99 2.5L Poor Fuel Mileage and Lack of Power

[Matt715]

Hi everyone, first post here.

I have a 1999 Mazda B2500 (5 speed) with a little over 180,000 miles on it. I bought it 8 years ago with only 30,000 miles on it and with every passing year my gas mileage has been going down hill.

When I first bought it I would get 27-28 mpg highway during the summer and around 24-25 in the winter. It also had alright power for what it was. I could go up hills without having to downshift too often and pull my small boat or ATV around without any problem.

Fast forward 8 years and 150,000 miles and now I am getting 21-22 mpg in the summer and my recent Thanksgiving travels netted me a really crappy 18 mpg. That's horrible since I don't like stopping for gas on long trips.

I also have lost a ton of power over the last two years. It's getting so bad that I have trouble using 5th gear pulling my boat or ATV even on flat ground. There is no way I could pull a 700 lb load at freeway speeds and not have the pedal on the floor the whole time. I also have to downshift frequently when climbing small hills.

I try to keep up on my maintenance. It got new plugs and fluids about 15,000 mi ago, a fuel filter about 5,000 mi ago, and other general maintenance as it comes up. The last time I put a timing belt in was at around 110,000 miles.

The engine runs pretty good but I notice that it gets a slightly rough idle when it tries to idle below 950 rpm (1,000 rpm is normal for it). I don't have any valve noise and I clean my intake about every 15K or when the plate starts to get sticky. My check engine light is not on either.

I would like to keep this truck another 3 -5 years since the body is still decent but if it loses much more power, it's going to have to go down the road.

Anyone have any ideas?

Edit: I forgot to mention that the whole time I have owned this truck that every once in a great while under acceleration from a slow speed I would get a sudden lack of power (and I mean it won't accelerate no matter how hard you hit the gas) and then it's fine 2-4 seconds later. It's very rare when this happens so I never did anything about it.

 

[shovelmike]

Do a compression test wet and dry, that will tell you the general condition of the engine, and whether it is a concern with rings or valves. A squirt of oil in the spark plug hole will temporarily raise compression if the problem is rings, not so for valves.
A leak down test can nail it down more precisely, but costs a considerable amount for something a backyard mechanic will only occasionally use.
The tendency to stall at idle points to a valve leaking a bit.
If the problem is indeed valves, I would advise grinding only the valves which are obviously defective, a full valve job can put a strain on the high mileage rings and cause excessive oil consumption.

 

[tomw]

+1 on the compression test.
I would also think about replacing the thermostat as you may not be getting the coolant fully warmed up as quickly as before if it has a 'hairline' opening when cold. Some stats get a little lazy and won't close all the way when cold.
I would also check the ECT and ACT sensors, as coolant and incoming air temp signals the computer what to do mix-wise. I would also clean the MAF, as it would affect how much fuel is provided based on what the MAF 'thinks' the airflow into the engine is. If the MAF is dirty, with even just a speck, it can throw off the air flow readings.
You can't fiddle with timing, so that limits ignition fiddling to checking &/or replacing the spark plugs.
You can check the short and long-term O2 sensor trims, and look at the O2 'flipping' as the engine runs with a scan tool. If it is lazy, or non-responsive, it can affect the amount of fuel provided and secondarily the power produced.
tom

 

[tbirdsps]

I agree with what everyone says but also check fuel pressure. With the OBDII you should be getting some codes if lean or rich. I had a bad fuel pump in my 92 2.3 causing the low power and real bad acceleration problems but it did not affect my fuel mileage so certainly with the lack of check engine light check compression, MAF, ACT and ECT along with the O2 sensors. With 180,000 miles the O2 sensors can be suspect just due to age.

Also consider the catalytic converter. There are tests for that too. If the engine has been running rich it could be getting plugged causing a lack of power. If it is bad don't consider replacing it until the other problems are identified and solved. It doesn't take long to melt one due to excessive fuel.

 

[Matt715]

Excellent advice guys! I will check into some of those things when I have some time and daylight.

I do have access to pretty much any kind of tool I need as I have a few friends who are auto mechanics and I am a power sports mechanic. I was more curious if there was a typical problem that this motor had but it pretty much sounds like I'll have to do some diagnostic work to it.

Thanks!

 

[ranger85]

I have a friend with the same truck but its the ford ranger. Could a bad clutch be causing bad fuel mileage? Not the fan clutch the drivetrain clutch. Thanks