plummeting mpg
08-27-14, 05:27 PM
#1
plummeting mpg
Recorded fill-ups since I've had the car ('94 SC400):
17.55 mpg (16.87 ga)
15.05 mpg (5.72 ga)
18.09 mpg (16.25 ga)
15.56 mpg (14.59 ga)
13.69 mpg (6.94 ga)
I missed a couple when I didn't write down mileage numbers and forgot that the digital trip odometer resets when disconnecting the battery... This is mostly around-town driving, with no crazy aggressive throttle stomping action, and no dramatic change there that would correlate to the drop in mpg. Actually, the best mpg came during the period when I did the A/T solenoid bypass and was driving harder to test out the newfound acceleration capabilities.
The 15.05 mpg #2 was negatively influenced by some fuel leakage while testing stuff. I had the ECU repaired late in the 15.56 mpg #4. Oddly, since then my fuel gauge seems to be perfectly calibrated. Prior to that, it consistently read 3-4 gallons low. Is it even possible for the ECU to recalibrate the gauge, or am I losing my mind?
And can an ECU that now seems to be working great cause the car to run richer than it did before, like it was compensating for something and is now thrown out of whack? I had the capacitors replaced, as well as traces/pins repaired that directly affected the O2 sensor and knock sensor inputs. (No more knock sensor trips since then, yay!)
Other things I've done during the last few fill-ups include: EGR delete (late in the 18.09 mpg fill-up), new Denso upstream O2 sensors (midway through the 15.56 mpg fill-up), and new fuel pressure regulator (early in the 15.56 mpg fill-up). The old O2 sensors were reading a bit lazy, but not too terribly far out of spec. I'm not getting any CEL codes.
The most likely culprit, IMO, is a trick I've been employing to side step the long-crank start-up problem I've yet to fully solve. I believe it's a fuel pressure thing, but the new FPR didn't do the trick, so it's probably leaky injectors or bad tank venting, creating excessive tank pressure. I've been bumping the starter very briefly, waiting a second, then hitting it again and it fires up immediately every time. Just gotta build pressure first. I figure that'll keep the starter happier until I get at the heart of the matter, rather than cranking it for 3-4 seconds every time.. I can't find in my notes when exactly I started doing that, so I'm not sure which fill-up it corresponds to. Probably the last two directly. Could that be creating a situation where the engine is being double-flooded in cold start mode or something?
I've got new plugs/wires -- haven't pulled them again to check for black fouling, but last time I looked to check timing (less cylinder pressure) early in the 18.09 mpg fill-up they looked good -- new caps/rotors, new vacuum hoses, replaced the MAF, cleaned the IAC valve, cats not glowing red last time I checked...
There's a pretty strong gas smell, but how do I check for a leak, other than looking for obvious fluid? I haven't seen anything on the ground or around the engine bay. I wouldn't be shocked to find something iffy going on with the injectors, though.
17.55 mpg (16.87 ga)
15.05 mpg (5.72 ga)
18.09 mpg (16.25 ga)
15.56 mpg (14.59 ga)
13.69 mpg (6.94 ga)
I missed a couple when I didn't write down mileage numbers and forgot that the digital trip odometer resets when disconnecting the battery... This is mostly around-town driving, with no crazy aggressive throttle stomping action, and no dramatic change there that would correlate to the drop in mpg. Actually, the best mpg came during the period when I did the A/T solenoid bypass and was driving harder to test out the newfound acceleration capabilities.
The 15.05 mpg #2 was negatively influenced by some fuel leakage while testing stuff. I had the ECU repaired late in the 15.56 mpg #4. Oddly, since then my fuel gauge seems to be perfectly calibrated. Prior to that, it consistently read 3-4 gallons low. Is it even possible for the ECU to recalibrate the gauge, or am I losing my mind?
