worse MPG after coil & plug swap

steagle

steagle

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it's only been two days so maybe this is premature, but i am getting 1-2 MPG less on average than i did when i had failing coils and old spark plugs. that doesn't seem to make much sense to me so maybe there is something else at work here. my CEL is still on, after at least 10 starts. the last CEL pulled was P0430, for one of the cats operating below threshold; i thought this issue would be solved by fixing the misfire issue, but apparently it's still around. anyways have any of you guys heard of someone getting worse mileage after doing this repair? i sure haven't...
 
oh and a little backstory, i swapped all 8 coils & spark plugs, but left the VCGs as there was no oil leak and all coil boots were dry.
 
So you've ran through a full tank of gas, and did the math to figure your actual mileage? That average on the message center can be very misleading. I think just having your car running in park can have a severe effect on the average MPG.
 
yea, to really tell you gotta do the math(not saying you haven't) but on my cars if you fill up and just sit there after the mileage does not go up, you can drive a few miles have like 30mpg and than stop and it can drop a good 5mpg in some cases(never sat that long to really see just how far it goes). Many other factors too.
 
if you drove long enough with a mis-fire - you may have damaged the cat.

another small thing - but did it just get colder there? it doesn;t take much for a small mpg change.
 
If the CEL is still on then the Cat isn't working right. Operating below threshold would mean that it is plugged up. That creates excess back pressure in the system, which makes it harder for the engine to work, which means poor fuel mileage.
 
Quik - what's strange is I got the P0430 code over almost two years ago, but it went away on its own and recently reappeared (at least in terms of the CEL, not that the problem fixed itself). i'd say about a year ago i first started noticing very minor acceleration problems and eventually small shakes. then it was serious misfires for at least a few months before i got it fixed. I know, that's poor caretaking on my part, so I probably did do further damage to the cat seeing as how that code has been in there for ages.

still, why would i see a significant drop in MPG right after swapping the coils & plugs. i was averaging 11mpg all day today; in city traffic i usually get 14 (due to the 20"s, not the slowest driving habits, etc). after 15 minutes of highway travel, i averaged 13mpg. i'm thinking maybe it's taking awhile for the PCM to calibrate everything now that the cylinders are firing properly, but i dunno, it's been nearly a full tank of gas and the numbers haven't changed much.
 
97stscaddy said:
If the CEL is still on then the Cat isn't working right. Operating below threshold would mean that it is plugged up. That creates excess back pressure in the system, which makes it harder for the engine to work, which means poor fuel mileage.
Click to expand...

yeah you're absolutely right, and i was under the assumption that if i fixed my cylinder misfire issue, that i wouldn't be running as rich and the cats would have a chance to breath again. unfortunately that wasn't the case... yeah the shakes are gone (thank god!) but you can still smell tons of richness outside of the car, my friend smelled it in his Audi when he was trailing me last night and i accelerated to about 60mph. i'll probably have to replace that cat; but i'm still unsure why i saw such a big drop in MPG the last two days, you'd think at the very least i would have a slight improvement after fixing the misfires.
 
@ pektel & Beamer -
no I haven't done specific calculations on it, just been reading the dash, i know it can be misleading, but in its defense it has been quite accurate on MPG with me the last few years. i mean maybe off a couple miles on a full tank but nothing noticeable. today was the first time i saw a very significant change in my MPG - even when i had a clogged fuel filter it wasn't this bad. i'm wondering if because all 8 cylinders are firing now, that is taking more gas out of the tank... though i always thought it was the other way around... the less cylinders the more your engine is struggling.
 
Quik LS said:
another small thing - but did it just get colder there? it doesn;t take much for a small mpg change.
Click to expand...

x2 - I've already started to notice the cold weather affecting my MPG slightly. But about the cat, that would be a good thing to check up on. Once it gets plugged up, it's not going to unplug itself after the engine starts running right.
 
it could be the winter blend of gas because the mpg is usually less in the winter. I gotta 09 camry that went from 28mpg to 24mpg just in the past few weeks. Its the stuff they add to the gas that brings down our ave.
 
also it's not exactly cold out here in LA. it has been in the 70s and 80s the last few weeks, so i don't think that's the reason.
 
steagle said:
also it's not exactly cold out here in LA. it has been in the 70s and 80s the last few weeks, so i don't think that's the reason.
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Yeah, I guess I should have looked at where you were from before posting that link. LOL. It was 13 *F when I left for work this morning.
 
If you can smell gas like woah, it might have a bad O2 sensor. If those get covered in carbon and crud they will give false readings of a lean condition so the ECM enriches the mixture. What I'd do first, is what's called an Italian tune up. If you've ever seen how Italians drive, then you know how to do it. :D
 
id love to do an italian tune up, but kinda hard here in LA where there are ten billion cars everywhere at all hours of the day :rolleyes: fortunately it looks like i'm getting normal mpg again, i guess it just needed a full tank to burn through, now on my second tank after the repair it seems to be fine. doesn't mean i won't do an italian tune up at the next availability though :D
 
Just go for a country drive one day and do it there.

It's kinda like a woman, more fun to do when there are curves involved. :D
 
Steve
Got the gaskets today and did the credit. It should show in a day or so on your card. Happy to hear that the car is now back to normal.

Max
 
hey thanks a million Max. yes, it was a relief that i didn't have to do the VCG replacement. i was surprised to say the least that there was no oil in the coil boots. guess it doesn't happen to everyone.

now i just gotta get my passenger side front door checked out, because the body shop that replaced it earlier this spring (drunk driver hit me) didn't do a great job and i'm getting all sorts of rattles. once that is fixed - the car will feel like it did when i first bought it :)
 
I have been told you can run a car at about 70 to 80 MPH on the Freeway for about 50 to 100 miles and it "COULD" clear out a somewhat clogged cat!
 
lincoln00 said:
I have been told you can run a car at about 70 to 80 MPH on the Freeway for about 50 to 100 miles and it "COULD" clear out a somewhat clogged cat!
Click to expand...

A clogged cat is usually form age or from over-fueling. The unburnt fuel gets into the exhaust, into the cat, which then gets the cat super-heated. So hot it actually melts the internal structure of the cat - melting it into a solid mass. Once the cat is in the state - you must replace it.

The exhaust does not flow properly through it, and you could really hurt the engine with that kind of back-pressure - taking it up in rpm would certianly not be a good idea.

even if you were able to 'blow out' the melted inners, basically hollowing out the cat, the LS would light up a CEL quickly since the monitor post cat could tell that the exhaust was not being cleaned any longer.
 
I've been told that the likelihood of the 02 sensors going bad is much less than a cat going bad. What's your take on that Quik? I don't know if you remember, but this was the very first issue I brought up when I got my LS and started posting on this forum. I just ignored the problem but now it's at the point where I need to do something so I'm not running so rich, and I don't want to do trial-and-error repairs, really want to nip this in the bud. The P0430 code keeps coming back but no one seems to be able to tell me if it's a bad sensor or a bad cat. My mechanic tested the cat earlier this year and told me it's OK and I ended up passing smog check. So I'm leaning towards sensors at this point.
 
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