Lukewarm heat and bad gas mileage

I have a '91 Briarwood, 4.0 with AW4, factory transmission cooler. Bone stock, only upgrade is '95 vacuum booster.

I bought the Jeep 2 years ago with 65K on it, 1 owner, dealer maintained its entire life with receipts to highlight work done to it. Since I've had it it has been in a fender bender, and has been overheated since I blew a radiator hose off due to a loose clamp. It currently has 75K on it.

Problems:

• My heat is lukewarm, but also inconsistent.
• My Jeep takes a lot longer to get up to temperature than it used to, and seems to fluctuate temp.
• Doesn't idle the smoothest, its not shaking all over the place, but there is a vibration at idle, and I can notice a harmonic vibration when stopped in gear.
• I can't get better than 12mpg and that is driving like an old lady

Items replaced/checked:

• Complete tune up when bough it
  1. Plugs (Champions)
  2. Wires
  3. Distributor cap
  4. Distributor rotor
  5. Air filter
  6. Replaced CCV tubes after finding oil covering original air filter
  
  
• Almost entire cooling system: When I drained the system the coolant was bright green and clean from the dealer
  1. 
     Napa water pump less than 2 years old
  2. Hoses done and heater valve bypassed at same time
  3. Stant superstat in the entire time this has been going on, just replaced again with superstat 2 weeks ago, symptoms persisted
  4. New thermostat housing at time of water pump
  5. CSF 3 row 1 year old
  6. ZJ fan clutch 6 months old
  7. Heater core not clogged, flushes clear every time with no blockage
  8. Coolant temp sensor repaced 6 months ago from Autozone, fan still does not turn on though
  9. New 16psi cap
  
  
• Compression test performed roughly 10 months ago after overheat. No oil in coolant, no coolant in oil. I have not noticed drop in coolant levels
  List from Cylinders (round numbers, don't have actual in front of me at moment)
  1. 
     130psi
  2. 130psi
  3. 150psi
  4. 150psi
  5. 150psi
  6. 150psi
  
  
• Throttle body removed and sensors cleaned with sensor safe spray.
  
  
• TPS tested fine according to FSM test procedures.
  
  
• All new exhaust: Walker replacement exhaust pieces
  1. 
     Exhaust header not factory, replaced by dealer. Is the header with belows on it, haven't seen any cracks.
  2. Muffler and tail pipe replaced 1.5 years ago
  3. Down pipe and Cat replaced 2 months ago.
  4. Bosch o2 sensor
  
  
• Harmonic balancer, idler bearing and belt changed 1.5 years ago
  
  
• Dealer converted to R134a AC system.

Heat: At times it comes out barely warm at all, other times a little better. I'm used to my '99 and father's '01 where you can't hold your hand in front of the vent for an extended period of time, and can cook up a sweat if you leave the heat on for a while. I'm still shivering if I have the heat blasting on hi many times.

I've always driven it nicely, but 1 day last week I was trying to diagnose some issues, and ended up doing some brake stands and drove it really hard. Heat was noticeably hotter after this, confirmed by another Jeep friend.

Temperature:The Jeep has almost always ran hot since before the overheat. It runs at 210+ on roads/highway, and when crawling around the city the temperatures gets up there (approx 220-240). My e-fan has never come on due to temperature, but has been verified to work by the AC circuit and unplugging CTS. When it started getting colder, I was amazed that it would take so long to get up to temp, as the past 2 winters it has not failed to run hot. I will drive, it won't want to get up to 210ish, will rise when I'm stopped at a light, but will noticeably dip a good amount when I start moving again. I've smelled coolant many times after shutting the vehicle off and walking in front of grill, but it has never lost fluid and there are no leaks. Just a hot coolant smell.


Does anyone have any ideas on what I could be experiencing? I know the heater core is not clogged, it flows freely, but I'm can't definitely say that it is good or not. I would hate to go through the hassle of changing it and losing the charge on the AC system just to replace parts.

