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99+ intake manifold hurt my mileage

D

dhreeves

NAXJA Forum User
Location
Aurora
Two weeks ago I replaced my leaky exhaust manifold on my 96 with an exhaust header. I figured since I had the intake off why not install a 99+ manifold at the same time. It was acutally a much easier swap than I expected but after two weeks and two tanks of gas.....

There was a noticable improvment in performance and just overall driveability but my mileage has suffered greatly. I used to get a respectable 17.2 - 17.8 mpg but now I am getting 15.3 - 15.5. Thats a full 13% decrease! Has anyone else experience similar results?
 
Have you verified you do not have slight vacuum leak in the intake manifold gasket?
When you extended the wiring for the Intake Air Temperature sensor; did you ensure you have a good connection? Cold engine warmup loop will kill your gas mileage.
 
How have your driving habits been since the swap? Are you driving the same as you did before the swap, or are you pushing it a little more to feel that increase in performance? Another could be that you're motor is getting more air movement now so the ecu adjusted the fuel accordingly?
 
Maybe the first week I rode it harder enjoying the new power but this last week I have been very careful to not do that.

How do I test for the cold start loop and how do I test for a vacuum leak? One thing I just thought of, it has a cold air intake that the vacuum connection was capped off and the one on the old manifold was capped (but leaking badly) When I did the new one, I ran a vacuum line from the CAI to the manifold. And I am sure the extened sensor connections are solid.
 
Last edited:
How have your driving habits been since the swap?
==========================================
There was a noticable improvment in performance
------------------------------------
Thats a full 13% decrease!
======================================
To me it sound like your having 13% more fun to.
I say recheck you work.
Give it a little time.

You now are running winter gas (switched last month) and a drop of about that much isn't that uncommon on winter gas.
 
I've heard this before. JP just did an article about 4.0L myths, and they mention that the old style manifold makes better economy. But less power.
 
dhreeves said:
One thing I just thought of, it has a cold air intake that the vacuum connection was capped off and the one on the old manifold was capped (but leaking badly) When I did the new one, I ran a vacuum line from the CAI to the manifold. And I am sure the extened sensor connections are solid.
Click to expand...

I believe that line on the intake connects to the CCV line at the front of the valve cover (it does on my 97)- running a hose from the CAI to the manifold... the CAI is already connected to the manifold with a big tube. I think you may have some things hooked up incorrectly, but I don't know if that would hurt a great deal.
 
DOH!, you're right, that is supposed to go to the CCV. Mine has one of those little filters on the CCV. Guess I need to cap the CAI and manifold one off...
 
dhreeves said:
Semi off-topic, how big of a difference will a regear make on MPG? I am on 31's with the stock 3.55 gears
Click to expand...

with just 31's, maybe 1-3 mpg possibly.

when i regeared for 31's, i was more concerned about performance, but mine is a 5 speed with 3.07 gearing stock.
 
yeah, I bit the bullet some time ago and paid the 80 bucks for the correct speedo gear.

I can't imagine it would make much difference but we reused the injector O rings as dealerships here are closed on Sundays and I wasnt prepared enough.
 
badron said:
You now are running winter gas (switched last month) and a drop of about that much isn't that uncommon on winter gas.
Click to expand...

Is this for real? i have never heard of this before. however, i seriously noticed about 2-3 weeks ago that my mileage went from ~18.8 to ~17.4 consistently over the past 3-4 tanks worth of gas.

thanks:)
 
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