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Thread: 2003 MK4 VW Golf GTI 1.8T ECU Replacement (OEM)

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    Question 2003 MK4 VW Golf GTI 1.8T ECU Replacement (OEM)

    Hello everyone, I am terribly new to this forum, so I apologize in advance for not being descriptive enough of my issue.

    I'm looking to replace the ECU in my 2003 VW MK4 GTI 1.8T Standard Transmission, however I have a few questions regarding what to look for when purchasing a OEM used/refurbished one. Here are some of my general questions:

    1.) Can any Mk4 1.8T ECU be used in my GTI? For example, will an ECU marketed for Jetta 1.8T Standard Transmission work?
    2.) What is a general price that I should be looking for? I have seen some ECUs range from $100 to $500.
    3.) Is there anything else I should be looking our for when buying an ECU? Maybe something I am forgetting or red flags?

    My mechanic is planning to swap the ECU in my car as long as I purchase a replacement one myself. Since they were discontinued for the mk4 in 2018, I am doing my best to find a sufficient replacement for a new one.

    Here are some of the ones I am looking at:

    https://www.ebay.com/itm/2004-Volksw...oAAOSwwUJd1Hla

    https://www.ebay.com/itm/Programmed-...MAAOSwvudeFj0I

    I appreciate any thoughts/advice. Thank you!

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    Benevolent Dictator Uwe's Avatar
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    Why do you want to replace the ECU?

    Your best bet is one with the correct part number, or a part number that directly supersedes it.

    Does your mechanic have the ability to match the replacement ECU to your immobilizer? Doing so correctly requires the security code from your car and from the donor car.

    -Uwe-
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    Ok, here goes nothing

    For 6 years, since I have bought my used 2003 Mk4 VW Golf GTI, I have continually fixed/had mechanics repair, several issues with the car. I really love this car and the mk4 platform, so I will stop for nothing to make sure my car preforms nearly as well as it did the day it rolled off the assembly line.

    Since purchasing the GTI, the OEM turbo had many issues. One problem that has persisted throughout the years is an issue related to boost pressure consistency. Here's what happens:

    The boost will preform adequately for several hours/many miles IF the car is kept in 1st to 4th gear. (boost pushing ~25psi)

    Once in 5th gear, the GTI will push ~25psi of boost on level ground or a downhill slope at any speed. However, if the car is going up a slight incline, after a few seconds of boosting, the car will limit its self to a maximum of ~12psi. Once the boost is limited, it becomes limited in EVERY GEAR, until I pull over and restart the car, then everything is back to normal pushing ~25psi. THERE ARE NO CODES BEFORE, AFTER, OR DURING THIS ISSUE


    Here is what I have diagnosed while being in 5th gear.

    No matter how much throttle I give the car, at any speed in 5th gear, the turbo will always limit itself if it is going up a slight incline. The only way to stop the psi limitation is to restart the car.
    If I go up a slope in 5th gear, without pushing any psi of boost, the turbo wont limit itself, and continue to push ~25psi after the slope has been crested.
    If I have the car in any other gear going up an incline, at any amount of throttle, the car will not limit the boost.


    My mechanic says the only thing left to replace is the ECU in the car. I have already had the:

    OEM turbo replaced with a Borg Warner Turbo
    Vacuum tests preformed on every hose related to the turbo resulting in a large amount of elbows, hoses, and valves replaced
    Turbo Recirculation Valve Control Solenoid replaced
    Turbo Charger Bypass Valve replaced x2

    My mechanic says since there are no codes, and everything else has been replaced, the last thing left to replace is the ECU. He told me specifically to post on this forum to figure out how to obtain the right ECU for my GTI. I am open to any other suggestions you guys might have in order to fix this problem. It is driving me mad.

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    Verified VCDS User PetrolDave's Avatar
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    We REALLY need to see a full unedited VCDS Auto-Scan of your Mk4 GTI, so we can see any fault codes and how it is equipped (SW & HW versions of the modules).

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    Quote Originally Posted by PetrolDave View Post
    We REALLY need to see a full unedited VCDS Auto-Scan of your Mk4 GTI, so we can see any fault codes and how it is equipped (SW & HW versions of the modules).
    I'm going to see if my mechanic has a VCDS or if the local dealer has one, then I will get back to you. Do you recommend that I have it scanned once the turbo boost is limited, before it is limited, or if the timing of the scan matters? Thanks

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    Benevolent Dictator Uwe's Avatar
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    I recommend you get VCDS an log specified/requested vs. actual manifold pressure.

    Oh, and 25 psi doesn't sound stock. How is that being measured?

    -Uwe-
    Lock-down of sick people: Quarantine
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