And can an ECU that now seems to be working great cause the car to run richer than it did before, like it was compensating for something and is now thrown out of whack? I had the capacitors replaced, as well as traces/pins repaired that directly affected the O2 sensor and knock sensor inputs. (No more knock sensor trips since then, yay!)Other things I've done during the last few fill-ups include: EGR delete (late in the 18.09 mpg fill-up), new Denso upstream O2 sensors (midway through the 15.56 mpg fill-up), and new fuel pressure regulator (early in the 15.56 mpg fill-up). The old O2 sensors were reading a bit lazy, but not too terribly far out of spec. I'm not getting any CEL codes.
The most likely culprit, IMO, is a trick I've been employing to side step the long-crank start-up problem I've yet to fully solve. I believe it's a fuel pressure thing, but the new FPR didn't do the trick, so it's probably leaky injectors or bad tank venting, creating excessive tank pressure. I've been bumping the starter very briefly, waiting a second, then hitting it again and it fires up immediately every time. Just gotta build pressure first. I figure that'll keep the starter happier until I get at the heart of the matter, rather than cranking it for 3-4 seconds every time.. I can't find in my notes when exactly I started doing that, so I'm not sure which fill-up it corresponds to. Probably the last two directly. Could that be creating a situation where the engine is being double-flooded in cold start mode or something?
I've got new plugs/wires -- haven't pulled them again to check for black fouling, but last time I looked to check timing (less cylinder pressure) early in the 18.09 mpg fill-up they looked good -- new caps/rotors, new vacuum hoses, replaced the MAF, cleaned the IAC valve, cats not glowing red last time I checked...
There's a pretty strong gas smell, but how do I check for a leak, other than looking for obvious fluid? I haven't seen anything on the ground or around the engine bay. I wouldn't be shocked to find something iffy going on with the injectors, though.
Last edited by t2d2; 08-27-14 at 06:22 PM. Reason: clarified a few things
08-27-14, 07:04 PM
#2
I would pull the air intake off and look to make sure there was no blockage by the trac. control butterfly being stuck, just makes me think something is blocking air flow into engine. Have you changed the impossible fuel filter?
08-27-14, 07:21 PM
#3
Thanks. Forgot to mention those...
Changing the fuel filter was one of the very first things I did, and boy was that a nightmare. 5+ gallons of gas never stopped draining down my arm (extensive burnt skin for a couple weeks), and the factory threads' near-impossibility of removal was exceeded only by their complete impossibility in putting the new filter on. There wasn't any cross-threading. The mechanic figured the factory threads must be filled with some sort of sealant goop that becomes cement over time. Same thing happened with my fuel supply line fitting in the engine bay. He snapped a wrench in half trying to loosen it and gave up before badly damaging another line. The one by the filter had to be cut out and replaced by a usable line with a new flange.
My car doesn't have TRAC, so only the one throttle plate. I would have to pull the air hose off to check for a blockage there, but wouldn't that be pretty obvious with the car's performance?
I was thinking about buying a new air filter soon just for the heck of it. I have two decent ones, having blown them out with compressed air and putting one back in after taking the K&N filter out that may have contributed to the "early" demise of my MAF. Everything is clean and dry in the airbox. And I've had the throttle body "opened up" for cleaning not too far back, so it's doubtful anything has gotten in there recently with the filter still intact.
One other thing I did after the EGR delete was remove the fuel pressure VSV. No real reason, other than the Lextreme instructions saying it can be done and I couldn't find any compelling arguments not to, so I figured I would give it a try in case it related to my fuel pressure (long crank) issue. That only affects warm re-starts, though, right? I wouldn't think it has anything to do with the mpg.
Changing the fuel filter was one of the very first things I did, and boy was that a nightmare. 5+ gallons of gas never stopped draining down my arm (extensive burnt skin for a couple weeks), and the factory threads' near-impossibility of removal was exceeded only by their complete impossibility in putting the new filter on. There wasn't any cross-threading. The mechanic figured the factory threads must be filled with some sort of sealant goop that becomes cement over time. Same thing happened with my fuel supply line fitting in the engine bay. He snapped a wrench in half trying to loosen it and gave up before badly damaging another line. The one by the filter had to be cut out and replaced by a usable line with a new flange.
