troubleshooting lower fuel economy

Hey all,

2001 XJ 330,000 miles (mini-cat). Starting middle of last month I noticed poor fuel economy. It was pretty obvious as it was on a cross country trip. I had expected 17-19 MPG and was recently getting 15.5 MPG except on several tanks in Eastern KY, NC, MD, and VA. The worst fuel economy was seen on fuel purchased in CO, KS, MO, and Western KY. Normally in the summer time I'd expect up to 22 MPG in Colorado but I would take anything over 18 MPG. I know in the winter fuel economy is less. In NC, my first stop, I had codes checked and there were none except for the bank 2 catalytic efficiency which has been there for about 230,000 miles (long enough for the CEL to burn out). I had ruled out driving into a headwind by the time I got to Missouri. Power has been fantastic and the engine does not smoke or run rough. Sometimes the exhaust rattles a little more at a stop light.

I performed some diagnostics with more to come:

- Compression check (adjusted for altitude @6500 ft): 157, 166, 160, 159, 162, 163
- Spark plugs: #6 was perfect, #2 had a small amount of carbon on it, all other were more heavily carboned
- MAP sensor gave an accurate voltage reading testing for current barometric pressure
- IAT gave an accurate resistance based on current temperature
- Fuel pressure with ignition power on goes to 45 psi and then drops slowly. After 30 minutes it was still at 20 psi.

Could this be a case of worn out fuel injectors?

Winter fuel has additional ethanol in it, and other stuff. My fuel mileage in my heep always bombs a bit while driving during winter.
With that many miles though, some 784 injectors would pep it up a bit.
You may need to replace the bank 2 O2 sensor, that can/will affect mileage.

I'm not expecting summer fuel economy, but 15.5 is as low as its ever gotten on the highway unless I'm towing. I've had this Jeep for 10 years so I'm pretty confident it should be doing better. Looking back, it may have been getting lower for a while but last month was the first time in a while I took it steady on the highway for prolonged periods, so this problem could have started a bit earlier in the year.

Not model specific, but in general, when my mileage lowers enough to notice I check for vacuum leaks, O2 sensors and temperature sensors (engine, MAT and battery).

Thanks... this afternoon I dropped some fuel injector cleaner in the tank and also went through the intake/exhaust manifold bolts and was able to snug up a few of them. I'd steer more quickly towards the O2 sensors if all plugs looked the same. I'll see if there is any improvement and go from there.

Have you ever run Seafoam/Berryman's B12 through your intake via the brake booster vac line? This helps clean out your intake and such. If there is a leak large enough it might even help you find that.

No, not yet.

I tightened up the manifold bolts. When tightening two of them, I heard the manifolds shift like they were seating. So I may have had an air leak. I also removed the O2 sensors and tested them with a butane torch. I damaged one on removal and it never made any voltage with a butane torch. The others seemed to operate as well as a new one with a butane torch, reaching 0.90V and quickly going back to zero with the torch removed. I put in the three new ones that I since I already had them out and that lower one by the oil pan is tough to get at. I also threw in 2 bottles of Techron.

md21722 said:

I tightened up the manifold bolts. When tightening two of them, I heard the manifolds shift like they were seating. So I may have had an air leak. I also removed the O2 sensors and tested them with a butane torch. I damaged one on removal and it never made any voltage with a butane torch. The others seemed to operate as well as a new one with a butane torch, reaching 0.90V and quickly going back to zero with the torch removed. I put in the three new ones that I since I already had them out and that lower one by the oil pan is tough to get at. I also threw in 2 bottles of Techron.

Click to expand...

and your results were ...

Good call on the vacuum leaks. Get some BG 44K and add the can to a full tank of gas. Run it to 1/2 tank and refill. Run that batch down to 1/8 and refill.

I did not drive the Jeep much right after tightening the manifold bolts and testing the O2 sensors because the blower motor was also on its way out. I fixed that about 6 weeks ago and have been driving it again. Fuel economy where I expect it to be. In my original posts, I was looking at where on my trip was poor, but I think ambient temperature, not location, may have had more to do with it. Whenever fuel economy was best, ambient temperature was probably the highest. Since the O2 sensors generally tested fine, I think the manifold bolts were the more likely cause.

Thank you for the update.