Engine idles rough, poor gas mileage

Tiny
JFAZIO5188
  • MEMBER
  • 2002 JEEP GRAND CHEROKEE
  • 143,000 MILES
Laredo 4.0 liter engine. Brand new re-manufactured engine. Gas mileage went from 17.8 to 7.4 mpg. I am assuming this has to do with the reason the engine idles rough and worse when there is a load put on it. (D or R and/or when the AC is turned on max). Tested and checked IAC, TPS, and the coil rail. Has brand new plugs (Champion RC12ECC). I also cleaned the throttle body and IAC real good. Also tested all the injectors today.
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Thursday, January 12th, 2012 AT 11:04 PM

11 Replies

Tiny
CARADIODOC
  • EXPERT
You are really barking up the wrong tree. First you have to determine whether there is a misfire and fix the cause or there is way too much fuel going in causing the engine to run rough in a flooded condition. That can be done by viewing fuel trim numbers.

The engine computer uses the automatic idle speed (AIS) motor, also know as the idle air control, (IAC) to control idle speed when your foot is off the gas pedal, that is all. It has nothing to do with other running problems. The throttle position sensor just tells the computer which way and how fast the gas pedal is moving. They cause very little trouble and definitely not what you are describing. Injectors can be tested electrically on the vehicle but that will not show if one is sticking open and spraying fuel constantly instead of in short pulses. Chrysler has extremely little trouble with injectors so they should be the last thing to look at. You can check for that by monitoring fuel pressure when the engine is off. It should hold steady for weeks. If an injector is leaking badly, that pressure will drop off within a couple of seconds.

One misfiring spark plug can cause a lot of fuel consumption but not nearly enough to cut the fuel mileage in half. With a misfire, unburned fuel and air go into the exhaust where the oxygen is detected by the oxygen sensors. They do not detect unburned fuel. All the computer knows is that side of the engine is too lean. It does not know why or if it is being caused by just one cylinder. It responds by requesting more fuel on that side from all of the injectors. No matter how much extra fuel goes in, there will still always be that unburned oxygen from the misfiring cylinder. It is an never-ending cycle, but the computer can only add or subtract fuel by about ten percent.

That is where long and short-term fuel trim numbers come in. You need a scanner that can display live data to see what the computer is doing. High positive numbers means the computer is requesting more fuel in response to an incorrect sensor value, typically the MAP sensor. He has the biggest say in how much fuel is needed. If the numbers are high negative, the computer is trying to cut back on fuel metering, obviously without success. That is where you have to look at things beyond its control. That includes an injector sticking open, (very rare), or high fuel pressure. Look for a leaking or disconnected vacuum hose going to the fuel pressure regulator, or a crushed fuel return line to the tank. Also, look in that hose for signs of wetness. Your problem is real common on GM trucks due to a leaking regulator. I have only heard of that once on a Chrysler product.

A lesser-known problem that causes poor fuel mileage is an exhaust leak ahead of the first oxygen sensor(s). Between the pulses of exhaust flow, the momentum creates little pulses of vacuum that can draw in outside air. The oxygen will be detected as a lean condition that the computer will try to correct by adding more fuel. You will often smell the unburned fuel at the tail pipe but the computer will be showing a lean condition.

Cleaning the throttle body can solve some hesitation or stumbling complaints, again, more common on GM vehicles, but that will slightly restrict air flow if severe enough. You have way too much fuel going in, and that has nothing to do with the throttle body.

If none of these things seems to point to the cause, you are going to need a scanner to see what the engine computer sees. If you unplug the MAP sensor's electrical connector and the engine runs better, (not well), that is a clue that it is providing incorrect information. The main fuel delivery calculation starts with load on the engine which is evidenced by a drop in intake manifold vacuum and is measured by the MAP sensor. All the computer cares about when self-testing that sensor is its signal voltage must be between 0.5 and 4.5 volts. The readings can be way wrong, but as long as they remain within that range, no fault code will be set in memory.

