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ctlucas93 06-27-2010 04:59 AM

Extremely bad milage!!!
Well i've finally had my jeep outa the shop from an engine overhaul and I ran slap outa gas on 146 miles on the odo, I have the 20 gallon tank and it was full when I took off. New fuel pump, air filter, plugs, wires, oil, oil filter, distributer cap and button, and cooling system clean and flush. I just dont understand how a 2.5 4 cyl can get that bad of milage. I have 31.10.50 that are on wide rims that hang pronly 4 inches past factory flares, By the way its a 1993 model 5 speed(like it ever sees 5th gear). Would the CAT being half stopped up and having a big hole knocked in the side have anything to do with this?(something the PO did). Im stumped here I get code 51 and 52 from the computer but I think its from where I broke a rocker bolt and went without a cylinder, just got it back from the shop to have that drilled and tapped? ... any advice/answers to my crappy milage would be greatly appreaciated. I run 87 octane gas with 10-30 castrol oil. I figured I would have gotten around 15 mpg.

RhcSavage21 06-27-2010 07:01 AM

id replace the cat and check your o2 sensors. how long were you driving without the cylinder? and did the rocker happen before or after the engine overhaul?

Gonda 06-27-2010 08:59 AM

Try unplugging your O2 sensor and disconnecting battery for 5 minutes (should clear the adaption stored in the ECU). Leave sensor unplugged and drive for half a tank to see if this improves your mileage. If it does, you likely have an exhaust leak (maybe that cat, but more likely upstream of sensor) that is tricking the O2 sensor into thinking you are running too lean. Could be the sensor too, but less likely. The 02 sensor tells the ECU to add more fuel if it thinks the engine is running too lean.

jpdocdave 06-27-2010 09:12 AM

when you say overhaul is the engine rebuilt? if its all rebuilt, its breaking in. fresh engines get poor mileage during break in.

and you had an engine overhaul, but didn't replace the cat with a hole knocked through it?

Snuffy115 06-27-2010 09:29 AM

Check your fuel pressure. If the return line is blocked it will increase you fuel pressure and consumption.

hannahuffster 06-27-2010 09:52 AM

I get horrible gas mileage in my 93 2.5 - I couldnt tell you exactly because my odom doesnt work but I go through at least 8 to 10 gal of gas a week & only put on approx 40 miles a week (on average). I only live 1.5 miles from work & the trail is only 3.5 miles from my house...

bck4more 06-27-2010 10:14 AM

I get 9mpg in my 87 with 33's

ctlucas93 06-27-2010 01:22 PM

As in engine overhaul everything was replaced exept for piston, rings, crank, and cam, everything else is new, waiting for te money to just have a staight pipe and muffler put on(dont have to have a CAT around here, I have no exaust leaks upstream of the CAT. The bolt that held the rocker in broke after the overhaul and I maybe drove it 5 miles because I didnt know that was the problem then it was towed when oil pressure bottomed out because of the lifter popping out and not controling oil pressure. I didn't know It took that long to clear codes from the computer on these things either, I did unhook it but it was only off maybe 5 seconds, when the lifter broke it was smoking black now it isnt burning rich but still get the codes, fuel presure regulator have anything to do with milage, because it does take you having to turn the engine over, wait and turn it over agian when its cold to crank.

MR-Fixit 07-10-2010 01:08 PM

Case Study: 1995 Jeep Wrangler 2.5L Problems
Ok, then…

Case Study: Overheating, Low Oil Pressure, Poor Fuel Economy, Open Loop Mode, Lacking Power, Rich Fuel Mixture, Drivability Issues, Emissions, Timing, Map Sensor

Here is a 1995 Jeep Wrangler SE 2.5L engine and AX5 transmission without A/C

Presenting problem: Engine seized

1st differential diagnosis: Improper oil maintenance

Troubleshooting revealed: Cylinder #4 Piston shattered sending debris into intake, breaking valves off, and finally jammed the crankshaft with a large chunk of debris.

Cleaned and degreased engine compartment and removed the defective engine block.

*Disassembled all accessories cleaning them thoroughly, degreasing then painting in high temperature enamel.

*Assembled prepared parts on a remanufactured engine block primed and painted with high temperature engine enamel, with new ignition wires, distributor cap, rotor, spark plugs, water pump, thermostat, serpentine belt, temperature switch (broke apart easily) and installed assembly into vehicle.

