'98 GMC Suburban

Discussion in 'GM' started by t_haynes, Jan 2, 2013.

  1. t_haynes

    t_haynes Member

    Hello all...

    I am driving a '98 Suburban 4WD with the 5.7 gas motor and 175K miles. I have owned it for the last 15K. This truck has the 3.73 final drive and is the 1/2 ton. I have been trying to improve fuel economy. I have tires at 40 psi front and rear. I got a tuneup with plugs, wires, rotor, cap. Fuel and air filters are new. The truck runs great.

    I have a couple of questions. First, I think the distributor was removed due to a broken retainer for the cap. Reinstalling a new one required drilling and tapping for a retaining bolt. I am not getting any codes, but I understand the cam retard offset can only be set with a fancy tool I do not have. I checked at AdvanceAuto and their loaner will not read this value. Is it worth taking it to the dealer to set? The ignition is run by the computer if I understand it all correctly. I have the Scangauge II and I can monitor the total ignition advance. Total advance is somewhere between 24 and 30 or so at highway speeds.

    FWIW, I got the Quadzilla Stealth2 programmer to adjust the speedo/odometer and try the different tunes. The tunes don't seem to make much difference in performance or fuel economy that I can tell.

    I have also watched temps and I am 188-191 or so whenever it is warmed up. This is summer, winter, whatever. Is this too cool for best economy? The thermostats available all seem to be 160, 180 and 195.

    Thanks... Tom
  2. Temps seem the same as my 99 Express van (8600 grs weight / 3/4 ton / 5.7L 4l80 / 3.73 / 8 lug wheels)

    What kind of tires do you have and what psi do they say is max? 40 seems low, but I have e rated tires at 80 psi.

    Never watched IGN on my van. Wont drive it till Friday, I'll watch it then.

    What is the part # on the Quadzilla? Was it a programmer that you plug in, download your stock tune, then upload the Quad tune?
  3. phoebeisis

    phoebeisis Well-Known Member

    My 1998 Chevy Suburban- much same truck-except I have a 3.42 rear end-and 2WD.
    Your 4x4 wheels??might be 16" and maybe 6 lugs??
    Mine are tiny 235/75 15" 5 lugs-think 4x4 got 16" wheels-and they are tall enough so you probably turn about the same number of RPMS at 60 mph-1720-that I do-or pretty close anyway.
    Think door jam recommends 32 41 odd -
    I currently run 42-43 psi both ends

    Thos temps-and advance-seem about right-I rarely chack advance- but I check it soon-let you know- but those numbers sure seem right.

    At sea level I get about 20-20.5 mpg at 60 mph-on long all hy trips 21mpg-but traeling out west and some is at altitude whee mpg improves.
    The older 4x4 are usually 1 mpg down hy-same 1 mpg down city.
    Once warm-in pure city driving - motor on pulse and glide- 16-17 mpg-not shutting down at lights
    The glide very well- 30-35 seconds from 36mph>27mph.
    Just lift in D- just like it is in N(with my 2wd-probably same with 4x4)
  4. t_haynes

    t_haynes Member

    Thanks for the replies.

    I have the six-lug wheels and 265/75/16 tires on it. The ones in back are fairly new C rated tires. The ones in front are p-metric tires that are worn on the sides of the tread and pretty meaty in the middle.

    The tuner is a S2TUNER-G. I got it from 4wheelonline. You plug it in, download your tune, upload the new one, etc.

    I checked my records, and back in the summer I had a couple of tanks just over 17 mpg. The last trip to the coast and back was about 15, but it was cold and so the transmission temps were down and I am sure frictional losses were higher. I ran it in on-demand 4WD mode for about 300 miles in snow.
  5. t_haynes

    t_haynes Member

    From reading through everything I could find, the Quadzilla and the Hypertech seemed like the ones people liked. The Hypertech only had one tune that required premium gas. The Quadzilla had a "Tow Performance" tune that was intended for regular gas. 4wheelonline had the best price...
  6. I know a guy that did the hypertech on a tahoe. Dont know if he is running premium gas tho. He claims mpg is better with it
  7. t_haynes

    t_haynes Member

    I have the Hypertech on a Mercury Grand Marquis. I track all the gas used and my commute is just under 80 miles round trip so very consistent driving patterns. I could not tell a difference in fuel economy. The car was more responsive with the Hypertech tune, and it pings badly on regular.
  8. 08EscapeHybrid

