So i finally have my Jimmy running and driving, (Build thread to be updated sometime soon) and I have done some tuning on it and my last tank i only got 13.8 mpg. thats terrable. thats back roads mostly flat 60 mph fairly normal driving.
heres the breakdown of the setup.
1999 GMC Jimmy 4 Door 4x4. front and rear axles only have 6k miles, both with syn fluid
2004 4.8 with ~70k when bought, running a 2002 12212156 OS in a 12200411 PCM
Comp XR259HR specs are 206/212 .515/.522 112 LSA CAM with LS6 valve springs.
2004 Trail Blazer 4L60e with some mods, stock converter
Hand built shorty headers 1 5/8" primarys 2.5" collectors
2.5" crossover in to a single 3" with a magnaflow cat and muffler.
3.42 geas
235/70R15 stock tires.

its a slead out of the hole and it feels like it lags really hard just going down the road. once the RPM get up there its not bad, but just driving on flat 60 mph roads its between 17-23% throttle and feels like i am towing a trailer all the time except when i am in it.
also i have a weird shifting problem thats random. some times it hits pretty hard and sometimes it slush shifts. also i have a problem with my WOT 1>2 shift. I set it at 6000 RPM and it hits the rev limiter on ooccasion (set at 7200) and thats also random. I had some issues with my yukon and it turned out i had a corrupt tune file, i might try to start over with a new file and re tune it. all I know is I expected more than 13.8mpg.

Yes, you need to tune some. Mine is also a bit of a slug out of the hole, thinking a small stall like 2000 to 2200 would help. I'm also thinking now that I either need to reduce my tire diameter by 3 inches or put 3.73s back in. I had been running 93 octane and it felt fairly powerful, but the last few tanks have been 87 octane so I'm on the 87 octane table. I can really feel the power difference so my 87 tuning needs help. I'm hoping to be able to do something about it in December, right now I'm working 7 days a week and only have the truck so I can't do a damned thing. Got everything I need to put electric fans in, for example, and all I can do is look at the fans every time I leave for work. But, I'm thankful I even have a job right now.

You should be getting better mileage though, as you should have a better wind profile than I do, and your trans uses less power than mine. I get about 14.5 on 87 and about 16 on 93.

Sweet cant wait to see pics

Speeder wrote:

Yes, you need to tune some. Mine is also a bit of a slug out of the hole, thinking a small stall like 2000 to 2200 would help. I'm also thinking now that I either need to reduce my tire diameter by 3 inches or put 3.73s back in. I had been running 93 octane and it felt fairly powerful, but the last few tanks have been 87 octane so I'm on the 87 octane table. I can really feel the power difference so my 87 tuning needs help. I'm hoping to be able to do something about it in December, right now I'm working 7 days a week and only have the truck so I can't do a damned thing. Got everything I need to put electric fans in, for example, and all I can do is look at the fans every time I leave for work. But, I'm thankful I even have a job right now.

You should be getting better mileage though, as you should have a better wind profile than I do, and your trans uses less power than mine. I get about 14.5 on 87 and about 16 on 93.

I sure would hope it will get better than 13.8. I have done some tuning, ran a few logs using the CALC.VE and did some smoothing of the VE table. and MAF table. it sure seems like its pig rich as a swipe of the finger in the tail pipe will get you a really black finger.

Hmm... did you replace the O2 sensors? Sounds like you are further along with the tune than I am. Mine has the same problem, heavy carbon in the exhaust. I have a wideband to install, eventually, then I can get it tuned right. In the meantime I'm running a Corvette spark map and some fueling parameters.

Im running brand new delphi o2 sensors, I have my wb02 hooked up and ist right around 14.2-14.8 afr pretty much all the time. I just did another log yesterday and finally have my fuel trims in the single digits, it seems to be a little better. Its almost as if it lags too much going down the road. When im going 60 and drop it back to 3rd then im using less throttle % and less on my map. I knew I should have put 3.73 gears in it.

