Timing belt - mileage

I recently had the timing belt changed on my 2004 Nissan Frontier at the dealership. Since then, I’ve noticed my mileage has decreased by 3-4 mpg. Could there be any connection between the belt changing and the reduced mileage? As far as engine performance goes, the engine seems to run and idle as well as previously.

I doubt it. If it were out of sync., you would notice, or they would immediately. Maybe the weather is colder. Give it some more time and miles.

They may have changed the belt and the timing may be off slightly…I would start by looking there.

About the only thing that would likely affect the mileage would be if the alignment was off by a notch or two. But experienced mechanics – especially at a dealership where they only basically work on this make – wouldn’t likely make this mistake. And if this mistake were made, you’d usually notice it, either as reduce acceleration, or possibly pinging, esp when going uphill.

One thing to consider: If this car has dual overhead camshafts, where each camshaft has its own pulley that the timing belt has to go around, then a misalignment during timing belt install is an easier mistake for a mechanic to make than if there’s only one camshaft pulley to deal with.

Unfortunately, it’s not a simple matter to check the alignment. As mentioned above, the best course of action – absent the symptoms above – is to monitor the mpg for a while, see if the readings you are getting aren’t representative. It may be your mpg had gone down even before the timing belt install, due to other as yet unresolved problems.

It seems as though being just one cog off would make the car run poorly immediately on start up if it started at all.
If the cam shaft sprocket had 36 teeth, that could be as much as ten degrees off. Even if it were just a few degrees which is more likely, I feel it still wouldn’t run very well if at all.

I suppose it might be a change to winter gasoline from summer gas if that happens in your neighborhood. How long ago was the work done? But you can always take issue with the repair shop.

I think jtsanders is on to something here. Winter gas always gets worse fuel economy than summer gas. I agree with dagosa as well. If the timing belt is off just one tooth then the engine would run poorly.

I’m not so sure that just one tooth would cause an obvious problems, cam timing can be adjusted over a range and affect efficiency but not driveability (many cars do that now, I could get an adustable timing gear for my GTI to do the same thing). So I’d have the dealer recheck the cam timing.

If it was just cam timing that was affected, but if the cam is off one tooth, then the cam position sensor would throw off the ignition timing. If this is a lost spark ignition system, which would use a crank position sensor, then maybe.

Which engine, the 4 cylinder or the V6?

Sometimes it depends where the crank sensor is located whether its aproblem or not. On the Toyota 2.2 its on the bottom crank timing belt gear. So either one tooth ahead or behind, the
crank sets the spark timing perfectly.

On a Toyota 2.2, I gained 10 mpg by installing the belt one tooth advanced from spec. I went from 28 to 38 mpg on trips. I imagine the opposite would happen if it were one tooth retarded one tooth from spec.

Now they have Variable valve timing on the new engines. It keeps the efficiency at max at all rpms.
So if your advanced one tooth, you get better low rpm performance and better gas mileage because thats where most of our driving occurs, at the bottom end. Might sacrifice some passing power having it set like this at the higher rpms. Although to me, the car feels like it has more power one tooth advanced.

@tech, if you can increase your mileage by ten mpg, by incorrectly installing the belt, there is an entire auto industry ( or more likely a psychiatrist) you need to have a conversation with.

Years ago I played with advancing and then retarding the valving timing of a VW by one tooth - mostly out of curiosity. In one of the off-by-one-tooth directions, I did get more high rpm performance, but at a cost to low rpm drivability. It was fun to experiment, but I was convinced it ran best at the factory timing marks.

Hence the reason why so many cars have gone to variable valve timing. I’m sure it was available to the manufacturer for years but not at a price they were willing to pay until now. It’s it experiments like yours @JoeMario that prove that motor manufacturers have a general idea of how their cars should be tuned and parts installed.

I had an adjustable cam pulley (and an aftermarket cam) on my '81 Accord.
I did try changes in timing, but kept a neutral setting for the compromise the cam maker intended.
The adjustable pulley was necessary because the re-worked non-CVCC head had it’s deck milled down an unknown amount.

To directly answer the OP’s question, yes, there could be a relationship between the new belt installation and the gas mileage.

However, based on your description of the engine operation and the small amount of mileage change, I’m inclined to think it’s more likely something else, such as perhaps an insufficient number of mileage checks (making even a different pump handle a variable) or perhaps a switch in gas.

There’s no question that valve timing changes can affect performance, but (with the exception of variable valve systems) always at a compromise. Such as more power at teh high end at the expense of poor low end performance and idling. But in this case you’re describing a change well within the realm of other variables and an engine that runs perfectly. You can take it back and ask the, to check it, but if it were me I’d be sure to eliminate all other possible variables first.