Sputtering on acceleration (fixed) now poor MPG. Any ideas?

Bennyggg

Bennyggg

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Joined
Nov 8, 2019
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Location
Vista CA
Successfully solved a low RPM acceleration sputter a few weeks ago but now I’ve been getting significantly lower gas mileage. Normal MPG is about 15-16 monitored for years, but it has dropped to 13 tops.

here’s a timeline of the issues and troubleshooting:

while stuck in a traffic jam I started to notice some sputtering at low rpm acceleration. It seemed to only show up after driving for awhile. No CEL or codes. Rich smelling exhaust.

After reading about probable causes I decided to take a shot in the dark and replace the upstream O2 (went with NGK). Unfortunately this didn’t solve the issue. So I started working though other possible causes.

Cleaned the throttle body along with the associated 3 sensors (TPS, MAP and IAC). It was pretty dirty but nothing outside the norm. No change in symptoms. Also checked TPS voltage (.96-3.9vdc).

Checked for vacuum leaks. Found a a few crusty vacuum 90°s and couplers so I replaced everything that looked suspect. Still no change. Figured a quick tuneup wouldn’t hurt.

Changed out ignition parts one at a time. Cap and rotor no change. Popped a new coil in and boom! All the sputtering is gone. Continued with the remaining ignition parts (APP985 .035 gap plugs, united Motor wires, cap, rotor and coil)

the jeep is running well but my mileage has really dropped off. Could it be the ignition parts? Or maybe some other issue? Any quick checks to rule out some of the pricy sensors?

Appreciate any advice.
Ben

99 TJ 4.0 manual 3” sus lift 32s stock gears
 
Sometimes the coolant temperature sender can be bad and give the computer erroneous temperature information. When that happens it can screw up the air-fuel mixture and cause those symptoms. It's the sender screwed into the thermostat housing.
 
Jerry Bransford said:
Sometimes the coolant temperature sender can be bad and give the computer erroneous temperature information. When that happens it can screw up the air-fuel mixture and cause those symptoms. It's the sender screwed into the thermostat housing.
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Thanks jerry. Your posts on this forum have helped me out for years.

Is there separate “computer” and “gauge” sensors? I’ve read that different year TJs have 1 or 2 temp sensors. The gauge appears to be working properly. I actually did a coolant / heater core flush along with a new thermostat, housing, water pump and fan clutch about 4 months ago. Reused the sensor though.
 
Could I be running a bit cold? This was at idle after a 30 min drive. 45°F Outside. Heater on low. The top of the radiator measured 167°F with an infrared thermometer.

57F1D85E-863F-4703-A93F-8E2A13CF7E24.jpeg
 
Bennyggg said:
Could I be running a bit cold? This was at idle after a 30 min drive. 45°F Outside. Heater on low. The top of the radiator measured 167°F with an infrared thermometer.

View attachment 127307
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I live in Escondido close by you, it hasn't been cold enough here to cause the Jeep to run that cool. I'd suspect either your thermostat is not the correct 195 model, or perhaps someone installed a "failsafe" thermostat which has stuck open which happens way too often with failsafe thermostats. I'd suspect a bad thermostat or incorrect thermostat and would install a fresh 195 NON-FAILSAFE thermostat like a Stant SuperStat.
 
Fughuert said:
That’s what mine reads, just under 210.
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Thanks for the reply. I’ve read that quite a few guys see the needle ride right at 210. I guess if I’m running low, especially below 180, the computer will run the engine rich. Have you had any issues with poor MPG?
 
Jerry Bransford said:
I live in Escondido close by you, it hasn't been cold enough here to cause the Jeep to run that cool. I'd suspect either your thermostat is not the correct 195 model, or perhaps someone installed a "failsafe" thermostat which has stuck open which happens way too often with failsafe thermostats. I'd suspect a bad thermostat or incorrect thermostat and would install a fresh 195 NON-FAILSAFE thermostat like a Stant SuperStat.
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Crazy. I put in a stant 45359. I have another coming in this week. I’ll give it a shot. Have you seen these stants DOA like this?
 
Bennyggg said:
Crazy. I put in a stant 45359. I have another coming in this week. I’ll give it a shot. Have you seen these stants DOA like this?
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I wouldn't expect a Stant to be DOA, that's a good brand. Maybe your temperature sender is defective as I mentioned up in post #2.
 
Replaced the temp sender/sensor with a Delphi. Symptoms remain. Replaced the thermostat with a super stant. Symptoms remain. Still running a little cool.
 
Bennyggg said:
Replaced the temp sender/sensor with a Delphi. Symptoms remain. Replaced the thermostat with a super stant. Symptoms remain. Still running a little cool.
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Do you have a code reader, you can plug in and see what the temp sensor is reading.
 
