4.0L fuel use at idle

railguy · **Posted:** Thu Apr 09, 2015 12:41 pm

My trusty '98 Cherokee is misbehaving a bit (I think) after years of faithful service (about 112,000 miles now). My problem is this: My fuel economy has suffered somewhat over the last few months. I hooked up my ScanGauge II to the Jeep and I found that the fuel consumption with a fully warmed up engine is from 0.58 to 0.62 gallons per hour at idle. That seems excessive to me--based on the displacement of the 4.0L, I would have expected it to be a little more than half that. The ScanGauge shows no codes. I did replace the rear oxygen sensor and catalytic converter and that had no effect on the fuel usage. I have also checked and I can detect no exhaust manifold leaks (no noticeable exhaust smell under the hood). The engine uses no oil, so I don't suspect any compression issues. Air cleaner is good. The MAP sensor was replaced about 10K miles ago, the plugs and plug wires about the same time. The engine idles and runs smoothly, with no misfires, so I think the injectors are fine. I've run some injector cleaner and Marvel Mystery Oil over the last couple of tanks of fuel with no effect. I'm using the same fuel source that I've used for the last three years. I've run out of ideas of what to look for. I would take it to a dealer for a thorough check-up, but the nearest dealer is 80 miles away and does not have a good service reputation. In the itty-bitty town where I live, there is no mechanic with a full diagnostic setup. I've checked for all the other usual stuff--sticking brakes, underinflated tires, etc., etc.

I would appreciate any suggestion for things that I might check. Thanks in advance.

P.S.--Meanwhile, my '05 CRD just keeps chugging away--thanks to a GDE Ecotune some years back.

lfhoward · **Posted:** Mon Apr 13, 2015 6:51 pm

Have you noticed if it is idling faster than usual? If so, a vacuum leak could be the culprit. The vacuum hoses on my '00 XJ started to get brittle and go bad after it was about 10 years old. In my case, I lost cruise control first.

railguy · **Posted:** Mon Apr 13, 2015 11:41 pm

Idle speed is dead-on to spec, around 750 RPM in Park.

4.0L fuel use at idle

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