Locating MAF sensor on 2003 Subaru Impreza 2.5 RS

[Syncert]

Also, something that's relevant to mention. This car is manual transmission. 

[Gloyale]

 

There's a problem though, the wiring on my new 02 sensor has 2 white wires, a black wire and a gray wire. The old 02 sensor has 2 black wires, a blue wire and a white wire. I havn't been able to find any wiring diagrams that indicate voltage on the 02 sensor wiring that would help me figure out which wire would go where...I'm kind of at a loss here. I'm wondering if they jacked the wiring on the 02 sensor and it's sending rump roast backwards readings to the ECU that's causing it to remain in open loop?

 

 

the 2 white wires will go to were the 2 black wires are (heater circuit)  The other 2.......try them one way and if it throws a code immediately switch them to the other way.

[Syncert]

Status update! Since the last post I
did take it into a mechanic. He has a subaru master tech
certification. In the past week that it's been in the shop there's
been a number of diagnostics ran. We both have our hands up in the
air saying what the codswallop, as such he's going to consult other techs
he knows to see if we can divulge. I decided I can also help by
consulting on forums.

 

The engine is running rich.
Codes
presenting are still misfire codes and rear O2 sensor.

In terms of driveability I've taken the
car back for the weekend, cleared ECU to try to get additional codes.
Driving the car it runs fine unless it's trying to idle. When the
engine is warmed up it stalls out at idle. To keep it from dying have
to rev the engine at stop lights.

 

Here's what's been additionally
done and diagnosed at this point.
-Timing. All timing has been
looked at except for removing passenger side timing cover and
determining if it's skipped ahead a little bit.

-Fuel pressure reads at 38 idle w/
pressure regulator disconnected

-Replaced front 02 sensor

-Replaced Fuel Pressure regulator

-Replaced Cam and Crank sensors

-Replaced Knock sensor

-Map Sensor replaced

-ECM replaced with ECM stock unit for
another Impreza

-Injector Resistance 15 Ohm Hot, 14 Ohm
Cold

-Engine pressure 170-180 PSI

-Neutral safety switch reads out
successfully

 

The two standing theories (all long
shots at this point)

-Carbon buildup on engine heads. Fuel
injector is spraying but it isn't reaching the engine due to carbon
buildup.

-The timing belt is a tooth ahead.

 

I'm begging anyone with any additional
theories to suggest them.

[heartless]

you mentioned a couple of times that you replaced the plugs & wires, but failed to mention what you replaced them with.

 

these need to be NGK - both plugs AND wires, or NGK plugs and OEM wires - Nothing else! Anything else and you are just asking for problems.

 

Checking the timing would be a very good idea. In fact, I would do this BEFORE doing anything else, and do it ASAP. Make 100% sure it is exactly where it needs to be. Being an interference motor, you want to check this sooner rather than later, or you could find yourself with a major overhaul on your hands.

[Syncert]

you mentioned a couple of times that you replaced the plugs & wires, but failed to mention what you replaced them with.

 

these need to be NGK - both plugs AND wires, or NGK plugs and OEM wires - Nothing else! Anything else and you are just asking for problems.

 

Checking the timing would be a very good idea. In fact, I would do this BEFORE doing anything else, and do it ASAP. Make 100% sure it is exactly where it needs to be. Being an interference motor, you want to check this sooner rather than later, or you could find yourself with a major overhaul on your hands.

 

 

Plugs and wires are both NGK. The NGK double iridiums I threw in were sooted up by the time they were yanked again to check them 2 weeks later due to how rich the engine is running in idle. Otherwise it drives fine. This weekend I drove it and it stalled out at stoplights throwing misfire codes. 

 

Timing cover was pulled and timing is correct. Last ditch theory is the fuel injector is shot and that's going to be swapped next. Otherwise we can't think of anything else. 

[Fairtax4me]

At this point I would be looking at scanner data for throttle position, engine load calculation, fuel trims, etc.

Did you ever get the TPS adjusted properly? Scanner data would show that. It should be at 0% with the throttle closed.