I've run a number of tanks of gas, always filling up from the same gas station, and have been recording the mileage. 12 when ever I'm doing city driving (which is how I get to work), and 15 when on the highway. The transmission "power/comfort" button does not make a difference. Have run injector cleaner through before and seafoamed.

I just don't know with the temperature issue anymore now that it switch from running hot to not warming up.

Any help/suggestions/questions are appreciated.

maybee you got a bad tstat? and maybee the 3 row rad is cooling to good?

With two different t-stats, there was no change between the heating. 3 row was in last winter, didn't have this issue with the running cooler. One thing I can think of that I didn't have last winter was the zj fan clutch.

maybee the flan clutch is bad does it sound like its locked up?

ROBZ95Xj said:

maybee you got a bad tstat? and maybee the 3 row rad is cooling to good?

Click to expand...

The heater core bypasses the thermostat, so as long as the stat is closed, all coolant goes through the heater. Once the stat opens, only a portion of the coolant goes to the heater. How much is determined by any flow restrictions in the heater circuit. I doubt the radiator is a problem. I am running the same radiator in my XJ and my heater works well.
Two bad superstats is possible, but highly unlikely. You did install a 195* F thermostat, right? If you play with lower temperature thermostats, your heater output will suffer. You should not lower the stat tempurature in an attempt to fix an overheating problem. Find the true cause of an overheat and fix that.

Are you sure that the heater control valve is opening ? It is vac operated.And you can by-pass if necessary,,,this was done on later models anyway.

Are the heater hoses burning hot? they should be. Is the little piece of the heater core that sticks out hot it should be. Sounds like the heater valve is not opening all the way.
Ron

MoFo said:

The heater core bypasses the thermostat, so as long as the stat is closed, all coolant goes through the heater. Once the stat opens, only a portion of the coolant goes to the heater. How much is determined by any flow restrictions in the heater circuit. I doubt the radiator is a problem. I am running the same radiator in my XJ and my heater works well.
Two bad superstats is possible, but highly unlikely. You did install a 195* F thermostat, right? If you play with lower temperature thermostats, your heater output will suffer. You should not lower the stat tempurature in an attempt to fix an overheating problem. Find the true cause of an overheat and fix that.

Click to expand...

Yes, 195* thermostat. Trying to find the cause, which is why I replaced bad/suspected bad components.

Wayne Sihler said:

Are you sure that the heater control valve is opening ? It is vac operated.And you can by-pass if necessary,,,this was done on later models anyway.

Click to expand...

As I mentioned, bypassed the heater valve when I did the water pump 2 years ago.

MAY POP said:

Are the heater hoses burning hot? they should be. Is the little piece of the heater core that sticks out hot it should be. Sounds like the heater valve is not opening all the way.
Ron

Click to expand...

No heater valve, but yes the hoses do get hot. I can reconfirm this weekend since the Jeep is currently at in my parent's garage after a brake job didn't get finished due being given incorrect parts.

deleted. poor timing.

Just throwing this against the wall, but I'm assuming a Stant brand T-stat? I don't want to open Pandora's box in your thread, but I've used aftermarket and dealer-bought T-stats, and I've had better luck with the dealer 'stat, with the check valve ball, installed at 12 o'clock. Possibly the system is air-bound? Either that or there's a major clog somewhere? I've seen majorly clogged plenum chambers, usually leaves, that had prevented just about all airflow. How's your air volume? You mentioned heater hoses getting hot. Supply and return? Possibly is the return much cooler?

I'm going to give you a worst-case scenario. Seven years ago, I pulled the head off my '92, to replace some ticking lifters. Never had an overheating problem. As I was cleaning the head, I noticed the rearmost coolant passage, in the head and block, was just about completely clogged with dried, coked-up coolant, like green rock. I cleaned it out good, and after driving it awhile, noticed that the temp was noticeably cooler, and that winter, it was like, WOW. Turkey roasting hot! I'd hate to see anyone yank a head chasing a possible clog, but my accidentally finding this was pure luck. My guess is that with high operating temps and poor heat output, there's most likely a clog somewhere.

heyhar said:

Just throwing this against the wall, but I'm assuming a Stant brand T-stat? I don't want to open Pandora's box in your thread, but I've used aftermarket and dealer-bought T-stats, and I've had better luck with the dealer 'stat, with the check valve ball, installed at 12 o'clock.