Sounds like the beginning of a clogged Catalytic Converter. Feel your floor board does it feel hotter than it use to. Your millage will start to drop even more and your starts will feel more lethargic. It just gets worse and anything near the converter will start to melt. All my rock light wiring under the jeep on that side suddenly went out when I looked the wiring jacket was melted off form the heat. Never did throw a code.
 
Dilly Dilly said:
Sounds like the beginning of a clogged Catalytic Converter. Feel your floor board does it feel hotter than it use to. Your millage will start to drop even more and your starts will feel more lethargic. It just gets worse and anything near the converter will start to melt. All my rock light wiring under the jeep on that side suddenly went out when I looked the wiring jacket was melted off form the heat. Never did throw a code.
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Thanks for the input. I’ll look into it. My floors do get damn hot but I don’t have carpet so it’s hard to tell if it’s getting hotter. How many miles did you have when the cat went? Im rolling 178k.

edit: I have definitely noticed a decline in power/torque when the original problem popped up. It’s a bit better now but still feels lacking.
 
Bennyggg said:
Thanks for the input. I’ll look into it. My floors do get damn hot but I don’t have carpet so it’s hard to tell if it’s getting hotter. How many miles did you have when the cat went? Im rolling 178k.

edit: I have definitely noticed a decline in power/torque when the original problem popped up. It’s a bit better now but still feels lacking.
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130 thousand , We were on our way to AZ in a convoy and it started to feel like I was having more problems keeping up on the hills, then I started to run warmer on the Temp gauge. on the way home I had to start stopping for gas a little sooner than everyone else. I didn't notice the increased heat until my wife said the floors were hot, We were in AZ so I didn't think much about that. Then my Gas millage took a dump and it was apparent something was wrong, I thought it was my trans I have never had that happen and I didn't have a clue what was going on, the old timers didn't know what it was the Trans guy that figured it out. I am just Throwing this out there.
 
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Dilly Dilly said:
130 thousand , We were on our way to AZ in a convoy and it started to feel like I was having more problems keeping up on the hills, then I started to run warmer on the Temp gauge. on the way home I had to start stopping for gas a little sooner than everyone else. I didn't notice the increased heat until my wife said the floors were hot, We were in AZ so I didn't think much about that. Then my Gas millage took a dump and it was apparent something was wrong, I thought it was my trans I have never had that happen and I didn't have a clue what was going on, the old timers didn't know what it was the Trans guy that figured it out. I am just Throwing this out there.
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I appreciate the info for sure. This is the first time I’ve had to troubleshoot a problem like this. Obviously the jeep isn’t running right but there’s no codes and no major problems. It’s just a little “off”. I’m trying to do this as intelligently as possible without throwing money at it. Do you know of any easy “check” to verify the cat is working right?
 
Bennyggg said:
I appreciate the info for sure. This is the first time I’ve had to troubleshoot a problem like this. Obviously the jeep isn’t running right but there’s no codes and no major problems. It’s just a little “off”. I’m trying to do this as intelligently as possible without throwing money at it. Do you know of any easy “check” to verify the cat is working
Lack of perfomance , basically what you described. Might smell funny from the excessive heat in the Cat. You have the millage for it. I was having problems passing smog the yr before and the shop replaced the upper cats which helped it pass smog, then this happened maybe 2-3 months later. I think it's much more apparent if you are on a longer drive it really heats up. Driving around town it may not get enough time to heat up from the un burnt gas so it feels and drives a little better than on a longer drive..
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Jerry Bransford said:
Sometimes the coolant temperature sender can be bad and give the computer erroneous temperature information. When that happens it can screw up the air-fuel mixture and cause those symptoms. It's the sender screwed into the thermostat housing.
Click to expand...
Would this also cause a slight smell of coolant in the interior? I never see any leaking, but my TJ usually smells just a bit of coolant while driving. I only mention it because I am also having significant gas mileage issues...filled up yesterday, and got 25 miles on a quarter tank this morning driving to work. Lost an eighth of a tank while idling for 15 minutes in my driveway warming up this morning
 
sandmc said:
Would this also cause a slight smell of coolant in the interior? I never see any leaking, but my TJ usually smells just a bit of coolant while driving. I only mention it because I am also having significant gas mileage issues...filled up yesterday, and got 25 miles on a quarter tank this morning driving to work. Lost an eighth of a tank while idling for 15 minutes in my driveway warming up this morning
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Your heater core might be starting to leak if you smell coolant in the cab but not under the hood. If you smell it outside as well it could be anything from water pump to radiator to a heater hose.
 
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