My 98 LT had a P0300 "Misfire" code and 12-13 mpg. After new AC Delco dist cap, rotor, plugs and a new spider fuel injector assembly, the code is gone. Today's fill-up and mileage calculation yielded 11.97 mpg with mostly around town driving - not much highway. It feels like it has more pep and power so why is the mpg still so bad?

Right after the spider was installed two new DTCs popped up -> (1) P0118 ECT Circuit Hi Input and (2) P0122 Throttle Posit'n Sens "A" Circuit Low Input. I tried scanning other forum posts and the DTC guide to learn a bit more about these codes and whether they might relate to poor mileage but I didn't find anything helpful yet.

Also, (pre-new Spider) it used to start up right away but now requires a bit more cranking and, sometimes, some throttle pumping. I drive slow and easy (mostly), keep tire pressure up, etc. I even matched mileage markers along the interstate to see if my odom was accurate. It was.

Anyone have any ideas how to get better MPG?

 

ect is coolant temp sensor,which tells the computer temps so it can adjust fuel mixtures. its a brass sensor beside the upper rad hose at the thermostat. the other code is the tps on the throttle body.

id replace both. they are about 20 each. the temp switch is probably the reason for bad mpgs.tps can also play in a effect,also controls shift points.

 

DTC's may have popped up if the ignition was turned on while the sensors were disconnected. Have you checked fuel pressure and leakdown for the hard starting? Could also have an affect on fuel mileage if it's too high or too low or the leakdown is excessive.

 

im having the same prob with MPG 11.7 today when i checked it iv replaced all the same parts the only prob i dont have is hard starting but the fuel mileage it killing me all fuel trims have been checked O2's sensors all working i just cant figure it out the cat is gutted because the rattle was annoying thats about the only thing i can think of that might be hurting my mpg.
gonna fallow this thread. . good luck

 

The rotor, plugs, etc., were done a month or so earlier than the spider. Still no MPG improvement and still having the P03XX code. So the spider was next and sorta rushed into as the engine had started sputtering and threatening to stall at times. The sputtering was still present after the work had been done at the garage but, conveniently, the tech had gone home. I was pissed as I made it clear that I wanted the sputtering problem fixed. So the manager volunteered the Diag Test and that's how the two new codes came to be.

BUT, the sputtering problem cleared up within about 1/2 mile away from the shop on my drive home. (!) I had read about new spiders sometimes taking a bit of time to work right while the sensors/computers/circuits/whatever figure out that it's been replaced. I'd hoped this was one of those cases and expected much better mileage. Not, I guess.

So, based on Hook's and chris015's responses, I'm thinking that a new Diag Test may be in order because the earlier test was done BEFORE things got sorted out. Maybe I'll get new/different codes now that it's running better. (?) And if I still have the same codes, its good to know that the needed modules are only about $20/each. Is this a DIY thing for a reasonably handy person?

Question to Scottl1346 - Is your SES light on? If so, wouldn't a reading point you to the problem area?

 

Ever had the camshaft retard checked?

 

no ses light just bad MPG use to get about 300-320ish to a tank now im getting maybe 200 a tank

 

To Burned: "No" to your camshaft retard question.

I checked the air filter and found it to be waaaay dirty. Will replace soon. Don't expect much improvement to mpg.

 

Thought about getting a K&N filter at Advance Auto since (a) the website said they were on sale (and in stock) for $39.50 and (b) I already have the oil & cleaner. Otherwise I wouldn't spend the extra $$. Turns out I have to buy it online to get the savings. Meantime I had them do another Diag. Test and only one code came up this time -> the P0118. They offered to sell me a new one (BWD-brand) for about $19. Told them I'd probably buy the filter and ECT on-line and be back to pick them both up later.

I'm a little skeptical about the one-code-only result. I have owned the car for about 14 months and the SES light has always been on. Before, it was the p0300 and p1860. When the p0300 disappeared after the spider install, it was the P0118, the P0122, and the p1860 (still). I have done nothing to address the p1860 (was told early on by a mech that it could wait) so why did it go away?

I guess I'll buy the new ECT and a filter (maybe K&N, maybe not) and see if I can figure out where it goes and whether I can swap it myself.

 

A friend of mine has a dakota and it all of a sudden started to get really bad MPG and no engine codes and it turned out his front axle was locked all the time w/o being in 4wd. Just a thought just not sure if yours is 4wd.