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RB25 will not rev past 2500 rpm at full throttle

maximajim

By maximajim,
in Engines & Forced Induction

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maximajim Community Regular

maximajim

Posted
Posted

Continuation of my previous

"rb25 refuses to start thread"

What I know:

1) old fuel has no effect

2) must hold throttle open to start only when cold, may take 4-5 attempts. When hot engine starts fine.

3) still getting codes for AFM and TPS, even after reset

4) AFM voltage reads fine at off,idle and reving- there are no spikes/dips

5) AFM wire was lengthend to adjust to 240sx bay. Shielded cabling was not used.

6) At part throttle will rev to redline.

I do not understand what is happening. I thought if the AFM was at fault the engine would not rev past 2000 rpm at all, but it does. I measured voltage at the ECU harness while going WOT. Voltage did not reach 5V at any time. The rpm would build to ~2500 rpm fall back to 1000 rpm and back to 2500 rpm, etc.

Would unshielded wiring cause this? I'm out of ideas..

Blitz Grand Master

Blitz

Posted
Posted

My moneys on an AFM wiring problem for sure if not the AFM itself. The fact that you have modified the wiring to the AFM makes it the number one suspect. If the connection to the AFM is faulty if can cause the ECU to go into limp home mode which in effect rev limits the car to around 2500-3000 rpm. This is something that I have experienced in the past after not checking that my AFM plug was fully clipped onto the AFM.

Hope this helps

Lithium Grand Master

Lithium

Posted
Posted

I know its clutching for straws, but check all your piping which gets pressurised - you could have blown an intercooler hose or something like that, car is throwing in fuel for air it isn't receiving when it comes on boost.

Just adding a possibility to the mix :headspin:

maximajim Community Regular

maximajim

Posted
  • Author
Posted

I forgot I had an issue with tps voltage. Since I'm using a q45 tps I had to splice wiring. Turns out I wired the power the wrong way- so Idle was reading 4.5v, WOT at .6v. I rewired it and adjusted to .6v at idle.

Guess what? That fixed it the rev limit problem. Strange..

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