1999 Century 3.1 Rough Idle Full Throttle ok - Page 2 - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 01-13-2015, 04:38 PM   #11
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Even though you checked the coil, should check coil output at end of each wire with a spark tester....

All you have to do is disconnect the injector harness coming out from under the plenum, to do a resistance check on each resistor....

The schematic will give you the colored wires to each injector.....pink is the common branch for all.....so #5 five circuit would be pink wire and black with white stripe....attach your ohm meter to those two wires in the harness going under the plenum.......can check the rest for comparison.....should all be around 12 ohms....

I agree with JW.....if you should think of swapping injectors, you should replace the o-rings.....

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Old 01-16-2015, 06:29 AM   #12
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Tech II
Thanks for the schematic. However, I can not find the connector for the injector harness. It must be under the plenum. I have seen suggestions to check injectors using a stethoscope. That probably the least invasive way, however, #5 looks like might be difficult to get to. That being said I agree the injector is not the most likely problem as under first start up and full throttle performs ok.

It is over a week of being driven every day and the I/M Readiness Monitor shows: CAT() EVAP() O2S() EGR() all INC.

The scanner I have is an Autel AL439. I have discovered I have to go through each test manually to get all the data (still doesn't) into it so that it can be printed/viewed on home computer, it easy to miss things.

No EGR off-idle test
ID() 0C MOD() $10 TEST() 255.824 MIN() --- MAX() 0.000 STS() Fail

EGR decel test **
ID() 0D MOD() $10 TEST() 44.938 MIN() ---- MAX() -44.997 STS() Fail

EVAP canister loading test
ID() 04 MOD() $10 TEST() 0000 MIN() 0000 MAX() ---- STS() Fail

Evap Purge Leak Vapor Fail ** Test 0 Max 0
Evap Purge Leak Vacuum Fail ** Test 3276.8 Max 3276.8

** = Engine Off

The EGR valve appears to be the older type i.e. the up to 1999 one. The only DTC code is P305, No EGR codes and the Idle is not "rough"

Do these shed any more light on the problem(*).

John
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Old 01-16-2015, 08:24 AM   #13
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no, it wont try those with the misfire code. the ready monitors go in a list. misfire is first. if any test fails it doesnt bother with the rest.

did you check the compression?
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Old 01-16-2015, 02:11 PM   #14
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The harness connector should be located right behind your alternator...
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Old 01-17-2015, 02:09 AM   #15
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Quote:
Originally Posted by jwfirebird View Post
no, it wont try those with the misfire code. the ready monitors go in a list. misfire is first. if any test fails it doesnt bother with the rest.

did you check the compression?
jw - ready monitors not being tested - seems valuable diagnostic info would not be available - not very helpful.

There are several monitors that are reported as passed i.e.:
MIS() OK
FUEL() OK
CCM() OK
CAT() INC
HCAT() N/A
EVAP() INC
AIR() N/A
O2S() INC
HTR() OK
EGR() INC

Compression - no. Will have to wait until I can take to shop - may be a week or two. At this time I cannot take it out of service. Yes I know it should only take about 1/2 hour BUT undoubtedly would be leaving over night and shops are 1/2 hour away.
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Old 01-17-2015, 02:19 AM   #16
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Quote:
Originally Posted by Tech II View Post
The harness connector should be located right behind your alternator...
Ah yes I see it now or about 3 connectors - a little awkward.

I still have trouble seeing how it can run great at start up and have a bad cylnder. I can shut the engine down at a red light - re-start and it will climb an incline fine at speed for about 30 seconds then it loses some power.

It would not surprise me there is more than one problem. The P305 was up with steady SES for months with little to no driveability issues and then the SES started to flash under load with loss of power but not at startup or at full throttle.

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Old 01-21-2015, 11:56 PM   #17
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I have recorded Live Data with my scanner to try to see why the engine runs good for maybe 30 seconds (I haven't timed it) and then loses power. I have two examples in the attached JPG.

To me the most notable is the O2 sensor at .9v (Rich I believe) at startup and then after a period of time the voltage drops. The Spark Adv also changes.

Does any of this add up?

Thanks
John
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Old 01-22-2015, 08:32 AM   #18
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your ect looks funny, should read engine temp not 80 or whatever that will cause fuel issues
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Old 01-22-2015, 08:33 AM   #19
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unless the scanner reads c
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Old 01-22-2015, 11:15 AM   #20
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The 80 probably is centigrade since the speed is km/hr.....

On startup, the car will run rich, as indicated by the O2 sensor, and then when it goes into close loop, you are getting a lean condition, probably from the misfire....

Funny thing about misfires....because of them, the O2 sensor will detect unburned oxygen, and because of that the PCM will command the injectors rich, which can cause a worse misfire, contaminate the O2 sensor, etc.

If the SES was flashing, that means you have a misfire that is bad enough to cause cat damage......first thing I wold do is fix the misfire....could be ICM, coil, wire , plug.......or it could be a cylinder problem......

Does your scan tool show current misfires or have a history of misfires in a cylinder?

Are you losing coolant?
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