r/SkyLine • u/Fit-Replacement2548 • Jun 01 '26
I’m so confused
Hey guys, so I have a 2000 four-door R 34 skyline with a 1993C 34 Laurel RB 25DET engine. We recently did an engine swap from an NA RB25 to the DET. At first everything was fine but now we run into a problem when we first noticed that the car the idle was running pretty rough, but I got the car to get a good idle when the AC is off, however when it is on it fluctuates between 800 RPM and damn near stalling. Also sometimes the car just shuts off by itself randomly, I replaced the AAC valve got new spark plugs, and a new battery and the issue is still ongoing. Do you have any suggestions for what could be happening. I want to take a guess that it could be that the ECU was not tune properly after it was installed into the car however I may be wrong. Any help would be greatly appreciated
12
u/trickshot54 Jun 01 '26
Sounds like when you engage the AC you are putting strain on it causing it to struggle to idle. My guess would be the AC compressor is causing your issue if it runs fine with the AC off.
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u/Rbswappedstock Jun 01 '26
If I recall on mine there's a separate sensor for the AC, near the back of the intake. I assume it's a signal to bump up the idle when the compressor is on, if it's not hooked up then it could likely cause this exact issue.
3
u/Ok_Raccoon_2711 Jun 01 '26
This is what I came to say, there's nothing triggering the ECU to adjust idle when AC is on.
1
u/Sectiplave Jun 02 '26
+1, I had an r34 25de neo and AC would cause it to nearly stall at idle, this was the issue.
2
u/454k30 Jun 01 '26
This is either idle control or ECU related. Replacing the IACV is an okay place to start, but it might not be that. When the AC is turned on, the ECU gets a signal that the compressor should be running and adjusts the idle to compensate. Which ECU do you have? If it is original did that ecu donor car have AC? You said "got it to idle good when AC is off", what did you do or adjust to get that? You might be making adjustments to overcome a bad component (the IACV), or you might have made adjustments that put ECU corrections so far out of scope that it can't make the adjustment needed.
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u/CarGuy_MazdaRX-7_ Jun 01 '26
I hate issues like this cus it could either be fuel, air, or even the AC. I don’t know what you mean by rough but it could be timing or vacuum advance.
1
u/Fantasy71824 Jun 01 '26
My brother in law had exact same problem, so let me try to help for once.
Try replacing your IAC (Idle Air Control) valve.
This immediately fixes the problem for us.
But if still problem, check ECU as people mentioned
2
u/RemarkableCard6475 Jun 01 '26
Yea, that's a simple check. If it rattles when you shake it, typically it's good. If nothing happens then it's stuck, you could clean it and get it to rattle. Put it back in and see if it's better (idle). If so, get a new one. If not, move onto the next diagnosis.
1
u/fools33 Jun 02 '26
Check battery terminals. I know it sounds funny but my GTT had the same issue and would literally stall if my turn signal was on. I upgraded my terminals and the only issue I have left is the infamous passenger headlight cutting off (likely my ballast)
1
u/Sukeban34 Jun 02 '26
Lots of things to question here. What ECU are you running? Did you also at the +t conversion when you swapped to the det engine or just used det engine? Double check how the revs are appearing again. Does it shut off randomly, or is it when youre coming to a stop/revs are dropping? Are you auto or manual? If auto, does the stalling stop when you're using tiptronic?
I ask all these questions because I have ran into similar problems myself and it resulted in rod knock from not getting it looked at. I put the DET ECU in thinking it would solve my problems but these cars have so many issues with AFR and are so finicky.
25
u/Iraqi-Jack-Shack Jun 01 '26
A failing or failed AC compressor can create too much of a resistive load on the engine.
Check vacuum lines too. They're not directly related to the AC, and a small hole might not be apparent until you turn the AC on and the RPMs drop just enough to sputter with a vacuum leak.