R34 GTT RB rough idle problem with weird RPM gauge fluctuations

[Joni Boi]

Thanks everyone for the replies/suggestions on my issue here. I'm still trying to diagnose the root cause of this.

Things I have done so far:

A week ago, I disconnected my 12v constant relay mod and returned it to the stock FPCM wiring: I could hear the sound of the fuel priming was different because of the FPCM limiting the amount of volts going to the fuel pump. From memory I think it also primed to 43psi. It was very hard to start it. I don't think from memory I could start it at all. I had to then ground the fuel pump to the chassie and then it could start after a couple of cranks. Fuel pressure did drop a bit to about 45psi (i think). It was idling even worse than before. 🤔 It was also doing the same weird RPM tacho thing. So I've returned it back to the constant 12v relay mod for now as I investigate more.

Right now, I'm in the process of deleting my aftermarket immobilizer, an old Japanese turbo timer, and another aftermarket transponder immobilizer that the previous owners put in years ago (from sexspec days). 

Just curious to see if this whole bird nest would cause some weird electrical problems. The Viper 5900 immobilizer stopped working a few years ago, and I pulled out the main fuse on it years ago to stop it from being a parasitic battery drain and I haven't bothered to clean up the mess there, so now is a good time to. There was a RPM signal wire that spliced into the RPM tacho signal from the ECU harness. I've taken it out.

I've already pulled out the Viper immobilizer and returned all the joints wires/splices back to factory. I let the car idle and it's still the same idle and RPM tacho idle issue.

Now I'm going to delete the turbo timer and the other key transponder immobilizer (autowatch) and return splicing/wiring back to factory setup. Just to see if this might be causing some electrical issues. It might not, but I just want to get rid of it anyway.

On 16/10/2024 at 6:26 PM, luke gtr said:

TL;DR

Not sure if checked, but confirm no blockage in the vac/boost reference line to the ecu. 

Have had rich and fluctuating idle from a blocked line and ecu could read that it was under vacuum(no MAF).

True!, I was actually thinking this the other day. I will check with another new vac line to make sure this is not the issue.

 

On 17/10/2024 at 9:48 AM, MBS206 said:

Go get some fresh gas from a totally different servo, jerry rig a fuel pump, and run the new tank supply to the fuel input at the motor.

 

There's every possibility you've got really shit fuel, thats possibly full of water etc.

This is actually on my to-do list, and you might be right, this might be a shit batch of fuel because I filled the tank up to 3/4 full from near empty, then I went to the shops and it happened straight after that. I've always filled up from this servo for years with no problems, but yeah might be my first batch ever batch of shit fuel from them. Will be doing this next when I get my hands on a container/jerry can. I just didn't think to do this straight away because it doesn't really explain why the tacho RPM thing is happening. Maybe I'll chuck some injector cleaner in the new batch too?