Poor MPG's causes list. Why I am woried!

Here is the list first. Because you prolly don't care why I'm worried. There is a lot to look at here as possible reasons for poor gas mileage.

Here are a few engine problems that hurt fuel economy:

- missing or defective thermostat, causing low operating temperature

- clogged intake manifold exhaust crossover passage (for heating intake manifold)

- engine misfire caused by bad spark plug, ignition wire, coil, dist. cap or rotor

- late ignition timing

- bad vacuum advance unit (common in old cars)

- stuck centrifugal advance mechanism in distributor (needs oil occasionally)

- bad tension adjustment of centrifugal advance springs

- defective oxygen sensor (causes rich mixture)

- defective coolant temp. sensor for engine computer (causes rich mixture)

- worn timing belt (may cause rich fuel mixture in fuel injected cars with a MAP sensor)

- incorrect timing belt installation (timing marks should line up)

- sticking or poorly adjusted carburetor choke (causes rich mixture)

- dirty air filter on an engine with a carburetor (causes rich mixture at high power)

- clogged or defective PCV valve

- incorrect valve clearance (especially if too tight)

- high performance camshaft installed (not efficient at low power)

- worn valve guides (makes fuel mixture too lean at low power)

- vacuum leak from bad vacuum hoses or intake system gaskets (causes lean mixture)

- dirty fuel injectors

- bad fuel pressure regulator

- bad MAP sensor for fuel injection

- leaking intake duct between air flow sensor and throttle body

- too much alcohol in gasoline with old car (requires carb. modification for correct mixture)

- rusted or missing exhaust heat stove or pipe for heated air intake

- loss of compression pressure due to worn rings or leaking valves

- sticking EGR valve (causes rough idling and rich mixture on some engines)

- exhaust backpressure caused by clogged catalytic converter, muffler or crushed pipe

- stuck exhaust manifold valve for some V type engines. Intended for fast warm up.
More engine problems:

- high idle speed

- high fast idle speed when cold

- incorrect idle mixture adjustment (carburetor or fuel injection)

- leaking fuel line or carburetor (dangerous too)

- clogged jets (calibrated holes) inside carburetor

- defective power valve in carburetor

- worn camshaft lobes

- high alternator voltage (should be about 14 volts when warm)

- high oil level (interferes with crankshaft movement)

- high oil pressure (relief valve stuck or has wrong spring)

- stuck cooling fan clutch (runs fan when not needed)

Problems that increase rolling resistance:

- sticking disk brake calipers

- tight drum brake adjustment

- warped brake rotor or drum

- sticking parking brake cable

- incorrect wheel alignment, front or rear (check tire wear)

- worn suspension joints (ball joints, steering joints)

- worn control arm bushings (changes wheel alignment)

- worn or incorrectly adjusted wheel bearings

- leaking grease seals for wheel bearings

- use of snow tires when not needed

- leaking tires, causing loss of pressure

- warped tires

- bent rims, causing car to shake

- poor wheel balance, causing shaking at high speeds

- worn shocks or springs (forces driver to brake more)

Automatic transmission problems:

- wrong fluid level

- incorrect throttle valve cable adjustment

- bad vacuum modulator or vacuum hose to modulator

- sticking valves in valve body, causing delayed shifts; changing transmission fluid may help

- failure of torque converter to lock up (if equipped with lock-up torque converter); it should engage at 30-55 MPH, reducing engine speed about 10%

- stuck stator clutch in torque converter

Manual transmission problems:

- wrong fluid level

- excessive oil viscosity

- bad bearings

- failure to engage one or more gears

Differential problems:

- wrong fluid level

- excessive oil viscosity

- bad bearings

- high ratio ring and pinion gear set installed for racing.