*Primed the oil pump

*Burped the air from the engine block (filling through water pump outlet hose disconnected and raised above block and radiator level and without the temperature sensor removed as suggested because had installed a radiator flush tee in the heater hose and simply unscrewed the cap to let air escape)- note, Duralast thermostat from AutoZone with jiggle valve works better than other aftermarkets tested to relieve air bubbles inside engine block. The original thermostat had a very small v-notch on the edge of the valve lip to bleed the system). Did not use a 180 degree because it would cause an open loop mode. Did not drill hole in thermostat because it would raise the cold side of the cooling system’s base temperature so when or if the thermostat did open the cooling system would not have been as cool.

*Started engine briefly to verify oil pressure was sustainable- noted slight rattle at top of engine, intermittent misfire, differential diagnosis: (rocker chatter), hydraulic lifters - no rocker arm adjustments so incorrect timing and ignition.

*Shut down engine after brief startup to confirm there were no oil leaks and that all was in order

*Finished fastening the system into place.

*Restart engine and allow to warm up to normal operating temperature and troubleshoot rattle. Trouble noted: missing not smooth idle, minor exhaust leak from muffler drain hole, backfire on revving engine out tailpipe - slight popping sound after releasing throttle on slight rev-up. Occasional stalling after revving engine slightly. Drives good but not powerful on acceleration as would expect, as if it was geared very low and seemingly the need to downshift to get up to speed. Fuel economy very poor ~ 150 per tank full. Differential diagnosis: catalytic converter, ignition and timing, vacuum leak, fuel mixture rich, muffler. The distributer timing was correct but the wires were arcing to the dip stick tube. The rotor contact tab had arc burn and inspection revealed the plastic holding the the tip was malformed allowing it to rise leading to the contact tab going off center thus creating a bad connection to arc and burn the metal tab and also the cap. Replaced with different aftermarket manufacturers - all were replaced rotor, cap, wires, and it resolved most of the missing and rough idle.


*Temperature Gauge Reading ~ 210 degree Fahrenheit, notable swing when thermostat opens and closes during idle

*Oil Pressure Gauge Reading ~ 50 psi at cold idle, drops to 40 psi when warming up at idle, as low as ~ 20 psi when hot idle, differential diagnosis rich fuel mixture causing "hot" running engine - open loop limp-in mode.

Troubleshooting Procedures:
*Verified there was no air in cooling system by continuously squeezing and releasing the water outlet hose to jiggle thermostat valve for air release. Measured the resistance of the temperature sensor @ ~ 4.3K Ohm cold and ~ 800 Ohm hot, research reveals temperature sensor may have increased in value - expected values could range as low as ~ 100 Ohms or so. The temperature sensor was replaced as the new aftermarket measured @ ~ 3K Ohm cold. This may explain the approximately 1K Ohm variant - increase measured cold and the 800 Ohm reading when hot causing open loop mode.

*Vacuum tested @ 16-17 in Hg, determined no vacuum leaks - very steady reading with only a fluctuation of ~ 1 in Hg

*Revved engine @ 2000 RPM and vacuum did not drop - in fact it increased a bit, determined catalytic converter was not restricting exhaust.

*Measured supply voltage to MAP sensor @ 4~ 5V, measured output from MAP sensor @ ~ 1.5V and would vary on idle change, did notice the system to enter closed loop mode at least for a short time - idle changed, rattle disappeared from valve cover area, test drive had power and appeared fixed, however after ~ 10 -15 minutes the trouble reappeared. Differential diagnosis, timing advancing/retarding relating to MAP sensor vacuum, MAP sensor, MAP Sensor wiring, or the computer. Possible to be electrical ground due to engine being painted.

Research reveals common trouble with Jeep computers having problems with solder joints on the input side (ring cracks, cold solder joints, etc)

Before replacing the MAP sensor or the computer, the wiring from the MAP sensor to the Computer needs verified. Groundings and signal routes need explored, only three wires for MAP sensor and two grounds (engine and body) so should be simple to troubleshoot except the computer needs disassembled for inspection and repair of solder joints if present.

So what do you think happened to the original engine? At this point I believe the first engine failure was due to several problems leading the engine to run rich fuel mixture thinning the oil, deteriorating the piston wrist pin until it broke and the rest is evident. The thermostat causing open loop mode and the MAP sensor related circuitry disrupting the timing are the most likely culprits.

I am certain there is someone who would have done this differently, I will post the final diagnosis soon. I hope this helps some of those gas guzzling Jeeps before they get glazed from oil and toasted. It kinda seems as though there are many Jeeps out there prematurely failing due to the computer program running them. Sure, limp home mode helps but why Jeeps do not flag a code when computer is in that mode instead of no codes and poor gas mileage is strange.

