Help me troubleshoot poor fuel economy
 

The last 2 tanks I've averaged 18 and 19 mpg, respectively, with about 70/30 highway/city. From what I gather, it's about 7-8 mpg difference which is pretty significant with a 6 speed. I typically cruise around 75 mph on the highway and I'm not hooning around in the city.

Mods that I know of to the car are just a sprintex (69mm pulley) and CAI which wouldn't do much. I do have the torque app which I suppose could help track fuel usage, but I don't know how to read the logs because I'm a noob. I'd be willing to save some logs and post them here if someone could get anything useful out of it.

The only things I can think of are I either have a leak or bigger injectors. What other variables are there? I've never had a fuel issue like this before so how do I troubleshoot from here?

Need more info.

-- Has the check engine light reared its ugly self? If so, any particular codes?

-- Fuel. Are you fueling up in the same gas station?

-- Air filter. Usually won't cause 30% change, but doesn't help if the filter is loaded up with crud.

-- When were the plugs, coils last changed?

-- How's the idle? Lumpy, hear a miss?

-- How's the exhaust? Smell rich? Sooty rear muffler / hatch area?

-- Are the brakes dragging? Sometimes the rear calipers can be warm due to pistons getting a little seized up in the bores. (That's not just a mini thing, it can happen to any car.)

-- Tire pressures? Up to spec? I'm always nagging my son to check his R53 since it's such an easy thing to do.

My son's R53 is near 222K miles and is currently averaging about 26 mpg, about average for the vintage from what I gather. Sounds like something is definitely amiss with yours.

Answers in red

I think something is definitely amiss. Brake is something that hadn't even crossed my mind and I guess is something I'll need to check. I don't think the caliper is actually sticking as I don't have a warped rotor yet, but will give things a once over this weekend.

Even with that, I think something is still wrong. I was planning to run some seafoam through the tank and hope it cleans the fuel system, but I don't expect much change since I don't smell anything and there's no obvious signs of running really rich.

Wonder if leaky fuel injectors would impact mpg that much? 30% drop is a lot! You'd have to be running rich for the efficiency to drop that much I would think.

I don't think it'll change anything, but I have seafoam sitting around so it can't really hurt anything, right?

My issue with not doing the parking brake is a stripped set screw for the rotor, so I'll pick up an ez out kit this weekend and see if I can't get that done. I would expect a change there, although I still don't expect that to increase my economy by 8mpg.

I don't see anything within torque for AFR... Is there anything that can monitor AFR, not even real time, so that I can crowdsource some help with that? Is my only option in regards to fuel management to get properly tuned?

Edit: The more I am thinking about this, the more I think the fuel filter is a big issue due to the lack of other symptoms. I did add AFR and manifold pressure to my torque app so I will try to monitor/log those when I drive to make sure everything is copacetic. I'd still love to hear everyone else's thoughts.

If everything else is fine, check the 02 sensor, when the mileages drops the 02 sensors are going bad, then after a while, you get a CEL for the oxygen sensor. That's what mine did. 11780872674

https://www.northamericanmotoring.co...placement.html


https://cimg7.ibsrv.net/gimg/www.nor...c670712b39.jpg

Wouldn't bad o2 sensors be causing it to run rich, still? I'm not noticing any significant black smoke or anything from the exhaust. I'm not opposed to spending the $150 for new sensors but would prefer not to just toss money at it. There has to be a logical conclusion somewhere here. I did order plugs because I don't know when the PO last changed them. I'll be doing a fuel filter as well because I assume it's original with 144k on the clock.

When mine went bad there was not any noticeable black smoke or anything, the big factory was it was higher milage and the MPG started to drop like crazy. I check the filter, plugs, coil ...etc and then I got the CEL that was on the front sensor going bad, then on the rear (later down the road). Once I saw the same drop in MPG and I knew it was time to change it. Once this was done my MPG bumped up to around ~34 avg mpg on my R52 Cooper and has been good ever since.

I'll put them on the list right after I do plugs and filter. Thanks ECS.

Welcome. Get the socket and watch taking it off, sometimes they are rusted pretty bad. Also, make sure on the install the anti-seize on the thread does not touch the head of the sensor by accident. So it does not contaminate anything.

Socket: SLY88750B

https://www.ecstuning.com/Search/SiteSearch/ES2949705/


https://cimg4.ibsrv.net/gimg/www.nor...0b488e785b.jpg



I used it so I would not twist/ press down the wires (like on a long closed socket with the side open), also used it on my R56 when i did the oxygen sensor. Also with R55-R61 models it was easier to do the job with this.

Do you drive with a heavy foot? You said you don't hoon around, but I think they can be very different things. I've been getting 19-20 mpg combined in my 2006 but I know I drive with a heavy foot. I had considered that pretty normal for a forced induction engine.

