BadFish523

Sounds like a vacuum leak to me. The truck creates more vacuum at low RPM, when you're accelerating there is less available vacuum so the hubs are slightly trying to engage. You need to fix this or you will be replacing IWEs again. Borrow a vacuum gauge and check all the lines and check valves down from the solenoid to the IWEs.

nddragon01

Another option (because I just got finished a 4-month war with looking for vacuum leaks, replacing actuators, hub assemblies, vacuum lines, vacuum box, and the solenoid...)

You can replace the actuators (and hub assemblies if the gearing in them is bad as well) on your own if you're fairly handy with tools and have jack stands, etc. But leave them fully engaged all the time (i.e. - rip out the entire vacuum system related to the 4WD setup and plug the inlet for vacuum to the engine - leave the solenoid in place and plugged in electrically). No more worrying about vacuum leaks, no more destroying actuators or hubs, and perfectly safe to drive with "live axles" engaged all the time in 2WD. The axles will spin with the wheels and the differential inside will turn, but no power to them from the transfer case until 4HI or 4LO is used. When you need 4WD, switch the dashknob as normal and there you go.

I got tired of half-engaging actuators and destroyed both side actuators and hub assembly gears enough that I'd had it with trying to find the vacuum leak. Replaced the vacuum lines to the actuators, the solenoid, the double line running from the solenoid, the vacuum box behind the battery, the vacuum line from the box to the solenoid line, the check valves... so I decided to say fvkk it and just run live axles for the cost of 1-2mpgs and a working system that isn't going to grind out the actuators or hub assemblies. This also solved my CEL issue since the vacuum leak was affecting not only the 4WD system but the engine running itself.

Just a thought and another way to go if you don't want to hunt down a vacuum leak in the system.

I'd recommend not engaging 4HI at 70mph. I'm sure once won't hurt anything, but repeatedly over time can really muck up your transfer case. I'd also recommend getting ready to replace both side actuators and possibly the hub assemblies if you've been driving around with the grinding noise. It' doesn't take much of that grinding to chew down the gearing to the point of unusable.

Good luck!

gtrivette1

Thank you for all the replies. I changed the check valves out yesterday and today NO NOISE! Only 5.49 a piece! The one from the intake manifold was the culprit. I could blow through both ends with no resistance on the old one.

I do still have the loud pop when accelerating in 4wd however. It kinda puts me in mind of 2 pieces of metal breaking free from each other

nddragon01

Most likely the loud pop is one side (or both) actuator and hub assembly pushing apart. The gearing is probably worn down to prevent them from fully engaging. That's what the grinding was doing that you heard before.

With truck off, Jack up each side one at a time and spin the front tire with your hand as fast as you can. If the axle spins with the tire, you're good. If you hear any grinding, popping, or the axle doesn't spin with the tire, that side is bad.

Also, don't throw it in 4wd at highway speeds if that's what you're still doing. The system is meant to be able to take up to 55mph once engaged, but not to go from zero to highway speed instantaneously.

gtrivette1

I have only driven above 55 in 4wd once and it was to see if the grinding was still taking place.

The popping takes place under normal acceleration. It doesn't do it at a constant speed. I'm sure one of the vacuum hubs are going bad probably the driver side since that is where the noise and popping is/was coming from. I have jacked up and both driver and passenger side axles spin without noise. I will probably just continue to drive until repair is necessary, unless it will cause harm to other parts