I cant my 99 envoy to "lock" into 4wd! I found a broke vacuum hose on a sensor the firewall and a check valve with a peice of hose on it too, by the driver side fender. I also want to add that this suv has been the biggest piece of **** EVER! I'ts literally falling apart around me, and every "true blood" american wonders why people by foreign( which most are made inthe states anyways) cars instead. American automakers... quit producing GARBAGE!

Check that your getting vacuum to the front wheel drive assembly. The vacuum is what locks the drive unit. Vacuum from the engine to the trans switch must be there when placed in FWD. Then from the trans vacuum switch/valve to the front differential. If you have vaccum and all lines are good and not graced up and leaking, it will work properly.

no not normal. check center of firewall the is a switch with 3 vacuum line CHECK THEM ALL this is your 4wd selector. if thats okay you might need the switch on the front trasfer case about $20 just unscrews easy fix.

There are so many things this could be, Your code is for a misfire and this could be cause by plugs, carburetor, timing belt jump or timing, crack distributor cap, loose plug wire, pcv valve. I suggest having a mechanic diagnose the problem so your not chasing the issue with money. After he finds the problem, you can decide to do the repair yourself or have them do the repairs.

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In the engine compartment, locate black vacuum hose that runs from brake booster check valve to intake manifold, there will be a small vacuum hose attached to check valve at the power brake booster check valve. This is the vacuum line going to the heater control unit,check the hose all the way to the firewall, repair any leaks you find. And that should take care of your problem.......GOOD LUCK

A frequent problem with these trucks is the vacuum switch on the transfer case.It is a three port switch screwed in above where the shift lever comes out of the t/case.The switch just screws in,7/8 wrench if I remember correctly.When you remove the vacuum hose connector from the switch,you should hear a vacuum noise for a few seconds.If you don't check for a defective vacuum check valve near the firewall behind the engine,or a cracked vacuum resevoir.One other area to inspect is below the battery. Sometimes battery acid will leak onto the vacuum hose that goes to the actuator causing a leak in the hose,which will disable the four wheel drive.To check for a broken actuator cable,jack a front wheel,put your t/case in 4wd, move the diaphragm manually you should feel the axle lock in...the front tire should not turn by hand

When you say hose - is it a large water type hose? The hoses that connect near the firewall would be heater hoses. Is there a piece stuck in the end of the hose that came off? That would indicate a damaged heater core and the only repair is to replace it from inside the vehicle.

If it is a smaller hose it could be vacuum tubing that operates equipment inside the cab. A white piece on that would indicate the vacuum port that the tube was connected to and that part would likely need to be replaced.

If your luck it's going to be a $15.00 vacuum switch on the transfer case that takes about 15 min to replace, if it's the electronic solenoid to the transfer case then it's going to be cost a little more and take a little longer. If your try switching from 4WD to 2WD and hear the solenoid working by hearing the cluck's when it engages and disengage then it's going to be the vacuum switch on top of the transfer case on the drivers side. Auto Zone Sell's them for $15.00 and they will have the step by step instructions to replacing the switch. They will also have the transfer case solenoid  too, if you don't hear the solenoid. Check the fuses, the fuse may have gone bad. Good luck and hope this helps.

Typical ford Check on the firewall for all your rubber vacuum hoses where the heater hose goes into the firewall if one is broke as you accelerate the vacuum makes the heater switch open giving you heat

check vacuum hoses to shift motors .check the shift linkages. also check shift motors.