RB20DET wont idle

[Ian-187]

Hey everybody, this is my first post on the forums, but i'm not new to 240sx's. i've owned a s13 hatch with a stock ka24e for almost 2 years but i recently bought a hatch with a RB20DET swapped in, which brings me to my problem.

My RB20 wont idle, it starts up just fine, but immediately revs up high then either dies or revs really low then back to high over and over and eventually it dies. sometimes it will idle fine and if i play with the gas pedal a little bit i can make it idle, but as soon as i touch the pedal again the idle goes back to messing up. otherwise it runs pretty strong, i've driven it on the interstate for an hour and a half no problems except the car wont idle and it dies if i put it in neutral to slow down like at a stop light, stop sign, etc.!! it also has a pretty strong odor of gas at times, as well as absolutely shitty gas mileage. I think it has a bit of hesitation under hard acceleration sometimes as well, but the main problem is the lack of idleing (if thats a word). I already tried running it with the MAF unplugged to test that and it didnt help. I also tried unplugging the TPS to test that but it had two plugs so i wasnt sure which to pull. Sorry for the long post but i wanted to provide as much info as possible and thanks in advance for any help you guys can provide!!!

[Ian-187]

also forgot to add the motor still has the stock FPR, which i've heard goes out on older motors. are these symptoms of a bad FPR? thinking about ordering a nismo AFPR tomorrow but wanted some opinions from people with more experience with RBs.

[budderigs]

To check the fprs unplug both of them and try to start it. If either one leaks gas out of the vacuum port it is bad and needs to be replaced. Only by an adjustable if you have a fuel pressure gauge to set it.

Yes your symptoms do sound like an fpr issue.

[Ian-187]

I just tried taking the vacuum line off of the back FPR (the one that comes off the send line from the fuel pump) and it ran a lot better. The idle went up a little higher, but it didnt die nearly as easily as with the line hooked up. does this mean the FPR is bad? could it mean i have a vacuum leak somewhere?

[budderigs]

was the inside of the line wet or smell like gas?

[Ian-187]

the lines didnt have anything in them and didnt really smell like gas BUT there is a definite gas odor from somewhere in the bay. i also unplugged the tps and adjusted the IAC valve and it would adjust but it didnt help it from dying but it seemed to work fine. i went ahead and ordered a nismo FPR and a fuel pressure gauge though.

[Ian-187]

i switched out the FPR for the Nismo unit and put in a fuel pressure gauge. i have it adjusted correctly, but the car still is acting funny. its almost like a vacuum leak or something. i think i've ruled out the IAC valve as well because it adjusts the idle just fine. checked spark plugs and they all seem to be fine. How do i check my MAF? what about the AAC valve? i'm stumped. if anyone could chime in with some feedback i would appreciate it tons. it still has the same problem, hunting for idle, then dying when it dips too low.

[Ian-187]

i forgot to mention the motor has a rb25 turbo on it. would that affect any of these problems?

[3v1Ls14]

shouldnt do lots of people slap those on and doesnt change a thing but i dont know that for certain. shouldnt mess with the idle anyhow and shouldnt be on boost. whats your fuel pressure set at on the gauge?

[bkt808]

do you have a walbro in their? if so i believe the stock fpr cant handle the pressure of the walbro.
im also having trouble with idling with my rb25, having to feather the throttle to start.

[Ian-187]

i think its the stock s13 fuel pump but im not sure. i installed the adjustable nismo FPR and it still wont idle all of the time. i can pull off vacuum hoses and it runs better for a minute but then runs too lean i think. have you checked your IAC valve?

[3v1Ls14]

pulling vac hoses off sucks un-metered air in so yea would spike idle and lean out the mixture a bit

[Ian-187]

shouldnt do lots of people slap those on and doesnt change a thing but i dont know that for certain. shouldnt mess with the idle anyhow and shouldnt be on boost. whats your fuel pressure set at on the gauge?

its set around 35psi. but its weird because it goes from rich to lean once its warmed up.

[3v1Ls14]

35 at idle or when it primes? and is that reading on an aftermarket afr gauge? how many times will it go from rich to lean or vise versa within 10 seconds?

[Ian-187]

i have it around 35 at idle. and thats coming off of an aftermarket fuel pressure gauge. i dont have an afr. im saying it goes lean but im just assuming because it gets really sluggish and the exhaust note changes. but once it swaps over to being sluggish it stays that way pretty much. it will try to go back to normal but it goes right back to lean. thanks for your help man im new to RB's and it has me scratching my head!

[ultimateirving]

I know u said the IACV changes the idle, but try unplugging it and seeing if it still dies. you may need to screw the screw all the way in to make sure it keeps its own idle when its unplugged

[Ian-187]

thanks for the input. i just went and tried screwing the IAC valve all the way in, still having problems with idle. does that mean the IAC is bad or working correctly?

[Ian-187]

i also don't know if i mentioned it yet, but when i unplug the MAF, the car goes dead. AND i found that my o2 sensor wasnt even plugged up so i plugged it up, no difference.

[ultimateirving]

If you screwed the screw all the way in, and unplugged teh iacv then started the car and it made no difference in the idle then i would say the iacv is not the problem.

[Ian-187]

hmm i hooked up a boost gauge and went out for a run and at idle i have around -20psi and during what should be full boost i never get over 5psi and sometimes it doesnt hold that. does this point towards a vacuum leak? what is the correct vacuum at idle?

[Ian-187]

just ran diagnostics off ECU and it showed code 55 no problems. so im thinking maybe o2 sensor?

[91ka24]

Do you have a multimeter? I would start checking voltage at tps and iacv. A faulty tps can cause weird things, like at idle the voltage is like .9 if I remember correctly, well if its reading say 4v the ecu thinks the throttle is open and sends more fuel thus running rich. Do u have to pump the gas to get it to start? Have u tried unplugging your coolant temp sensor and see if that changes anything?

[ultimateirving]

Never reaching full boost points to boost leak. Did you pressurize the system to check for boost leaks

[emericanz]

check for leaks!
my s13 swapped rb20det im in the middle of refuses to idle until i unplug the maf n o2 sensor yet back fires constantly. im about to go check for leaks i suggest you do the same.

also ur fuel pressure seems fine i did the test on mine n when the pump primed it hit 50 and then dropped to 40 while cranking but good look im gonna be watchin this thread might give me lots of help