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2001 GMC Yukon XL 191K Mile Tune-up

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Last summer I purchased a 2001 Yukon XL with the 5.3L engine, replaced the rear rotor/pads and air filter, then went on a 13,000 mile 8-month road trip starting in Boston, going through Maine, Atlantic Canada, back to Boston for a seasonable job, then to New Mexico via South, back again to Boston through the midwest and finally Florida to help my folks through covid (all safe). 


The truck now has 191K miles, but the original title indicated the mileage was not accurate (which is how I got it for  $1,200 on eBay), so, who knows.   The mechanic who did the rear brakes was surprised how little rust was in the undercarriage.  I removed the rear seats, added a mattress and three milkcrates for my stuff, installed a cheap aftermarket radio with bluetooth and a rearview mirror camera.  Brake and ABS warning lights were skillfully covered with duct tape by the previous owner, I removed the ABS fuse because ABS kicked in without need when breaking at low speeds (dirty sensors?).


Over the 13K miles  I had 2 oil changes + tire rotation at Wally, and poured  Bar’s Leaks into the radiator to stop a slow coolant leak out of the intake manifold.


I drove full tanks at different speeds on cruise and averaged:
 - ~13 MPG city
 - 20.44 MPG at 55  highway (sorry truckers, thanks for not throwing your pee bottles on my windshield)
 - 19.53 MPG at 60  highway
 - 18.81 MPG at 65  highway

It’s been a hell of a truck, and now in Florida I have a garage + time, and will do some maintenance.  Here is my list, I appreciate any suggestions (or requests):

  • driver side rear caliper/pads/hose (the wheel is a little warmer than the others after driving, and it just started to whistle though pads/rotors are only 13K miles)
  • front rotors and pads, total brake fluid flush
  • check if front hubs need replacement (I occasionally hear a noise when turning left at very low speeds)
  • oil change w/ amazon synthetic  + mobil 1 filter
  • trans fluid flush w/ Dexron VI, new filter, and tighten valve body bolts (sometimes tranny thumps from a stop after highway driving, but the fluid is still pink enough)
  • spark plugs and wires (just tested resistance, 7 AC Delco wires are perfect, at about 660 ohms but the 1 shorter Denso wire is at 505 ohms; double platinum AC Delco 41-962s have some gray and orange and could probably go back, but I’ll change just because I went through the trauma of removing them - BTW, three out of the eight plugs have oil in the threads, should I be happy?)
  • fan clutch (not a lot of resistance, and my ac belt “jackhammers” when moving from idle - which according to some of the forums might be due to higher freon temps at idle due to a dying fan clutch)
  • fuel filter
  • clean and seal all grounds
  • both downstream oz sensors (voltage is getting lazy)


I’m predicting that between the freed caliper, new plugs/wires, fuel filter, trans fluid  and oz sensors I will see some mpg improvement over the next 10K miles, which will hopefully cover the Pacific Southwest, California and Pacific Canada.  Thank you all for sharing you knowledge here, I look forward to putting it all into practice.


Edited by Ramblin Man
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These trucks have dry intakes, there is no coolant flow through them. Your leak could have been from a steam lines under the intake, head gasket or your truck may have had a coolant line going to the throttle body.


The downstream o2 sensors should not change the fuel mileage, they only monitor the cats and don't directly change the fueling like the upstream o2 sensors. Downstream sensors when working properly will likely only move from a small voltage swing. Maybe like .500-.700 where as the upstream sensors will jump between .100 and .900 constantly. When a cat converter goes bad, then the rear sensors will toss a code and the voltage swing of the sensor will be more wide spread.

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I looked at it more carefully - the coolant was leaking out of the passenger side valve cover ?. (see arrow)




Bar's Leak was added 3,500 miles ago and I have not lost a drop of coolant since. No evidence of coolant in the oil. Removing the valve cover should give a more solid idea of what is happening, but it might also undo whatever Bar's Leak did, so I'm tempted to leave it alone and enjoy all the borrowed time I have.  


On the bright side, I collected 15 minutes of O2 sensor voltage data at 65mpg and it matches your description  (ignore the occasional dips to 0 volts, that's the bluetooth connection, the same happens to my coolant temp readings), no need to change those.



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Did it - I pulled the passenger side cover (the one leaking coolant before Bar's Leak), verified that it is not a Castech, and was pleasantly surprised that it looked fairly clean.  Is there anything I am looking for besides evidence of a hairline crack? Are those orange marks on the springs rust? I was going to replace the gasket, but decided to keep the existing as an early warning system in case the leak returns. 








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A spark plug and wire update on the 5.3l 2001 Yukon XL.  There is no telling how old these plugs are, but about 90K miles is a good bet.  I measured resistance on each plug and wire, as well as the plug gaps.


AC Delco 41-962 platinums.   AC Delco 7mm wires, except for a two-inches shorter denso wire that does not belong to the set.  Sorted by gap size:




Evidently there is more to the story (was the short wire installed on day one, are there any issues with the coils, what do the valves and pistons look like given oil on the threads, did I have a crack on the passenger side cylinder head - which had the coolant leak - leading to more heat and larger gaps), but generally:

  • though the second row is an anomaly, roughly the more resistance, the larger the gap
  • only one plug stayed at the factory gap, despite claims on the box that "gap remains virtually unchanged for the life of the plug"; perhaps this makes a case for iridiums
  • tempted to get a complete wire set that is two inches shorter than recommended


Wish I had checked them when I first got the truck 13k mikes ago.



I measured the length of the wires - ONLY THE SHORT WIRE IS THE CORRECT LENGTH!  Each of the other seven wires is two inches longer than spec.  While I still believe there are other factors at play (especially the passenger cylinder head running warmer), this has to be the primary issue.  

Edited by Ramblin Man
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  • 7 months later...

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