Horrible gas mileage, humming 4x4, and no ABS

[johnjk07]

Hey everyone,

picked up a 99 Yukon for $4k with 134k miles. Very clean truck, especially for the midwest.

I noticed the 4x4 was humming in direct proportion to the speed I was going. Wouldn't do it at first, but after driving for a minute, it would do it. Not horrible, but sounds like loud road noise, and it instantly goes away when I shut it off. Noticed a leak from the pinion seal on the front drive shaft, so I got that replaced and put synthentic in it. 4x4 still hums pretty loud, and it didn't do that when I first got it.

The biggest problem is the horrible gas mileage. We're talking 5-7mpg. At first I thought it was simply due to that I was tromping on it. But last couple tanks, I've been babying it and only letting it warm up for <5 min, only got 6mpg! I replaced the spark plugs and wires, made no difference. Going to replace the fuel filter next. This is all in 2wd - doesn't matter if I baby it, put high octane in it, or whatever. Still horrible MPG! What is weird it it has great acceleration, runs very smooth.

I have a coworker with a 99 Tahoe that runs like crap, backfires and stuff, and he gets the same MPG. WTF?

Finally, the ABS doesn't work. If you hit the brakes, it just locks up and slides. Checked the ABS fuse, looked good...

 

[McFizzl]

On your mpg: Check the air filter. Check your oil. Dont drive in 4wd(possibly check to make sure its fully disengaging). Check all tire pressures.

On your humming again, check to make sure the actuator is working properly. If its not fully disengaging or engaging you could be doing serious damage.

ABS...If fuse is good I would say start with the sensors. Dont know much about the sensors. I believe there is one on each wheel that keeps them from locking the brakes. Dont quote me on that. They went bad in one of our trucks. We never have fixed it. Just learned to not lock up the brakes.

 

[98 2-DOOR]

it sounds like your transfer case is locked in 4wd to me. what you describe sounds like what happens when i drive in 4wd. my actuator did go out and would not allow me shift from 4wd to 2wd. Even your MPG sounds that way...man i get 10MPG in my rig with 3.73 gears. ABS is definitely a wheel sensor. i just had that changed as well. had a to have a local mechanic do it all though.

 

[SunlitComet]

you tcase requires autotrac 2 fluid which is blue in color verify condition and level first.

as far as gas mileage check for any codes and if crappy mileage with back firing and such you may have a problems with the injection spider poppets leaking. there is a new improved assembly to fix problems like that. it would not hurt to inspect and clean the distributor cap and rotor and redo the valve adjustment to make sure it is in good shape.

for the abs system are you saying that when you slow down the brakes apply and dont slip as needed to prevent lockup or the just grab real easily and dont let go?. there is no chattering at all? is you abs light on at all?

 

[johnjk07]

you tcase requires autotrac 2 fluid which is blue in color verify condition and level first.

as far as gas mileage check for any codes and if crappy mileage with back firing and such you may have a problems with the injection spider poppets leaking. there is a new improved assembly to fix problems like that. it would not hurt to inspect and clean the distributor cap and rotor and redo the valve adjustment to make sure it is in good shape.

for the abs system are you saying that when you slow down the brakes apply and dont slip as needed to prevent lockup or the just grab real easily and dont let go?. there is no chattering at all? is you abs light on at all?

Pretty sure it's NOT in 4x4. I know because I hit a patch of ice last night and ran into a snow bank. Took me a 1/2hr to get out, but it was definitely switching between 4hi/lo and 2x2 just fine. It actually never made any noise that time, seems like I have to get into higher RPM for it do to it. I'll look into the autrak II fluid. Any instructions on simply checking the fluid in the T case? Also, the front wheels don't fight me unless I am in 4x4 - it should do that all the time if I was in 4x4. Plus I noticed I get like 4mpg if I'm actually in 4x4, so I tend to think it's engaging/disengaging pretty good.

Any additional info on the better injection spider setup?

It's not throwing any codes. When I bought it, it was throwing a O2 sensor code, I replaced the culprit and it hasn't come back.

Also, it's not my truck that is back firing, it's a buddy of mines. I was just referencing that his sounds like it runs like crap, mine runs great, we both get the same mileage.

I've never cleaned the distro cap or rotor before - any guides on this?