Yup sounds like you're undergeared, esp. for a GEN 3 4.8. A nice 3400rpm torque converter would help your 1/4 mile times the most.

peace
Hog

I'm thinking the same. My tires are about worn out, looking at dropping the diameter a bit this time. Switching from 265/70R17s (31.7 in diameter) to 255/65R17s (30.1 in diameter) will lower the truck about an inch and will be the equivalent of swapping from 3.42s to 3.60 rear gears. Hoping this will help mine a bit. Unfortunately, you won't be able to do much on tire diameter with yours as they are already pretty short. A stall converter will help acceleration but won't do anything for cruising.

I think I am going to drive it till spring the way it is, and then make some changes, I know I need a converter, but I may swap to at least 3.73 gears as well, maybe I will just upgrade to a 5.3 or a 6.0. idk, its not bad but i am not really that happy with it. if i'm not going to get that good of milage then i might as well go bigger engine.

Sorry to hear that. Mine's not perfect but it is a faster engine than the 5.3 it replaced. Of course, I never intended on it being a race truck. Since you're waiting till next spring, if I can get a decent mileage tune on mine I'll shoot it over to you to see how you do with it.

I am not sure if my overhead display is close or correct yet, but on my way in today after loading my log on sat and doing a little cruise timing tuning it was up to 16.2 avg. hopefully its pretty close. I never intended to have a race truck either, i wanted a nice daily driver that had some get up and go, i went with the 4.8 so i could get good milage, but if i cant get good milage, then i should have dropped in a 6.0 and had some more power.

Hmm, have you calculated your mileage using the fill and divide method, where you top off the tank, drive it down, top it off again, then divide the miles traveled by the second fill gallons? Those in-dash mileage things are usually not that reliable even with the engine they were installed with. Unless it was calibrated along with the new engine I don't think it would be reliable at all. I know EFI Live has an input that looks like a calibrator for a mileage output, be worth looking into.

Thats how i did it the first time and the in dash one was pretty close, (I keep resetting it after loading a new tune) I just filled it up sunday after doing some tuning sat so I need to drive it down some and get a calculation. i will probably fill it back up later this week and see what it is at. we only have a 3 day week so I wont be able to run a full tank out of it.

I put that very same Bravada driver information center in the overhead console for my truck (1998 GMT400 RCSB 4x4). I connected the class 2 data bus to the newtork and and I am still using the original black box. The mileage is nats a$$ dead on when I do the drive and fill method. I took it on a 1500 mile trip and it was acurate even by the gallons used. I have the 26 gal tank but my gas gauge reads empty at about 20 gallons. I have never been able to pump more than 20 into the tank until I started using that information center readout.

TJ

98Blackss wrote:

I put that very same Bravada driver information center in the overhead console for my truck (1998 GMT400 RCSB 4x4). I connected the class 2 data bus to the newtork and and I am still using the original black box.TJ

Installed in a GMT 400? Haven't heard of this mod. Got any pics or part numbers on what's involved? Sounds cool.

I know I wanna do it to but I was unsure if the milage computer would calculate correctly. Wonder if its any different with the 411 pcm then the black box

Whipped383 wrote:

I know I wanna do it to but I was unsure if the milage computer would calculate correctly. Wonder if its any different with the 411 pcm then the black box

I was also wondering if this would work with the 411. Sounds like a worthwhile mod.

Well ya got my thinker thinkin I'm gonna do it I allready have the OHC just need the DIC I'm running COS3 well see if it works

My guess is that it the DIC will work with the 411. The 2001 Bravada has both.
I will work on some pic's, today is a rather slow day at work.

I will put these in a different thread so that I dont totally hijack this one any more than I have!

TJ

Some of the overheads in the GMT 400 trucks were smaller and will fit the DIC form the Jimmy/Bravada/Blazer. . there are a few different ones, the other ones are thicker for the HVAC control. some only have comp and temp and some have all the MPG etc. I put the comp temp one in my dads old tahoe and i had to hook up a speed siginal to make the compass work.