G20: why sudden mpg decline?

[chuckm]

A puzzle for G20 friends: my beloved '93 used to deliver 30-38 mpg in my driving cycle--always. A while ago, it dropped to ca. 24-28, for no perceptible reason, with no loss of power or definable symptom. I've checked all performance-related items in/with FSM--MAF, TPS, ignition/timing, injectors, vacuum lines, PAIR, EGR, FPR, etc. No sign of glitch. Replaced O2 sensor, rotor/cap, plugs for hell of it. Checked it all again. No sign of fuel leakage, all components/systems apparently within FSM specs. Car still runs FINE--but mileage crappy anyway.I must be missing something, but have run out of possibilities. If anyone has a clue to a solution, I'll name my next grandchild after you!Thanks in advance for your kind assistance.Chuckm

[aebstract]

Okay this may sound silly and very basic.. but you didn't mention oil? Has it had its last oil change recently and/or is it about ready for one? I know that mpg rate starts to drop around the time an oil change is needed on a car. Like I said, very basic.. but if this was the problem then that would be awesome

[chuckm]

Hey, thanks for your response. You're right: oil and most everything about it is important to an engine. But probably not a factor here. I change O/F pretty religiously @ 3K--and watch level/condition in between--and did so just a few hundred ago. I stick with Valvoline 10/30, and on this car have seen virtually no oil 'consumption' between changes in the 7 years I've owned it (am 2nd owner). It's been hands down the best, most reliable and troublefree car I've ever owned...hence my upset about fuel consumption, which just plain mystifies me. Have gone thru FSM diagnostics at least 2x completely, in some parts more often. Nothing shows up 'wrong'--yet something obviously isn't right. I even went so far as to switch to premium gas (tho it's made for 87 octane). I can't see any improvement with that either (well, maybe a % or so more pep--but not enough to justify continuing the extra cost indefinitely.)

If you have any further ideas, I welcome them. Thanks again, chuck

[ABNRML4U]

Ummm, How many miles do you have on your Nissan? http://cars.about.com/od/helpf...e.htm^^^ That might help or try to Google.com the problem that will help too.Have you checked your fuel filter and air Filter you might want to re-place those. Also do you have a SR20 engine?

[chuckm]

Thanks to kind replies to my problem. Car now has ca. 174K (carefully driven, well-maintained) miles on the clock. A lot--but not so much for a good SR20DE. It still has good, even compression, burns no measurable oil in the 3K it gets between changes, will still go 80+ all day thru the mountains, quite happily, just as years ago--but, as said, with poorer mpg. Trust me, I've tried/checked everything cited anywhere in the FSM and found no emissions/ignition/fuel system part out of specs. Filters are recent (and old ones showed little sign of crud). Someone asked about plugs. I've had best luck with NGK Lasers. Tried Bosch (2-3 years ago) & Autolite platinum (this summer), but got poorer performance, so went back to NGKs. So, I'm still mystified. I did have trouble ca. 5 years ago with the PAIR system--which will mess you up. But replaced all those bits--and have re-checked them recently & found them still in good order & functioning. It's really weird. Car starts first time every time, idles evenly @ ca. 800, revs smartly, as it should, accelerates about as it did years ago, has no trouble at speed--just crappy mpg. And yes, I am aware of tire pressures, tire size, tread pattern, etc., as possible factors. These are not an issue.Well, thanks for trying. chuck

[nametakennow]

Air filter? IACV? Also pretty basic, but worth a check. Read this thread a few days ago and have pondered it on and off since. It's a strange one for sure.

[RED_DET]

What did you gap the plugs at?

[chuckm]

The Lasers? if I remember correctly, what NGK recommends--0.44. Should it be otherwise? chuck

[chuckm]

THanks. Air filter is recent and still clean. IACV ... well, I guess it could be wonky. It's so hard to get to on a '93, I really haven't studied it much, must admit--just took for granted that if idle is where it should be, it must be more or less OK. Also admit I don't know how this item affects fuel consumption: FSM doesn't seem to address this consideration, so far as I can tell.And...I must also say, the FSM, even tho I guess it's the last, best word on this vehicle, well, it's a weird set of documents. ah well. life isn't perfect.chuck

[RED_DET]

.44 is correct. I assume you have checked the timing?