    08EscapeHybrid Moderator

    I have a 98 GMC K1500 (extended cab pickup version of your Suburban) with 184,000 miles. I run E rated tires at 70 PSI, and I get 13-14 MPG city, 18 highway on ethanol free. Subtract 2 MPG for E10. The temps and ignition readings are pretty normal. If you're not getting a code, leave the distributor alone. The best mileage will be obtained using the 195* thermostat. Make sure your throttle body is clean, that can be pretty nasty after 15 years if its never been cleaned before. Make sure you get throttle body cleaner, old fashioned carb cleaner will mess it up. A bottle of BG 44k in the tank wouldn't hurt either. Make sure you have the proper OEM AC Delco platinum plugs in it. Mileage will suffer if you put anything other than the proper plugs in it.
  9. t_haynes

    t_haynes Member

    The gas with alcohol in it is had to avoid, and I know I was using it on the recent trip to the coast and back. I also remember that the plugs my mechanic said he used in the tuneup were NGK. What size e-rated tire are you using in that K1500?
  10. 08EscapeHybrid

    08EscapeHybrid Moderator

    I'm using Michelin LTX A/S 265/75/R16. My mileage took a hit of about 1 MPG with the E rated tires, but I do load it to capacity and take it on trips. I'm beating the EPA numbers. (12/13/16) The tires are also wearing very well. My old tires were C rated Coopers, and they barely lasted me 40,000 miles. I currently have 35,000 miles on the E rated Michelins, and they still look like new. I rotate the tires every 5,000 miles when I do my oil change. From talking to others that have put E rated Michelins on K1500's, I expect to get about 100,000 miles out of the tires if they don't dry rot first. (Its not my primary vehicle anymore, and I'm only putting about 3,000 miles/yr on it.)

    I use Mobil1 0W30 with a WIX oil filter. I change it every 5,000 miles. I use nothing but AC Delco cap, rotor, plug wires, and plugs. If any of your ignition parts aren't Delco, I'd get rid of them. My truck had Champion plugs in it when I bought it, and it runs so much nicer with the OEM Delco's.
  11. I've been running a Lucas additive in the gas in the van. It has always been picky on gas since I bought it in 09. Would run like Crap most of the summer. The ethanol counteractant seems to help a bit.
  12. t_haynes

    t_haynes Member

    I have wondered if there might be an advantage to the 235/85/16 tires. They seem to have the same rolling diameter in a narrower tire. They only seem to come in the E rating.
  13. 08EscapeHybrid

    08EscapeHybrid Moderator

    I would not put a narrower tire on it. It would make the truck less stable, and more likely to break traction. The 350 is a strong motor. If you had the 305, I'd say maybe, but not with the 350. Just last week I was merging onto the highway, going about 45-50 MPH when I floored it, and the rear wheels broke traction. Do you regularly tow or haul heavy loads at or beyond the vehicle's rated capacity? If not, then E rated tires are not for you. You'd get better mileage with C or D range tires. Yes, the E's have a lower rolling resistance, but that's counteracted by the fact that they're heavier than their C and D range equivalents.
  14. t_haynes

    t_haynes Member

    I have towed small things from time to time, but I am not approaching load capacity. I don't plan to tow a camper or anything.
  15. 08EscapeHybrid

    08EscapeHybrid Moderator

    Unless you're looking for increased value by having a tire that will last about 100,000 miles, then stick with C's or D's. The E's won't get you better mileage unless you're actually working the truck hard. I'll load my Harley and a bunch of luggage and gear in mine, and drive 1,000 miles straight, only stopping to get gas. For me, the E's are that extra bit of overkill that makes me feel better when weighing the truck down, and going on road trips.
  16. t_haynes

    t_haynes Member

    FWIW, I found a deal on some factory steel wheels. I have been wanting to put some proper snow tires on this for the winter. I am getting some 235/85/16 M&S Cooper tires, so I wil get a feel for the tire height (should be about the same on paper) and handling, etc. I talked to a local guy whose father ran the "pizza cutter" 235/85/16 tires on a K2500 and loved them.
    Last edited: Jan 4, 2013
  17. Ect @ 185. 55-60 mph. 37-38 ign

    Stock tuning.

    If you were closer I could plug in and log what yours is doing.
  18. t_haynes

    t_haynes Member

    Mine will jump up just for a split second into the low 30's but it stays much lower most of the time. I have the SGii, so i can watch it from the corner of my eye, but I don't have logging. I think I don't have enough advance. This is all a computer thing, so I am not sure the cam retard setting would make any difference as long as it is close enough to keep from throwing a code.
  19. 08EscapeHybrid

    08EscapeHybrid Moderator

    Yeah, mine generally stays in the 20's too.

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