My car doesn't have TRAC, so only the one throttle plate. I would have to pull the air hose off to check for a blockage there, but wouldn't that be pretty obvious with the car's performance?
I was thinking about buying a new air filter soon just for the heck of it. I have two decent ones, having blown them out with compressed air and putting one back in after taking the K&N filter out that may have contributed to the "early" demise of my MAF. Everything is clean and dry in the airbox. And I've had the throttle body "opened up" for cleaning not too far back, so it's doubtful anything has gotten in there recently with the filter still intact.
One other thing I did after the EGR delete was remove the fuel pressure VSV. No real reason, other than the Lextreme instructions saying it can be done and I couldn't find any compelling arguments not to, so I figured I would give it a try in case it related to my fuel pressure (long crank) issue. That only affects warm re-starts, though, right? I wouldn't think it has anything to do with the mpg.
08-27-14, 07:32 PM
#4
BTW, I wasn't a complete moron in trying to change the fuel filter with 1/4 tank of gas. That was shortly after I bought the car, before I knew that the gauge read ~4 gallons low! I had it right on empty and figured there was at most 1 ga left.
Trending Topics
08-27-14, 08:24 PM
#8
Oh, haha, I see what you meant now. Low tire pressure would equate to bad mpg.
I'll re-check all my hoses with daylight tomorrow. They're all new and very recently added (or capped, after the EGR delete), so I'd be very surprised if that's where the problem is. Well, all the small (5/8") lines are new. I didn't replace the big ones that connect up to the intake hose right after the MAF.
I'll re-check all my hoses with daylight tomorrow. They're all new and very recently added (or capped, after the EGR delete), so I'd be very surprised if that's where the problem is. Well, all the small (5/8") lines are new. I didn't replace the big ones that connect up to the intake hose right after the MAF.
08-28-14, 11:24 AM
#9
I don't see any issues with the vacuum hoses. I would need to get some brake cleaner to spray on the hoses while the car is running and see if that causes a change in idle to inspect that angle any further.
I did remove my recently cleaned IAC valve and run the checks for resistance between terminals and open/close under battery power, neither of which I had done previously. It passed both tests just fine.
As for the air intake post-MAF, I don't think I have any issues there. I've previously tried OLT's suggestion of pulling up on the black box below the throttle body end of the intake, but mine was already snug and it made no difference. I also wrapped the accordion flex hose with duct tape awhile back, since it is quite dry and brittle feeling. I'll replace it eventually, but I wanted to see if sealing it up would highlight that there was a leak previously. Again, no difference.
I'd go ahead and do the injectors, but man, I don't want to have to try getting those EGR pipe bolts off the back of the manifold again... One of them I think was a bit cross threaded and super difficult to work with. If I could have accessed the bottom end of the pipe, I would have removed it altogether and never have to deal with it again, rather than just capping the top end and reattaching it.
I did remove my recently cleaned IAC valve and run the checks for resistance between terminals and open/close under battery power, neither of which I had done previously. It passed both tests just fine.
As for the air intake post-MAF, I don't think I have any issues there. I've previously tried OLT's suggestion of pulling up on the black box below the throttle body end of the intake, but mine was already snug and it made no difference. I also wrapped the accordion flex hose with duct tape awhile back, since it is quite dry and brittle feeling. I'll replace it eventually, but I wanted to see if sealing it up would highlight that there was a leak previously. Again, no difference.
I'd go ahead and do the injectors, but man, I don't want to have to try getting those EGR pipe bolts off the back of the manifold again... One of them I think was a bit cross threaded and super difficult to work with. If I could have accessed the bottom end of the pipe, I would have removed it altogether and never have to deal with it again, rather than just capping the top end and reattaching it.
08-28-14, 01:03 PM
#11
Ha, I wish it could be attributed to smoking the tires and winning pink slips. 
Nothing has changed in that respect (for better or worse), though.
