Trying to figure out a mpg thing here...

[Myco]

After all I've done for gas milage in my car I'm always trying to squeek out more.

Over the past week (about 300 miles) I drove about 270 miles in-city and highway driving (mostly highway) and the MPG leveled out to 16.5mpg which is pretty poor. I reset the trip and did strictly in-city driving and got 19.5 after driving 10 miles in 45mph zones/lights etc. Then I jump on the freeway and the MPG went down.

In a recent trip to New Orleans I got 25.5mpg.

I'm trying to figure out what this could be. Could the EGR tube being clogged cause the 'cruzing mode' to never engage? Could gunked up IMRC's be causing it. I don't think it's the IMRC's as that would mean my low-end torque is gone.

But you do you guys think?

 

[Green Machine]

Im assuming you are using a 91 octane or higher premium gasoline, correctomundo?
running low octane, (87-90) to save a buck, will drop your mpg by at least 4 mpg. it does to my car anyway.

 

[Green Machine]

code scan would be in order. DPFE sensor is a possibility. what gear ratio is in your Mark?

 

[unity]

A vacuum leak can cause a similar issue, but you would have other issues also. At higher RPM there is less vacuum, thus leaks have a greater effect. But I thought I would toss that out there.

You did not mention year or maintenance. But cleaning the MAF and checking out the plugs are a good start. Even if the plugs are new, pulling them may indicate the issue since difference colored tips can indicate certain issues. You may also want to consider the O2 sensors. Avoid BOSCH sensors if you can, they are not well made it seems.

But I would go over basic maintenance first before moving on to other things.

 

[kaha]

Im assuming you are using a 91 octane or higher premium gasoline, correctomundo?
running low octane, (87-90) to save a buck, will drop your mpg by at least 4 mpg. it does to my car anyway.

Here's a question that I think is related. I burn only 91 or better in AL (my car, not the State), and a guy at the gas station told me that there is no need to because the higher the elevation (I'm in the Rockies) the higher the actual octane is. I've pretty much ignored that advice so far because the cynical little douchebag inside of me sez "if he knows so damn much, why is he pumping gas for 8 bux an hour?".

But I'm curious: is there any truth to that?

 

[kaha]

That said, he might be right. Because on a trip to Helena I set the cruise to 80mph, reset the fuel economy checker, and wound up with an average of just a hair over 30mpg.
No passenger, no cargo.

 

[chris2523]

At higher RPM there is less vacuum, thus leaks have a greater effect.

you sure?
throttle blades closed, high vacuum, pulls lots of air through the leak.
throttle blades open, lower vacuum, pulls less air through leak cuz it can get through the throttle body.
wouldn't that have a lesser effect overall?

 

[chris2523]

After all I've done for gas milage in my car I'm always trying to squeek out more.

Over the past week (about 300 miles) I drove about 270 miles in-city and highway driving (mostly highway) and the MPG leveled out to 16.5mpg which is pretty poor. I reset the trip and did strictly in-city driving and got 19.5 after driving 10 miles in 45mph zones/lights etc. Then I jump on the freeway and the MPG went down.

In a recent trip to New Orleans I got 25.5mpg.

I'm trying to figure out what this could be. Could the EGR tube being clogged cause the 'cruzing mode' to never engage? Could gunked up IMRC's be causing it. I don't think it's the IMRC's as that would mean my low-end torque is gone.

But you do you guys think?

gearing? mileage? octane of gas? any CELs? what year car? routine maintenance performed recently?

you'd feel stuck imrcs i bet.
how are your plugs?
high mileage could have a clogged egr. i did.
broken egr tube? that will cause a vac leak. i had one of those too.

 

[Myco]

Sorry for not responding earlier I was at greg's all day working on his son's mark.

The COP's have been tested and bad ones have been replaced.
spark plug boots are new.
The plugs are under 20k (just over a year).
The fuel filter is just over a year old.
I've done sea-foam through the brake booster in the last year and done an injector cleaning.
Tires are up to pressure.
Gearing is stock.
Tires are the widest/lowest profile you can put on a stock 10spoke rim.
Maf has been cleaned in the last year,
Air filter is brand new (under a month).
I get no check engine light but haven't read the codes in a while. There might be one in there not throwing a light. Need to get that checked.
Octane is the premium stuff.
Vacume lines have been smogged. No leaks.


Chris: Your thread has me thinking I want to do what you did.

I tried to reach around and find the lever for the IMRC's but didn't find them.

high mileage could have a clogged egr. i did.
broken egr tube? that will cause a vac leak. i had one of those too.

Where was the egr clogged? The tube itself? Is there a way to test this? Also, was the egr tube just cracked or completely broken? Is there a way to get under the car and check this out?


Milage is a bit of a guess. When I got the car it had 150k but the guy said that the engine had been replaced under warranty at 90k with an engine with 50k on it which would make the engine 'now' at 130k or if this was a lie 170k. (I'm at 170k now). So I just go with that the car has 170k on it as most of the components (egr etc) were probably re-used anyhow.

 

[chris2523]

disclaimer! gen 1 mark viii! i dunno what year you have.

at 180k, my egr was clogged here:

here is one imrc actuator:

the ufo looking thing with numbers on top is the egr valve. the egr tube bolts to it and heads down under the car.

the actual tube was cracked. a few inches down from the top.

 

[unity]

you sure?
throttle blades closed, high vacuum, pulls lots of air through the leak.
throttle blades open, lower vacuum, pulls less air through leak cuz it can get through the throttle body.
wouldn't that have a lesser effect overall?

Pretty sure. Think about a straw for drinking. If it has a small hole in the side but you are consuming fast, it will hinder but not be a huge deal. Then try sucking a drink slowly with the same hole size, it will be much harder and there will be a higher air to fluid ratio.

 

[chris2523]

Think about a straw for drinking. If it has a small hole in the side but you are consuming fast, it will hinder but not be a huge deal.

throttle blades open. lots of air, "consuming fast." not a big deal.

Then try sucking a drink slowly with the same hole size, it will be much harder and there will be a higher air to fluid ratio.

throttle blades closed. idle. "drinking slowly." higher air to vac leak ratio. bigger deal.

this is why vac leaks rear their ugly head with bad idle issues.

 

[unity]

If thats how you want to interpret it, sure. But I will stay content with knowing what I am talking about.