2003 Envoy SLT 4.2 - Help


Original poster
Dec 8, 2011
I have just purchased a 2003 Envoy SLT and would like to start off on the right foot; it has 170,000 on it so I would like to change all the fluids, plugs, wires, fuel filter and anything else that any of you would recommend. I have a couple questions for anyone willing to take a few moments of their time to make my new Envoy ownership a happy one.

1. Where is the fuel filter located? Is it easy to remove and re-install?
2. Do I need to remove the rear dif cover to drain the rear dif fluid?
3. What other fluids should I change, where are they located and how hard are they to change?
4. What else do I need to change? Air Filter, Plugs, PCV value, Plug Wires... did I miss anything?
5. It needs a new radio, can I simply pick one up at the wrecking yard and plop it in (same model)?

Thanks again for any help you can provide.
2003 GMC Envoy SLT 170,000 km
1998 Chevy Blazer 2 Door Five Speed - 446,000 km


Dec 5, 2011
As far as the radio they are not plug and play. You will need to take it to GM and have them use a Tech2 tool to program it (about $85.00). You might be better off looking at an aftermarket unit.


Lifetime VIP Donor
Nov 18, 2011


Dec 8, 2011
Not sure about the '03's specifically, but I had an '05 4.2 and the fuel filter was in the gas-tank....considered "non-serviceable".


Dec 5, 2011
If you don't know the service history, probably a good idea to change all of the fluids. Transfer case every 50K if you have a 4WD, front & rear diff fluid every 100K. I think some of the 2002's had a rear diff drain plug- if you don't, you'll have to loosent the diff cover to drain. Just make sure you can get the fill plug out of the diff's & transfer case before you drain them.

Some people may tell you to not change the trans fluid with that amount of mileage especially if you do not know the history.

Only use AC delco plugs. There are no wires- we have coil packs with the boots connected directly to the packs.



Dec 4, 2011
Ottawa, ON
I basically went through this when I bought my TB (and an engine swap). At this point, you should change all your fluids:
- engine oil and filter
- tranny oil flush and filter
- Front and rear diffs
- Transfer case (very important and usually neglected)
- Brake fluid (if it looks funky. Test it for moisture)
- Engine coolant (if it's suspect)

For the rear diff, my '02 has a drain plug but read that later years, GM got cheap again and removed it, forcing the removal of the cover again.

And as mentioned, you will get naysayers to not flush the tranny. You can do it yourself (hopefully the article to do that will come over here soon). As long as you replace the filter with it, it's all good. Lube shops don't replace the filter when they flush so not recommended. Use Dexron VI fluid. GM doesn't even use Dexron III anymore plus it's synthetic. Mine was brown and funky. Did my own filter replacement and flush, no problem in 5 months.

Brake fluid could still be original if the system was never opened so may be a good idea to flush it.

The transfer case oil has a fairly short service span (50k miles if I remember correctly) and it can be damaged if not done. It is usually neglected as most still use the older service intervals for them or they're just neglected, like the diffs.

And recommend using good synthetic oils in the engine and diffs. No choice for the transfer case though as that lube is only available at the stealership.

For how to do these (except the tranny flush), just follow the link in my sig for the manuals.


Nov 20, 2011
Milwaukee, Wisconsin
:tiphat: Welcome:tiphat:

In case you haven't

My 2003 still has the rear diff. drain plug.But, you may want to remove the cover for inspection.

The cover gasket is reusable.

The fuel filter is below the dr.seat, in-board of the frame rail.

The filters were put in the tank starting with the 2005 models.

Spray the filter holding screw ahead of time with penetrating oil.


Lifetime VIP Donor
Nov 18, 2011
Wooluf1952 said:
Spray the filter holding screw ahead of time with penetrating oil.

:yes: Spray it alot.....GM uses a generous amount of rust to ensure that they would never come loose by themselves. :biggrin:


Original poster
Dec 8, 2011
Just picked up the Envoy last night and it's so sweet!!!! I love it. Thank you all for your comments and help. It will come in handy this weekend when I give this thing a good over hall.
Take care


Original poster
Dec 8, 2011
I got down and dirty and changed the front and rear dif oil as well as the transfer case fluid... broke my baby finger getting the front dif fill plug out (watch out for that one, its in there good).

View attachment 19424

I also changed the plugs, fuel filter, air filter... then decided to clean the throttle body. Incase you haven’t, you should. Mine was black inside with a thick residue.

View attachment 19426

View attachment 19425

She is running great now and I'm really happy I bought her... only thing left is the Transmission Fluid and filter...

Thanks again to everyone here, I would never have attempted this job if it was not for you guys.


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