 

The rear O2 sensor may not be causing the whole problem but it isn't helping. Gloyales advice on that above is exactly right.

It sounds like a universal sensor was installed, check on the side of the transmission just above the rear crossmember for the harness plug. Hopefully that plug wasn't cut out. If it was you can splice straight to the factory harness, but IIRC the wire colors in the harness are different.

Worst case, the harness that runs along the trans to the rear sensor is replaceable. It should have another plug up near the top passenger side of the bellhousing.

 

After that, and you've verified TPS position is correct, my next steps would be to remove the intake manifold and check it for carbon/sludge, check the intake ports and valves for buildup, and probably replace fuel injectors or have them rebuilt by a professional rebuilder service.

[Gloyale]

Plugs and wires are both NGK. The NGK double iridiums I threw in were sooted up by the time they were yanked again to check them 2 weeks later due to how rich the engine is running in idle. Otherwise it drives fine. This weekend I drove it and it stalled out at stoplights throwing misfire codes. 

 

Timing cover was pulled and timing is correct. Last ditch theory is the fuel injector is shot and that's going to be swapped next. Otherwise we can't think of anything else. 

 

Get the stupid double iridiums outta there and put a standard single iridium or better yet standard copper electrode in there.

 

Gimmick plugs suck for these engines.  The tiny iridium tip can't produce a large enough spark and get's worse as it burns back into the ceramic tip.  JUNK.

 

I don't see anything on here about the TPS.  Gotta check it and it's adjustment.

 

Main things tied to idle :

 

IAC and associated hoses

MAF or MAP

TPS.....Must be properly adjusted

VSS

Neutral switch  (it's not a safety switch, doesn't have anything to do with starter.  It's only job is to tell ECU when needed to idle rather than fuel cut for engine braking in conjunction with VSS.)

[Syncert]

I did adjust the TPS correctly. Intake was pulled, there isn't any build up or sludge affecting injectors.

 

Plugs are a consideration, didn't realize double iridium could cause those issues with idle.

 

IAC and associated hoses have been checked.

MAP sensor was swapped with functioning one to verify, no difference.

Grounds have been checked.

Neutral switch was checked and verified to be signalling correctly.

 

Fuel trims are indicating the engine running rich at idle otherwise it's running normally. The problem is idle trims and maintaining idle. It's sooting up plugs and chugging fuel at that point. After engine warms up Tachyometer drops to near 0 when it surges then it eventually regains 700 rpm or dies.

 

What I'm gathering from this is the injectors are a very real possibility. Spark plugs are something to be addressed, and that rear O2 sensor is not a negligible factor. Correct?

[heartless]

Do one thing at a time here...

 

start with the plugs first - just basic NGK plugs at the proper gap spec is all that are needed here.

(i completely agree with Gloyale - get the stupid double iridium plugs out of there - they are just a gimmick to part you from more of your money)

 

then address the rear O2 sensor.

 

If there are still issues after both of those - then look into the injectors.

[Fairtax4me]

There's really only two reasons for fouling plugs at idle. It's spraying too much fuel, or it can't breath.

 

Actually I suppose a third reason could be that the plugs heat range is too cold, but this usually isn't a problem once the engine is warm.

 

The ECU uses a number called Engine load calculation to determine fuel input at a predetermined rate. This is based on ECT, MAF values, RPM, and throttle position.

The few times I've seen Subarus run so rich they foul plugs at idle and hunt up and down for idle speed, the MAF sensor was always bad and was telling the ECU that air flow was higher than it actually was. High engine load calculation was a sign of that, due to the falsely higher airflow rate.

You don't have a MAF sensor, but you will have a calculated/estimated MAF value in the scanner data from the ECU. It calculates this based on throttle position and MAP sensor values.

If you have an incorrect MAP value, either because of a leak or a plugged orifice or vacuum port for the sensor, that can cause the ECU to add or remove fuel trying to compensate.

 

Are your fuel trims high or low at idle? Basically, high positive trims show the ECU is adding fuel. Negative trims show it is trying to deliver less fuel, but can't or can't adjust the mixture any farther.

