I have a 97 with an L36 and 252k. When I first bought the car two years ago, it got a solid 19 to low-20s for city economy, but as of right now, I am seeing anywhere from 11 to low 14s.
Recent work done to the vehicle includes upper intake, PCV valve, air filter, fuel filter, plugs (AC Delco), plug wires, ignition coils, ICM, Denso O2 sensor (was getting a frequent code for O2 circuit and was replaced with Bosch that lasted ~6 months before the code came back, and replaced it with a Denso after some more research), replaced the o rings at the fuel rail inlet (3/8 line on the front of the motor), and a transmission filter+fluid. All done within the last four thousand miles. FPR replaced less than 300 miles ago, as it was suggested from one of the service coordinators for the shop next door (he also has a Bonneville and recommended doing since they are a "common" failure point on these and it's only a $30 part).
I have over 4,200 miles of recorded bad fuel economy getting an average 14.4 mpg, seeing anywhere from 172 to 230 miles per tank (~15-16 gallon per tank), done using the miles driven per tank divided by gallons at the pump (at first shut off)...compared to the first 2200 miles an average of 19.4, this is a MASSIVE drop.
Only CEL is for a faulty heater circuit in the down stream O2 sensor (won't ever effect fuel economy since downstream only measure cat's condition); this code only first appeared today (10/08). Only other code I get is an occasional super generic P04xx code. I don't remember which one, I haven't exactly had it pop up in the last 3 months. I have been inconclusive in chasing down the source of this as well.
Other information I have: -Don't ever hit above 15% throttle when accelerating in my NORMAL daily driving conditions (shifts out of 1st around 2300-2500 rpm and all other gears in the 1900-2100 rpm ranges), if I give any less throttle, it takes a WHILE to get to even 25mph. -Performance is beyond exceptional when doing heavy throttle. -Short Term Fuel Trim bounces from -2 to around 8 at idle. -Long Term Fuel Trim hovers in the 8-10 area when idling. (NOTE: I don't have 2k or 3.5k rpm trims yet as I always forget to check while engine is warm) -Ignition timing ranges anywhere from -35 degrees (13⁰F cold start) to 0 degrees (pretty much WOT), warm idle is normally around the -22 ish area. -Vacuum is normally in the 22-23psi range with no clear signs of a leak. -Intake pressure is normally a positive 5-6psi, which confuses me as it counteracts the vacuum reading -ECTS is reading fine and is telling the ECU temperatures up to 210 degrees (this is when idling for a while like when sitting at a drive thru), normally sits around 197 when driving. -my torque pro app is not picking up a MAF signal. Not sure if this is an app issue or not. I have unplugged it when warm, and the engine DOES begin to stumble after unplugging, which makes me think it's at least functional, so again, no idea why no signal according to Torque. -Tires are normally around 35psi on a brand new set of all seasons (maybe 600 miles on them) -Oil pressure sits 25-ish PSI warm idle, 50 ish cold idle, around 40-45 ish warm above idle. (Not sure if this matters, just feel like sharing) -Takes a while to finally turn off converter lock up when climbing hills -Has an OCCASIONAL shutter / miss at idle, but normally only occurs if I idle for more than 5-10 minutes. Solving this is just as inconclusive as both my other issues. Never had a code pop up for it either. -On a COLD start (below 40⁰F), I get a hammering sound accompanied by a hazy white/gray smoke that lasts anywhere from 10 seconds to 3 minutes depending on how cold it is when it happens before the symptoms COMPLETELY disappear without a trace. Confirmed not a head gasket (combustion leak test). Smoke / Haze does not smells sweet, maybe slightly carbon rich aka rich?
At this point my 4x4 Ranger is getting better fuel economy (3.0L V6 5 speed @ 18.7 city average), which I find sad since it weighs the same but has significantly less power "per rpm" and the fact that I drive it MUCH harder than I do my Bonneville.
Last two points of my other info are making me think either injectors or some weird PCM issue, ESPECIALLY the last one. Cuz at this point, I'm at a loss for words on what else could be causing this without tripping a hardcore code. I will get a compression test on Tuesday for shoots and giggles, but I don't think it'll be compression related, not with the power I have when I do decide to be a little spirited (which isn't that often at all as I don't want to risk tearing my transmission a new one).
Any other suggestions or people who have had similar occurances that they have since resolved?
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