Oh, I found in my notes where I started doing the bumping the starter trick for long cranks at start-up. It was somewhat late in the 18.09 mpg fill-up. If it's to blame, then that tank would have been 20+ mpg. Also, it would have affected the subsequent 15.56 mpg one just as much as the most recent 13.69 mpg one, so that could probably be eliminated as the prime suspect.
One thing that I didn't consider initially is the A/C servicing last week prior to the 13.69 mpg fill-up. At only 6.9 ga, it may have been overly influenced by however long the car was running on the lift while they were inspecting for refrigerant leaks, burning fuel but racking up no mileage.
I've got a few hours on the highway coming up tomorrow or Saturday, so if I don't do much around town driving of the SC before then, I can do an almost strictly highway fill-up calculation with very few starts and stops. That should give a good baseline number and eliminate several unknowns.
Nothing has changed in that respect (for better or worse), though.Oh, I found in my notes where I started doing the bumping the starter trick for long cranks at start-up. It was somewhat late in the 18.09 mpg fill-up. If it's to blame, then that tank would have been 20+ mpg. Also, it would have affected the subsequent 15.56 mpg one just as much as the most recent 13.69 mpg one, so that could probably be eliminated as the prime suspect.
One thing that I didn't consider initially is the A/C servicing last week prior to the 13.69 mpg fill-up. At only 6.9 ga, it may have been overly influenced by however long the car was running on the lift while they were inspecting for refrigerant leaks, burning fuel but racking up no mileage.
I've got a few hours on the highway coming up tomorrow or Saturday, so if I don't do much around town driving of the SC before then, I can do an almost strictly highway fill-up calculation with very few starts and stops. That should give a good baseline number and eliminate several unknowns.
08-31-14, 08:16 AM
#12
I didn't have time to fill up before dropping the car off yesterday with the mechanic to work on A/C leaks and a few little things, but it was at 130 miles of mostly highway driving on what looked like ~6 gallons. That would put it in the 22 mpg range, so pretty close to spec. In which case, it's just around-town use and frequent starts (relative to miles driven) that need closer inspection for low mpg.
08-31-14, 12:07 PM
#13
Fill your tank up to the max every time you put gas in your car. It helps prevent you from going to the gas station to get gas, therefore saving gas.
It's always good to hop on the highway and give it a good test. That's the only way your going to figure out if it's correct or not.
It's always good to hop on the highway and give it a good test. That's the only way your going to figure out if it's correct or not.
09-14-14, 01:39 PM
#15
I've got a few more data points.
1) 13.939 ga @ 255 miles = 18.29 mpg
2) 5.364 ga @ 121 mies = 22.56 mpg
#1 was 130 miles of highway driving initially and looked to be in the 22-23 mpg range, but the rest of the tank was mostly city. #2 was all highway, with half of it being with the A/C on, plus 4 miles of city driving at the tail end.
So, I know the highway mileage is right around the rated 23 mpg. The big issue appears to be city driving, with the last half of fill-up #1's around-town driving pulling the average all the way down to the rated 18 mpg for city. That means it was averaging ~14 mpg city, which is in line with the prior fill-ups that were all city driving.
Any idea why the highway mileage would be spot on but the city mileage so low? Or is the 18 mpg city rating a myth?
1) 13.939 ga @ 255 miles = 18.29 mpg
2) 5.364 ga @ 121 mies = 22.56 mpg
#1 was 130 miles of highway driving initially and looked to be in the 22-23 mpg range, but the rest of the tank was mostly city. #2 was all highway, with half of it being with the A/C on, plus 4 miles of city driving at the tail end.
So, I know the highway mileage is right around the rated 23 mpg. The big issue appears to be city driving, with the last half of fill-up #1's around-town driving pulling the average all the way down to the rated 18 mpg for city. That means it was averaging ~14 mpg city, which is in line with the prior fill-ups that were all city driving.
Any idea why the highway mileage would be spot on but the city mileage so low? Or is the 18 mpg city rating a myth?