New to the forum but hi. I have a 96 sierra 1500 sle with a 350 tbi in it. Was in it when I got it but it flutters like it's going to die after i let off the gas. There's a strong gas smell too only when that happens. What do I do to fix it. Thanks.
350 tbi fluttering after letting off gas
- Thread starter montysdad
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Molson02536
New member
This is kinda confusing, 1996 Sierra is a Vortec engine and don't have a TBI (Throttle Body Injection) but uses a CSFI (Central Sequential Fuel Injection) system.
1996-2002 Chevrolet and GMC C/K, M/L, G, P Models with 4.3L, 5.0L or 5.7L Engine (VINs W, X, M, R -- RPOs L35, LF6, L30, L31) All run the CSFI system. A deposit build-up on the CSFI poppet valve ball and/or seat may cause the poppet ball to stick open or closed. In either case, the specific cylinder will be mis-fueled, resulting in a cylinder mis-fire condition. These symptoms may be intermittent. The Service Engine Soon (SES) light may also be illuminated. Current misfire or history misfire codes may be detected with the Tech 2 scan tool.
Cleaning the CSFI injectors is the preferred repair, the CSFI unit should not be replaced until cleaning has been attempted. If this did not help fix the issues, then the CSFI unit can be replaced with an MFI unit.
The first step of this process is to use the J 41413 Evap Pressure/Purge Station, in conjunction with the J 44466-10 pressure regulator/hose assembly, to provide the required high pressure (150 psi) source to perform the unsticking procedure. The J 39021 fuel injector tester and accessories provide the means to "energize" the injector. Some later model-year vehicles may utilize the Tech2 to "energize" the injectors.
There is a new MFI fuel system upgrade to replace the CSFI fuel system, the MFI fuel system eliminates the CSFI poppet valve ball and seat. The CSFI fuel system is replaced as a unit with the new MFI system and you can pick it up at Auto Zone, Napa, and the Dealer of course, and takes about half the day to do it.
The other issues with the 96-97 vortec (un-confirmed other years as well) GM engines have a problem with false codes and misfires, blown coils, arcing and alike. The problem with the dealer is the data they get or don't get is WRONG because the scanner only shows DTC codes. When when these engines reach 80,000 or so miles the "Knock" sensor retards the timing (thinking it's pinging) due to increased engine noise up to 28 deg thus causing cross firing inside the dist cap and a slew of other issues.
In my case, the new expensive coil wire arced to the PCM sensor wires causing may sensors to indicate random readings such as TPS at 80% during idle. There is a GM Tech Bulletin #76-65-04 regarding a software update, the MIL light and DTC codes MAY or MAY NOT show, and WITH or WITHOUT real faults.
Start with the simple and try cleaning the poppet injectors first, you can try running fuel injector cleaner through your fuel system for a few fill ups. If that is not working, you can try the old school way of cleaning the system and getting a higher octane rating by adding Acetone.
I have found that adding Acetone actuall DOES increase gas milage by at least 20% when used in both carbuerated and fuel-injected engines. Make sure it gets well mixed into the fuel tank. I have not NO discernable degrading in either the stock fuel lines (GM in my case) or aftermarket fuel lines and hoses. This is most likely because the ratio of Acetone Gasoline is very small 3oz to 10 gallons of gas is so small, you need no more then that, Any more and you may be asking for issues.
Used to add Acetone to my old school S10 that we installed a 98 Vortec 5.7L 350 V8 motor and hit the drag strip on the weekends 
OK got carried away, but start with the inspection of the fuel system and then check with your timing after you find out what year your Sierra is and what type of fuel system it's running
Molson02536
New member
Going to be hard to trouble shoot this one, the OBDI which runs on the 1995 engine and the 1996 is a OBDII system and will not run on the 1995 TBI system. OBDII system does not support the TBI system to operate the engine.
If you do have a OBDI system in your Suburban, here is a link for you to retrieve the DTC codes that may indicate why your engine is running poorly the way it is. Keep me posted on what you have for operating system
http://www.troublecodes.net/GM/
Molson02536
New member
Start with the simple and replace your fuel filter, The other thing also to note is to check your vacuum lines to your fuel pressure regulator and inspect your regulator for any signs of gas leak.
Your 5.7 TBI engine runs it's best at 14 PSI, so you may want to replace your fuel pressure regulator with a adjustable fuel pressure regulator to fine tune your fuel pressure to ensure you are at 14 PSI. Any more or less will then 14 PSI will cause your engine not to perform at it's peak performance.
Keeping your TBI also cleaned will help smoothen out your engine's idle and give you better responsive to your throttle inputs. To give you the most out of your 5.7 in performance and economy, do these simple things.
- Replace your fuel filter.
- Replace your fuel pressure regulator and ensure you have 14 PSI.
- Install a TBI spacer to give your throttle body a better fuel atomization before entering the intake.
- Clean your throttle body and upper intake with throttle body cleaner and Seafoam for your upper intake.
- Install and better free flow intake. K&N air filter or cold air intake.
- Install a good Cat Back Exhaust
- Install a performance PCM power chip.
Custom http://wait4meperformance.com/ http://www.pcmforless.com/index.php?option=com_wrapper&Itemid=324
These small thins should get you around an extra 40 HP and better fuel economy if you can keep your foot off the skinny peddle.
As mentioned, start with the simple and replace your fuel filter. A restrictive fuel filter will cause your engine to have issues as your have describe, since your fuel pressure is less then 12 PSI and the TBI as very sensitive to fuel pressure changes. As mentioned, the 5.7 TBI runs beat at 14 PSI and anything more or less with cause you to have issues as your having and will show no DTC codes.
Good luck and keep us posted.