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Tune up problems

W

Wheels

Full Member
Joined
Aug 31, 2016
Messages
9
I need some help figuring out what went wrong here.. change plugs and wires, changed oil from whatever the basic conventional oil Monro muffler uses to the penzoil premium full synthetic and put in one bottle (15oz) of the STP oil treatment and somehow I went from 23mpg highway with cruise at 55 down to 17... I took a look at the plugs and didnt realize I swapped out best for good quality so I cleaned the old ones real well and put them back in and just took it for a ride on the highway again same conditions and I got up to 19mpg occasionally 20 but still worse off than before the tune up... could it be the wires? I figured I'd swap those back to the old ones next and try again.  The new one are AC Delco that I picked up from advanced auto parts.  I wouldnt think making the switch to better oil would do that?  What could have gone wrong here?  Im no master mechanic so it could be human error.. Any info helps I'm averaging 25k miles/year so every extra mile goes a long way for the bottom line.

Ps 5.3L 146k miles

Thanks!
 
Are you using the DIC on your dash for MPG figures? If so they are just a guideline and not very accurate.
There can be many variables to affect short term gas mileage figures from your old readings such as quality of gas, summer gas vs. winter gas, tire pressure and weather conditions, dirty MAF sensor, etc.
Also did you change oil with same viscosity or was it thicker?
Just my .02 based on many years of driving....there is no need for oil treatments if you use good quality oil either dino or synthetic. Also what oil filter did you use?
Several members have complained about problems when using fancy aftermarket spark plugs and most find good results with GM recommended type of plugs.
For best mileage you need to drive the AV like an old geezer and very gentle on the brakes and throttle and also understand that the AV is a 6,000 lb. vehicle with the aerodynamics of a brick so it will never be great at gas mileage.
If you don't know the maintenance history of your AV you may want to change all fluids from the radiator,transmission and rear diff too. In addition to checking tire pressure and other maintenance things spelled out in your manual for a vehicle with 146K miles.
Good luck with your efforts.
 
Which AC delco plugs did you get.
 
I went with NGK V-power plugs and the standard ACdelco oil filter (PF46).  The only reason I added the oil treatment was because the truck was off the road for about 9 months in storage so I figured it could only help.  I was using the dash for the estimated MPG.. I know it may not be the most accurate buT shouldn't it at least be consistent with the estimates even if they're off?
 
I missed what year your truck was?

Only use the standard AC-Delco platinum plugs. Other plugs may cause issues. Can be a truck to truck issue, some work fine some don't. But if you go AC Delco for plugs and critical sensors such as O2 sensors you can't go wrong.
 
What year is your Avy is it a 2nd gen with VFM? Would being stored that long be a learning curve for the truck? Did you clean the MAF with proper cleaner? Like posted the average on the truck takes in many different variables. I would do the old fashion manual calculation the next few tanks. 
All pugs are not created equal.                                                   
I have used NGK?s for years on all types of vehicles with good results. But I will experiment with different brands of plugs on every engine I have owned. I started using NGK?s when I first bought my Avy. With the wisdom on here and the results I have had I went back to factor recommended Ac Delco with the Avy.  The wires you can Ohm them, when it is dark with truck running and hood up check for any spark around the wires and plugs. Did you use any anti-seizes on the plugs? There is a good post about it on the site. I still use it but I?m more aware how. 
 
The truck is an 03 z71 mostly stock other than a K&N air filter put in before I purchased the truck.  I took the NGK plugs out and put the old ones back in after I cleaned them ( autolite xp5245) and that got me up to 19-20mpg with cruise at 55 on flat ground so I know it runs better with the XP plugs vs the NGK but I'm still worse off Than before the tune up.. I still have the old wires and thinking about putting them back on to see what happens?
 
Is your K&N dry or oiled? If oiled did you oil it recently?
My .02 if you have not cleaned your MAF and throttle body I would clean them also.
 
I have not oiled the air filter since I purchased the truck a little over a year ago.. what oil should I use?
 
The reason I asked about the oil is it can get on the MAF sensor and cause problems. At this time I would not oil but clean the MAF and throttle body at this time. Use only K&N cleaner and oil it comes in a kit.
 
wind blowing?
Up hill or down hill?
Same place on interstate?
cruise control on or off?
AC on or off?
ambient air the same?
weight in truck the same?
same fuel load?

Following someone before or after?

so many variables...

instant or avg mpg?

Same weight of oil at change?

Why STP? it is thick and makes engine work harder IE waste gas unless you have some sort of leak or burn oil.

Fuel from same source?

To me your original 23mpg is crazy and 17-18mpg is about normal

I sugget driving an entire tank and comparing...
 
Did you know what specific cylinder is misfiring?
compression test it and go from there
 
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