99 K3500 7.4 missing under load

[ShawnN]

Hello, new member first post. I have a 99 K3500 crew cab dually with the 7.4 Vortec and 4:56 gears. It only has 88000 miles but was obviously worked hard as a farm truck. It runs good but when towing a heavy trailer I am getting a dead miss/cutting out. I just had a new Carter fuel pump & fuel filter installed less than 500 miles ago. No miss unloaded, runs like a scalded dog. I've read about the OE injectors & fuel pressure regulator both being problems and plan to change soon, but I wanted to post and ask if i should check anything else before doing the injector swap. TIA for any input.

 

[Schurkey]

I'd be looking at the ignition system--wiped distributor bushings, weak ignition coil, faulty cap 'n' rotor, faulty plug wires, faulty plugs.

Heavy load increases cylinder pressure, voltage to the plugs increases--and if the ignition insulation has any fault, the voltage will escape somewhere other than the plug gap.

A scan tool might tell you which cylinder(s) are misfiring the most.

 

[83GMCK2500]

Can confirm, oem injectors are steaming piles of canine feces. Get you some of these and don't look back: https://www.fiveomotorsport.com/bosch-454-j-code-024-blue-knight/

Another 4.56 geared truck! Mine too! So many rippems on the highway, but so nice with the 5-speed.

 

[88GMCtruck]

As everyone said, injectors and FPR are high on the list, and a general overall tune up (plugs, wires, cap, rotor). If you have any way of watching data you can see how balanced the engine is based on the oxygen sensors. Lower intake gaskets are notorious for failing around the coolant passages at the age these trucks are, you may have a internal vac leak (would be able to tell if you watched the data) but it's also good preventive maintenance if you are already pulling the top half of the intake for the injectors, you just pull the distributor and lower intake and good to go. Plus you get to inspect the distributor and get a coolant flush out of the deal. When you put the distributor back in you'll want to get it close as possible to 0 TDC and then perform a CASE relearn with a scantool, to sync up the crank and cam for ignition timing.

 

[ShawnN]

I'm planning to replace injectors, FPR, base intake gasket etc later on. I've read a lot about the OE injector problems and figured it would be good to preempt those issues. Doing the major tuneup first. The truck runs good other than under heavy load, and gets better MPG than the 94 2wd CCLB DRW 7.4l I had previously. The previous owner told me he put a new distributor in it chasing a miss but I guess I'll find out once I get the cap off & check for play.