1996 Chevy 350 Misses at idle

[Schnikke]

I recently bought a 96 Chevy, it has a miss fire at an idle and throws a p0300, and a p0303 codes. All the plugs, wires, cap, and rotor are new. It has 129000 miles and compression is good. I would think it would then be a fuel delivery problem at an idle? Any clues as to what it could be? Thanks

[1997SierraSLT]

oh boy....where do we start with the first generation Vortec engine?! All I can say is get used to seeing that bright orange check engine light and the P0300 code (random misfire). First off, GM thought it would be a great idea to come up with the CSFI system coupled to poppet valves located at each cylinder. These poppet valves are notorious for getting stuck. As a result, years later, GM came out with a TSB to completely remove poppet valve design but still incorporate the use of those goofy plastic fuel line hoses coming from the center mounted fuel cluster converting it to a MPFI assembly. The retrofit now placed the injector at each cylinder. I installed this kit about 6 years ago. While it did significantly help, it did not completely solve the problem with P0300 codes. Low end torque however greatly improved.

 

http://gmpartsdirect.com/results.cfm?singlepart=1&partnumber=PK93441235

 

At around 140,000 miles on the odometer, my truck would start to miss terribly at speeds greater than 65 MPH and throw the P0300 code. I did fuel pressure tests and everything came out ok. I finally decided to change out the fuel pump with a new one and that made all the difference in the world. Contrary to popular believe, a fuel pump actually can just wear out. My conclusion was the old fuel pump could develop enough system pressure (55 psi) but just failed to deliver adequate flow in GPM.

 

If you are lucky you won't have the dreaded intake manifold coolant leak issues that have plagued this first generation Vortec engine. Again, not sure what GM engineers were thinking by only using 8 bolts (2 at each corner) to secure this manifold to the engine block using special plastic intake manifold gaskets. I think after about 4 attempts, Felpro finally came out with a decent set that seemed to solve this issue.

 

Good luck!

[mike94silverado]

SCW1991 Covered it pretty well..... I gave up trying to "perfect" my old Vortec..... I just kept updating and it ran pretty well.

[Schnikke]

The intake manifold gaskets, and valve cover gaskets both have been done, what would be the most cost efficient way to prevent damage to the cats? On the highway when I drive it goes perfectly, and wont throw a code, but in town when the rpm drops below a certain point, the truck starts to miss, mostly just sitting at a stoplight. I guess I dont know how to go about troubleshooting it.

[1997SierraSLT]

My truck was a CA truck and the P0300 code was notorious partly due to the fact that there are so many additives in the gasoline to reduce emissions. I'd have the fuel injection system/poppet valves flushed at the GM dealer under warranty and it would solve the random misfire for about 3 city blocks and the check engine light would come right back on.

 

I have gotten into the habit of using a fuel injector cleaner every few months and it seems to help a bit. Costco sells a 6 pack of it pretty cheap. Description below.

 

http://www.amazon.com/Chevron-PRO-GARD-Injector-Cleaner-6-bottle/dp/B002SWM6P0

 

I'd change out the fuel filter as well every 30,000 miles or so. It's located along the frame (inside) in the center of the truck (drivers side). It costs about $10 and is simple to install. Using a set of flare wrenches is preferred.

 

The fuel regulator located at the fuel injector cluster assembly has been a culprit as well either going bad or prone to leaking.

 

With as many miles as you have on the truck, I'd start with fuel filter, fuel injector cleaner additive, fuel pump, then perhaps as a last resort converting to the CSFI to MPFI assembly.

[Schnikke]

Thanks for the advice, Ill give it a try, I can just take it to the dealer and it'd be under warranty to flush it? Ill also get the other things serviced, its not a huge concern now as it only really misses at an idle and has no loss of power. I mainly bought the truck because it has such low miles compared to most 17 year old trucks, and I've had good luck in the past with 350 motors.

[1997SierraSLT]

I think GM warranted the OEM fuel injection system up to 200,000 miles or 10 years whichever comes first.

 

I've never owned anything but GM V8 engines and have sold about 4 cars in my life with 200,000 miles on the odometer without ever having to break deep into the engine. Intake manifold gaskets, timing chain cover seal, and rear main seal replacement is about it.

 

The misfire at low engine speed perplexes me.

 

just thought of another component I changed out which helped alleviate the misfire problem and that was replacing the MAP sensor located on the upper plastic manifold in front of the distributor.

[Schnikke]

OK thanks for all the help! Ill keep you posted on progress