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2003 GMC Sonoma

Recently bought a 2003 GMC Sonoma Crew Cab SLS, 6 cyl., 4WD and auto trans with only 29K miles on it. Already had to back flush the heater core as it had partial blockage. The truck is in excellent shape and appears to have been well maintained but in the back of my mind I’m wondering if I should do any other fluid checks or changes because of age.

Anytime I purchase a used vehicle without a repair history the first thing I do is service all the fluids.


And all rubber parts under the hood (belts and hoses).


Transmission fluid and filter service using Dexron 3 . . . NO FLUSH
Fuel filter
Air filter
Coolant drain and refill with Dexcool
Brake fluid flush
Check out those tires . . . make sure they’re not dry rotted with age
Make sure the frame isn’t rusting away
Transfer case fluid change
Differential fluid change . . . front AND rear

Thanks folks for the “Honey Dew” list, looks like I’ll be busy for the next few weekends.

Just a note on the trans fluid. Trans techs and GM both would tell you to change over to Dexron VI. It’s supposed to be completely backward compatible, but a better fluid. Dexron III technically doesn’t exist anymore because the designation is basically a GM licensing thing and GM doesn’t license III anymore - only VI. So you’ll have to make a decision about whether to go to VI or not. If not, in the AP stores you’ll just have to read the bottle to stay with the Dex III spec - most of them will be labeled Dex/Merc because back then (early '00s) Ford & GM were using the same spec. No longer though - Merc & Dex have parted ways.

Others will disagree, but I wouldn’t’ stay on DexCool.


Be aware that Dexron 6 is synthetic . . . and very expensive

If you’re going to use Dexron 6, I advise you to use the real thing, NOT some fluid that claims to be compatible

I feel DexCool is perfectly fine, as long as it’s serviced every 5 years . . . and the previous owners OBVIOUSLY did not do this

I’ll go a little further. I’m a professional mechanic, and we have tons of GM vehicles in our fleet. The ones with the plugged heater cores are ALWAYS the ones that haven’t had their coolant systems serviced on time. That also goes for Ford, by the way. We also have lots of Fords in our fleet.

@db4690, “Be aware that Dexron 6 is synthetic . . . and very expensive”

?? It costs no more than Mercon V, Toyota T-IV or WS, ATF+4, or any other trans fluid. What do you mean by very expensive? Genuine AC Delco Dexron6 is barely $2/qt more than the plain Dex/Merc III they sell.

I can only think of one transmission fluid that I’ve encountered that is very expensive. The Esso trans fluid that went into a Jaguar that had a leaky cooler line was $53/qt. And yet the customer chose (on my advice) to use the fluid from the dealer rather than Valvoline or some other fluid that supposedly met the same specs. And even at that I would think that the correct fluid vs. a costly transmission repair isn’t really expensive?


Most of my colleagues consider synthetic ATF to be very expensive

I also consider it to be expensive, but I’m not complaining

There’s a difference between saying you find something expensive versus griping about it

I don’t think $6/qt for a proper transmission fluid to be expensive.

Not trying to pick a fight or anything, just expressing things from my perspective…a quart of Dexron6 costs about 2/3 of what a pack of cigarettes costs. One when used properly will keep your transmission functioning properly. The other when used properly can kill you. Is transmission fluid expensive? No it is not.


I’ll go off topic here

I no longer smoke, but I have absolutely no problems with those that still do

That would make me a hypocrite

We both know there are plenty of things about our job that might kill us . . . slowly or instantly

In fact, I had an EXTREMELY close call 15 years ago or so, while I was still working at the US Army depot, as a heavy equipment mechanic

A colleague of mine dropped the header board . . . headache board, apparently . . . on me

Those of you in the heavy line/trucking industry might know what I’m referring to

It landed flat on my back and then bounced off my work boot

I got lucky . . . a swollen foot for about a week

If it had landed on me any other way, I probably would have fared MUCH worse

The guy that dropped the header board didn’t even get into trouble. No write, not even a verbal reprimand.

@db4690, If it had been me that the headerboard hit and I was still conscious, the guy that dropped it would have gotten into trouble avoiding every last thing in my reach thrown at his head. Glad you escaped that one with only swelling. For all the safety devices we have now nothing can compensate for stupid carelessness.

The guy that dropped the header board on me was careless, alright

One time, he somehow managed to step INTO the solvent tank . . .