Updates to follow.


MR-Fixit 07-10-2010 02:19 PM

Correction to above:

This line is wrong: The thermostat causing open loop mode and the MAP sensor related circuitry disrupting the timing are the most likely culprits.

Should read: The temperature sensor causing open loop mode and the MAP sensor related circuitry disrupting the timing are the most likely culprits.

MR-Fixit 07-10-2010 04:36 PM

MAP Sensor Test
Supply Voltage: 4.9 V
Ground Pin to Body/Battery Ground Resistance: < .5 Ohm
Applied Vacuum (in. Hg) / Expected Voltage Drop
3.94 / 0.4 V
7.87 / 0.6 V
11.81 / 1.2 V
15.75 / 1.6 V
19.69 / 2.0 V

Measured Idle Vacuum (in. Hg): 16.5 V
Measured Idle Voltage (VDC): 1.7 V
Expected Voltage Drop 1.6 VDC (+/- 0.1 V)

MR-Fixit 07-11-2010 04:53 PM

Fault Found:

I stated above there was not a vacuum leak measureable with the vacuum gauge - the MAP Sensor vacuum end fitting was not cracked or dry rotten but was deformed slightly intermittently allowing unmetered air into the system. Most noticeable when the engine and transmission would twist during rev-up to take off. Regarding the computer, it was potted in a jell like substance so I did not pursue troubleshooting that further. The wiring from the MAP sensor to the computer was tested and was not the cause. The ground lug for the body did have a 1 Ohm change when loosened-cleaned-retightened but it was not significant to the solution. It was during the testing of the MAP sensor when the trouble resolved for a short period of time that lead to re-checking throughly the connections surrounding the MAP sensor.

It was not posted in previous posts but the O2 sensor was also replaced along with the vacuum lines' end adapters to the valve cover, fuel rail regulator valve vacuum end and the other end of the line to the intake manifold.

The slight fluctuation of < 1 in. Hg is due to the ignition system (cap and rotor fouling again but probably due to ignition advance- have not installed replacement yet but inspection reveals the trouble is most likely there).

It was easier to put a timing light on the vehicle to observe fluctuation of the ignition timing from the MAP sensor vacuum leak to find the problem.

Note: The vehicle would also jerk during take off unless the engine was revved which was also associated with the inttermittent vacuum leak to the MAP sensor.


Bad Fuel Economy:
*Coolant Temperature Sensor: defective causing open loop limp-home mode and poor fuel economy.

Vacuum Leaks: Primarily to the MAP sensor but also the valve cover vacuum line and fuel rail regulator line causing loss of power on acceleration and poor fuel economy.

Cause & Effect: The coolant temperature sensor and unmetered air leaking into the system setup the condition for high fuel consumption, loss of power, and enriching the fuel mixture leading to a "Hot" running engine- thinning the oil and and leading to premature failure of the engine. It could be argued that the engine failure was due to oil maintenance but I disagree and believe it was from running too rich and for too long - generally from lack of proper service.

Test drive confirms fuel economy restored ~ 100 miles and used less than 1/4 tank of gas.

... also another correction to my previous post, the line that reads: Measured Idle Vacuum (in. Hg): 16.5 V
should not have the "V" at the end because it is not a voltage but a vacuum.

In short, check and replace coolant temperature sensor - be sure to get all air from the block or it will be a problem!!! Verify MAP sensor's proper operation by issolating vacuum leaks.

Feedback is welcome regarding my posts.

Good luck!

MR-Fixit 07-11-2010 05:03 PM

It's as easy as that!

ENIX89YJ 07-11-2010 11:22 PM

i get right at 16mpg with my 89 yj with 2.5 5spd and 31-10.50.... but for some reason i think i am getting worse now.... gas type???

MR-Fixit 07-12-2010 10:37 AM

Re: MPG getting worse now...
The rubber vacuum componets may be starting to deteriorate on the vacuum lines end fittings and the coolant temperature sensor is probably gradually increasing in resistance. Heat causes resistors to increase in value - resistors never decrease in value! So as the coolant temperature sensor gradually increases in value because it keeps getting hotter from the engine running hotter and hotter each time it is driven (fuel getting richer), it is reporting to the computer that the engine is getting colder and colder until the computer is dumping all the fuel it can into the system. And the hotter engine compartment leads to faster breakdown of the rubber stuff under the hood.

Of course it could be other stuff causing it all but... I like my version best. Post some measurements here and let's compare results. State what you are measuring, where your test probes are connected, and the lowest reading, average reading, and highest reading. Include operating parameters such as, idle, hot or cold or warm engine, raining or dry day, and whatever other details you would think could be informative. It is much cheaper to test and measure than throw parts at the vehicle until you end up replacing everything but what needed replacing.

