Drop in MPG after new plugs and wires

[TheJMan]

I just put new spark plugs and wires on my X about 9 days ago and I have noticed a drop of 1.5 mpg in the city and about 2 on the highway. I used to get 13/18 now I get about 11.5/16. The plugs I took out had large gaps and were very worn out. I have noticed a gain in power, but the gas mileage has fallen off. Would disconnecting the negative battery cable help any? The engine runs great, but the mileage has fallen off.

The plugs are Bosch +4 and the wires are Borg-Warner.

 

[2001ExpSport]

You should have stayed with motorcraft plugs, those plugs were in the enigine design. Also, when a plug sparks it only sparks to one post, not all four. I would take them back, just a good gimmick(no offense). Having the correct gap is important and should be set properly. I have also read stories of post falling off the plug and then bouncing around in the combustion chamber.

If not, you can reset the computer by disconnecting the batt and following the re-learning sequence in the op manual.

 

[Brandons]

Heres a good story for yall

1994, we get the truck brand new.
later on, a mechanic tells us we have platinum plugs, he said dont change em till 160000km.

now last spring, i check the spark plugs (due to really poor performance/idle speed), pull one out, and discover that they were basic spark plugs, with 120000km wear on them, or 9 years.

So there was a definate increase in power/ mpg after we changed them with champion plugs. Then i got Niehoff (SP) plug wires and a performance muffler and a K&N drop in filter, and its running way better.

 

[X~FACTOR]

Maybe since you found yourself with more power you have been punching the gas a lot more than usual. I know I have when I installed my KKM. My mpg dropped by 1.5-2 mpg. But I know why... I enjoy reving my SOHC now.

 

[Brandons]

i enjoy reving my OHV ever since the mods, but i do about 600km on a tank of gas

 

[TheJMan]

It has been somewhat hot here lately, but my thermostat is partly stuck open, could that be the problem?

 

[SWLathrop]

Open stat? Yep, that can cause excessive fuel consumption because the computer sees the engine not up to operating temp. This can lead to CAT failure as excessive fuel in the exhaust will cause the CATS to run hot and melt the catalyst causing a plug up. Fix the stat.

 

[X~FACTOR]

Originally posted by TheJMan
It has been somewhat hot here lately, but my thermostat is partly stuck open, could that be the problem?

How do you figure its half open?

 

[TPLYNCH]

Open Stat, engine remains in startup mode - running rich - eating gas and sensors and cats.....

Also, based on a lot of threads on this site, the Bosch plugs are NOT worth the $$$.

 

[TheJMan]

How do you figure its half open?

I'm not 100% sure that it's partly open, but when it is near 100 degrees here, the engine gets to operating temperature, it just takes awhile. In the wintertime, the gauge barely moves off the C for cold. Most of the time the needle is in between C and the N in normal.

 

[bluestream]

I have had to put a new thermostat in my truck every three years or so, after noticing the temp needle didn't go as far up in the temp range as it used to. I have always used Motorcraft thermostats. This year I also replaced the temperature sending unit after noticing that the gauge did not change much after putting in the new thermostat. Now my temp needle goes right up into the normal range. Those sending units should be replaced on a periodic basis as well.

 

[Brandons]

Yeah my thermostat i think is completely stuck open, cause our temperature gauge does change, but usually sticks hardly away from cold.