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1997 305 vortec cant fix miss!!!

cameronc

By cameronc
in Ask A GM Technician

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cameronc Enthusiast

cameronc

Posted
Posted

1997 305 vortec 5 speed 2wd. The truck starts and idles good, it's runs good going through gears and at wide open throttle. When I try to maintain a steady speed it starts loading up and popping through intake. I have replaced the distributor, tune up, spider injector, fuel pump, mass air, coolant temp sensor, tps and iac. I have a fortune in this truck and it still has same problem. Please give me some ideas guys I'm about to pull my hair out!!!

P.s. if I would get more help in another thread on this site please inform me thanks!

Jsdirt Senior Enthusiast

Jsdirt

Posted
Posted

That is a classic symptom of intake manifold gasket leakage.

cameronc Enthusiast

cameronc

Posted
  • Author
Posted

I've replaced the upper set and I sprayed 2 cans of starting fluid around the intake and it won't pick it up. Do u still think it could be lower intake even if it won't pick up the starting fluid?

Jsdirt Senior Enthusiast

Jsdirt

Posted
Posted

More than likely. At cruise, the engine is at a high vacuum state. Full throttle is next to zero, and idle is somewhere in between. It probably only leaks during high vacuum.

 

You don't have to use something that flammable (and explosive ...) to check for air leaks. Water works just fine - if there's a leak, water will find it. You'll hear the change. The leak won't be large enough there to worry about any damage. Back in the day, we used to pour a thin stream of water right in the carburetor to soften excessive carbon up.

MMC Enthusiast

MMC

Posted
Posted

Hate to ask but did you replace plugs and wires in the tune up?

cameronc Enthusiast

cameronc

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New info today, I double checked for intake leak and some other things and noticed 2 days in a row it runs great when cold. Once it warms up it starts the poor running at light load. Also notices that once it starts the stumbling the idle drops down and when I drive it the rims won't drop between shifts, of I run it up to 2500 to make shift it stays at 2500 when I change gears

Jsdirt Senior Enthusiast

Jsdirt

Posted
Posted

Ain't electronics fun?

 

Usually the opposite happens with leaky intake gaskets - it'll get a little better as the metal expands to seal the leak. Looks like we can possibly scratch that ...

 

 

At this stage you'll have to post some scan tool data - with the electronics on these vehicles, it literally could be one of 10,000 things ... and without the data, we're all just peeing in the breeze.

cameronc Enthusiast

cameronc

Posted
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OK I have some videos and snapshots of it now. If anyone would like to help me I will gladly email them to you, I don't know how to upload them on here. I did notice a few things. When driving and it starts the miss it's in closed loop, I can floor it and it goes to open loop and miss goes away. It has 4 o2's, two of them are reading good but two of them are not reading much at all. The timing advance doesn't seem to change during miss, the maf, map are reading normal. Only other thing is when I'm setting still I can hold rpm around 2000 and it will gradually start to load up and miss, when it starts the miss the timing advance goes up quite a bit like its trying to compensate, but it doesn't change when I'm driving under a load.

Jsdirt Senior Enthusiast

Jsdirt

Posted
Posted

Just go to Photobucket, upload them there, then hit the little picture square right above where you type (below and to the left of the smilie button, in the second row) and paste the direct link to the pic in the box.

 

You can also hit "More Reply Options" in the lower right corner and upload directly to the site, but I have had intermittent luck doing this. Was never able to upload directly ... then suddenly I was .... and now I'm back to Photobucket again. Even the smallest pic gets rejected on this site. I don't get it.

MikeInCtown Enthusiast

MikeInCtown

Posted
Posted

Since you mentioned how it shifts when it's warm, and how the RPMs don't drop when it shifts, etc I am wondering if maybe it actually isn't a trans problem with a solenoid or a converter or something. The fluid will act differently when cold than when it is warm or hot. I only mention this because I had a 1996 4.3 S-10 which had what felt like stumbling, hesitation, shudder when I hit the gas, you name it. Turned out to be the damn center support bearing and had nothing to do with the engine or any of the sensors people kept suggesting.

cameronc Enthusiast

cameronc

Posted
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Those are 3 different videos I recorded, it's hard to hear the miss but if u pay attention to the throttle when it starts missing the worse I will open throttle do 100% and it will start running perfect, if I hold it steady at any rpm it will gradually start to load up and miss. The only thing I can see from the live data is I have a couple 02 sensors not reading well.

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