015, 08:29 AM   #10
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compression is alot easier to check than injectors plus you subject yourself to leaks once disturb the orings
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