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What would cause gas mileage to drop from 26 to 22 (in town) after a tune up?

The car is a ?98 Corolla, 93k, and car has always been reliable and consistent in mpg.

I check the mpg at each fill up and always go to P66 or PB for gas.

I got it tuned up (including new AC Delco plugs, wires), oil change, serpentine belt, rear brake pads. I filled up that day to check mpg which I expected at worst to be the same as before the tune up. The gas mileage was 22! Four miles per gallon less than typical.

I noticed the overdrive was off so I thought maybe that was the cause. I turned it back on and checked the next tank of gas, but it was 22 mpg again. I called the shop. They checked the vacuum, double checked the plugs were within specs for the car, etc. They went ahead and switched out the plugs to NKG saying they were hotter plugs and may improve mpg. They also added Seafoam to a vacuum hose to clean out any carbon build up and put the rest in the gas tank. They did not charge me for the second set of plugs and the Seafoam.

Within 5 miles (and after the shop had closed for the day) the check engine light came on. The next morning they said that could be because of a loosened piece of carbon blocking a sensor. Within in a few more miles the check engine light went back off.

I ran the tank almost down to empty to flush out all the Seafoam and filled it up again. For kicks I checked the gas mileage of the Seafoam tank because this time about half the tank was highway mileage. I was hoping it would be a little higher but it was still 22 mpg.

The weather?s been great so I haven?t used heat or AC during all this. The car otherwise seems to run great.

What could cause this sudden drop in mpg? I don?t know what to do next.

Maybe your looking at the wrong end of the car. Check for drag on the rear wheels, maybe the brakes are too tight or the calipers are no longer floating.

Other than that, it could be the resistance of the plug wires. They should be around 4k ohms each in order to get the correct duration of spark.

I know this is never what people want to hear, but the drop in MPG’s really isn’t significant enough to really point to a mechanical problem. When you’re driving mostly city miles, your mileage should vary quite a bit based on how you hit the lights, traffic conditions, etc. Also, many parts of the country will be switching over to the winter blend gas, which will usually yield worse gas mileage. Also consider that the actual difference in number of gallons dispensed per fill up is probably something like 1-2 gallons and when you’re dealing with such a small difference a lot of generally minor factors can make a noticeable difference. For example, a few extra cold starts or prolonged idling (both of which are likely to occur in a shop) will easily burn a gallon or so of gas and the minor variations in accuracy and cut-off point with gas pumps can also amount to maybe a quart or two, which along with normal variation in driving conditions could easily explain the discrepancy.

I’d keep driving it normally for a few more tanks and I suspect it will get back to normal, though checking that the wheels are all turning freely and that the alignment is on certainly aren’t bad ideas.

Have you checked your tire pressure?

Yes. I check the tire pressure regularly. The tires are new as of the 2nd tank of gas after the tune up.

The thing is the mpg has always been consistent with only a 1/2 mpg variance on any fill up so I have a hard time believing the day of the tune up I suddenly drove differently or started hitting more red lights et al. It is possible they ran the engine a while in the shop but there has be 2 fill ups now since that tune up. Still 22 mpg.

You don’t think a sudden 16% drop in mpg is significant? hmmmm.

Actually I had thought of this. The car doesn’t ~seem~ to be dragging but it may be worth having someone else check the brake job. I’ll look into that this week. I’ll get back to you and let you know how it turns out.

Try the next tank of gas. Your rear brake pads were changed and if they ere on disc brakes, the mechanic did some hard stops as a break-in measure. The car has probably been running a lot without putting miles on it. You know what to do. You have to wait until the next tankfull. Then you can worry about things but it’s too early to start now. Your computer is relearning how to control the engine.

By the way, my rear brakes (which were the ones worked on) are drum brakes not disk. If the new pads were dragging on the drum wouldn’t they make noise? Thanks!

When the rear drums were dragging on my 95 Dakota (stuck e-brake) there was no noise. However, I noticed that the truck slowed down quicker when it was coasting and the rear wheels were very hot after the drive.

Another thought, have the gas stations switched over to a winter blend (E10 perhaps)? The mpg on my truck would drop about 10% after the switchover.

Ed B.

You said they did the ‘serpentine belt’ - the belt that runs the accessories? Not the timing belt? If they did the timing belt, they might have gotten it off a tooth.

The thing that bothers me is not so much that you’re seeing a drop in mpg…but the fact that all the previous years it was so consistent. I’ve NEVER seen a car that consistent…no matter how I drove. There have always been factors from one tank to the next that my gas mileage would fluctuate ±4mpg. I find it amazing that it’s been so consistent. If I took the average over 3-5 tank fulls it would be with within 1mpg. But from tank to tank it would fluctuate a lot.

Just depends on how it’s driven. My ES get 20 mpg for my daily commute, which is 90% of its driving. If it gets 19, it means I forgot to check the tire pressures, they’re low. My old GTI got 25 mpg regardless, city or highway, it was geared so ‘short’.

Hard to believe or not the car has always gotten 25.8 to 26.2 with every single tank. I do not drive much (about 500 miles per month) and no matter the car I’ve always gotten what the mileage the sales sticker says if not better. I am a consistent driver. What can I say.

It was the serpentine (drive) belt not the timing belt. The one that is on the exterior and weaves around several parts hence the name serpentine I would guess.

I asked a local mechanic an easy way to check the rear drum breaks for this. He said jack up the tire in question and spin it without the parking break on of course. I did and the driver’s side rear tire spun noticeably slower and had a rubbing sound with each go around. Perhaps the freshly machined drum wasnt done right. I’m in the process of getting that looked at.

I hope this is the problem. Good idea to check this Keith. Thanks