Now why I'm worried.
I think I may have screwed something up. Having installed an AEM system i added seafoam to the gas and oil as prescribed by the bottle. I then proceeded to an "Italian tuneup" Heres where I think I broke something. I didn't really go nuts. Just a couple of WOT's and 80 +/- and not for to long. During this no cel or lights, just a brief blink of the stability light. But I think that i still may have burned up theO2's or a spark plug or 2 or something. Or got some crud lodged somewhere that may be causing the low mpg's I went from around 16 city / 20+ highway, to 15 city / 18 highway. The only other thing is the AEM,( I only paid & 200. for it) I wonder if the temp sensor is misreading air temp being only an inch from the TB now. I don't think it's the kit itself cus I would expect that to at the very least stay at the same mpg's. Plus I added pipe insulation to the top of the heat shield to insure a good seal with the hood. So I gotta look into all the things listed above.


Still think I broke something during the WOT. 98k ya know. :shrug:

You have some things on that list like distributor/springs, do not apply to KJ.
This is not an old Hei ignition system.
To take the vehicle on WOT runs especially with a new chip is bad news unless you knew what all the correct perimeters should have been, and confirmed the motor & cooling was running inside with correct timing advance/lean rich settings. Seafoam..dont even want to get into that with your chip tune.
The first thing I would do is to return the engine to stock chip and start by pulling the dipstick to confirm no water in oil/overheat/blown gaskets. Then remove rocker covers to ensure all is well in there. Missing thermostat? Replace with new one. Actually with that entire list is is sounds like another KJ may be a better idea, it is huge as to problems associated driveabilty and overall condition. Sounds like someone really beat on it.

seafoam thins oil out beyond levels you want in your truck. you should use it on your intake and gas, if you put it in the oil you need to dump and refill the oil.

the aem could be throwing the computer off and screwing with fuel mapping. between oiled air filters screwing with MAFs to intakes screwing with air/fuel mix, I just stay away from it.

Last plug change?

Besides that change that oil ASAP!!!!!!!!!!!!!!!!!

Seafoam should only be run in the oil for 10-15mins max then drained out.Any longer and you can "wash" the cylinder walls and main bearings which is very bad.

Other then that your PCM is a learning computer,drive it hard and it adjusts for your driving and you will get bad mpg's if you start driving nice till the PCM relearns your driving habits.

Just to be a bit more clear on a couple things.
This list was pulled from the internet, and is for any vehicle where those parts apply. It's not what was diagnosed by me and my KJ.
I don't have a chip tune. I couldn't afford the gas.
as for the intake, duely noted.
Spark plugs were changed about a month ago.
The seafoam... it didn't say to dump and re fill. That I will do ASAP.

Thanks for your comments so far I appreciate it Immensely.:smokin:

offrovering said:

seafoam thins oil out beyond levels you want in your truck. you should use it on your intake and gas

Click to expand...

I was told about using it on the intake.... the procedure seemed kinda scary.
How do you do it? I wanna see if everyone does it the same way and how 'dangerous' it is. I was told not to rev too high or I could screw things up big time.

The same person told me to add it to the crankcase but not to run it too long which is one thing it DOES NOT state on the bottle.... then to change the oil..which I did.

chill said:

Just to be a bit more clear on a couple things.
This list was pulled from the internet, and is for any vehicle where those parts apply. It's not what was diagnosed by me and my KJ.
I don't have a chip tune. I couldn't afford the gas.
as for the intake, duely noted.
Spark plugs were changed about a month ago.
The seafoam... it didn't say to dump and re fill. That I will do ASAP.

Thanks for your comments so far I appreciate it Immensely.:smokin:

Click to expand...

your oil is going to be as thin as water, hopefully you havent driven or cold started too many times since dumping that stuff in.

i think your issue is just air/fuel mix and computer

BjBnet said:

I was told about using it on the intake.... the procedure seemed kinda scary.
How do you do it? I wanna see if everyone does it the same way and how 'dangerous' it is. I was told not to rev too high or I could screw things up big time.

The same person told me to add it to the crankcase but not to run it too long which is one thing it DOES NOT state on the bottle.... then to change the oil..which I did.

Click to expand...

It's to harsh for your plastic intake,I would skip it and just use throttle body cleaner through the throttle body itself.

