Gas mileage had gone down in the last 2 weeks!

[Tim_K]

The gas mileage in my '89 2.5 Turbo Spirit has really gone downhill in the last 2 weeks. I used to get 20-21 mpg, but the last 2 tanks of gas barely lasted 200 miles each!

I haven't seen or smelled any fuel leaks, I checked the plugs and they are tan colored, and the car recently got a new air filter.

I did some searching here, and found a post that said a bad MAP or bad coolant sensor can kill mileage. So I'm going to try and test them today.

Are there any other things I should look at to fix the mileage problem?

This car doesn't have a catalytic converter, so it's not a clogged cat.

[scott_s]

check the tire pressure, higher pressure is better for mileage. Bad O2 can make a difference too.

[jonnyb]

Wow - yeah - that's a pretty big difference...

Does the car run OK? If it does, then it's probably not the MAP because if that goes bad the car will usually run pretty crappy.

Is the fan always on - or kick on immediately after a cold start? If it's not always on, then it's most likely not the coolant sensor (bad coolant sensor triggers an always on fan condition).

A faulty O2 sensor will allow the car to run OK, but will cause the computer to add more fuel as a safety measure. My bro's Lebaron O2 went bad and mileage went from 320-350 a tank to 200-225. The only thing is that you say the plugs are tan colored - that doesn't indicate a rich condition - my brothers were black.

Have you checked the codes?

[Tim_K]

Yes, I checked the codes several times. None, only 55 showed up.

The car runs very well. A slightly rough idle, but it runs very well other than that. I have 12 degrees timing in it and haven't changed any parts lately, the mileage just dropped off. Right after I mileage tested a few tanks of gas and had consistent 20-21 mpg readings, too. About the time I noticed the mileage drop off, I also noticed that the Dawes Device would snap up to the green / blue lights much more easily when I closed the throttle from light acceleration.

The fan isn't always on. I'm going to check the coolant sensor with a multimeter today and see what resistance values it gives when hot and cold. I saw the specs on thedodgegarage.com . I think it does kick on after a cold start. I'll have to look again.

I run 35 psi in the front tires and 40 in the rear. Been doing that for quite a while now.

Even though the plugs were tan, they had quite a bit of buildup on them. The buildup was also tan colored. #1 was the worst, then #2 was a little less, then #3 was even less, and #4 was nearly normal. The plugs had been installed in early April and have no more than 2000 miles on them at the most.

I can't see how the engine could possibly be rich without having black spark plugs. The tan color looks normal, maybe a tiny bit lean if anything.

I'm running 55 psi on stock 33# injectors with a Walbro 255 pump.

I hope it isn't the O2 sensor, $75 is one thing I can't spend on an O2 sensor right now.

[22dodge]

don't bother measuring the coolant sensor. start with Oxygen sensor. Hook up your scanner and read the data stream. the O2 sensor must go from .9v to .1 at a fast rate with the engine fully warmed up. if it doesn't, 99% chance the sensor is bad. When an oxygen sensor fails, it usually puts out a constant reading below .2v. telling the PCM the exhaust is lean. so it dumps in more fuel. the fact you state the plugs are brown tells me you are running rich. if you keep driving that way, you WILL melt down the converter!!!!! if you can't afford a $75 sensor you surely cannot afford a converter.

[22turbo]

Did you switch the brand of gas? I have had bad luck as far as mileage goes with certian "Cheaper" Brands.

[Tim_K]

22dodge - I have a Dawes Device hooked up, and it regularly lights all its lights. So the O2 sensor should be working, right? I don't have a catalytic converter on the car, so there's no chance of screwing it up.

If the plugs were black, I'd think it's rich. But the plugs are a light brown (tan).

22turbo - No, same brands of gas I've been using for many years. BP, Sunoco, Citgo. It's been mostly Sunoco lately.

[22dodge]

you need to actually read the O2 sensor. your idiot light don't show the true movement of the sensor. you need to see that the sensor really moves from .1v to .9v. if your sensor is lazy it may take a while to go over .5v or it just barely goes over .5v and stays mainly between .1 and .45v

[Tim_K]

I see. Well, I can hook up a digital multimeter, I've done that before. In my pre-Dawes Device days.

[Tim_K]

According to my regular (analog, pointer needle) multimeter, the coolant sensor shows 11,500 ohms at 75 degrees. A bit higher than it should be. It should be 9120-10880 ohms. Could be telling the computer to add more fuel than needed...

I have a 180 thermostat, if that matters.

The MAP sensor showed 2.3 volts with key on, engine off (should be 2.4 to 2.5), and with the engine idling at 16 inches vaccum, 0.78 volts.

I need to go and hot test the coolant sensor now.

[Tim_K]

Coolant sensor tested at 700 ohms hot. So that's not the problem.

[22dodge]

so what does the O2 sensor show?

[Tim_K]

Haven't checked it yet. Tomorrow.

[Tim_K]

I don't think it's the O2 sensor. It shows a wide range of numbers, although I didn't see much over .9 volts.

At idle, it'll bounce around a lot from 0.00 to the 0.700's.

While driving, the number changes all over the place.

I didn't do much WOT driving, but it works properly then also.

One of my little tests was to rev the engine to 3000 rpm, then watch the multimeter. It would spend 3-4 seconds below 0.1 volts, then go back to bouncing around.

I changed the MAP sensor for another one I had. A stock Turbo 2 bar one. BOTH MAPS are stock Turbo 2 bar units. With that other MAP in, the 3000 rpm to idle test would show only about 3 seconds on >0.1 volt readings before going back to the normal bouncing around.

And the spark plugs were black. In less than 200 miles since their last wire brush cleaning. The ends of the plug threads had the most black on them, there was some on the side electrode also, but not completely covered. As before, #1 plug was worst, and they gradually improved as I went across the engine. #4 was the cleanest but still had more black on it than it should have had.

After I changed the MAP I cleaned the plugs again.

So if it's not the coolant sensor, and it's not the O2 sensor, what's next on the list? I have the other MAP in it now.

[22dodge]

oxygen sensor should never stay .1 v for 3-4 sec. that's telling the computer you are LEAN and for that long of a time period you have a lot of fuel trying to richen the a/f mixture.

But you REALLY need a scanner on the pcm to see what the computer sees. About 12 years ago I had a customer bring a 1986 K car that got horible gas mileage (14-16mpg). another shop put in 2 oxygen sensors 2 separate times because that's what the codes told them. anyway I get the car to my shop, up the scanner and get a O2 code. so replace the sensor right? NOPE, I looked at the data stream and found a wierd value in the Logic module. with some investigating and diagnostics, I determined the module was bad. so I installed a new module... that fixed the problem and the customer is still driving that car today.