low vac, boost cut, etc... help please! - Zilvia.net Forums | Nissan 240SX (Silvia) and Z (Fairlady) Car Forum

inlineesiixxx · 02-19-2015, 10:41 AM

i know there are many similar threads, but none exactly the same as mine that i have found. I have an rb25det with fmic, plenum manifold, coil packs, stock turbo, stock ecu...
the problem has always been a low vac reading at idle, idle hunting, and a lean idle... when i cold start my vac reads at 10 and the a/f is leaned out. when it warms up it usually sits at 13 with a normal a/f range (still hunts occasionally). even though i still had the same problems i just mentioned i never had a problem with boost cuts until recently when i put a manual boost controller on. I set the boost at first to around 10psi, when it cut i tuned it down to 6-8 and it still would cut, sometimes as early as 3 psi. when slowly accelerating it feels r&r to me. i recently replaced all clamps, couplers and vac lines and nothing changed. i have not tested the maf or tps yet. I've also read it can be the bov. the bov was used on a previously blown motor and had some oil mixture blow off through it. not sure if that matters. when i turn my car off and go under the hood to inspect it there is a high pitched air leak sounding noise near the turbo for a couple seconds (wastegate?). also if my car is warmed up and the vac is at 13 like normal, i sometimes can rev my motor to 4k or so and let it drop which lowers my vac to 19-20 like it should be. I've also read it could be the ecu.. i have an auto trans ecu i can throw in. I'm going to test the voltage on my tps today and i also removed the manual boost controller to hopefully eliminate the boost cuts although I'm sure that will, best case, take care of the boost cut and nothing else... hoping someone has had the same exact problem and can pinpoint a solution for me so i can get back to boosting hard

Kingtal0n · 02-19-2015, 01:25 PM

2. stock turbo boost cut is via maf voltage. You can either verify the maf voltage is an arbitrary high number (4.5v+ during symptom) or start diagnosing the maf situation (do a boost leak check, clean maf, change maf, block off the blow-off valve)

1. you must be 100% sure the bypass isnt leaking during boost (or anything else) so do a boost leak test by filling the plumbing with compressed air. My initial guess/hope/simplest solution is that you have an enormous air leak somewhere.

inlineesiixxx · 02-20-2015, 07:00 AM

Thank you for the help. i will see if i can find someone who has a leak tester and ill try to get my hand on another maf to test out.

Kingtal0n · 02-20-2015, 10:37 AM

Quote:

Originally Posted by inlineesiixxx

Thank you for the help. i will see if i can find someone who has a leak tester and ill try to get my hand on another maf to test out.

you can make a leak tester out of a pvc end cap from home depot and a tire valve from pep boys (google boost leak tester)

Josh400 · 03-15-2015, 11:34 AM

Bumping this thread for updates. I'm going through the same thing and my rb feels slow but it's on 10 psi. I can't figured out that high pitch sound, that I'm hearing around the maf area. Any thoughts?

02-19-2015, 10:41 AM	#1
inlineesiixxx Join Date: Apr 2014 Location: Florida Age: 30 Posts: 15 Trader Rating: (0) Feedback Score: 0 reviews	low vac, boost cut, etc... help please! i know there are many similar threads, but none exactly the same as mine that i have found. I have an rb25det with fmic, plenum manifold, coil packs, stock turbo, stock ecu... the problem has always been a low vac reading at idle, idle hunting, and a lean idle... when i cold start my vac reads at 10 and the a/f is leaned out. when it warms up it usually sits at 13 with a normal a/f range (still hunts occasionally). even though i still had the same problems i just mentioned i never had a problem with boost cuts until recently when i put a manual boost controller on. I set the boost at first to around 10psi, when it cut i tuned it down to 6-8 and it still would cut, sometimes as early as 3 psi. when slowly accelerating it feels r&r to me. i recently replaced all clamps, couplers and vac lines and nothing changed. i have not tested the maf or tps yet. I've also read it can be the bov. the bov was used on a previously blown motor and had some oil mixture blow off through it. not sure if that matters. when i turn my car off and go under the hood to inspect it there is a high pitched air leak sounding noise near the turbo for a couple seconds (wastegate?). also if my car is warmed up and the vac is at 13 like normal, i sometimes can rev my motor to 4k or so and let it drop which lowers my vac to 19-20 like it should be. I've also read it could be the ecu.. i have an auto trans ecu i can throw in. I'm going to test the voltage on my tps today and i also removed the manual boost controller to hopefully eliminate the boost cuts although I'm sure that will, best case, take care of the boost cut and nothing else... hoping someone has had the same exact problem and can pinpoint a solution for me so i can get back to boosting hard

02-19-2015, 01:25 PM	#2
Kingtal0n Post Whore! Join Date: Jul 2005 Location: South Florida Age: 41 Posts: 4,828 Trader Rating: (17) Feedback Score: 17 reviews	2. stock turbo boost cut is via maf voltage. You can either verify the maf voltage is an arbitrary high number (4.5v+ during symptom) or start diagnosing the maf situation (do a boost leak check, clean maf, change maf, block off the blow-off valve) 1. you must be 100% sure the bypass isnt leaking during boost (or anything else) so do a boost leak test by filling the plumbing with compressed air. My initial guess/hope/simplest solution is that you have an enormous air leak somewhere.

02-20-2015, 07:00 AM	#3
inlineesiixxx Join Date: Apr 2014 Location: Florida Age: 30 Posts: 15 Trader Rating: (0) Feedback Score: 0 reviews	Thank you for the help. i will see if i can find someone who has a leak tester and ill try to get my hand on another maf to test out.

03-15-2015, 11:34 AM	#5
Josh400 Zilvia Junkie Join Date: Dec 2011 Location: Las Vegas Posts: 340 Trader Rating: (3) Feedback Score: 3 reviews	Bumping this thread for updates. I'm going through the same thing and my rb feels slow but it's on 10 psi. I can't figured out that high pitch sound, that I'm hearing around the maf area. Any thoughts?