Truck starts only when starved of air

I have a 1998 Chevy 1500 2wd 1/2 Ton pickup with a 4.3 ltr vortec V6. The engine will turn over and I get spark on the plugs. I have fuel in the carb and when I turn the vehicle over the engine will crank but not start. As a test we covered the carb opening completely and cranked the engine, after a few cycles it started and would stay started as long as we feathered air through a small opening, if we let too much air in then it will die.

I am not getting any check engine lights. When the key is turned on the fuel guage will go all the way around to about 4 oclock and then back to the fuel level with a vibration in the dial indicator. I do hear a noise that sounds like the system is being pressurized.

I am leaning toward a O2 sensor or Catalytic converter but do not have the indicator lights or the rotten egg smell. Any ideas?

Thanks in advance,
Jeff Did you put a carb on the truck? That year should be fuel injected with a central multiport fuel injection system. It is the original equipment. I call it a carb but what you say is probably really what it is. Large opening with a silver valve set inside that levers when pedal is pushed. Also to ammend getting the truck to start we had to have the pedal floored.

Thanks,
Jeff What you are referring to is the throttle body. Inside the plastic manifold there is a fuel injection unit that supplies the engine with fuel. The fuel injection unit is controlled by the engine controller or PCM ( Powertrain control module). Putting your hand over the throttle body opening restricts the air going into the engine. Unfortenetly you have to take the air ducting off to do this. In the air duct there is a sensor called a MAF (Mass Air Flow) sensor. The sensor is designed to measure the amount of air flowing into the engine and sends the signal to the PCM for fuel and ignition calculations. The engine will not run or run really badly without the MAF sensor and ducting connected. With the ducting and sensor connected how does the engine run? Also make sure that all hoses are connected to the air duct. When all hoses and the air ducting is connected to the throttle body the truck will not start. It will crank, and fuel is being delivered, (I can see the wetness within the throttle body), but will not start at all. Prior to the truck not starting it seemed to have a miss occassionally at highway speed. Can you smell raw fuel inside the throttle body? Sounds like it is flooded. That would explain why it will run when you hold the pedal to the floor. Yes there is a strong fuel aroma. If you can smell raw fuel then you have a fuel leaking from the fuel pressure regulator, injector unit, or pintle hoses. You will have to take the upper intake manifold off to verify this. Take note of the o-rings, washers, and spacers when you take the fuel pipes off of the top of the manifold. Once you get the manifold off then reconnect the fuel pipes to the injector unit and turn the key on but not to start. The fuel pump should come on for a few secs. You should be able to determine where the fuel leaking is. The regulator and pintle hoses are serviceable and are mounted on the fuel injector unit. The fuel injector unit has one fuel injector per cylinder to feed gas to all the cylinders through the injector hoses. The hoses have a pintle valve on the end that are designed to open at a specific pressure, just as a diesel engine fuel injector does. The pintle poppet design was common for clogging up with carbon and causing multiple driveablility problems, such as misfires. There is a new design injector unit that now has the injectors on the end of the hoses that eliminates the poppet valve design and is a whole lot more reliable. If you plan on keeping the truck and don't mind spending some extra money, it wouldn't be a bad idea to convert it.

Newly designed injector unit with injectors for each cylinder


Fuel pressure regulator


One of the old style pintle hoses and the injector that goes inside the unit..there are 6 on your unit
I am leaning toward the MAF since the truck will start and run when I restrict airflow through throttle body. If it is the injector unit then I will take it to a shop.

Thank you,
Jeff What do the spark plugs look like? If you have a fuel pressure gauge, you can verify if you have a fuel leak without pulling the manifold off. That is strange that the engine will run if you have you hand over the throttle body. How does it run when you do this? Is there black smoke coming out the tail pipe? Usually on a carb, if it ran with your hand over it that meant that you have a vacuum leak. What happened when it failed? Were you driving and it quit or did it just not happen to start one day? MAF sensors are pretty robust and rarely ever fail. The only problem that I have seen on MAFs are the little resistors get dirty or the diffusor screen gets garbage on it. I changed the plugs first thing, they were dark but not really too bad. I rented a code machine and replaced the throttle position sensor today since it was throwing a code. The fuel I am smelling may be coming from the exhaust. What was the code for the throttle position sensor. Were there any other codes?