changed spark plug gap, lost MPG's
Thread Starter
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Laughing Gas
Joined: Nov 2002
Posts: 1,143
Likes: 18
changed spark plug gap, lost MPG's
Truck listed in signature. 90K original miles.
I had been getting roughly 10-12 mpg in town before doing a service to the truck. After the service, I'm down to 7-9 mpg in town. Had a chance to finally get it out on the highway this weekend and only got 14.5 mpg (mostly highway, in OD, at 68 mph). Was my G-pa's truck before and, supposedly, it was able to get 18 mpg on the highway.
Here's what I did and found at the time I serviced the truck ... Removed Motorcraft plugs gapped at .35. Replaced with Motorcraft plugs gapped at .44. Replaced original plug wires with 9mm Ford Racing wires. Also replaced: distributor cap and rotor, fuel filter, and oxygen sensor.
Truck runs great in town and on the highway, just burning fuel. Even ran great before I serviced it.
Could the plug gap be the culprit? Is it possible that the .35 plug gap that it had was a "fix" for another issue? What should I be looking at? All info is appreciated.
I had been getting roughly 10-12 mpg in town before doing a service to the truck. After the service, I'm down to 7-9 mpg in town. Had a chance to finally get it out on the highway this weekend and only got 14.5 mpg (mostly highway, in OD, at 68 mph). Was my G-pa's truck before and, supposedly, it was able to get 18 mpg on the highway.
Here's what I did and found at the time I serviced the truck ... Removed Motorcraft plugs gapped at .35. Replaced with Motorcraft plugs gapped at .44. Replaced original plug wires with 9mm Ford Racing wires. Also replaced: distributor cap and rotor, fuel filter, and oxygen sensor.
Truck runs great in town and on the highway, just burning fuel. Even ran great before I serviced it.
Could the plug gap be the culprit? Is it possible that the .35 plug gap that it had was a "fix" for another issue? What should I be looking at? All info is appreciated.
Thread Starter
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Laughing Gas
Joined: Nov 2002
Posts: 1,143
Likes: 18
Was going to regap them when I got time, but figured I'd ask anyway.
No, I did not reset the computer. Does it make a difference with the EFI351? Most of my experience is with EFI302's and I've never had to reset the ECM after a plug change. I did, however, pull codes afterwards and all was good.
I recheck plug wires until I go crazy over it.
Plugs are torqued properly.
I believe the O2 sensor is a Bosch.
No, I did not reset the computer. Does it make a difference with the EFI351? Most of my experience is with EFI302's and I've never had to reset the ECM after a plug change. I did, however, pull codes afterwards and all was good.
I recheck plug wires until I go crazy over it.
Plugs are torqued properly.
I believe the O2 sensor is a Bosch.
Senior User
Joined: Jan 2008
Posts: 273
Likes: 1
While I have not had a problem with the single wire style bosch sensors. I have had a problem with the multi wire sensors with fuel milage loss. my wifes regal went from 23 city down to 14 city after installing a bosch 02 sensor swapped the 100k mile gm sensor and it went right back to normal.From a quick search i found this to be normal for alot of people.If it was mine I would swap the 02 sensor back for the factory one and do a milage comparison if it goes back up I would look into a better quality sensor.
Just my take on things
Just my take on things
Thread Starter
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Laughing Gas
Joined: Nov 2002
Posts: 1,143
Likes: 18
While I have not had a problem with the single wire style bosch sensors. I have had a problem with the multi wire sensors with fuel milage loss. my wifes regal went from 23 city down to 14 city after installing a bosch 02 sensor swapped the 100k mile gm sensor and it went right back to normal.From a quick search i found this to be normal for alot of people.If it was mine I would swap the 02 sensor back for the factory one and do a milage comparison if it goes back up I would look into a better quality sensor.
Just my take on things
Just my take on things
Senior User
Joined: Jan 2008
Posts: 273
Likes: 1
I would price one from ford dealer,but Ive heard good things about NTK/NGK and denso sensors.
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Lead Driver
Joined: Nov 2002
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From: Concord, NC
The OEM O2 sensor is a Bosch unit, so you won't gain anything by replacing what you have with a sensor from the dealer. If you have a timing light, it wouldn't be a bad idea to check the timing. If for some reason it lost advance, mileage would suffer.
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Senior User
Joined: Jan 2008
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You positive on that? nothing else he has done would result in a milage loss IMO i speak from direct experiance with the sensor.
Fleet Mechanic
Joined: Mar 2005
Posts: 1,603
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From: St Louis
I agree with EPNCSU2006 on this one that the original factory O2 is a bosch unit.
Bosch is an OEM for some other Manufacturers as well. With that said it is possible to get a 'junk' sensor from the aftermarket autoparts stores.
Bosch is an OEM for some other Manufacturers as well. With that said it is possible to get a 'junk' sensor from the aftermarket autoparts stores.
Senior User
Joined: Sep 2007
Posts: 336
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From: Iowa
Does seem odd to have lost MPG's. If something suddenly changed as a result of your work, then recheck the work. Maybe gap was read wrong initially (would explain the difference between the old and new ones too)? Bosch has a great track record for service parts like the O2 sensor, but there is always a possibility of a dud (or perhaps damaged/contaminated on install...did you use anti-seize on the threads when you installed it and get sloppy or accidentally bang it going in?). Timing check would be a good idea, but since this mileage drop occurred after tune-up and you didn't change timing setting, it's a stretch. Don't remember, but where's the spout connector-off the dizzy or separate on the fenderwell? Any chance that is out of place? Vacuum line broken/knocked off reaching around? Dunno, just a few thoughts....check, check and recheck...chances are it is something simple that changed as part of the tune...something got bumped or busted or whatever. Recheck your steps.