Fuel economy issues!?

[ryanc5413]

Hey guys. I'm Ryan I am a professional mechanic and I own a 98 Yukon. I'm having insane fuel usage problems. Last year I was getting 9 mpg of 93 and today I ran out of gas with 145 miles on the trip odo. Calculated out to 6 mpg. I need some advice.

The truck is on 285s with a tuned hot computer from wait4me
Removed converters
Short tube headers
Flow master exhaust
K and n full intake
Bored maf
Built trans with stock stall.
E fans
186 t stat tuned for closed loop at 180
Throttle blade mod

All of these mods have been done long before the fuel usage problem started.

No check engine lights or pending codes. Compared maf and map and all other sensor readings to a know good running truck. You can smell the fuel from the pipes. I'm thinking poppets are not closing but is there any way to narrow it down to the injectors before throwing that 400$ at it? I can't stand 6mpg its killing me. That was in town empty.

Thanks in advance

 

[beardeddummy]

Open the throttle blade and smell for raw gas

 

[michael hurd]

Hey guys. I'm Ryan I am a professional mechanic and I own a 98 Yukon. I'm having insane fuel usage problems. Last year I was getting 9 mpg of 93 and today I ran out of gas with 145 miles on the trip odo. Calculated out to 6 mpg. I need some advice.

The truck is on 285s with a tuned hot computer from wait4me
Removed converters
Short tube headers
Flow master exhaust
K and n full intake
Bored maf
Built trans with stock stall.
E fans
186 t stat tuned for closed loop at 180
Throttle blade mod

All of these mods have been done long before the fuel usage problem started.

No check engine lights or pending codes. Compared maf and map and all other sensor readings to a know good running truck. You can smell the fuel from the pipes. I'm thinking poppets are not closing but is there any way to narrow it down to the injectors before throwing that 400$ at it? I can't stand 6mpg its killing me. That was in town empty.

Thanks in advance

First, get rid of big tires, that's easily 2-3 mpg, especially in town with lots of stop and go, and turning. Low pressure and alignment issues will also cause low mileage.

Second, the oiled K&N isn't doing you any favors, especially if over-oiled. It will coat the MAF sensor wires, leading to inaccurate readings.

Replace the bored out MAF with a STOCK MAF. The fuel maps are likely calculated on the stock MAF. Same with throttle body.

Replace the thermostat with a factory replacement.

Check engine grounds, especially the one at the rear of the cylinder head. Not a visual check, a check with a multimeter to determine resistance.

You won't have a check engine light for converter efficiency, since they would have been tuned out in the computer, however data from the post converter O2's are also used in fuel trim.

Check for dragging brakes, especially the return springs in the rear drum brakes. A non contact digital temp gun will help out here.

 

[df2x4]

^ Some good info there.

One thing to note though, unless he specifically requested that the rear 02 sensors were tuned out, he'd probably still get a CEL for cat efficiency. I never mentioned it when I got my tune from Black Bear and they left it alone. I don't mind running cats.

EDIT - Just noticed he said cats were removed. Guess that means he probably had the rear 02's tuned out. Carry on!

 

[slowburb]

There ought to be a flashing MIL on the instrument cluster, for the dumping of raw fuel in at least one hole. Does it still work or is it tuned out?
How's the idle?
Is overall engine output down?
Have you pulled the plugs?
Fuel leaking anywhere?

 

[kennythewelder]

It sounds like a fuel leak. If you pull the throttle body, you can take an inspection mirror, and a flash light, and see all of the fuel lines feeding the popet valves, and most of the CPI unit it self. If you jump the FP relay, and make the FP run while you look at the fuel system, any leak can be seen. Here is a good video on jumping the FP relay. He jumps the relay ay about 17 mins in the video. It is a good idea to watch the whole video though.

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[beardeddummy]

If you have the computer tune for the descreened maf and you go back stock you will loose mpg and power i know from experience. I would imagine the same holds true for the thermostat once you tune for something as far as the truck knows it's stock.

 

[skylark]

Ummm. A fuel pressure test will easily show the fuel pressure dropping with the key turned off.

 

[kennythewelder]

Ummm. A fuel pressure test will easily show the fuel pressure dropping with the key turned off.

This is true, but it won't show where the leak is. If you jump the FP relay, you can check the system under pressure, and any leak will be easy to find.

 

[ryanc5413]

First, get rid of big tires, that's easily 2-3 mpg, especially in town with lots of stop and go, and turning. Low pressure and alignment issues will also cause low mileage.

Second, the oiled K&N isn't doing you any favors, especially if over-oiled. It will coat the MAF sensor wires, leading to inaccurate readings.

Replace the bored out MAF with a STOCK MAF. The fuel maps are likely calculated on the stock MAF. Same with throttle body.

Replace the thermostat with a factory replacement.

Check engine grounds, especially the one at the rear of the cylinder head. Not a visual check, a check with a multimeter to determine resistance.

You won't have a check engine light for converter efficiency, since they would have been tuned out in the computer, however data from the post converter O2's are also used in fuel trim.

Check for dragging brakes, especially the return springs in the rear drum brakes. A non contact digital temp gun will help out here.

Truck is tuned for the ported maf,lower temp thermostat. I am a profession mechanic. Its not brakes or grounds. It has no converters and tuned for no converters. Tires again not going to cause this. 7 months ago same tires it was giving 8 mpg. Still way low but now 6. Nothing has changed. I have owned 57 gmt400s and I know every nut and bolt on them. Thanks but the things you pointed out were checked a year ago.