List of common mechanical problems that hurt fuel economy

[Andyman]

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.

[bgd73]

that same crossover goes nuclear. yep. thats a bad one, its called egr.
The same crud from the same egr eats plugs, dirties pistons, and retarding timing is retarding power after the dirty perma egr pings start, and that equals fuel consumption. Let the engine stay noisy until a longer warmup. its quite ok. A thing called reality. It might even have tapping valves...

Engine cleaner following instructions closely and the miracle goops for fuels does function it back to normal. At most, given normal miles, valve seals may need help from the same torture the egr gave everything else.

Cold air for fuel injecteds, being sure to have plastioc tubes where metal may hurt electronic meters. Carb version can do ANYTHING. play games with it or not...same results, unless fixing an extreme rarity (they all do)

[Andyman]

Here are some additional problems that hurt fuel economy:

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

[ebouwman]

I'm pretty new to this forum.

But i'd like to point out what a massive difference proper maintenance can make. I have a toyota mr2, i'm not really into driving super carefully to save gas, i'm sorry people, i own a mid engine sports car, it's hard to not drive fast at least once and a while

But on to my point, i've been watching my nasty fuel consumption (don't have numbers but it was worse than my moms minivan, way worse) and considering the fact that no matter HOW FAST i drive it, a 1.6 litre engine does NOT use that much gas.

I finally went in and plugged a big vacuum leak, that helped a bit, but i was still looking at a horrible consumption rate. I then realized what it was, the o2 sensor!!

You see, since i have a lead foot, that means the ecu is using the o2 sensor more of the time since i'm on the throttle a lot, and litteraly pouring gas down the pipe (the exhaust pipe )

So when i finally put in my new O2 sensor (ok i admit i was having my header put in and new bigger exhaust as well) it made a MASSIVE difference, so much so that i will quite literally recoupe what i spent within a month or two at most. Another plus side is that i'll be prolonging the life of the engine (less carbon build ups) and getting more power!

So even though i still like to rev the nuts off the engine sometimes, i'm getting WAY better numbers in terms of fuel economy, power, and fun (knowing i'm not burning my cash)

[Frank Lee]

Hmmm... I'm wondering about such a list cuz darn near anything that's out of kilter can hurt fe.

[ebouwman]

Originally Posted by Frank Lee

Hmmm... I'm wondering about such a list cuz darn near anything that's out of kilter can hurt fe.

It can thats true, but different things make different impacts, with the o2 sensor for instance, the ECU is only using it part of the time, and it'll dump extra fuel in, but only to a maximum amount allowed when comparing with the air flow meter reading. At least that is my understanding.

So if the difference in afm readings is too far from the o2 sensor the engine should want to go with the o2 sensor when it's at higher rpm but not straying too far from afm readings.

Lots of problems like vacuum lines will cause noticeable differences in the feel on the engine at low rpm (where you want to be for best fuel economy)

I also have the issue of a non-working t-vis (closes off part of the intake manifold to get better power at low rpm) so it really makes me want to rev the nuts off the engine since i'm only starting to make power at 3500 rpm!

So all those little things that make a difference at low rpm, like spark cables plugs and dist cap, will help a LOT by keeping you in the green zone if nothing else

[99LeCouch]

Or even rubber engine hoses/gaskets developing cracks from age.

[datsundude91]

Thanks for this post. Now all I need to do is go through the list to see if there iis anything I can improve.

[radioranger]

Well , here's one for older 90's vintage Honda owners, a friends 1989 civic was not staring as quickly , this was back in 91 when it was only 2 years old, so I figured i would change the plugs, since it was only thing i could figure would make it a bit more hard starting, it had about 30 k miles, it worked and all was well, i called the local Honda dealer about it explaining how my Ford van had 50 k and no issues whatsoever, anyway he told me Honda's need new plugs every 15 K miles or mileage suffers, but nobody changed them !, my next girlfriend had an Accord , she started complaining about mileage, in went new plugs and 5 more MPG. change plugs more often and if you gap them , open them up a few thousands more for a fatter spark,

[radioranger]

Originally Posted by Andyman

Here are some additional problems that hurt fuel economy:

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

I read that in WW2 some pilots found by running a bit less oil in their planes a good amount of HP was gained , in my own Ranger i started blowing valve cover gaskets and for one thing was always running a tad overfull and also PCV valve was stuck, big improvement with a tad under filling the oil and freeing up , replacing the PCV, never had one ever stick before