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93 TT falls on its face thru most the RPMs, fine unless in boost


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I have a 93 that when i got it was running great.  In driving it for a year several issues arose. I did the 130k service and loads of other maintenance. Car doesnt run right any more. What it does is starts and runs fine, then when i drive it its fine, until i put my foot into it then it sputters and runs incredibly rough until i back off the pedal.  When not in boost i can redline it and its smooth as glass the whole way. Not until i get into boost does it fall on its face. I dont even know where to start with this. Ive had local mechanic tell me compression is fine, coils are fine, injectors are fine(i replaced one suspect one even), ive replaced knock sensor, temp sensor, fuel filter, and more. I jsut need a direction! I can post a video of the issue this weekend once i can start and drive it. HELP!

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Could be a number of things.

first I would check your throttle position sensor is set around 0.40-0.45v when closed. It’s easy to knock them when servicing an engine and out them out of adjustment.


if it’s worse under load, it’s likely to  be air or fuel related.... I’d then suspect either the Fuel Pump Control Unit or the MAF. FPCU is easy to bypass and is a common failure point.... MAF is a bit trickier to diagnose as they can play up without being “fully” dead...


there’s plenty of articles on the forum on adjusting/testing TPS on the forum aswell as how to bypass the FPCU.


good luck


Edited by nickz32
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Firstly, welcome to the Club!

Firstly, have you conducted an ECU diagnostics to see if there are any error codes? 


As you have done a 120K service was the cambelt correctly installed on the timing marks and tensioner adjusted?

https://conceptzperformance.com/wiki/index.php/Timing_Belt_Service (scroll down to timing mark photos)

and the Ignition timing and CAS setup?


You should also check for boost leaks (engine under load and positive manifold pressure).


Throttle Position Sensor checking and adjustment (see Nick's comment above).


BTW, there are two coolant temperature sensors, which one did you replace?


Also, how was the knock sensor replaced - engine in situ or removed?


Edited by AndrewG


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On 08/01/2021 at 11:43, Dazsw said:

If the car was fine, you then worked on it an it's now broken, then you need to re-check every single thing you have touched, as something is incorrect.


Agreed - and did you use OEM parts....?

After-market cambelts are really a big no-no on these cars. ALWAYS use a Nissan belt - and take the utmost care in lining it up correctly. So many garages get this wrong, so it's easy for you or me to reassemble it incorrectly.... Even more so, with non-OEM belts.

I have something to say............ It's better to burn out than to fade away..... :tt2:

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