[68lemans462]

Guys,
I just bought a really cherry '98 2 door tahoe with 350 engine. This thing has 92K and is in immaculate shape. This truck replaced my '99 4 door with the same drivetrain. My '99 was getting around 14mpg in town and 18-20 hwy. I had replaced all 8 injectors and did regular tuneups in the '99.

THe '98 runs flawlessly, but is only getting 11mpg around town. What can I do to improve gas mileage? It appears to have been tuned up recently (new air cleaner) and the engine runs very smooth. I was thinking about a cold air intake. What else can I do to improve mileage?

[mitchgto]

Does this model have the throttle body injection? I had one with a 350 and TB injection. I changed the injectors, throttle body, computer. DIdnt make much of a difference.
If its the model with the vortec engine then you should be getting a helluva lot better mileage than that.

[68lemans462]

Originally Posted by mitchgto

Does this model have the throttle body injection? I had one with a 350 and TB injection. I changed the injectors, throttle body, computer. DIdnt make much of a difference.
If its the model with the vortec engine then you should be getting a helluva lot better mileage than that.

This is a vortec 350 which has the throttle body injection.

[68lemans462]

I was just doing some reading and some guys have said to replace the 02 sensors. Also they said that if the engine is not running hot enough, poor fuel economy can happen. I did notice that this engine runs REALLY cool. Ideas?

[sleepy]

If the motors runing cold the computer will make it run rich. Try replacing the thermostat. My 99 CMC PU ran about 200 degrease. Running cold will also keep the torque converter from locking up.

[PonchoV8]

Mine is the same truck and probably gets 12-14, and my O2 sensor has set off a code a long time ago. I should repalce it and see if I get better mileage. I just assumed that's all it's going to get. I have 193K miles though.

[yak]

a 99 chevy will have had multi port fuel injection with 8 seperate injectors as long as it had a 5.3 or 4.8 engine, your 98 tahoe will have a 350(5.7) vortec. o2 sensors could be slow causing your problem, but that vortec engine has a garbage fuel injection system on it. it has 8 seperate fuel injectors located underneath where that big wire plug in is on top the manifold right behing the throttle body, from there fuel flows through tubes to poppet valves located at the intake ports. it looks like a spider. there is a new kit out that replaces the poppet valves with injectors. i have done this and you will definately have more power and a smoother running engine and an increase in fuel mileage.....

[68lemans462]

Yak,
I'm familiar with the "spider," I replaced the one in my old truck. Does the conversion include a different intake manifold?

[yak]

http://www.theautoshop.net/VortecMPFI.htm

[67drake]

Are you sure they are geared the same? Just checking. I had a 99' 350 Suburban with 3.73's that got around 11-12 around town,but my current 99' 350 Suburban with 3.42's gets around 13-14 in town.

[TheMonkey]

best truck i owned was '00 tahoe old body style with the 5.7l vortec. '00 tahoe came in both new and old. it had the shorter gear set (4.11 i think) and got 13-14 mpg. my '05 suburban now has the shorter gear set too and gets 12ish.

IIRC, the temp gauge was not symmetrical. it was 100 - 210 - 260, so maybe it had a curve to it, but it always ran at the 210 mark. i had an '00 chevy truck that had a similar gauge, but it read 160 - 210 - 260. never figured out why they were like that; one symmetrical, one linear. i'll take a look at my '05 in the morning.

i put a K&N cold air on my 5.7l vortec and it did not improve MPG, only hissed louder at idle.

[68lemans462]

my old truck had 3.73's and it was also heavier (it was a 4 door, this is a 2 door). I havent had the diff cover off of this one yet. I'm really suspicious of that thermostat. What should I replace it with, a 180? I think its running at about 140/160. I'm gonna replace the tstat tomorrow and cross my fingers.

I checked out that MFI conversion as well. Rock auto has it for $300! Seems like a no-brainer. I'll definitely do this when I get some time. My last truck had 110K and I pulled the intake to fix the standard coolant leak. I also replaced all 8 injectors (the spider) and couldnt believe how gunked up it was in there.

[69lm69gp]

Yak,

How much of a mileage increase do you see.

[67drake]

68lemans,this will tell you what gear you have.
http://www.fullsizechevy.com/forums/...odes-here.html

[yak]

Originally Posted by 69lm69gp

Yak,

How much of a mileage increase do you see.

i don't own the vehicles that we have put those updates spiders on, i would never drive a chevy!, i think one customes said maybe 2 more miles a gallon for all round driving....if you took one out of a perfectly running vehicle and put the updated spider in i'm not sure if you'd notice a heck of a lot of difference

[mike nixon]

i'd change the thermostat first.

the next thing i'd check is gearing. i had an 87 blazer, great truck, 350tbi, 700r4, 3:42's with 31"tires. i lost my job at the time and got nervous about the payments and sold it.

a couple months later i got the itch again and bought a 91. 350, 700r4, 31" tires, 4:10 gears.

the 87 would run a consistant 20mpg under 70 mph.

the 91 ran a rock steady 18 mpg. i did exhaust, timing changes, stat,verified the map and temps sensors. 18 mpg is all she'd do.


mike

[TheMonkey]

Originally Posted by 68lemans462

... What should I replace it with, a 180? ...

i think stock (which ECU will be happiest with) is hotter than that. 195?

[The Boss]

I had a 95 and a 98 Tahoes - both 2 doors.

All I ever got from them were 12-13 mpg, sometimes 14 if I was doing straight highway miles. I replaced the inside driver's door handle on both of them, and had to put on new front pads every 18-20k miles.

I loved the looks of them, but was not sad when the leases were up.

[68lemans462]

Originally Posted by The Boss

I had a 95 and a 98 Tahoes - both 2 doors.

All I ever got from them were 12-13 mpg, sometimes 14 if I was doing straight highway miles. I replaced the inside driver's door handle on both of them, and had to put on new front pads every 18-20k miles.

I loved the looks of them, but was not sad when the leases were up.

I'm guessing your trucks were not running very well because my '99 4 door was pulling consistent 18MPG on hwy, 14 around town. Going out to replace the t-stat now. I'll post results in a week or so.

[zbuickman]

Originally Posted by 68lemans462

I'm guessing your trucks were not running very well because my '99 4 door was pulling consistent 18MPG on hwy, 14 around town. Going out to replace the t-stat now. I'll post results in a week or so.

Wow you guys have some milage issues. Ive got a 93 3/4 ton 4x4 suburban w/ 454 and Im getting those milages. I would completely expect 18 out of a small block truck