swampass2
NAXJA Forum User
- Location
- Boston, MA
So I've had my 99 4.0 XJ AW4 for a year and a half and gas mileage has always been terrible (10 city, maybe 14-15 highway bone stock). Needless to say I've read every gas mileage thread on here. I'm investigating everything I possibly can to bump my mileage and lately I've been focusing on what I think is odd engine behavior.
When I lift off the throttle completely, I feel like my RPM's drop much faster than they should. For example, if I push it a little to about 3,000 RPM's and then let off the gas completely before the engine can upshift, RPM's drop to around 800-900 in at most 1 second, followed by a blip up to about 1,000-1,100 where it stays steady until I slow to a complete idle. She idles around 750 pretty smoothly but with what I think is probably the characteristic 4.0 occasional stutter. With the Jeep parked and the hood up, I can blip the throttle by hand and notice this same type of behavior (after letting off the throttle, the engine slows and then "rebounds" a bit, noticeable both by sound and a by a bit of a shudder as the RPMs rise again).
I've been meaning to take a video of the tach while driving to demonstrate just how quickly the RPM's drop. I'm thinking something like a vacuum issue/exhaust leak/backpressure buildup might be to blame, but I can't find any vacuum leaks though I do hear a sort of low "airy" howl when letting off the throttle (sounds like it's coming from just in front of the passenger side firewall, but strangely more noticeable from the cabin than from under the hood.)
I have no engine codes. Here's a list of all gas mileage related things I've done since I've owned it (within the last 10,000 miles). Many of these things were probably ok but with 106,000 miles on her, I changed them anyway:
new Champion Truck plugs, Mopar wires, cap, & rotor
new O2 sensors
new Coolant Temp Sensor
new Intake Air Temp Sensor
new stock Thermostat
new Magnaflow Direct-Fit Cat
new OEM exhaust manifold (w/ the accordion-style joints)
new CCV orifices and grommets
cleaned Throttle Body
multiple Sea Foam treatments
new Revo's at 36 PSI
Mobil 1 in the diffs
Mobil 1 High Mileage in the engine
The only things I haven't done are cleaned the IAC, and haven't checked the MAP. In general, it runs pretty smoothly but seems down on power (my old girlfriend had a 5-speed 4.0 wrangler, which is my only base for comparison.) Any ideas on what might be causing RPM's to drop so quickly? Call it naivete or blind optimism, but I'm hoping that solving this would get me closer to the 13/14 city and 18/19 highway that I'm expecting out of the Jeep. I'll try to take a video today to demonstrate.
When I lift off the throttle completely, I feel like my RPM's drop much faster than they should. For example, if I push it a little to about 3,000 RPM's and then let off the gas completely before the engine can upshift, RPM's drop to around 800-900 in at most 1 second, followed by a blip up to about 1,000-1,100 where it stays steady until I slow to a complete idle. She idles around 750 pretty smoothly but with what I think is probably the characteristic 4.0 occasional stutter. With the Jeep parked and the hood up, I can blip the throttle by hand and notice this same type of behavior (after letting off the throttle, the engine slows and then "rebounds" a bit, noticeable both by sound and a by a bit of a shudder as the RPMs rise again).
I've been meaning to take a video of the tach while driving to demonstrate just how quickly the RPM's drop. I'm thinking something like a vacuum issue/exhaust leak/backpressure buildup might be to blame, but I can't find any vacuum leaks though I do hear a sort of low "airy" howl when letting off the throttle (sounds like it's coming from just in front of the passenger side firewall, but strangely more noticeable from the cabin than from under the hood.)
I have no engine codes. Here's a list of all gas mileage related things I've done since I've owned it (within the last 10,000 miles). Many of these things were probably ok but with 106,000 miles on her, I changed them anyway:
new Champion Truck plugs, Mopar wires, cap, & rotor
new O2 sensors
new Coolant Temp Sensor
new Intake Air Temp Sensor
new stock Thermostat
new Magnaflow Direct-Fit Cat
new OEM exhaust manifold (w/ the accordion-style joints)
new CCV orifices and grommets
cleaned Throttle Body
multiple Sea Foam treatments
new Revo's at 36 PSI
Mobil 1 in the diffs
Mobil 1 High Mileage in the engine
The only things I haven't done are cleaned the IAC, and haven't checked the MAP. In general, it runs pretty smoothly but seems down on power (my old girlfriend had a 5-speed 4.0 wrangler, which is my only base for comparison.) Any ideas on what might be causing RPM's to drop so quickly? Call it naivete or blind optimism, but I'm hoping that solving this would get me closer to the 13/14 city and 18/19 highway that I'm expecting out of the Jeep. I'll try to take a video today to demonstrate.