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Old 12-02-2001, 10:47 PM   #1
2.5GT
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Default Subie mechanics/techs please read - LONG

Hello,

My mechanic is currently in the process of dropping a 2001 legacy motor into my 97 GT. The swap is going well, just needed to rebend the power steering lines to accomodate the different fuel rails. Other than that, opretty straight forward. However, a couple questions were raised.

1. EGR valve: 01 motor does not have an egr valve. My mechanic doesn't expect this to be a problem since the ecu won't realize the egr valve is not in place given that the computer will still read vaccum pressure into it. Is he right?

2. Throttle Position Sensor: the 97 style is much more complex. Are there any issues that might arise with using the newer version with the older (97) ecu?

I'm sorry if these questions may seem a little screwy. I'm paraphrasing what my mechanic asked me to convey to you guys. I hope someone out there can answer these questions.

Thanks,

Jason
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Old 12-02-2001, 10:59 PM   #2
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Old 12-02-2001, 11:03 PM   #3
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When there is any doubt, you should always match the electronics. I.E. use the 97 tps for the motor, because the ecu is for a 97 model. I honestly don't know if the ecu will have a problem with eliminating the EGR or not, it probably will though, as it is an emissions component, and is therefore monitored by the ecu as mandated by obd-ii rules. It may be better to use all the old electronics and manifold on the phase-II motor if it will fit.
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Old 12-03-2001, 12:18 AM   #4
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The ECU will try to test the EGR and will set a code when it doesn't get the result it wants. It needs to see a corosponding change in MAP when it tests the EGR.
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Old 12-03-2001, 01:50 AM   #5
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My question to you is : "why on earth would you want to go from DOHC -> SOHC"?

If your block was the problem, you should've just swapped out the short block. However, the only advantage I see that you'll have is lowering your torque band, as well as saving 20lbs in weight. I guess if you're not swapping over ECU's you may be able to still have the 6500rpm redline.
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Old 12-03-2001, 05:14 AM   #6
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Quote:
Originally posted by SubyTechMaster
The ECU will try to test the EGR and will set a code when it doesn't get the result it wants. It needs to see a corosponding change in MAP when it tests the EGR.
\

He is right...the ECU will also throw a code for EGR LOw Flow....Even with the EGR Valve unplugged it will throw a code...this is the Theory behind OBD-II stringient emissions controls.

jeremy
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Old 12-03-2001, 09:59 AM   #7
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Quote:
The ECU will try to test the EGR and will set a code when it doesn't get the result it wants. It needs to see a corosponding change in MAP when it tests the EGR.
I guess I've answered my own question. Upon looking at the EGR valve we have noticed that it is a mechanical sensor. The ecu has no way of knowing that the egr is nonexisitant. The ecu can only determine whether there is flow through the egr valve. If the ecu reads flow (ie: just tie in the two ends with a vaccum hose) there shouldn't be a problem.

Quote:
My question to you is : "why on earth would you want to go from DOHC -> SOHC"?
i-Club Forums > i-Club Technical > Normally Aspirated > SOHC vs DOHC battles

I'm not sure if this link works but search under SOHC vs DOHC battles, and you'll see why I DIDN'T go with the DOHC heads. Plus, I'd rather end up paying $400 for cams instead of $800

Jason
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Old 12-03-2001, 10:31 AM   #8
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the ECU calculates flow thru the MAP sensor...NOT thru any kind of vacuum.

Jeremy
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Old 12-03-2001, 10:41 AM   #9
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How does my 97 ecu calculate flow through the MAP sensor when it doesn't have one?

Jason
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Old 12-03-2001, 02:23 PM   #10
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okay my bad...wasn't paying attention...but i can tell you that the ECU is programmed to monitor EGR flow....not sure what it would use to do it on a MAF based system...maybe my SubyTechMaster can help out here.

Jeremy
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Old 12-03-2001, 10:33 PM   #11
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Every vehicle with OBDII monitors Manifold Absolute Pressure (MAP). On most Subarus ('98 and earlier) it is done with the Pressure Sensor which is located on the right front strut tower. The ECU uses this Pressure Sensor to monitor both Barometric Pressure and Manifold Pressure. Below the Pressure Sensor location is a Pressure Sources Switching Solenoid (PSSS) which primarily looks at Manifold Pressure but switches over to sample Barometric Pressure.
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Old 12-04-2001, 02:25 AM   #12
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see i knew there had to be a manifold pressure sensor of some sort.

Jeremy
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Old 12-04-2001, 06:17 AM   #13
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The EGR may eb a mechanical valve, but trust me the ECU KNOWS if its working or not. Since my 85 brat has no ECU and still manages to light up a EGR light since its not working......

Have you tired the intake manifold that matches your ECU? You do not want to swap sensors if at all possible.
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Old 12-04-2001, 03:40 PM   #14
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XT6- He'd have to stick with a DOHC EJ25 which he already said he's not...he's going to the SOHC EJ25 because parts are cheaper.

Jeremy
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Old 12-04-2001, 04:15 PM   #15
2.5GT
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I tried swapping intake manifolds but the DOHC manifold doesn't bolt up to the SOHC heads. I also tried to use the DOHC throttle body but found that the older does not have a provision for the idle air control sensor (it's on the intake manifold on the DOHC engine). So, no go. Looks as though I'll have to resurface my old heads and use them with the DOHC intake manifold with the new block if I can't get any definite answers as to whether the 01 TPS works with the 97 ecu.

As for the EGR valve. I was under the assumption that the ecu just monitors flow rates. If there is a blockage in the EGR valve, the ecu will throw a code. I remember when I changed my hoses to the silicone variety, the vaccum hoses to the EGR were a smaller diameter than the stock hoses. The ecu sensed a decrease in flow and threw an EGR code, even though the EGR was perfectly functional. As soon as I swapped the hoses back to stock, the code disappeared.

HndaTch627, you are right. Parts are cheaper

Jason
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