MPG drop after tuneup/timing belt?

[sidecardog]

I've got a weird question for the group:

I have a Gen3 2002 Montero Limited and I was wondering if anyone has seen their MPG go DOWN after a timing belt and/or plugs+wires change. I got my timing belt changed at the dealership recently and my mileage has gone from 18mpg consistently to 12mpg consistently. I also had plugs, wires and valve cover gaskets done there since I didn't 'have time to do it myself. I am at 117K miles and have been running tires at 40psi both before and after the service. All I can think is that the timing is off but power, acceleration and idle rates seem to be the same as before. I'm going to take it back to the dealership but thought I'd check in here to see if anyone has any ideas or suggestions beforehand.

Is it possible that they did a software update that would have affected mileage? I have a tuned Audi and I know that the software in it is for high octane gas exclusivly and putting anything else in it will affect performance and aftermarket components. I have always run regular gas in my Montero and have always gotten 18mpg or better.

Lift and tires are coming next but I need to get the issue sorted out first.

Thanks
Matt

 

[Rambo Penguin]

How many tanks have you run though it since getting the service done? You should be able to check the timing on the 3.5 without having to do too much work, so that could put an ease to the question of being mistimed.

 

[RyanY]

There are no software updates done by the dealer - our Monteros predate that technology by a number of years.

I'm at a loss to explain the mileage loss when everything else seems normal. I would run through a couple tankfuls before being too concerned.

BTW, 40 psi is way too much tire pressure - you'll wear out the middle of the tire tread prematurely rather than wearing evenly across the tires. Check the driver's door jamb for the recommended tire pressure sticker, likely 28-30 psi.

 

[jeep-N-montero]

Are you sure they used the OEM Iridium spec plugs?

 

[sidecardog]

Are you sure they used the OEM Iridium spec plugs?

I'm not sure what they used. I'll be sure to ask.

 

[sidecardog]

There are no software updates done by the dealer - our Monteros predate that technology by a number of years.

I'm at a loss to explain the mileage loss when everything else seems normal. I would run through a couple tankfuls before being too concerned.

BTW, 40 psi is way too much tire pressure - you'll wear out the middle of the tire tread prematurely rather than wearing evenly across the tires. Check the driver's door jamb for the recommended tire pressure sticker, likely 28-30 psi.

Thanks for the info. I'm aware of the middle wear from higher pressure tires but I usually run around 38psi, including on my previous Montero and it's never been an issue for me. I just find that the vehicle handles so much better on the pavement (in my opinion) and I think it's a much safer on the road as a result. Of course I switch it up based on terrain.

 

[sidecardog]

How many tanks have you run though it since getting the service done? You should be able to check the timing on the 3.5 without having to do too much work, so that could put an ease to the question of being mistimed.

I haven't counted tanks but it's probably around 10. I noticed the mpg change earlier on but was too busy to give it the proper attention. My calculations of 12mpg have been based on the last 3 tanks. I've had it for three years and it's always given me 18mpg or slightly better in the same driving conditions.

 

[coffeegoat]

I wonder if it is something unrelated except you have to remove it when you do the timing belt. Basically the top and front of the engine comes off to get to everything which means lots of unplugged sensors.

 

[normal_dave]

So, I had somewhat of a similar experience with my '02 Montero. I did the timing belt job on my "new to me" Montero. Discovered the passenger side cam had been running ~3 teeth advanced. It had a slightly different exhaust note/idle sound than my wife's '03 6G74, but I had blamed it on the exhaust system.

Once the job was finished correctly, I noticed a drop in average mpg, not as bad as yours, but I recall 18-19 average was pretty easy to get, now that everything is timed correctly, 15-16, now, and only 18 on the highway taking it easy. I'll take correct timing over a few mpg any day. I'm fortunate to not have had an issue with whatever prior mechanic did the last job. Always rotate two complete turns, wait a few minutes, check the tensioner pin, verify the marks again. Also, the exhaust note smoothed out, and now sounds just like the wife's truck.

Before pic, passenger side at TDC.

Here's the post:
6G74 timing belt fun

Anyhow if you're down around 12 mpg, something is still amiss. I thinking a cup of coffee with the shop manager, friendly conversation with the mechanic that did your job.

I find it harder and harder to source techs that actually "speak" fluent Mitsubishi. The 6G74 3.5L has been out of production here so long, I wonder if the dealers still have folks that really know their way around these engines?

 

[plh]

following

 

[ChrisCosta416]

Try replacing your PCV valve, its a few bucks for an OEM valve and takes less than 5 minutes.

I would also recommend running a chevron fuel system cleaner - 15$ and a tank of premium.

 

[BayMonty]

FWIW I just had the major maintenance done on my 97 Montero, which has the 3.5. I’ve noticed a slight drop in MPG too. I’ve only had it for about a month now and went only through maybe a tank and a half as it was at the shop for the last week. Just got it back yesterday and although it runs super smooth, I did notice the fuel level dip a little more when driving my commute to work and back today.

 

[BayMonty]

but I did just throw on some 31’s