So I've had my '88 convertible for over 2 years now, and I love it, despite the many issues I've encountered and fixed, but one thing that really bothers me is the low MPG's I'm constantly getting and I really want to see if anyone might know why. Naturally the MPG's vary a bit depending on weather and traffic, but on average I'm getting around 12-13, which seems ridiculously low to me. My buddy has an S5 'vert in stock trim and he was averaging more 17-19 mpg city, and that's really where I would like to be.

For reference, here are the mods/replacements/fixes I've done:
-New Water Pump
-130A Taurus Alternator & Mercury Villager E-Fan Swap (drive belt replaced/upgraded)
-New Battery
-Oil Changes, Coolant Flush, Transmission Flush, Fuel Injector Cleaner/MMO
-New Clutch
-New Tires (stock wheels)
-New Starter

Any ideas? :/

 

I don't see air filter or fuel filter, and spark plugs in that list? Riding with windows down or the top will effect mpg. Unnecessary weight in the car also factors in to the equation. Make sure the tires are at its suggested psi. Cold air has the tendacy of losing air in the tires, so that doesn't help. Those are some things I would start with should help. Unfortunately the convertible was the heaviest rx7 made.

 

You should be happy it runs if it's the original engine. Like all engines, as the internals wear the engine becomes less efficient. If you want a 20+ year old engine to run like new, the internal seals and sealing surfaces should be returned to like-new specs.

 

How did you determine that?

Things that cause bad gas mileage:
- Tires not inflated correctly or tires wider than stock.
- Steering not aligned or other suspension problems.
- Carrying a lot of weight.
- Engine worn.
- O2 sensor bad.
- Leaky fuel injectors.
- Using ethanol-enriched fuel or "winter" fuel.
- Driving with the AC on or the windows down.
- Rapid acceleration.
- Driving over 55mph.
- Excessive idling (for example, getting stuck in traffic).

If your car is low on power, then also check the condition of the cat, mufflers, and air filter.

 

The mechanic that originally serviced the car for me said that he believed that the engine was either rebuilt, or replaced entirely, so probably not the first engine. I determine mpg's by keeping track of mileage between each fill up, and the amount of gas filled each time, as well as octane of the gas.

All basic maintenance has been done - air filter, fuel filter, transmission fluid, oil changes, spark plugs, etc. Tires are fine, alignment is fine, suspension is fine, no extra weight (less weight even). Driving style doesn't tend to make a huge difference oddly enough, maybe +1 mpg for very conservative driving. I guess I'll check through injectors, and sensors then.

 

When you fill up the tank, do you fill it completely?

The octane rating of the gas isn't necessarily going to make any difference because it is simply an anti-knock rating. The chemical composition is what makes a difference in power and economy, but unfortunately you will probably never know this information other than maybe the ethanol content. Avoid ethanol whenever possible.

Checking the injectors and sensors is a good idea. Be sure to check the fuel lines for cracks. The fuel lines have nothing to do with your problem, but you should check them while you are at it because they will be in scary shape if they are original. When was the catalytic converter last replaced?

Driving style should make a huge difference. With my 88 convertible I get about 12mpg autocrossing, 17mpg city, 21mpg highway. My engine is low on compression, so those numbers are a little low, but at least you can see the difference that driving style makes.

 

I bet your coolant temp sensor is not being seen by the ecu. You broke the 20 year old connector when u did the waterpump like most do. Myself included.


Start by back probing the ecu when the engine is hot.

:-)

 

Always fill it completely. Not necessarily it won't no, but I've found that for my car it typically does. I believe the injectors have been replaced, but I'll need to check. I'll go through the sensors too once I get a chance. I don't have a cat anymore, my previous one was clogged, so I removed it and went with a straight-pipe set-up.

Should make a huge difference in theory, yes; in practice for my car, not so much unfortunately.

Possible I suppose, but the issue existed before the replacement, and my gauges for temperature work properly, so I'm very doubtful.

 

If you can tell any difference between high and low octane fuel, then your engine is knocking. Make sure you have the correct spark plugs and check the ignition timing.

 

The difference is in mileage, not performance necessarily. Spark plugs are good, timing is good.