Have you even checked for diagnostic fault codes? If the check engine light is on, and it should be, there will definitely be at least one code. The light is only required to be turned on when the problem detected could adversely affect emissions. There can be other codes that do not turn the light on.
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Friday, January 13th, 2012 AT 12:26 AM
Tiny
JFAZIO5188
  • MEMBER
Thank you so much for your information. I have done some test and I will list the results. There is still no change in my problem. Cleared the PCM and rechecked for codes a couple days later. I had no codes stored. I replaced the fuel filter just to rule it out because it was cheap enough. I re-torqued the exhaust and intake manifolds to ensure that there is no leak. I also replaced both upstream O2 sensors because they were the originals. I clean and cleared the vacuum on the MAP sensor. I also tested the map sensor and the connector. At the connector with the key in the "run" position it measured 5.0 volts which checks out. I then probed the MAP sensor, with the key in the "run" position it was reading 4.7 volts. With the engine on it was bouncing back and forth between 2.3 volts and 2.0 volts.
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Wednesday, January 18th, 2012 AT 7:33 PM
Tiny
CARADIODOC
  • EXPERT
Now we have some more variables to confuse the computer. All sensors are different. The computer learns their characteristics as you drive by comparing their signals to other sensors and known conditions. I doubt replacing the O2 sensors is going cause problems, but randomly replacing parts is the least effective and most costly way to diagnose this type of problem. You will be money ahead to just have it professionally diagnosed.

I would have told you also to not waste your money on a new fuel filter. Except for diesel trucks, they last the life of the vehicle on Chrysler products unless they start to leak like mine did at 180,000 miles. The pickup screen in the tank will collapse or become plugged before the filter will, and even that is not very uncommon. Replacing the filter will not have any effect on the computer. He will not notice any change from that. Also, remember we are dealing with way too much fuel. A plugged filter or screen will reduce volume and pressure. The engine would run poorly but the fuel mileage would not get worse.

Two volts on the MAP sensor at idle seems a little high from the experimenting I did when building "bugged" cars for my students to troubleshoot, but it is within the acceptable range to not set a code. The higher the voltage, the higher the load and the more fuel is needed. Since that voltage could be incorrect, the fastest and easiest way to know if it is causing the problem is to pop a new one or known-good used one on and try it. If I had to guess, I would have suggested around one volt at idle would be normal but if I found two volts on a good-running engine, I would accept it.

How did you "reset" the computer? If you disconnected the battery, that erased all of the stored fuel trim data it had learned during previous drive cycles. Knowing that is what helps it figure out when there is a problem, and why. Once the battery is reconnected, it has to relearn all of the sensors while there is still a problem present. Any fault codes that were in memory are lost too. That can provide real valuable clues. Part of the strategy the computer uses is to constantly compare sensor readings with each other. As an example, it knows that when the engine has been off for more than six hours, the coolant temperature sensor and ambient air temperature sensor, (or battery temperature sensor), had better be reading the same temperature. If there is a fault code stored for one of them, the computer knows it cannot rely on its reading to compare to the other one, so it will not set a fault code for it except for extreme conditions such as being unplugged. Every fault code has a long list of conditions that must be met for that code to be set in memory. Every list usually starts with a whole bunch of "xxx code must not be set". That is where mechanics got the notion to disconnect the battery to erase existing fault codes but that is counter-productive. Those original codes that set will be related to the current problem. Erasing them allows the conditions to exist that allows the other codes to set. The computer could have been watching the MAP sensor and trying to figure out which related code to set. Now that all the stored data is wiped out, all the computer might be able to figure out is the exhaust gas is staying too rich too long. That does not indicate a problem with the oxygen sensors; they are just the messengers. I used to read a lot about mechanics resetting GM computers by disconnecting the battery, but we never did that at the Chrysler dealership I worked at. Those 1990's GM computers had a lot of issues, but on other brands nothing is going to be magically fixed by erasing the memory and starting over. The fuel trim numbers the computer learns over time are what it bases initial fuel metering on. Those numbers will tell you whether it is trying to reduce fuel without success or whether it is adding fuel in response to incorrect sensor information. As soon as the problem is repaired and the sensor information is correct, the short-term fuel trim numbers will begin to update immediately. You do not have to do anything except drive the vehicle.

As another point of interest, MAP sensors are so sensitive, they could be used to measure engine speed by counting the pulses of intake manifold vacuum each time a piston takes a gulp of air. If you watch the signal voltage on an oscilloscope, you would see four little voltage ripples for each crankshaft revolution on a V-8 engine. Those ripples are a part of how the computer figures out which fault code to set. Electrical codes are easier to understand. If the 5.0 volt supply is good and the ground wire has 0.2 volts, but the signal voltage is wrong, the sensor is suspect. But the computer can also detect pneumatic codes. On older engines that meant a leak in the vacuum hose or a defective sensor. On newer engines the sensor's vacuum connection plugs right into the intake manifold. No vacuum hose is used. While experimenting on prepared cars in the shop, we used to disconnect that vacuum hose and run the sensor with a hand vacuum pump to see how the engine responded. It was very common to set a "MAP Pneumatic" code even though we kept the signal voltage at the proper level. The computer set that code because it saw there were no ripples in the voltage and it knew that was not proper operation.

