This weekly Q&A session answers your questions about cars and trucks. This week explores a 1992 Chevrolet Half-ton Pickup that has a miss/stumble problem.
Question: 1992 Chevrolet Half-ton Pickup
1992 Chevrolet Half-ton pickup 350 engine, throttle body fuel injection. Miss or stumble then sometimes "burp" back thru throttle body. Computer not coding. This weekend drove it 60 miles then started light miss, then about ten miles did the "burp". At this point goes from light miss or stumble to a hard miss every once in awhile. Times when you get back in after your trip will be all right then other times have that little miss/stumble.
Compression good/new wires/new plugs/cap and rotor replaced. Note: truck is second hand and has had engine work (for some reason harness moved from original attach points on motor. Wire harness is double backed on itself / tied, could this cause insulation damage and/or EMI distortion?
I have not checked coil / distributor.
Note: replaced fuel pump last week hard to start (like an old Chrysler) (built up to 12 psi then drop down to zero) when pump pulled found that hose in tank was "std" external" gas line and had delaminated and swelled, just sitting on pump nipple, went ahead and replaced pump/sock/hose and wire harness. Feel that one thing need to do is check pressure again, not starting right off wants to crank some before firing off.
I am at loss, read every text book / manual can get my hands on, talked to a couple dudes that I know that work for shops (both certified). Their comment is basically start back at tank and start checking/rechecking each component.
Ramblerman
Answer:
Rechecking the fuel pressure is the first thing to do. We need to know that the pressure is good before doing anything else. With the Ignition ON, Engine OFF fuel pressure should be 9-13 psi. Fuel pressure should increase by at least 5 psi when throttle moves from idle to part throttle with the engine running.
Past that you will need to check the ignition coil and distributor cap and rotor. To see if you have any spark 'leaks' get a spray bottle filled with water and a few drops of dish soap. Now spray the ignition wires, distributor cap and ignition coil with the engine running. If you hear or see any arcing then you have spark 'leaks'. If you do this at night it will be much easier to see any stray sparks. Look inside the distributor cap and rotor for any evidence of arcing or carbon tracks.
Then you will need to, very carefully, inspect the wiring harness for cracks, breaks or chafing. Check for vacuum leaks and loose or broken harness connectors. Connectors with broken locks will wiggle loose and not be immediately obvious, so unplug them, make sure they are clean and plug them back in. If you find some with broken locks an appropriate sized wire tie makes a good substitution.
Look into the throttle body and observe the fuel spray. It should look like a constant fine, cone shaped mist coming out of the injector. If you see any drops or a misshapen cone pattern then the injector(s) need to be cleaned or replaced.
With no codes in the computer this is saying it is a mechanical problem and not an electrical problem.
That’s all for this week. Feel free to browse the archives for past topics. And, as always, if you are stuck and need an immediate answer you can always call Vince on the phone.
