1998 Chevy Silverado 3500 7.4 starts hard when warm
Recently purchased this vehicle and have replaced the fuel pump, spark plug wires and plugs, cap, and oil pressure sender. We have had it into the shop and on a computer but no codes come up. It acts like it is flooding when you try to start it while the motor is hot. I am at a loss. Thanks!
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Re: 1998 Chevy Silverado 3500 7.4 starts hard when warm
I'd check coolant temperature sensor, it should be at center, front of engine. Not 100% sure on your vehicle, but on some I've worked on, the coolant temperature sensor was faulty, basically telling vehicle computer that engine was still cold, so dump more fuel.
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What's missing ? Fuel ,spark , compression ? How many miles on the vehicle ? Does it have spark , pull a plug wire while cranking an hold it near a good ground . Put a fuel pressure gauge on the vehicle , is there fuel pressure ? Do a compression test , 150 psi . engine mechanical problem . timing chain .
Very old vehicle, and highly likely it needs a new fuel filter and major tune-up (including all new spark plug wires, spark plugs, cap, rotor, and ignition coil).
Verify proper fuel pressure at the fuel rail, and service fuel pump and fuel pressure regulator as necessary. If you have been using ethanol blend fuel in the vehicle over the past few years, I recommend removing ALL FUEL from the vehicle and replacing it with new. Please read my "Tip" on Fixya regarding the problems with ethanol blend fuels in older vehicles (such as yours).
You've already eliminated the spark plug & wireset, distributor cap & rotor. Sounds like your car needs a tune-up, specifically, the PCV (positive crankcase ventilation) valve, and/of the EGR (Exhaust Gas Recirculation) valve.
Components checked/replaced during a tune-up are: air filter motor oil motor oil filter breather filter fuel filter(s) fuel strainer PCV valve EGR valve spark plugs spark plug wires ECM or distributor cap & rotor cabin air filter transmission oil filter transmission oil engine coolant check all vacuum hoses and lines
Troubleshooting the no-start condition - from the 1990 Mitsu Mighty Max Repair Manual:
Priority Action Part Type -- Cause 1 Inspect Battery -- Battery Discharged or Faulty. 2 Inspect Battery Cable -- Corroded, Broken, Shorted or Poorly Connected Battery Cable. 3 Inspect Distributor Cap -- Distributor Cap Cracked or Burned. 3 Inspect Fuel Filter -- Clogged or Dirty Fuel Filter. 5 Inspect Ignition Coil -- Faulty Ignition Coil. 6 Inspect Wireset -- Worn, Damaged or Faulty Spark Plug Wire(s). 7 Inspect Spark Plug -- Incorrectly Gapped or Fouled Spark Plug(s). 8 Inspect Starter -- Starter Motor Faulty. 9 Inspect Fuel Pump -- Low Fuel Pressure. 10 Inspect Solenoid Switch -- Faulty Starter Solenoid Switch. 11 Inspect Fuel Pump Relay -- Faulty Fuel Pump Relay. 12 Inspect Fuel Injector Pressure Regulator -- Faulty Fuel Injector Pressure Regulator. 13 Inspect Ignition Switch -- Improperly Connected or Faulty Ignition Switch. 14 Inspect Starter Drive -- Faulty Starter Drive. 15 Inspect Fuel Injector -- Dirty or Worn Fuel Injectors. 16 Inspect Fuel Pump Strainer -- Clogged or Dirty Fuel Pump Strainerat. 17 Inspect Engine Control Computer -- Incorrect Operating Information Being Delivered and Sent From The EEC. 18 Inspect Fuel Tank -- Fuel Tank Empty. 19 Inspect Carburetor -- Carburetor Flooded or Faulty Choke. 20 Inspect Distributor -- Worn, Loose, or Incorrectly Installed Distributor.
Carburated vehicles ether have an automatic choke, or the very old ones, a hand choke. Yours has an automatic choke. The starting problem likely is a problem with the choke. The problem is almost certainly not the fuel pump.To set the automatic choke, push the accelerator to the floor, and RELEASE. Then start the engine just like you would a fuel injected vehicle. If this doesn't solve the problem, then the automatic choke needs to be repaired.
You have three choices. First, check the spark and coil system to ensure you are getting a spark. Pull off a plug wire from the coil pack and crank. Have someone watch to see sparks snapping off the coil pack. If you don't have any replace the control module under the coil packs.
Second, you may not be getting enough fuel pressure. Put a gauge on the fuel pressure test port. You may have a bad oil pressure sensor, located down above the oil filter. Check that oil pressure sender and make sure the connector is pushed in. The power to the fuel pump runs through the oil sender. If the engine starts and oil pressure doesn't come up quickly, the computer will shut off the fuel pump. Check this and you can also manually engage the fuel pump from the relay block down by the passenger feet.
Third choice, and it's a long shot, check the harmonic balancer and the crank position sensor. If the sensor is bad, the computer has no idea when to fire the spark plugs. A magnet is on the harmonic balancer and flys by a sensor to induce a voltage in the coil and tell the computer what position the crank is in.
It sounds like you already solved your problem. Attach fuel gauge to fuel rail pressure line and monitor for fuel pressur (above 30 p.s.i. @ crank). If pressure low suspect pump. If pressure o.k. but dies out and pressure drops but restarts, check oil sending switch as this controls fuel pump. Computer primes pump for start, then oil sender powers up pump for normal operation. Check for oil leaking at terminals on sender. Also check plug connector at fuel sender on tank as they are known to melt and fail. If fuel not problem, check for spark as ignition module failure is common. Have obd2 diagnostic run to check for codes as this may help with diagnosis.