The reason your meter reading was bouncing around was due to those ripples. The meter takes a reading, thinks about it, displays it, then leaves it there while it goes back for another reading to think about. That one might have been taken, (like a snapshot), just when the voltage was at a high spot of that ripple. If you were to read that voltage on a scanner you would see it holding steady.

What you need to do is either drive it long enough to get a code to set or you will need a scanner to view live data to see what is going on. If the MAP sensor is indeed reporting a load higher than actual, the computer is going to request more fuel. That one can easily double fuel consumption.

No matter what the oxygen sensors see, the computer can only modify that initial calculation plus or minus about ten percent based on their readings. If it subtracts fuel, you will have a noticeable stumble or hesitation on acceleration. If I is adding fuel, you likely will not even notice unless you carefully keep track of your fuel mileage. Another clue when oxygen sensors or their readings are related to the problem is they are not even in the fuel metering calculation until they get to 600 degrees. For many years the O2 sensors have had electric heaters that turn on only when the engine is running to get them up to that temperature faster. Until that happens, the engine will run differently for that minute or two.
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Wednesday, January 18th, 2012 AT 11:12 PM
Tiny
MDHOLT21
  • MEMBER
I am experiencing the same problems with my 2000 Cherokee. It has the L6 4.0L in it. I am getting misfire codes on cylinders two and three and fuel injector open circuit codes on the same cylinders, along with a random multiple misfire code.

I replaced the coil packs and spark plugs and the cylinder three fuel injector just about 2,000 miles ago because I was experiencing the same codes, but just on cylinder three.

It runs extremely rough and is running very rich and is burning a ton of gas. Would my next step be to put new O2 sensors in it? I am not the original owner but there is 150,000 miles on it and I am guessing they have not been replaced. I just figured that if they were bad, I would be seeing a CEL code for them too.
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Saturday, August 15th, 2015 AT 9:40 AM
Tiny
JFAZIO5188
  • MEMBER
Plugs have to be original copper. Seems simple but that fixed my issue back in 2012, lol.
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Saturday, August 15th, 2015 AT 2:47 PM
Tiny
SPARTANROGUE
  • MEMBER
CARADIODOC I am hoping to GOD you will reply, you seem like the only person who could help! I have a 2003 Jeep Grand Cherokee 4.0L with seemingly the same symptoms as JFAZIO5188!

2003 Jeep Grand Cherokee 4.0L 2WD

I work for a small repair shop so I have access to a full system scanner Autel Maxysis

Symptoms are:
*A rough idle. No stalling. No codes of any kind and I have run the misfire counter test which decreases the threshold for counting a misfire so you can see more of them and I only saw it count like 1 or 2 misses on random cylinders over multiple tests which lasted about 2 minutes each. Which I believe does not dignify a true miss.
* VERY bad mileage (8-10 mpg average, even though the INFO center tells me a much higher number (15-20mpg average). (Original specs list at 17/24mpg)
* VERY Negative Long Term Fuel trims (-15% @ idle and I've seen up to -27% upon acceleration), Short term seems like its normal, bouncing around -5 to +8%
* If this makes any sense, A STRONG idle, Not HIGH. It sits at about 500-600 in neutral or drive but in drive I have to apply a good amount of pressure to the brake to stay still. I have had a steel trailer attached to it with a few hundred pounds on it, and the truck will STILL climb up a small incline if I let off the brake and without touching the gas.
* if you TAP (almost like a flick) the gas pedal while in park/neutral, the RPM will drop for a split second as if its about to stall, before the RPM rises. Where as in customer s jeeps with same generation and engine, the idle comes up immediately, instead of acting like it s about to stall first
* I also noticed if I take a long highway trip, VERY rarely does it have a split second hiccup ( as if it misfires only once).