[Dantedel]

So what was it?? This cliff hanger has me eager to find out what’s now wrong with my car I’m having the same problem with, except mine has some knocking to go along with it.

[1197sts]

I am having a similar issue, 2003 OB MT, it has a very rough idle and a lot of vibration at idle. it runs a lot better at higher revs. No codes now but at one point had P0519, P0302, P0304.  I have verified timing, cleaned IACV, cleaned all the grounds, cleaned the electrical connections, swapped the coil pack, inspected plugs (fouled, looks like its burning rich), replaced the nuetral switch, replaced a couple of vacuum hoses, etc.

I have a cheap bluetooth scanner, I compared some of the parameters at idle with my other OB, 01 Auto Trans, the throttle position looks ok, but I am seeing higher readings on the intake man pressure, and lower readings on the timing. I have not done a lot of this, not sure if the 03 will read the same the 01 Auto since it uses a different style map sensor.

03 OB Man Trans

Press  6.2

Throttle 0.0

Timing 10.0

01 OB Auto Trans

Press 3.8

Throttle 0.0

Timing 15

I also compared the two at higher revs, the discrepancy in the timing was a lot less, although the press was still different but not as much. How likely is it that I have a bad map sensor? Another thing I am wondering about, these cars also have a barometric pressure sensor I would guess this is sending a signal to the ecu, could that be fouling up the map calculations in the ecu?

[1 Lucky Texan]

what codes come back (or show as 'pending') after the work that was done?

is the car new to you? if not, has it gradually gotten worse or was there a wreck or some work done just prior to the problem?

[1197sts]

The codes are not coming back, but for now I am only running it in the shop, not driving it.

The car is not new to us, it was driven for about 10k miles with no significant issues other than an EGR code due a replacement engine that was non egr . Despite the CEL it ran great with the egr port plugged on the manifold.

I just replaced the headgaskets with oem turbo gaskets, all new timing components, etc. As I mentioned I re checked the timing since this problem showed up and it was perfect. The heads were surfaced, leak checked, inspected, all good. No valve guide issues, a common issue on the SOHC heads. The cam gear and crank gear are correct, the timing belt is the same p/n I have used on dozens of these. The headgasket job went fine, I am 99.9% certain there is nothing internal that is causing the idle problem.

I was really hoping that the new neutral switch would fix it, it seems that fixed a lot of the mystery 0519 codes on manual transmission cars, but no luck.

I think I might change the cam sensor and crank sensor (one at a time, of course).

After that I guess I need to wring out each wire going to all the sensors and the ecu.

[GeneralDisorder]

Those numbers indicate the 03 isn't pulling the vacuum it should. 17.3 in/Hg vs. 22.2 on the 01 (generally see 22-23 on most stock NA Subaru engines). 

Do a compression test. 

GD

[1197sts]

GD, thanks for the info, do you think the map sensor could be bad on the 03?

[GeneralDisorder]

Seems unlikely given the running issues. More likely it's not pumping efficiently and actually has low vacuum. 

With the engine off, both should read atmospheric pressure. 

GF

[1 Lucky Texan]

maybe IACV hose is cracked or partly loose?

is that timing supposed to be -10 ?

[1197sts]

Update, I performed a compression check as GD suggested and found the #4 cyl reading very low, and #2 somewhat low as compared to #1 and #3 which were identical. I decided to yank the engine back out, when I took off the head I discovered bent exhaust valves, the worst was on #4 cyl.

To give some more background, previous to the headgasket job the water pump seized up and destroyed all of the timing stuff and bent a bunch of valves. After taking the heads off I found 7 out of 8 intake valves bent, some were pretty severely bent, so I replaced all of the intake valves. At that time all of the exhaust valves appeared to be seating correctly and passed a leak check. Also no valve stem problems. In retrospect I should have completely removed the exhaust valves to inspect them, maybe even replaced them just in case even if they looked good. I still don't know how it passed a leak check and then leaked when put back together.

The other takeaway regarding the water pump failure, the previous owner recently put in a timing kit with new water pump, don't trust cheap no name timing parts.