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Old 11-10-2004, 09:07 PM   #1
dreaminsubys
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Member#: 71579
Join Date: Sep 2004
Vehicle:
1998 Legacy Outback

Question Poor Gas Mileag and Trouble Codes

So my 98 Outback (2.5 DOHC) is getting poor gas mileage and showing trouble codes 303 and 304 which are misfires in cylinders 3 and 4. I just recently replaced the headgaskets and at the same time put in new Plugs (gapped to specs) and wires. I don't think my fuel filter is more than 10,000 miles old.
After resetting the computer, I'll get decent mileage out of the first tank, about 22MPG. This is still a bit lower than what I have consistently gotten in the past. By the end of the second tank, my mileage goes down to about 17 or 18MPG. My driving habits haven't changed at all, I always drive with my foot on the floor. If the tack hasn't reached 5.5 grand on a drive, then I probable haven't left the driveway yet.

So here are my ideas...

Faulty coil pack:
since cylinders 3 and 4 are served by the same coil.
(I tried taking resistance reading but don't know if I was using the multi meter correctly. All my reading were significantly below the range.)

Worn out O2 sensor:
I can plug into my lap top and monitor a handful of sensors. The front O2 sensors seems to be fluctuating back and forth regularly. The rear one stays pretty steady and only fluctuates once in a while.

I'm pretty sure the culprit is the coil pack. The readings I got from the Ohm meter just didn't make much sense. Coil packs are a bit spendy. I would really like to be positive before I start replacing parts.

Any insight will be greatly appreciated,
Pete
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Old 11-10-2004, 09:29 PM   #2
Mulder
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02 RAV4 MT
15 WRX/11 CRZ

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The 02 readings appear to be normal. The front 02 is supposed to fluctuate between .1-.9V as it is doing its job. The rear 02 will fluctuate less because it is after the cat and is only monitoring the cat's condition.
How did the coil resistance readings not make sense? Doesn't the service manual have the resistance specs? Aside from those readings, you can check for any cracks or carbon tracks anywhere on the coils, also the tower contacts should be clean and not corroded.
The poor gas mileage may or may not be related to the misfire codes. Other possible causes are an out-of-spec front 02 sensor or coolant temperature sensor.
Check your air filter if you haven't done so.
If the problem started since replacing the head gasket, check your valve timing to be sure it is correct.
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Old 11-10-2004, 10:37 PM   #3
dreaminsubys
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1998 Legacy Outback

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The air filter looks good.

The coil secondary resistance was around 14 K ohms (I think, if I was reading the meter right). Haynes manual gives a range of 17.9 to 24.5. I don't know how a coil works but I imagined if it was junk, resistance would increase?

The coolant temp sensor seems to be reading normal operating temp of around 196.

One of the terminals on the coil, #3 had some corrosion on it but I cleaned it up.

I had the same problem before doing the head gaskets but figured I'd do the gaskets first because I kney they were bunk. I'm also positive the timing is correct as far as the belt goes.
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Old 11-11-2004, 02:05 AM   #4
hartage
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Hmm my alldata shows different. For coil values it should be on secondary side 12.8kohms +/- 15% between the secondary towers (across from each other) For primary side .73 ohms +/- 10% between 1 & 2 2 & 3 with 1 being on the right most side 3 on the left with the parallel prongs pointed up.

You might wanna check spark plug wire values while your at it. 1 & 3 6.43-15.01kohms 2 & 4 6.67-15.57kohms.

Been thinking about what you said about the first tank being good milage but the second being junk. Do you have an auto trannie? if you do the car does learn shift-points both down and up and will taylor according to your habits. If you drive the car hard it will shift up later and down earlier and will downshift on lift-throttle at certain speeds. This could be another source of your milage woes.



Another suggestion would be to back-track. If cyl 3 & 4 are misfiring pull the plugs and read them. Do they look like they are sparking properly? Compare them to plugs from 1 & 3 look for the worn areas where sparks normaly occur does 1 & 2 have a spark wear pattern that is not found on 3 & 4? If you are confident that the plugs are telling you that 3 & 4 are getting as strong a spark as 1 & 2 then you need to start looking at fuel or compression. Pull the injector or the whole rail out (be sure you know what you are doing) and energize the fuel pump. Trip the injector with 12v but be careful some injectors may need a resistor to limit current. See if the spray pattern is good.

The simplest tests should be done first, read the plugs, test compression, do a vacum test. Read up on how to read a vacum gauge needle. How that needle moves can tell you a lot about what is going on in your motor.

Post what you find on this thread and I'm sure people here will do their best to point you in the right direction.
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Old 11-11-2004, 10:36 PM   #5
themike625
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Is anyone's 04 WRX fuel meter weird? from the full to 3/4, i get 80 miles. then from the 3/4 to 1/4, i get 120 miles, and fro the 1/4 to empty, i get between 60 and 80. i usually don't drive my car hard unless on the weekends, but that gets a separate tank of gas.

Mike
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