New Parts and Things I've done:
*Re-maned engine! This truck came into the shop with a totaled motor so I bought it off the guy and replaced it for myself *(see bottom for amazing story about why)*
* Cleaned throttle body and inside of intake thoroughly when engine was being replaced
* New plugs with engine (Autolite, I recently found out Champion is OEM, thinking about swapping?)
* New front O2s in attempts to fix this issue
* New fuel pump and filter/regulator (Pressure at 55psi key on OR running)
* New TPS because the connector was broken when I bought it
* I Cannot find any Vac leaks besides the throttle plate shaft which I m told is allowed to leak a bit, although with the throttle body in hand it feels like the shaft might have a very tiny amount of play in it
* I swapped the IAC with a used one AND a new one and no change so I put old one back on
* Cleaned MAP sensor hole and throttle body, and found a Voltage test procedure and used a hand vac pump and looked at V on scanner as I changed the vac
> (Target pressure -- test says should be VS what I got, respectively ).
@ 0 inHg -- 4.8V vs 4.65V (this one bothers me a bit because I cant blame the hand pump)
@ 5 inHg -- 3.9V vs 3.94V
@ 10inHg --3.1V vs 2.9V
@ 15inHg -- 2.1V vs 1.9V
> I feel like it got pretty close to the test values based on not knowing how accurate my hand pump gauge is. Should I still blame the MAP after these results?
* Tested and even Swapped the Purge Valve with a used one (test okay, clicks with scan tool at various speeds) and no change when swapped.
* Swapped the Coil rail with a known good one (no Change)
* Swapped all 6 injectors with Upgraded 12 port Bosch Injectors that are said to smooth idle, increase power and some cases increase fuel eco (no change, although at the time I did not know if the fuel eco was bad or fuel trims because I was going off the INFO center and was happy with the numbers, I was just trying to get rid of the rough idle)
* I had an EVAP leak code for a long time that I couldn't figure out but I just recently changed the Leak Detection Pump and the vent valve because the pump was leaking and the vent valve was so worn out that it was not working right. Since ive done those the EVAP code has not come back but fuel trims have stayed the same if not worse.
* one thing I find strange is that on the scan tool there is a test for the IAC where its supposed to raise the idle to a set amount (800, 900, 1,000, 1,100, . 2,300) whatever you choose, and I believe its supposed to hold it at that RPM untill you stop it. But I tried it and the only thing it does is raise the idle to about 800 for a split second and then it comes back down, no matter what selection I choose, even when I had the brand new OEM one in there. BUT the "out of engine" IAC range test ( back and forth) seems to work fine.
* one modification I did do is swap the OEM muffler with a masterflow to change the sound of the exhaust. Hopefully the change in back pressure isn t causing this issue?

***** NOW. Here is the KICKER! And dont think im crazy!

I live in south FL (elevation 0-15ft) where the temp doesn't really deviate from 80-95deg F too much and the humidity is never really below 70-80%.
I have noticed that the idle is Especially rough during a hot day (85-95) and seems a lot milder at night or on cooler days (65-75deg).
NOW. I just took a trip up to North Carolina and when I pulled into the campground, Elevation 6,600ft and about 30-50 deg F, the idle was so smooth I couldn't even tell the truck was running! And the "strong" idle I mentioned earlier was GONE, no heavy brake needed! I actually had to give it a little gas to get out of the mud/grass! Unfortunately I did not have the scanner to read Fuel Trims and the driving was so random that I could not calculate fuel eco. But On the way back down VERY FIRST STOP in Florida The idle was rough again! Which makes me think it is a MAP sensory or a temp sensor. P.S. After it sits over night, ambient temp, coolant temp, and intake temp read: 82, 82.4, 84.2 deg F respectively, on the scanner. Seems like the intake sensor might be off a few degrees?

*** Reason it needed an engine*** The previous owner neglected the leaking radiator and allowed the engine to overheat. So He finally brought it in to the shop for a loud clanking noise. Turned out that the engine got so hot that one of the piston heads broke off the arm and got stuck in the cylinder. This allowed the piston arm to whip around the crank case freely as the engine spun! We started that sucker 2x/day to pull it in and out of the shop for almost two weeks before the engine finally locked up! I heard that the Inline 6 4.0L jeep motor was reliable but I still cant believe that engine still ran for so long!
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Thursday, January 10th, 2019 AT 5:23 AM
Tiny
CARADIODOC
  • EXPERT
It would be a good idea to start a new question specific to your vehicle. When we add to this one, only the three of us involved will get e-mail notifications when a reply has been added. None of the other experts will see your addition or have a chance to reply to it. That may not get you the best help Here's the link:

https://www.2carpros.com/questions/new

I suggest you just copy and paste your description rather than trying to retype it.

