Hard starting 1996 GMC 2500 w/ 7.4l engine

Sbrake · May 23, 2019

Working on a Hard starting 1996 GMC 2500 w/ 7.4l engine. Disconnected battery, changed failing fuel pump & in line fuel filter. Cleaned MAS with MAS cleaner & cleaned Throttle body with Throttle body cleaner. Reconnected battery after about 30 mins. Now engine won't start. Checked for spark at coil ... good spark. Bled air from fuel lines under pressure from fuel pump. Verified fuel pressure at 60 psi. Still no Start. Connected scan tool. I have only one code -- PO338 - CKP Sensor Circuit High Frequency. I'm going to T/S the sensor & wiring. Probably replace the sensor.

Replaced Crank sensor with OEM Sensor, checked clearance between the crankshaft reluctor pawl and the sensor for .5 mm clearance, checked wiring & fuses. Cleared the code, then ran the OBDII again... No codes, but engine won't start. I noticed the Tach on the dashboard is not moving when I crank the engine. Possible Camshaft Position sensor or faulty distributor?

I checked spark at the #1 cylinder so its unlikely that the distributor is bad. Checked rotor, cap & the Cam Position Sensor. CPS was bad... replaced with OEM sensor. Rotor and Cap are good.

Engine will still now start, but have a new problem that I didn't have before. Engine runs as good as before, but after driving on the road for about a mile it starts to act like it is running out of gas. After it stalls out & sits for about 2 mins it starts back up and will drive for about another mile. I know that the tank has about 1/4 full, I checked the lines at the top of gas tank for crimps, but all ok. I put the scan tool back on but no codes. I checked the current for the fuel pump while running (not driving). If new fuel pump is faulty I think I'd see excessive Amp draw but it reads 1.5 Amp DC on a circuit with 20 amp fuse from factory, verifying good fuel pump. Any ideas??

Edited May 28, 2019 by Sbrake
Still no fix

riverbanks · May 23, 2019

Should you unscrew front o2 sensors and test drive, possible failing cat(s)?

Doug_Scott · May 24, 2019

Take a good look at the underside of the rotor. If there is any black, replace it. If the exhaust is blocked you should be able to get the truck to fail within seconds. Start up engine, pull straight out onto road and floor the gas pedal. A partially or mostly plugged exhaust will show up instantly.. To validate its exhaust, put fuel pressure gauge on it and watch fuel pressure when it starts running bad.

Sbrake · May 28, 2019

SAME 1996 GMC 2500 w/ 7.4l engine still stalls when putting on the brakes and decelerating. Just a note... {The brake system of this truck has a hydraulic booster instead of a vacuum booster and runs off of the power steering pump. It has no leaks and the pump is filled with power steering fluid. So no possibility of a vacuum booster leak} I replaced the PCV valve, Idle Air Control and Throttle Position Sensor. Test drove the truck. When I put the brake on, the RPMS slowly drop even after coming to a full stop and releasing the brake pedal. Pressing on the gas pedal does nothing and then the engine stalls. The Scan Tool was attached during the whole test drive.... No Trouble Codes. Still looking for the problem and the fix.