Author Topic: Sudden Drop in Fuel Economy? (Read 4982 times)

CO318is · « **on:** November 02, 2010, 11:12:35 AM »

I recently switched my stock plug wires to the 8mm performance wires suggested by the Grass Roots Motorsports article, at the same time I took the cheap "cold air intake" that was installed by the previous owner and swapped it over to my old stock air box that has a K/N filter installed. Nothing else has been done since then, and my fuel economy dropped from around 28-29 to 23-24. I have not been driving the car all that aggressively, so I am trying to figure this out. Is it the plug wires?

cristimm · « **Reply #1 on:** November 03, 2010, 02:32:58 AM »

I think plug wires have nothing to do with mileage.

DesktopDave · « **Reply #2 on:** November 03, 2010, 10:08:18 AM »

I'd check for a vac leak...moving all that stuff around might have left something loose. I'd agree that plug wires aren't significant to fuel mileage, but I'm no expert.

deekay · « **Reply #3 on:** November 03, 2010, 10:46:36 AM »

check for leaks/cracks in the AFM-TB tube, and try swapping out the K&N for a paper filter.

gearheadE30 · « **Reply #4 on:** November 05, 2010, 09:43:23 PM »

I'm going to guess vacuum leak or maybe something with the AFM didn't get hooked up right? A dirty filter could cause that as well...

B318M42W · « **Reply #5 on:** November 22, 2010, 11:08:13 PM »

might be worn out O2 sensor, faulty fuel regulator (increases fuel pressure), stuck brakes, worn plugs?

CO318is · « **Reply #6 on:** November 30, 2010, 10:13:29 AM »

I am thinking that the afm might have caused it, I am seeing more like a 15% decrease in mileage now, I am going to swap afm today and see if it helps. If that doesn't help I will order some new plugs. Frustrating to say the least. My hard starting has gone away though, so that helps.

DesktopDave · « **Reply #7 on:** November 30, 2010, 05:43:10 PM »

Might be a fuel leak too...I had an old Subie with a fuel leak. It was a pinhole in the metal feed line off the tank, very hard to see. It looked like a single strand of spider silk, but every time I tried to brush it away it was still there. The tank was rusted too, so the car always smelled faintly of petrol. My mileage went from 25mpg to 30+.

CO318is · « **Reply #8 on:** December 08, 2010, 01:28:30 PM »

Well, I reset the ecu, swapped out the afm for the one that I replaced when I swapped over the airbox. Changed the oil, cleaned the airfilter. My mileage increased about 3 mpg but is still about 4 mpg different than what it was before taking of the "cold air intake". Now the thing is idling from 1200-1500rpm. This happened after reseting the ecu. Now I have 2 problems. Wish I would have left the thing alone haha.

tjts1 · « **Reply #9 on:** December 08, 2010, 04:42:10 PM »

Did you fix the hoses under the intake manifold yet? I bet you have a lot of vac leaks if not.

gearheadE30 · « **Reply #10 on:** December 15, 2010, 09:01:12 PM »

Weird idles are almost always vacuum leak related. Might want to try cleaning out the ICV as well.

txleadfoot · « **Reply #11 on:** December 24, 2010, 01:30:13 PM »

More city driving? My city average is 21 mpg. How about the ECU chip, is it factory or a custom job?

CO318is · « **Reply #12 on:** January 12, 2011, 10:54:10 AM »

My fuel economy is back to where it was, I changed from 91 octane to 87 octane and the car seems to be running much better. I am wondering if it had something to do with the Ethanol levels in the fuel in Colorado, the decrease in fuel economy started right when they changed to winter fuel with a much higher percentage ethanol level. I am still trying to figure out my idle issue though. I peaked under the intake and it looks like the hoses had been changed relatively recently, they looked like they were in good shape.

The car starts up fine even in the -2 weather we have been having, but it doesn't seem to want to warm up very fast. After I start it it idles at 700 rpm's, as soon as I put it in gear and move it jumps up to a 1200 rpm idle, and will climb to 1500 rpm every once in awhile. Seems like it should be the opposite, warm up at 1200, and then drop down to 700 when it warms up... Any ideas?

DesktopDave · « **Reply #13 on:** January 12, 2011, 11:29:49 AM »

Alcohol will give at least a 10% reduction in mileage. Winter driving also has more idling, and lower loads on the engine. The engine is most efficient under heavy load. I experimented with 93 octane fuel and found an extra 2 MPG. I suspect the premium has far less alcohol, though I'd have figured the opposite.

You could try the old fashioned radiator block - get some cardboard & stick it on the front of rad. I'd experiment with about 25% blockage to start. I'd take it easy, our cars have zero tolerance for overheating, as opposed to say a school bus.

That high idle is DME controlled. You might want to test the coolant temp sensor for proper readings. Mine isn't all that stable either though, might be a few things not working right.