Introduction


so my girlfriend got a 2001 blazer 2 door 4x4 about two months ago and I knew it had issues going into it. Ive put a lot of time and work into it which was easy and cheap work for me because Im always working on my Chevy's and more recently blazers. The only problem left after all repairs ive done to eliminate other symptoms is a weird consistently occurring one I cant figure out.



The symptom

It only happens on the highway and I typically drive around town to the highway for a couple mins. Once on the highway and only after I reach 70+ MPH it will start. At this speed it will happen suddenly and randomly. I might drive 5 mins at that speed and its as smooth as can be and out of no where a pretty loud flapping/groaning noise from the front passenger side will happen. I haven't tried going past 80 MPH because it sounds terrible and has alittle shake to it but if I do decide to and I end up going back in time ill stop the gf from buying it. This noise sounds like a piece of cardboard is stuck up in the right front wheel well or like a WWII prop plane has flown under the truck. The sound does change with speed faster louder as I speed up and lower slower sound as I slow down till about 20 MPH and it will "turn off" until I repeat the cycle. I recently discovered a shortcut to turning the noise off. When the noise happens I lower my speed to about 55 MPH and hit the 4 HI button and instantly the noise disappears even after I click it back into 2 HI.

The logic

I cant really figure out what it can be as ive tried shaking and wiggling everything in the front end. Wheel is tight when I check for play nothing is rubbing. CV shaft still looks brand new the old ones boots were shreaded and joints were shot. The noise does not change as I turn on the highway so I think faulty wheel bearing is pretty much ruled out. The fact that the 4x4 engaging has an effect is interesting. Its almost like when the 4x4 is engaged its taking the load off of something that is faulty. This noise has happened since she got it before all the repairs. The blazer was parked for about a year before she got it. idk if that helps and the 4x4 works awesome kicks in and out smooth. If anyone has any suggestions please let me know. Ive read a lot of threads and saw some people had luck with somewhat similar issues by replacing the actuator. I don't think that's my issue since I can go in and out of 4x4 fine. I do wanna check for the "tranny fluid in the vac lines" issue that people find in the vac lines going from transfer case to the 4x4 bladder because the transfer case takes tranny fluid. I initially thought it was a heat/friction issue in front end because it seems to happen when the front differential would get the warmest at high speed. When the fluid was replaced and it still made the noise I kinda wrote it off as being the fault because it had a little more than a quart in it that I drained so it wasn't really low like some people find theirs to be. Tranny fluid in tranny is full, red and not burnt.


Things Replaced

Ive done the front end steering repair kit with balljoints and all links from tie rod to tie rod. replaced the rear end that clunked bad and leaked out the pinion, the u joints were shot, replaced them. Ive filled both differentials properly and monitored fluid level. Ive also replaced wheel bearings in front and only the right front cv axle. I challenge anyone to a fuel pump swapping race in these damn things. replaced plugs wires cap and rotor. Had CEL's and a really bad stumble at 2000RPMs and again at 3000 RPMs and no power and it wasn't fixed until the distributor was replaced. It was causing the rotor to arch on the sides of the posts in the cap. Very easy job I think most people should do before spider injectors are done because ive never had new spider injectors fix my blazers with stumble/random misfire issues. I replaced the commonly faulty ABS module that continuously pulses the brakes when the 60 AMP fuse for ABS is inserted which by the way I am ashamed at GM for not issuing a recall on all 2001-2002 blazers, jimmys, s10's for this known issue. The bad module causes abs and brake light illumination and brakes to essentially become useless unless fuse is removed, or you find a rear bumper first then you can skip the fuse removal. I know ive gone off course but I wanted everything out on the table to help others if they find this. Any help with my final issue with the gfs daily driver would be helpful. Next thing on the list is tires but I don't have a lot of faith in that being the miracle cure. I feel its something stupid or uncommon so im looking for any input from someone who has had and fixed something like this. Thank you for any advice.

 

My wifes is the EXACT same thing. Its the one pictured. It sat for over a year and everyone month or 2-3 it will happen after highway speeds. Thinking it is the 4x4 trying to engage on the passenger side. yep we engage 4x4 back up 20 ft disengage 4x4 noise stops

 

Here is the key clue: ..."hit the 4 HI button and instantly the noise disappears"... The vacuum switch on the transfer case is a known problem. If the O ring seal leaks inside the switch, it will partially engage the clutch (item #12 below) on the right front axle shaft. The noise you're hearing is the gears trying to mesh. If it grinds enough, it will destroy the two gears. When you select 4WD, the switch applies full manifold vacuum to the actuator under the battery tray allowing the gears to fully mesh, and the noise stops, instantly.


The two gears are items #9 & #23:


The switch is on the driver side of the transfer case, just above and inboard of the encoder motor, and has 3 vacuum lines connected to it, (about a 15 minute job to replace it).



The GM upgraded part # is: 89059420 and lists for ~$60. Here is a link to the Dorman part, cost is under $20 at most auto parts stores: Dorman Products - 600-500

 

Thank you sir. It hasn't happened in months next time BAMMMM im fixing that sucker

 

I got the same problem. I think it's a vacuum problem. When the ac or heater is not on it doesn't do it. I noticed the air coming out the face vents change to the floor and them the noise starts in the right front.