What are your must-do things on new-to-you gmt900?

[m1dn]

Just got myself 08 Yukon XL Denali with 130k miles, picking up in few weeks.

What are your must do recommendations when getting these used trucks?

I’m planning on going down regular oil change, filters and have shop check suspension.

Anything else i should include right away, such as tranny oil?
As far as I’ve read the 08 6.2 doesn’t have DoD, so won’t need to disable this.
What about fuel/oil treatment, anyone adds anything regularly? I liked to throw in some MMO on occasional fuel ups and oil changes on my older f150, not sure if it’s a good idea with 6.2 since it takes premium fuel?

 

[OR VietVet]

If you have no paperwork, IMO, do all fluids. diffs, transaxle, transmission, coolant, brake and power steering and oil change.

 

[diesel_dave]

Definitely check your brake fluid. Every other fluid in my 2007 looked brand new but the brake fluid looked like it was the same stuff the factory put in it way back in 2007. Brake fluid should not be dark green. The pedal feel improved substantially after flushing a few bottles of DOT 3 through.

 

[PatDTN]

Something odd I learned when I bought my '09 was that the cruise control didn't work because the secondary brake light switch for trailer brake lights was bad.

Also, the tires on mine were in poor condition and mismatched. When I tried to use the automatic 4 wheel drive selection the truck started bucking like crazy. If I'm not confused you should have AWD on your Yukon so if it's doing odd things check to see how well matched the tires and pressures are.

 

[Joseph Garcia]

Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

Pics of the truck, please.

You are already receiving sage advice from the knowledgeable folks on this Forum.

Be sure to change ALL cooling hoses, including the 2 plastic Tees near the fire wall on the passengers side, even if they look good.

 

[diesel_dave]

Something odd I learned when I bought my '09 was that the cruise control didn't work because the secondary brake light switch for trailer brake lights was bad.

Also, the tires on mine were in poor condition and mismatched. When I tried to use the automatic 4 wheel drive selection the truck started bucking like crazy. If I'm not confused you should have AWD on your Yukon so if it's doing odd things check to see how well matched the tires and pressures are.

Yep this! My cruise also didn't work when I bought the truck. I was at a loss and saw that the brake switch might be the issue so I took the original one apart to see what was broken. It all looked completely fine so I cleaned it and put it back in. Still broken. Bought a new one on Amazon for $10 and that fixed it right up. Makes no sense to me but the new switch was the trick.

 

[me51]

Coolant (rust inhibitors) and brake fluid (moisture content) go bad with age.

I'd leave the power steering fluid alone unless you hear a little bit of noise when under load (turning wheel at standstill). When changing it, either flush it through the cooling line or replace the oil in the reservoir a couple of times to get as much new fluid in as possible.

The front differential has conventional oil in it which should be changed regularly (50k?), the rear differential has synthetic oil in it good for 150k miles if it is equipped with the G80 locking differential. Look at the glovebox sticker and search for G80.

Transmission and transfer case use ATF (Dexron 6) if I remember correctly, which should be changed every 50k miles. GM says the ATF can go up to 100k, but the list of exemptions is huge so that doesn't really apply to any real-world conditions.

 

[GO0BER]

Yep this! My cruise also didn't work when I bought the truck. I was at a loss and saw that the brake switch might be the issue so I took the original one apart to see what was broken. It all looked completely fine so I cleaned it and put it back in. Still broken. Bought a new one on Amazon for $10 and that fixed it right up. Makes no sense to me but the new switch was the trick.

Funny story... I bought an 08 2500 by accident.. thought I was getting a Denali (the ad stated Denali and the only 2 things to tell me otherwise was selectable 4 wheel drive and 8 lugs instead of 6)... When I realized I got a 2500 and I could use it to tow my 28 ft toy hauler I immediately grabbed a brake controller and put it in. Hooked up to my trailer and immediately had issues in which my trailer brakes were getting low voltage signal even when the brakes were not applied.. turns out my brake switch was bad.. I didn't even think about that affecting cruise control. I'll tell my wife she should have working CC.

 

[intheburbs]

If you have no paperwork, IMO, do all fluids. diffs, transaxle, transmission, coolant, brake and power steering and oil change.

Regardless of service history, unless maybe it was done recently at a dealership, I change all fluids when I buy used. 5 of the 6 vehicles in my sig were bought used, all around 70k-80k miles.

Engine oil, transmission, transfer case, coolant, and both axles. Brake and PS either left alone or changed on a case-by-case inspection, though on my 2500 the PS was an automatic fluid change because of hydroboost.

 

[mountie]

I should change out my differential & rear.... Everything else was changed after I got my '05 Yukon XL 5 years ago.

 

[Kreativestreet]

Here’s my always gotta do to whatever new nnbs /gmt900
That I just bought .keep in mind that some of the things here may not be “legal” in your state so adjust accordingly
First and WAY first is a DOD delete /afm delete / GET RID OF THE busted afm/dod lifters.
People all say tune it out and while it does put a band aid on the issue the way that’s always made all the sense is get rid of it completely. Yes it isn’t cheap if done correctly “ls7 lifters ,resurface heads,new oil pump,timing set,cam plate,deleted cam”btr makes one “arp head bolts ,oil pan relief valve delete and valley plate/tower block off and tune .YET I can say from plenty of experience doing this can easily get any 5.3//6.2 to 250k miles or more with normal service .

