1996 G.M.C. 5.7 vortec SLT

kaliningJuly 21, 2008

John, and anyone please chim in. This vehicle has a po306

code very briefly on start up. goes away when warm. about

5 mins. comes back again for a few seconds when hot. noticable stumble both times. Will not always set a code

but you will hear it stumble and feel it when driving.

G.M. says an injector problem. They were all changed. Ign.

system, ok. pump pressure ok, voltage ok, Leak down test ok, compression ok, does not over heat. This truck has the two piece manifold aluminum and plastic. Isn't this vehical

also subject to manifold leaks like the smaller vortecs ?

Truck is not mine and it is not here for me to see. problem has been going on for some time. Nevers gets better

never gets worse. #6 plug will show some black fouling.

Entire ignition system has been gone through and obvious

parts changed out. Well guys, or gals. What do you think ?

Thank you for reporting this comment. Undo
john_g

First, what does a vacuum gage show during the missfire?

The bottom line here is I can envision any number of reasons for this. Proper testing will reveal the cause.

    Bookmark   July 21, 2008 at 11:17AM
Thank you for reporting this comment. Undo
jemdandy

Trouble code PO 306 for 1996 reads:
Cylinder 6 Misfire Detected.

Could be a number of things. A couple of suspects are spark plug or spark plug lead.

Here is a link that might be useful: Code PO 306

    Bookmark   July 24, 2008 at 1:09AM
Thank you for reporting this comment. Undo
kalining

Everything has been changed including the injectors. scan
tool shows #6 misfired 3500 times. Other cylinders misfired
any where from 35 - 100 times also. there is nothing left to fix. The engine light comes on at first start, sometimes. The engine stumbles, light comes on. engine light slowly goes out. Yes it dims out. as engine warms to
normal it will stumble again. Light dimly goes to brighter
Never seen that. Light usually come on or goes off. this
truck has a centerlized injector. Looks like a spider sitting on the manifold. replaced by G.M. they said that was the problem. NOT. I guess this guy will have to live with it.

    Bookmark   July 24, 2008 at 10:18AM
Thank you for reporting this comment. Undo
jemdandy

By centralized injector, are you referring to a throttle body injector?

Does this engine have an aluminum intake manifold nested between the two v-banks of cylinders? If so, check the tightness of the mainfold bolts. Put a wrench (or socket) on a few of these bolt and attemp to turn in the tightneing direction. Are these easy to turn, i.e., not tight? During this test, do not turn the bolts more than 1.4 turn else you may warp the manifold. It has a specific tightening sequence much like an engine head. If you find that more than one bolt is not tight, it was likely leaking and needs a new gasket set.

Refer to a repair manual for installing a gasket set.

I believe there are coolant passages in this manifold. check the condition of your engine oil. Pull the dipstick and look at the oil. Does it have normal color and appearance, or is it sort of brown and a little syurp-like and has the oil level risen? - if so, coolant may be leaking into the crankcase via the intake manifold gasket.

On an engine of this age, check all of the vaccuum lines and fittings. Do not interchange fittings because some of these may be ported (have built-in orifices of specific restrictions).

[John_G would likely make a variety of vacuum tests as part of his diagnostics. Me - I'm only making blind suggestions based on what might be the matter. It is too difficult for me to know what to look for without seeing the engine and its behavor, and without a repair manual that shows the details of the engine control layout.]

    Bookmark   July 25, 2008 at 12:23AM
Thank you for reporting this comment. Undo
john_g

Hi Jem.

Centralized injector means this has a single fuel injector, and then eight spiders with poppet injectors coming off of that to direct the fuel right to each cylinder. The poppets require a pressure differential of 56PSI to open. One of them could stick open or closed, and have an impact on that cylinder, so this engine could be lean on just one cylinder in this case, which would be #6.

The reason I asked for a vacuum gage is I want to know if the valve train is truly functioning correctly or not. A valve can close fast enough to give good compression test readings when cranking the engine. But when it runs just a little faster, and heat from combustion comes into play the valves can stick ever so slightly. It';s quite common to occur in the center cylinders on each bank, and will occasionally move around the engine as the "missfire" causes exhaust gases to be leaked back into the intake and have an effect on a companion cylinder.

As always, test, don't guess. There is no sense pulling the heads righ now and sending them to a machine shop unless this issue is completely confirmed.

    Bookmark   July 25, 2008 at 9:05AM
Sign Up to comment
More Discussions
Honda Odyssey 2005 EX-L Fire
Hi All, My Honda Odyssey 2005 EX-L recently totaled...
mhs21
pump keeps stopping when trying to fill gas tank
Hi! Can someone give me a clue what might be suddenly...
twinklenose
Blown tire on a dying car........
Well, in my other post I talked about how my transmission...
catherinet
Warning:Quaker State Oil Filters
Better check your filters really close. I used to simply...
andyf
Power window goes down, but not up
The driver's side window on my 98 Chevy Venture would...
hardlyclever
Sponsored Products
Tamburo Pendant by Besa Lighting
$558.00 | Lumens
Kenroy Home Outdoor Lanterns. Custom Fit 1-Light Adjustable Height White Wall La
$68.40 | Home Depot
Queeny Matte Chrome One-Light Diamond Wall Sconce with Red Glossy Glass
$240.75 | Bellacor
Emerson Ivory/ Smoke Rug (5'0 x 7'6)
Overstock.com
Tall Ombari Honeycomb Vase - WHITE
$149.90 | Horchow
Feiss Geary 32" Wide Framed Round Wall Mirror
Lamps Plus
© 2015 Houzz Inc. Houzz® The new way to design your home™