MR-Fixit 07-12-2010 10:50 AM

Does the engine start with a tap of the key or does it take two or three revolutions to purge the flooded cylinders of excess fuel before it starts up? If it don't start at a tap of the key, engine was probably left in a flooded state from the last time engine was run due to the enriched fuel mixture from at least one malfunction in the system.

MR-Fixit 07-12-2010 11:19 AM

... if you have read this post and your Jeep is running good again regardless if these postings helped find or resolve the trouble... each and every reader should post the solution they found here, even if someone already wrote it!

ENIX89YJ 07-12-2010 09:28 PM

my jeep you have to crank it 2 to 4 times and then pump the gas to get it to start and then it seems to flood itself out when I keep the gas pedal is kept at one point... what could be the problem????

MR-Fixit 07-18-2010 02:00 PM

Fuel pressure test the system is it low or high? If the fuel filter is clogged causing ~ 30 LBS pressure drop and most likely the fuel return line is too clogged ~ 30 LBS. The fuel filter would lower the pressure comming in and the blocked return line will override the fuel regulator causing a buildup of pressure. You may get an near normal reading but just low 1-3 LBS. Once the filter is replaced the blockage in the return line will be obvious because the fuel pressure will be restored to ~ 50-60 LBS unregulated. To confirm blockage, a hose from the fuel rail return line to a 5 gallon gas can will bypass the blockage in the return line. Be cautious fuel is under pressure and will fill up the gas can fast in as few 2 minutes! The pressure gauge will read a regulated pressure once the excess fuel is allowed to be dumped into the 5 gallon gas can. So... no tester? You may as well start dropping the gas tank and buy a good fuel pump while your in there cleaning out the tank and changing the filters. Before installing those new parts it is the proper time to use fuel system cleaner and clean the injectors.

Bet ya get better mileage from this tip! Why? Because when the injectors are under the extra pressure of fuel, once they open all the excess fuel is pumped into the intake during the time the injector is pulsed open - whatever the pulse time may be there is just much more fuel being delivered. So the O2 sensor detects it, computer adjust the pulse width and still the excess fuel is pumped into the intake. It will send the system chasing it's tail and maybe you too.

MR-Fixit 07-18-2010 03:02 PM

I have also used a small adjustable air compressor to pressurize the fuel return line to the gas tank, it should have a properly working relief valve but be cautious not to over do it. I have used maximum ~ 20 psi to clear a blocked line but dont go near the operating pressure of the fuel pump! Removing the fuel fill cap to vent, bubbles may or may not be heard as air enters gas tank. Bleed excess fuel pressure before disconnecting fuel lines covering connection with a rag. The connector is usually the bottom fuel line in front of drivers door along the frame, A rubber hose over the end of the line connected to the air line, and slowly increase regulator valve for air pressure.

MR-Fixit 07-19-2010 11:05 AM

Regarding using the air compressor to clear a fuel return line... Dont even try it, leave the job to a professional! There is risk of fire and injury or accident. and Don't try using the compressor anywhere around the gas lines or vapor area. I had the compressor placed about 25' away, the hose length I used was 50' long. Adjusting the pressure would place me 25 feet away from the vehicle at the air compressor. The motor makes sparks and to avoid any potential mishap-... the vehicle battery is also removed along with keys to ignition before working with the fuel lines - just in case.

I just wanted to say that stuff.
Don't hold me responsible if you have a catastrophy because monkey see so monkey do.

rusty-shackleford 07-19-2010 06:23 PM

I, too, just had my 2.5l completly gone through: bored .30 over, new everything, etc. I think MrFixit was right on his first post w/ the thermostat. When I picked up my Jeep from the engine builder (Bunn Engines, Donaldson AR, GREAT PEOPLE!!!! & engine builders) I didn't like how HIGH the temp gauge ran on the 1st cycle of the thermostat. After about 5 miles, the temp would settle down, but that first cycle scared the sh!t out of me. I called Darren Bunn (the engine builder) and he said give it time and yes he had installed a factory recommended 195 degree thermostat. I had noticed that I was getting better mileage than before, which is ,IMO, not typical of a new(rebuilt) motor. The temp getting higher than I liked got the better of me, and I got a 180 dgree themostat and installed it. The temp reading was where I liked it, but after a couple of times to the lake, I noticed fuel mileage sucked. Then I talked to my Pop and some other folks smarter than me, I bought another 195 thermostat(new) and installed it. Guess what, mileage went back up and temp runs more like normal. Maybe I had a bad thermostat(I have come to the conclusion that it was STICKING on the first warm up cycle), but my mileage is way better with the correct thermostat. Basically that sensor that is screwed into the thermostat housing was not reading the temp correctly, due to the 180 thermostat, and was telling the computer that the engine wasn't at operating temp; thus sending more fuel all the time. Maybe your engine builder put a 180 thermostat in because that's what he had(???) For $10-$12 for a new 195 thermostat & gasket and 20 minutes to install, it's worth a try. It fixed 2 of my problems. I hope this helps and I'm sorry for the "sands through the hourglass" story.