But the proper way is 12oz through a vacuum line over a 30-45min time span with the engine turning about 1200rpm's.You want it to go in at almost a vapor.Letting the engine suck it in while liquid and to fast may end up with a seized engine or damaged cat's.And yes it can damage the cat's if sucked in to fast even though it say's it's cat safe.

if sucked in too fast the engine would die anyhow from low air flow.

best way I have done it would be to keep engine running, connect a tube to intake, put your finger over the end of the tube to keep vacuum and keep car running, then just "sip" the seafoam up. once youve taken in the seafoam, shut the engine off and let it sit for 10 mins, then go fire it up.

I do not think it is too harsh for the plastic intake but I have only done it on the jeep once, I run it through my rover intake more frequently, but that thing is known for carbon buildup.

the jeep just needs some throttle body cleaner, wipe the inside of the throttle body down and youll be good. refresh the PVC and air filter frequently and it seems to run fairly clean. I switched the jeep to high mileage oil and have noticed cleaner oil and longer runs between changes.

tjkj2002 said:

It's to harsh for your plastic intake,I would skip it and just use throttle body cleaner through the throttle body itself.

But the proper way is 12oz through a vacuum line over a 30-45min time span with the engine turning about 1200rpm's.You want it to go in at almost a vapor.Letting the engine suck it in while liquid and to fast may end up with a seized engine or damaged cat's.And yes it can damage the cat's if sucked in to fast even though it say's it's cat safe.

Click to expand...

That's the way he told me. He said to keep the RPMs around 1k...thanks Troy, just wanted to confirm what I heard. probably won't do it though

Found this on seafoams website.


Sea Foam® Motor Treatment may be used in an oil crankcase 2 separate ways, depending on your needs and expectations. Sea Foam® Motor Treatment is most commonly used as a pre-service, old oil residue re-liquefier / cleaner and moisture drier. It can also be used as an after-service oil additive. In either application, Sea Foam® Motor Treatment will safely and slowly re-liquefy old oil residue. Each method is described below:

1. As a pre service cleaner for old oil residue, sticky rings or valve train noise, pour 1 ½ ounces of Sea Foam® Motor Treatment into the engine oil crankcase for EACH quart of oil capacity, including filter. Drive a MINIMUM of 30 minutes/miles, MAXIMUM 100 miles, and then do your oil change service – Lube, Oil & Filter (LOF). This begins the process of safely/slowly re liquefying the old oil residue so contaminants may flow and be filtered. This also makes your old oil dirtier, quickly, so a LOF service is necessary when the oil gets dirty. Great for Turbocharged & Supercharged applications where oils deteriorate so quickly due to heat, and leave those residues that NEED CLEANING.
2. As an after service additive into fresh oil, nearly fresh oil, or oil (used condition) that is NOT ready to be changed (based on mileage since last oil change), put the same amount of Sea Foam® Motor Treatment into the crankcase as described above, and then regularly monitor your oil for color and clarity. Set a predetermined schedule for checking the oil condition on a mileage, timed, or event basis (like every time you add fuel, etc.) to determine when an oil service is necessary. Monitoring of the oil for color and clarity will tell you when it is time to do an oil change service. NOTE: Do not exceed 3,000 miles without changing the oil.

Sea Foam® is safe to use with all synthetic oils. 100% synthetic oils, and blends of synthetic and petroleum-based oils, were engineered and are manufactured to be 100% compatible with petroleum based oils, all brands, and vice/versa. Without this compatibility, oil manufacturers and engineers would be liable for the results of mixing non-compatible lubricants. Since Sea Foam® ONLY contains petroleum oils, it is entirely compatible with synthetic oils.

Remember! When Using Sea Foam® in Your Crankcase:

• Check your oil and monitor its color & clarity to determine need for LOF service!
• Change your oil when

At least with what their saying, it should be fine to use, at least for a little while. Mind you I'm noe a seafoam booster or anything. I wish the directions on the canister were like this.