fuel economy

[james1965]

so I have a 05 SLE and a few month back I was having an issue with the ICM. I was having issues with random misfires, dead spots in throttle, chugging ect. I took a ICM off a 03 and put it on my car and problem solved. no more issues, car runs great. However I do not drive the car much , maybe 50 miles in the last 2 months. last Saturday I took it on a short trip, around 300 miles and checked the gas mileage. car was getting 17mpg on the interstate. car has new tires, new air filter, new cat converter, new plugs, wires, coil packs, oil change. last fall and for the last 7 years I've gotten 21.4 on average. Any thoughts? I have no codes and it runs like new even though it has 156k miles on it. Not sure what to check from here.

[Roadrunner]

Oxygen sensor upstream of cat would be my next go to.

[maxi426]

Can you monitor coolant temp with a scanner while going down the road?. Thermostat opening too soon, or stuck open will drop mileage, or keep car from going into closed loop.

[james1965]

Maxi, on my trip I use a program called Torque, it records all vital data as i drive. the temp seems to rise like normal. and the car runs at 198f the entire trip.

RR, not sure on the upstream but if it was failing wouldnt it give a code? I can watch the data from both o2 sensors and they both seem to be giving out good numbers?

here is a link to a short data clip

https://youtu.be/RDgrAge1cGg

[nos4blood70]

How fast were you going and are there many hills?

[96 SSEi]

I would unplug the maf and get the same readings - your mpg should drop - if it rises, check the maf.

[james1965]

average 70 mph. pretty level most of the way. can see in the short video it shows max spd, max tach, ect. was 90% interstate driving.

[maxi426]

Coolant temp sounds good. Would like to confirm accuracy of coolant temp sensor with an inferred temp gun if available, or check it before a overnight cold start to see if it agrees with ambient temp.. Upstream O2 could definitely deteriorate, and not set a code . Seems like the programing on newer cars is more sensitive to this than older cars. Are you looking at instant MPG, or average? If average, have you reset it? The more miles it has between a resets, the longer it takes to respond. Hope that makes sense.

[MKMike]

Assuming your new plugs are ACDelcos, properly gapped.....
As O2 sensors age, they become slow to respond, which negatively impacts fuel economy.
Contaminants from rich fuel mixtures, coolant, etc. also damage O2 sensors.
If you replace the sensor, ACDelco OE and Denso are the best replacements.
Forget the "bargains" and avoid Bosch sensors (also true of spark plugs.).

[james1965]

Maxi, I've reset that on the information center before. and checked it several times last year. it was getting a good solid 21 mpg. it sat most of the winter as i was getting wierd things to happen only to find the ICM was doing it. replaced it and the car runs like it should. I replaced the Cat because i had thought after 156k miles it could be plugged. The Cat was a Walker one that i put back on it. Plugs are AC delco irridium and gapped correctly, wires Ac delco pro. I would not have a Bosch sticker on the bottom of my toilet. I have never liked their products. Hmm.. I suppose o2 sensors next on the list. Love the car too much to have it not run correctly.

[redfury]

Make sure that your transmission is locking the torque converter at speed. Easy test, get up to cruising speed, tap the brake enough to disengage the TCC lock up solenoid. You should see the engine RPM raise a bit, and then drop as it locks up the torque converter again.

[james1965]

will try that tomorrow redfury..