MR-Fixit 07-20-2010 12:45 AM

Hey Rusty,

You most likely had to burp your Jeep is all it needed. If coolant runs low or radiator cap sucks air - your engine block will trap air between the two temperature sensors and not allow the coolant to rise to the level of the temperature sensor in the thermostat housing. The engine sits down in the back so the temperature gauge sensor is getting a reading but the computer wont get the reading from the sensor in the thermostat housing because air tends to rise... Jack up the passenger's side front or the right rear, do the front for burping and filling through outlet hose, and do the rear for burping through temperature gauge hole. As you stop and go the coolant sloshes front and back giving wacky readings to both the computer and the temperature gauge. If coolant ever gets low... burp the thing again and again. The thermostat I prefer has a built-in jiggle valve to help burp it.

MR-Fixit 07-20-2010 01:02 AM

Re: Hard Starts... Flooded?

Maybe the fuel pressure is leaking through an injector or through the check valve in the fuel pump. If it is really flooding, I would think pressure leak or pressure too high, if it is because the pressure leaked out through the fuel pump it is because the pressure needs to build back up. By the time fuel pressure builds back up, the computer is dumping additional fuel into the intake to get the engine to start... as if it would do when it is cold ... and when the pressure does finally returns to normal the computer will be dumping enough fuel to flood the engine.

It's like this when you start it up...

computer squirts a bit of fuel in but barely a drop is dispensed because low pressure.

computer sees no emissions output and so it intends to squirt out even more fuel, but fuel pressure is still climbing.

computer sees little emissions output so trys to squirt a mega load to really get er goin... but fuel pressure back to normal so the mega load turns out to be a gusher and floods the engine.

Solution to flooded fuel injection engine.... Push the pedal to the floor and start the engine. The computer sees a wide open throttle whenever the engine is not running as it's instruction to stop dumping fuel because the engine is flooded. You could try it if you want to... floor the gas pedal and see if it starts.... if it don't start... it wasn't flooded and it will start as soon as you release the gas pedal, but if it does start before you lift off the gas pedal then it most likely was indeed flooded.

MR-Fixit 07-20-2010 01:14 AM

I should proof read what I post before posting but here is another correction to burping... drivers side rear gets jacked up to raise temperature gauge sensor's threaded hole for air to escape. The previous post said right rear which is incorrect.

MR-Fixit 07-20-2010 01:21 AM

Am I permitted to accept donations? Think of the savings not having to throw lots of parts at your vehicles.

Rule 1: If it costs more than $50 at the parts store then it's probably not the part that is needed.

Rule 2: If the parts store says it is a dealer item only or special order then it's probably not the part that is needed.

Rule 3: Check the last thing that you would ever think to check before you replace the first thing.

MR-Fixit 07-20-2010 01:29 AM

Oil pressure... if you don't burp the Jeep, it will run hot and cold... engine will be hot... very hot, but the computer will think the engine is cold and there goes your economy fuel mileage. If you let it go on long enough thinning the oil (low oil pressure symptom) it will ruin the engine.

There are other reasons for low oil pressure but other than bearings being worn it is most likely from rich fuel mixture or burping.

MR-Fixit 07-20-2010 01:47 AM

Did you know... a low battery voltage will cause the computer to think it needs to lengthen the duration the fuel injectors are triggered (dump more fuel) to adjust for the reduction in battery voltage supplied to the injectors in anticipation for a lowered fuel delivery? So, how can a low battery causes poor fuel economy. A bad alternator will do the same thing.

MR-Fixit 07-20-2010 01:55 AM

A defective brake switch can interfere with the computer knowing when the brakes are applied to adjust idle control motor. So it too can cause fuel economy problems.

MR-Fixit 07-20-2010 01:58 AM

A defective power steering pressure switch and air conditioner both can influence the computer into thinking more load is applied to the engine so dump more fuel.

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