Any concerns replacing just the front sensor first and seeing how much it improves?

Is that a question for your car or a suggestion?

Well there's been a development. I finally got around to doing spark plugs and here's the results. One of these is not like the other:

https://cimg4.ibsrv.net/gimg/www.nor...212e94bb43.jpg

I guess next time I buy a car I should pull the spark plugs just to eliminate potential huge issues. The PO said he recently did timing chain so I have my fingers crossed that it's something residual from that. Maybe some oil got on the plug or in the cylinder when he was doing it? I'll pull it again after a few hundred miles and see what it looks like. I'm guessing a compression test is in order, too.

Yowza. The plugs look really old. Can't see electrode, but guessing the tips are equally sub-optimal. Slap some fresh NGKs in there.

How'd the plug wells look? Clean and dry?

Lets us know what you find out after the compression test.

Wells looked great from what I saw. This was the tip of the fouled plug. Not sure how much the shop down the street would charge for a test but I'll find out next week when they're open. Otherwise I guess I'll order a tester from harbor freight. https://cimg7.ibsrv.net/gimg/www.nor...bba8861ba0.jpg

That looks pretty worn. Too bad the PO didn't change the plugs when he had everything already apart!

Is it oil and fuel on the plug? Looks like It wasn't firing at all. Coil okay? plug wire?

Everything looked A-Okay aside from the oil. I think it's just oil on the plug. Coil was clean, wire looks almost new - although they're 7.xmm wires. The coil is MSD and was replaced within the last few years. Maybe I'll toss new wires on there just for peace of mind for $25.

I am thinking the opposite of you. I think the tip looks like it was firing which is making me ever so hopeful it's an isolated issue and won't require pulling everything apart. Plus it hasn't thrown any codes. If it was completely fouled and not firing wouldn't I get a code

I'll post the rest of the tips in a bit if you guys want to try to read them.

https://cimg9.ibsrv.net/gimg/www.nor...f5ccb37b70.jpg

https://cimg0.ibsrv.net/gimg/www.nor...a935df6c39.jpg

https://cimg3.ibsrv.net/gimg/www.nor...bbbae681eb.jpg

https://cimg4.ibsrv.net/gimg/www.nor...20627317cd.jpg

You're right, it is firing, however, it may not be firing consistently. I'm finding conflicting in on whether or not the Bosch Plat +4 (4417) is the right plug for the R53. Saw a couple of threads about this being a better plug for the cooper...

FWIW, I put in NGK BKR6EQUP in my son's car. Runs fine. No issues.

Let us know what plugs you end up putting in.

i used the same ngk that you did. So far no problems but no noticeable difference in power. I did pick up a compression tester from vatozone so I'll be testing it tomorrow. Economy seems improved though. Not quite through half a tank and I have 150 miles on this tank. That doesn't prove anything but it's better than it was before so I'm at least taking it as a good sign.

So today is the day of reckoning. I'll be doing my compression test tonight after work. Very anxious. One thing I noticed last night while observing the OBC - I'm aware that it can only be anecdotal evidence of something, but it can still bring things to light - is that my economy seemed to not be great while on the highway. While on the tollway home, my estimated range dropped 50 miles over a 30 mile trip. It usually goes the other way around. When I got off the highway and was back to city driving through Chicago, my range jumped back up an extra 10 miles. That makes very little sense to me. What can this point to?

Do let us know how the compression test goes. Only other thing I can think of is a leaky injector, but you said the plugs were wet with oil...so, not sure.

So I used an Autozone rent-a-compression-tester. It's cheap, long, rubber hose so I expected numbers to be low from the get-go, plus the whole 145k on the odometer thing.

1- 105
2- 100
3- 98
4- 108

I also snapped a pic of the plugs after ~200 miles this weekend, but the hosting is taking forever so I'll post those in the next post.

Things I learned/confirmed:
1. There's undeniably oil in cylinder 1. I could see the piston was black through the spark plug hole.
2. My compression test was probably flawed and it's probably worth it to take it for a leak down test. When I started that car up afterwards to make sure the fuel pump was working fine I noticed the temp was WAY back down.
3. I'm not sure how to proceed.
4. I'm not too concerned about cylinder 3 being low.

Questions: If you guys confirmed that there's oil in the cylinder, what would you do? I got the car cheap with the anticipation I'd drive it into the ground. It needed standard suspension refresh, but has a lot of good factory parts and some light modifications. The cylinder is holding pressure, but there's 100% oil in there.

edit: with these results, I'm probably going to pay for a leakdown test just to confirm everything. Probably worth it due to low numbers across the board.