Cowl Intake Effectiveness
CF Veteran
Joined: Oct 2011
Posts: 1,812
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From: South Mills, NC
Year: 1998
Model: Cherokee
Engine: Rebuilt 4.0L I6
Today, I monitored the Instantaneous mpg numbers returned by the Ultra-Gauge monitor. But first I should say that when I purchased this Cherokee about 1 year ago, I replaced the speedometer gear to correct the speedo/odometer for the 33" tires I am running. I checked and calibrated the speedometer with a GPS system and found an error of less than 0.1%. I also calibrated the odometer using a tracking program locked to the GPS signal (iPhone extravagance). The odometer error was more like 0.3%.
Anyway, I did several runs on flat stretches of the freeway, averaged two directions. The results: 23.9 mpg @ 55 mph; 22.7 mpg @ 60 mph and 20.8 mpg @ 65 mph. This represents about 2 mpg better than my numbers before the install. I then did my standard run up through the mountains to home. This is a 1,200 foot elevation change over about 2.8 miles. I run this at 45 mph as the Jeep can stay in the same gear over this run and I'm looking for systematic changes in coolant temperature. Here I recorded 10.9 mpg vs. my typical value of 9.3 mpg before the install. From these results, I conclude that there is no mpg reduction, and quite possibly a 6 to 10% gain in fuel economy.
Of course, the picture is a little more complicated. The day I did the install, I did it just after changing the oil to 5W-30 Full Synthetic (Castrol Edge) and flushing and re-lubricating the rear differential. The differential fluid before the change had an disturbingly gritty feel! Also the previous hill climb numbers were with a quarter tank of fuel and this latter run was done with a full 20 gallons. Also, today it was about 4 degrees cooler than yesterday.
My 45 mph testing gave a coolant temperature at the turnoff of 221 F. The run today after the install and oil service gave a peak temperature of 203.
As a result of all of this, your mpg and temperature problem can't be related to the cowl intake install unless there is a major air leak or the installation inadvertently triggered an electrical control system problem. To the best I can tell from here, you should have the same results I have.
Nothing is ever as simple as it seems!
CF Veteran
Joined: Dec 2011
Posts: 1,777
Likes: 22
From: PA KOTUFU!
Year: 1998
Engine: 4.0L 162,000
CF Veteran
Joined: May 2011
Posts: 6,098
Likes: 250
From: DE
Year: 1988
Model: Cherokee
Engine: I6 4.0
Thanks for the write up as I was worried about the results due to cooper, i just received my package of a 3" hole saw and the same spectre 3" hose but in red since it's on sale and was noticing that the sleeve at the end of the hose is about 3.25"s so i'll defiantly be buying the spectre adapter
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Junior Member
Joined: Apr 2011
Posts: 47
Likes: 5
From: Reno, NV
Year: 1996
Model: Cherokee
Engine: 4.0 L
I would think that the cam position sensor (and ignition timing) would be critical to mileage. The potential variation in sensor readings including the O2 sensor should send the ECU all over the place in compensation.
CF Veteran
Joined: Oct 2011
Posts: 1,812
Likes: 0
From: South Mills, NC
Year: 1998
Model: Cherokee
Engine: Rebuilt 4.0L I6
I've had a check engine light for the crank or cam sensor for a while now. I've replaced the crank sensor but maybe its time I replace the cam sensor.
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