1999 Century 3.1 Rough Idle Full Throttle ok - Page 3 - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 01-22-2015, 04:39 PM   #21
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he said he was, when i asked on the first page. he seems to be in denial though. as i said the next thing to be done is check compression to confirm or deny mechanical issues or head gaskets
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Old 01-23-2015, 07:09 AM   #22
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jw - The scanner - sorry - the readings are Metric. It will Display data in English or Metric but seems to Print Data in Metric only. In the attached I have converted the readings to "English' and show a little more of the data in closed loop.

Tech II - Thanks - I'm learning.
There has been a P305 for a long time however, any misfire was very slight and was not a concern to me. Emission testing has stopped where I am located. Only recently has the SES been flashing. Coil and plug changed, didn't make it better, perhaps somewhat worse. I don't believe the scanner will show history of misfires.
I read on one of the states (Arizona?) emissions pages that a 2% RPM difference on a cyl will bring up a steady SES light - fussy, fussy - and 10% a Flashing SES light.
Would those specs have tightened over the years since 1999?

When in open loop - startup and full throttle runs fine.

Coolant leak - The car only logs about 200 miles a month, all city driving and was using 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 couple of weeks 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. I check the recovery bottle before cold startup with wood dowel dip stick, it has been remaining steady.

I appreciate your time
Thank you
John
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Old 01-23-2015, 08:26 AM   #23
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gms misfire system is very good, doesnt matter how it works, just that there is always a problem that needs addressed. the reason it blinks is because you have cat damaging running conditions and you shouldnt be pushing it until you fix it. as i said the compression needs checked. and the p[lugs inspected for coolant. when i had the 3800 intake gasket problems it showed me cyl 6 mis. because coolant was just dripping in you could see it on the plugs. the misfire detection is good you just have to listen to it.

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Old 01-23-2015, 10:32 AM   #24
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Funny, more often than not, on 3.1/3.4 engines, if you have a head gasket problem, it'* usually #1 cylinder......on start up, coolant could pool overnight in a cylinder, causing a rough idle on initial start....but it should clear up after a few seconds ....

I think you are at the point where you have to fix that misfire....that means, determining if it'* spark, fuel or compression....I would use an ST-125 on the end of every ignition wire to check spark.......I would check compression on all cylinders......I doubt it is fuel pressure, but it could be an injector problem....

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Old 01-23-2015, 02:27 PM   #25
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they dont run good with coolant in the cylinders and on the plugs, they end up fouled
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Old 01-24-2015, 06:35 AM   #26
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Compression test on agenda - just have to get the opportunity. I guess this done with cold engine if it should be cold to remove plugs.

Fuel pressure was checked - 45 psi I believe.

ST-125 - 2/5 coil replaced, I was told 5 had weak spark. #5 plug replaced, although looked good, wires swapped at coil. 85,000 miles on engine.

Checked for Vacuum leaks. The shop I had it to has an ultrasound and a "smoke" machine. I don't know which they used. If low compression and oil doesn't bring up can smoke be useful injecting into spark plug hole? Leaking intake valve should show smoke out throttle, exhaust - would it make it out the tailpipe?. Is there an adapter for this? Way back when ..the 60'*.. I used a spark plug to air compressor adapter to hold valves in place to replace valve stem seals on my slant six Dodge. I don't remember if I made it or was off the shelf. Or would using smoke machine be worth the effort, have to rip the head off anyways. If doing one head - do both??
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Old 01-24-2015, 10:18 AM   #27
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valves and stuff like that normally make noise, and given its a pretty low mile engine and the amount of those engines that fail the gaskets i wouldnt bother paying a garage for all those tests when you can get the felpro kit they have for those engines for like a hundred give or take some. the kit comes with both gaskets, would definitely do both and check them for straitness some warp after you let the bolts loose and have to be machined to put back on. i have heard some on those if decked have pushrod length issues, you may have to get good used ones if yours warp

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Old 02-13-2015, 02:48 AM   #28
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It'* Fixed. Runs great.

What was the fix? A 12 gauge shotgun shell.

A brief recap. P305 but not affecting driveability. I bought 1 NGK spark plug and a ratchet strap and was going to replace #5 plug. I abandoned that idea, my abilities not what they used to be.

Several months later SES Light flashing and loss of power under load, ok in open loop mode - i.e. 1st 30 seconds after startup and full throttle. I took it to a small shop and gave them the new NGK spark plug, they said weak spark on #5 replaced 2/5 coil, suggested replacing fuel injector. I balked.

Suggested here next step compression test - I agreed. Took to shop #2. I was called and told compression 160 psi all cylinders, plugs and cables needed replacing - I consented. Next day ready to go. Also replaced #5 injector, had taken plenum off as easier to get at plugs might as well replace while there. Minor cleanup of MAF, throttle body and electrical connections.

I was shown plugs, all 6 AC Delco - no NGK?. The one from #5 had some build up of deposits. Attached 2 photos of this plug.

The bill was shell shocking.

Lesson learned - don't get old.
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Old 02-13-2015, 08:43 AM   #29
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oem is delco platinum, ngk in my experience doesnt last as long. i thought the first thing you did was replace the plugs?

those body styles are just hard to work on its much easier to work on the wbodies with the tilting engine or the h bodies that have a big enough gap back there you can get at stuff.
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