Click to expand...

X2. I've read that Stants are the best, but I've had problems with them. The dealer tstat is generally the way to go. Cheap and easy. I would start there since it's the basis of operating temps.

You've pretty much got your grounds covered by replacing everything else. I was going to say heater valve, but...did you remove it? If not, work it in the opposite direction to see if you get heat (obvious input but maybe you overlooked it?).

Just wanted to make sure you verified the heater control valve is working.

I cracked the vacuum fitting on my heater valve once.

Too much salad?

Have you verified the reading of the CTS with a multimeter? I know you said you replaced it with new, but ya never know. Does yours have the split sensors (one for computer, one for gauge cluster)?

Also, have you verified your coolant temp using an IR thermometer?

Have you checked the overflow bottle while it's running hot? Maybe it's boiling into the bottle, but not overfilling?

Your compression is not uniform in all cylinders. The 20 psi difference in 2 of your cylinders is pretty significant and will affect you mpg. Also what condition is your O2 sensor in? A worn or faulty O2 sensor can greatly affect mpg as well. Just because your check engine light is not on or there is no detected trouble code doesnt not mean the O2 is good.

Regarding the heat in the cab, Have you verified that the blend door is functioning properly?

Norsk62 said:

Your compression is not uniform in all cylinders. The 20 psi difference in 2 of your cylinders is pretty significant and will affect you mpg. Also what condition is your O2 sensor in? A worn or faulty O2 sensor can greatly affect mpg as well. Just because your check engine light is not on or there is no detected trouble code doesnt not mean the O2 is good.

Click to expand...

I am going to call BS on this. I am currently getting 15mpg on my 98 XJ. It has 150, 150, 30, 30, 150, 140 as its compression readings. The head gasket between 3 and 4 is gone, you can feel air rush out of 3 when 4 compresses and vise versa. I have verified my odometer and measured the amount of gas used down to the hundredth of a gallon, using the same pump each time. No, I don't understand it either, but compression isn't THAT big a deal as long as they are all sufficient for a clean burn.

Mark, it sure sounds like something inside the hvac box or dash to me. I know you said the coolant hoses are nice and hot to the touch but have you checked with an IR thermometer? Just curious, I expect them to be at the right temp but you can never be sure.

I would suggest the O2 sensor but you already replaced that. Have you checked the MAP sensor?

kastein said:

I am going to call BS on this. I am currently getting 15mpg on my 98 XJ. It has 150, 150, 30, 30, 150, 140 as its compression readings.

Click to expand...

Wow.

Id check your blend door operation as well

Jesh, I just spend a 1/2 hour answering this post, hit post and got a BS token expired error message, and now I need to start over, CHIT!!!:flamemad:

OK, first check the fresh air/recirculate air door under the passenger side dash, near the door, there is a spring loaded vacuum operated valve there that operates the door. Bad valve or no vacuum to the valve defaults to cold ass outside air!!!!!

Check each spark plug for bad combustion on one cylinder. Check all the fuel injectors, for leaks, too clean, too dirty, oil, and so on cause of bad idle.....Check the fuel pump pressure too.

You may have, a small exhaust - head gasket leak, pushing hot air into the coolant, which would not cause loss of coolant, would cause the odor as gas escapes the coolant overflow bottle, and the silicate-silicone in the coolant would ruin the O2 sensor. Test the O2 sensor, and it's electric heater, and power feed....See here:

http://ns2.naxja.org/forum/showthread.php?t=1012701&page=5

If I am right about a small head gasket leak, the gas can accumulate under the T-stat, under the Gauge temp sensor and in the heater core, and cause some of your problems.

Get an IR gauge and verify the temps under the hood. Check that the Gauge sensor is actually reporting the real temperatures!!!!