As for ABS, yes, if you slam on the brakes on ice, it just locks up and slides sideways. No slight chatter, no typical pedal resistance. No ABS lights either. Not that I slam on the brakes on ice, but I was testing it...


Thanks for your help.

 

[SunlitComet]

tell you friend about the back fire issue i mentioned in regards to injectors assembly. he might want to make sure his egr is cleaned up well too.

As for the spider assembly, read these threads:
http://www.tahoeyukonforum.com/forum/showthread.php?t=31405&highlight=mpfi
http://www.tahoeyukonforum.com/forum/showthread.php?t=37584&highlight=mpfi

as far as purchase these are usually the best deals. the spider is a direct oem part from delphi. it should already have at least the oval o-ring and bracket. but short of some other seals so get the felpro kit to get the extras you need. the acdelco version is just a renamed delphi unit with all the gaskets but way more expensive. so avoid that route.
http://www.amazon.com/Delphi-FJ10566-Fuel-Injector/dp/B000TYLIFS
http://www.amazon.com/Fel-Pro-MS90269-Plenum-Gasket-Set/dp/B000C2GE6K/

for the dizzy cleaning you will need a torx bit to get cap off. get it off and if needed use a small mini file to clean of the tips inside the cap. do no clean to much of the rotor tip. some rotor hav what looks like carbon build up just at the tip made by the factory. if you like amazon has replacement delphi parts for low prices. when reinstalling wiggle the cap as you seat it to make sure it is flat and SNUG the screws done. continuously overtightened screws can strip the dizzy body.

the 4x4 will fight anytime it has dryish payment on all four wheels usually or if the front and rear axle ratios are not matched. there are two screw in plugs in the back of the tcase lower is drain, upper is fill. fill to the bottom of the fill hole. if it is any color then a blue to deep blue color change it immediately. does not require more then 3 quarts.

what mode of 4x4 gives you the bad mileage? 4wd lo will do that because of the gear reduction. but at any rate any tires slipping in 4wd lo or hi will cut your mileage.

also get some maf cleaner if your maf sensor is really dirty and inspect your air filter indicator as well.

on key startup does your abs light come on at all then go off?

 

[johnjk07]

Should I just replace the distribution rotor, it's like $10?

This right http://amzn.com/B000BYGD2O ?

As for the spider conversion, should I do that on mine? A little pricey, but given my bad mpg seems like something is pretty messed up. I want to do k&n conversion, and do the black bear chip upgrade.

Other question. When I turn sharp either direction, it lopes. This isn't like the 4x4 lope, that feels different. This happens when I crank hard either direction. I thought the CVs were bad, but I had a mechanic glance at them and he said they were good. Though this mechanic isn't the best.. Boots looked ok though.

If I listen carefully, I can hear the humming even in 2x2. It sounds like a slightly flat tire. In 4x4 it gets loud. Could a bad CV do that? I was thinking when the torque is applied to it, maybe that's when it grinds. I had an old Pontiac that had the CV go, and it loped similar to what this is doing.

Any advice? Sorry for all the noob questions.

 

[SunlitComet]

you can do the cap and rotor if you want. in fact if you can not identify them as delphi or delco units it is probably best that you do. the crossfire cap is sensitive to electrical breakdown when things in the ignition system get out of hand. but at the same time if they are good right nw just leave it for now.

If you are passed 100,000 miles then change the spider if not done already. it is all internal rework so you will see no difference on the outside of the engine. get the engine back to normal thru needed repair before adding cai or pcm tunes(btw yours does not use a chip, you simply reprogram the pcm internal memory.).

are you saying the engine lopes or does it feel like you are losing traction when hard turning in 2wd?

give you tires a good one over inspection by spinning them to loo for any thing out of the ordinary. listen for the noises if they are there and wiggle you tires back and forth, up and down to check for bad bearings first.

 

[johnjk07]

Has 135k so I'll look into the spider conversion. I'll also look into cleaning the distro. So you're suggesting cleaning it over just replacing It?

The engine doesn't lope when turning, just feels like its binding. Someone told me its due to how tight of a turn these can do, but I kinda think that's not true. I had a silverado where the tires would slip/screech on a right turn, but never bind up and lope.

Will check the hubs too and post what I find.

 

[SunlitComet]

you can replace it but just trying to help you save a few bucks if the parts are in relatively good condition. is the binding effect feeling consistent with engine speed at all. make sure your tires are aired up cause that will wreck some havoc with the steering too.