Next is get either long tube headers and higher flow cats or shorty headers .can’t tell you how many cracked manifolds,broken studs come across my shop and it’s not for performance it’s to just avoid the future issue .
New intake manifold gaskets super cheap to get,easy to do and something people always overlook that can cause random misfire and poor performance and kills your mpg

Upstream and downstream oxygen sensors
NOBODY ever changes them and they should be replaced every 35-40k miles.i can’t stress how much these can help

If you have a 6l80 6 speed auto

TRANSMISSION COOLER is a must even if you don’t tow
A Tru kool 40k is cheap,works wonders and will pay for itself instantly keeping most transmissions below 150 deg temp regardless of use .if your feeling spendy,a new converter as well since 6l80s have had a know torque converter issue and are a cause a majority of transmission failures

Last but not least
Front wheel bearings and brakes

It’s not uncommon to need wheel bearings after only one year of normal use
And since your in there redo your pads and rotors or consider a upgrade “2020 calipers and rotors are a cheap way to go”

Of course fluids ,hoses belts etc as needed or if your budget allows should be changed,topped off and addressed but that goes for any vehicle

Hope this helps,gmt900s are nearly indestructible if attention is paid to small things and these common issue points addressed

 

[mountie]

A good write-up.......
But..... Our engines are reliable. Basic service & maintenance will allow for a long time use.

If it ain't broke, don't spend money to fix, " what might be"....since they are reliable as is.
Transmission cooler is always a good idea.

Headers? Unless you modify to race, headers will just sound different. ( Spend money elsewhere / as per NHRA tuners )

 

[Kreativestreet]

A good write-up.......
But..... Our engines are reliable. Basic service & maintenance will allow for a long time use.

If it ain't broke, don't spend money to fix, " what might be"....since they are reliable as is.
Transmission cooler is always a good idea.

Headers? Unless you modify to race, headers will just sound different. ( Spend money elsewhere / as per NHRA tuners )

Agree to disagree on the reliable part .all the junkyards full of failed afm/dod equipped 5.3/6.2 motors would attest to and it’s not a debate when the top complaints are lifter failure and low oil pressure .
Taking a proactive approach saves money longer term vs the short term cost but that’s up to the person paying .
And as far as headers ,it stops the biggest issue of exhaust manifold bolts breaking ,offers better clearance to get to plugs and not all affect sound in a bad way but if you say that to someone in California government they may go into shock .

 

[mountie]

I'll play the odds on a 5.3.
Only if I hear a lifter noise, will I replace them. I'll spend the money then.
Percentage of 5.3s breaking manifold bolts..... ( % ?? verses all the 5.3s running?? )
Headers....... For a stock set up...... money down the toilet. As per NHRA tuners, as in Roland Leong, Don Prudhomme, Keith Black, even Doug Thorley !!. They all said the same thing.
(( I had the exact conversation with them about an '88 GMC 5.7, I had when I had a cracked stock manifold )
Plugs are EASY to get to anyway..

 

[jnicks01]

I'd swap out the heater core hoses and T fittings, all fluids front to rear, bleed brakes really well, new plugs/wires, clean the MAF and throttle body. Unfortunately, these items are usually not routinely done and cause issues or inefficiency.

 

[Airman68]

Hands down, change all the fluids you can. I bought my 2008 with 189k on the clock and thought I had a transmission issue. Turned out to be a broken spark plug giving me a misfire. Change plugs and wires. Basically the easy stuff. It's worth it in the end. I also added a AFM disabler to my OBD port. Just a cheaper one from Amazon which works perfectly. Even with that, I get almost 20 MPG on the hwy with my 5.3. Wont cure the lifter issue but if it cant activate, might prolong it a little longer.

 

[jared999]

Just got myself 08 Yukon XL Denali with 130k miles, picking up in few weeks.

What are your must do recommendations when getting these used trucks?

I’m planning on going down regular oil change, filters and have shop check suspension.

Anything else i should include right away, such as tranny oil?
As far as I’ve read the 08 6.2 doesn’t have DoD, so won’t need to disable this.
What about fuel/oil treatment, anyone adds anything regularly? I liked to throw in some MMO on occasional fuel ups and oil changes on my older f150, not sure if it’s a good idea with 6.2 since it takes premium fuel?

seems like most people have given some good advice in here. Change all oils and fluids that you can.
I would inspect hoses and belts, tires, brakes. Make sure all of your door locks work properly. If not your motors may be going out and they are an easy fix.

 

[mountie]

seems like most people have given some good advice in here. Change all oils and fluids that you can.
I would inspect hoses and belts, tires, brakes. Make sure all of your door locks work properly. If not your motors may be going out and they are an easy fix.

When I bought my '05 Yukon XL w/ 140,000 on the clock, even thought the truck was WONDERFULLY maintained, I felt so much better when I knew the fluids, brakes, etc., were done when I did it. Now I knew the 'history' and now knew when to check everything.

 

[DBacon]

Check the mounting bracket for the drivers side rear lower trailing arm. In the rust belt, they rot out and it is a big deal to repair.

 

[jared999]

I think that checking belts and hoses, as well as battery cabling is a must an any pre or post purchase.
bad battery cables can cause all sorts of weird issues