One of our other experts, Wrenchtech, is in Florida, so he might have run into this problem already if it is altitude-related. Steve W. is another expert I'm asking for help. Both of these fellows are real good with engine performance issues.

I can offer a couple of suggestions. The first is if your scanner has "record" capabilities, making a recording of key engine sensors that you can play back slowly, later, may give some insight as to what the computer is seeing and responding to. Too bad you weren't able to do that on your trip so you had something to compare to.

Next, there is a service bulletin for rough running, but only after restarting from a hot soak condition. 18-031-03 refers to fuel vaporizing in injector number three from heat migrating up from the exhaust manifold. You would get a diagnostic fault code P0303 for that if it was bad enough. One of the contributing factors is outside temperature. The bulletin says this happens most commonly when it's above 90 degrees, but that misfire clears up in about 20 seconds when the vaporized gas is used up. On the off-chance your rough running is related to temperature, you might try pinching the fuel return line from the pressure regulator. That will raise fuel pressure which might reduce vaporizing of fuel. Check the fuel pressure too. It should be 49.2 pounds plus or minus 5 pounds. The regulator is inside the fuel filter. I have to switch over to Google Chrome to post a drawing of its location. I'll add that shortly.
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Thursday, January 10th, 2019 AT 4:00 PM
Tiny
CARADIODOC
  • EXPERT
Here's the drawing of the fuel filter / regulator.
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Thursday, January 10th, 2019 AT 4:07 PM
Tiny
HEYMAN1104
  • EXPERT
Well,while this is a little out of my wheelhouse, I'm joining in here, I actually had a 2000 Grand Cherokee 4.0L that did essentially the same thing. I went back and forth with it for weeks before I borrowed one of those fancy SnapOn scanners with a live data mode and figured out the one of the O2 sensors, while working, was giving inaccurate data that was still in the acceptable range. So, it wouldn't set a trouble code to indicate any problem. It just ran like crap. When I cleared the codes, even though there weren't any codes, the truck would run better. I assume it was because it was running in an open loop mode (computer was running the engine based on factory settings, instead of interpreting data and adjusting accordingly). I changed the upstream sensor and it was fine for a couple of years.

Later on, it would get to running like crap, even with clearing the codes. The catalytic converter was the cause of that. I ran the "seafoam down the intake" trick on it and it would go away for 3 or 4 months. Here's a link to a youtube video on it: https://www.youtube.com/watch?v=1WWqkDo7KwY

It's a seriously backyard trick taught to me by the best backyard mechanic in town. Seriously, he's better than just about anyone else in town.

Just my $.02.
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Thursday, January 10th, 2019 AT 8:01 PM
Tiny
SPARTANROGUE
  • MEMBER
Should I really try the seafoam trick with a new engine and clean intake ( I cleaned the inside of the intake by hand when I swapped it to the new engine)? Im fairly certain the cats are original though (145k miles on the truck, but the re-maned engine only has about 4k) but the video doesnt mention anything about the seafoam working to clean cats? And like I said, I also changed both upstream O2 sensors as well, and 6 new injectors. Should I risk damaging the new engine to clean just the cats?

I've been wanting to put headers on! Maybe ill just delete the cats! Haha just kidding that would be too loud for my wife and kids, maybe on my Off road build in a few years.

By the way, I spoke to a guy that hosts alot of the tech training (ATG) classes that we go to, and he seems to be kind of stumped as well. But he says he thinks it sounds like a vaccum leak. Considering its a "speed density system" he says that since it runs much better in colder climate it could be that the air thats leaking into it is denser causing it to not act up so much when temp is lower.? What are your thoughts? I smoke tested it many times but im going to try today to let it run and wave a "fuel sprayer" tool we have around and see if the idle changes.

Also! Thank you both for your response! Considering the last response was over 2 years ago I was worried no one was listening! But ill post my question on a new thread like you said as well!
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Friday, January 11th, 2019 AT 7:55 AM
Tiny
SPARTANROGUE
  • MEMBER
This is the new post in case you have any more info or want to transfer conversation:

https://www.2carpros.com/questions/rough-idle-poor-fuel-trims-very-low-mpg

i will be monitoring both.
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Friday, January 11th, 2019 AT 8:24 AM

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