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Bucking / Missing Bad At First Then.........


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#1 HDChuck01

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Posted 18 November 2009 - 12:20 PM

Hi All,

Here's the story..was getting code P0340 camshaft position sensor. Replaced it and then started getting P0300 a couple of times, cleared it off then started getting code P0303 consistently.....cleared those too. In the morning at first start I let it warm up a minute or so then drive and then it misses / bucks real bad.....about 8 -+ miles into my commute smooths out and seems to run fine, no codes, no misses, seems good. When I replaced the camshaft position sensor I also replaced the distributor and rotor. It was acting up after that with the P0303 so I figured cylinder 3 may be fouled. Replaced all plugs and they all looked a little worn none fouled up so she has new plugs, cap and rotor and has been doing the morning miss / buck and does it on the way home too. Next up are a new set of wires but I don't know if that will clear it up. Any ideas where to go from here? Fuel injection? O2 sensor? (never changed) WTF? Any ideas are always appreciated! Thanks! 1996 GMC Sierra 5.7L automatic trans 172K miles owned since new.
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#2 95Sierra2500

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Posted 18 November 2009 - 12:57 PM

Your motor has the CMPFI where you have all 8 injectors inside the intake plenem, then 8 poppet nozzles. Your poppet nozzles are probably getting gummed up. I had this same issue on a '99 Jimmy with the 4.3, identical setup minus 2 injectors. Go to your dealer and have them perform a 'top engine cleaning' service (costs about $150 or so and takes 1-2 hours). Cleared up all the problems I had with it.

Just so you know - this is a KNOWN ISSUE for GM and this injection system. However, ONLY in California is there a recall on it. Supposedly it 'doesn't affect' other vehicles, even though I've heard of it throughout the nation.
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#3 Draenor

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Posted 18 November 2009 - 12:59 PM

Change the wires. And let us know if it cleared it.


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#4 Draenor

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Posted 18 November 2009 - 01:03 PM

Your motor has the CMPFI where you have all 8 injectors inside the intake plenem, then 8 poppet nozzles. Your poppet nozzles are probably getting gummed up. I had this same issue on a '99 Jimmy with the 4.3, identical setup minus 2 injectors. Go to your dealer and have them perform a 'top engine cleaning' service (costs about $150 or so and takes 1-2 hours). Cleared up all the problems I had with it.

Just so you know - this is a KNOWN ISSUE for GM and this injection system. However, ONLY in California is there a recall on it. Supposedly it 'doesn't affect' other vehicles, even though I've heard of it throughout the nation.

^ was my next guess, but the plug wires were a cheaper solution. Plus if they were never changed, now would be a good time. As for the engine cleaning itself, I would try sea foaming it first before going to the dealer.


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#5 Rod R

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Posted 18 November 2009 - 01:09 PM

... your P0300 code indicates random misfires and is a symptom of malfunctioning Vortec injectors (Sequential Central Port Injection - SCPI)

... it affects all of the small block Vortec engines of those years (4.3L, 5.0L and 5.7L) .... no matter where you live in the US or Canada..

... it's just that when these poppet valves start acting up, then the engine cannot meet California emission regulations, and the State of California forced GM to pony up for the injector service under warranty, and ultimately for injector replacements if necessary....

... everywhere else, you are on your own... thanks GM!

... the injectors generally work well, until they start to develop deposit buildup... you just have to keep them clean, after getting a professional high pressure injector service flush, run a good injector cleaner through a tank of gas on a regular basis

Edited by Rod R, 19 November 2009 - 11:30 AM.

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#6 HDChuck01

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Posted 29 January 2010 - 10:55 AM

Well, still running bad / missing. I'm wondering if the timing is off? Did a search on here to see how to adjust the timing and or check the timing on these motors (1996 5.7) Is there a way to check the timing. I changed the camshaft position sensor and when i did I had to "jog" the distributor to get the sensor out because there are cutouts in the metal piece on top of the distributor and you have to fit that sensor in there thru the cutouts and jog it to get the old one out to. Could I have thrown the timing off doing that?? I ran a couple of cans of BG44K throught it too and no help. It's weird though I would think if it was timing it would be constant but when it is warm outside it will run like a champ. It seems like if the temp is above 50F it runs great. Some days it runs like there is nothing wrong with it at all and others it misses real bad :crackup: I'm totally stumped. I just think it should be a constant issue if it is timing, injectors etc.....Any help is appreciated
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#7 Mike G

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Posted 01 February 2010 - 11:20 AM

Timing could be off, just like an old-school 350. I am not too sure of the procedure on the Vortec motors, but there is an advance lead you need to unplug and then set to 0 degrees on the 92's motor. If you have a favorite garage in the neighborhood, they can have at it for you, probably free of charge. What part of town you located in?
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#8 HDChuck01

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Posted 01 February 2010 - 02:10 PM

Timing could be off, just like an old-school 350. I am not too sure of the procedure on the Vortec motors, but there is an advance lead you need to unplug and then set to 0 degrees on the 92's motor. If you have a favorite garage in the neighborhood, they can have at it for you, probably free of charge. What part of town you located in?
--Mike


Up north Berthoud. Wouldn't it be a constant issue though?? I'll try a garage but it seems like it could be a dozen different things?

Edited by HDChuck01, 01 February 2010 - 02:12 PM.

2000 Jeep Wrangler 4.0L
2007 Chevrolet Silverado 5.3L
2009 Scion tc 2.4L
2010 Toyota Rav4 V6

#9 Rod R

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Posted 01 February 2010 - 05:49 PM

.. to set the timing on these Vortec engines you need a scanner that will monitor the cam retard and then set the timing at a specified idle rpm (in my mind a job for a mechanic shop).... any other way and you are just guessing at the timing setting. (this is different from Mike's '92 TBI engine above)

.. if you think that something is temperature related then could could check (or just replace) the Coolant Temperature Sensor (CTS) located at the top front of the engine right beside the thermostat... it is not an expensive part... it tells the Powertrain Control Module (PCM) the temperature of the engine block so the PCM can vary the fuel mixture... although when they fail the engines will often run ok when cold and too rich when warm.

- you did not say whether you got a good injector cleaning... sometimes these injectors can get so gummed up a cleaning won't do the job and they need replacing...

.. the other thing to check is your fuel pressure ... these Vortec engines need 61-66 psi continuously for the injector poppet valves to function correctly

... just passing on some of my experience gained over the last year.

Edited by Rod R, 01 February 2010 - 05:49 PM.

Rod Ramage, Saskatoon, Saskatchewan CANADA
1997 Chevrolet Cheyenne C2500 xcsb w/ 5.7L Vortec & 4L60E; 3.73 gears w/ G80 (231,000 km)




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