boost failure under load

[karupt93240sx]

Specs : 1993 240sx KA motor super tech forged pistons and rings, 550cc injectors, walboro high volume fuel pump, top mount header, turbonetics t3/t4 bb, 38mm tial waist gate 15 lb spring, duel core alluminum radiator with duel electric fans, front mount intercooler, oil cooler, enthalpy rom ecu flash, msd blaster coil, 8mm ngk wires, bov, stock maf, ngk bkr7e gapped .28

Problem: The car has been running great for months.... one night driving home the car wont build boost in 1st or 2nd gear then third gear it will build 5-7 pounds befor cut out then 4th and 5th it will spike to 15 lbs? Then once in awhile it will turn over and not start but if i let it sit and come back 30 min later it starts up. I have changed plugs, plug wires, cap n rotor, coil, relocated the battery to trunk earlier in the year. fuel pressure stays fine and the af seem's to stay where i need it..... any ideas? im tired of waisting money and not fixing the problem!

[PapaSmurf2k3]

Is it just not building boost, or is the car actually misfiring?

[karupt93240sx]

It could be missfiring but im not sure. when it hits the cut out it sounds like its being choked or held back not like a miss fire but i could be wrong. it almost reminds me of limp mode or a rev limiter when it starts to build boost and then does the cut......1st and 2nd it wont build anything past 1 lb then 3rd under less load it will build up to 5 then almost flawless in boost 4 and 5th except when i get close to a full bar.

[karupt93240sx]

i'll try to get a video of what it's doing and post a link.

[mmm240]

I'm in a bit of the same boat. I'm not building a lot of boost before the waste gate opens up it seems.
I have to install a gauge and controller to find out for sure.

Are your controller or waste gate possibly failling?

[karupt93240sx]

im running a brand new tial waste gate it functions fine in 4th and 5th i can hear it dumping when its supose to and dont hear it opening prematurely. im honestly baffled.

[blkvrtswp]

Could it be a boost leak? Car feels like it is bucking, 'falling on it's face'? Very common, and the funny thing is most people say "I checked for a boost leak" but it is usually the absolute last coupling or hose you check.

Block off your hot pipe to turbo with duct tape and use a shop compressor thru a vac line (WG control line nipple is usually perfect). I have used a bicycle pump but it is a PITA. If you cannot see 15-20 psi on your boost gauge, you have a boost leak.

FYI the test using soap bubbles is useless and so is the test using starter fluid. On a turbo, the problem is the intake will hold boost up until a certain psi like 4. So you idle fine, no soap bubbles and no RPM increase from starting fluid because the leak only happens under boost not at idle.

Manually remove and inspect each coupling for tears, also check for worn thru spots on piping. Also verify your BOV is not leaking.

Good luck!

[mmm240]

The stock maf isn't good for a lot is it?

[karupt93240sx]

not sure what stock maf is good for i have the n61 maf to but it's gotta be wired in with an ecu flash as well. but it's been running fine with the maf that was in it. car ran for over 6 months strait with no poroblems on this same set up. i unplugged maf to see if car acted any different and it stumbled so im assuming the maff is still fine. i even took it out and cleaned it with maf cleaner.

[blkvrtswp]

Did you do a comprehensive boost leak test yet? Can you get your boost gauge to read 15 psi with the engine off?

I can't tell you how many times I have seen someone dismiss a boost leak over and over when that is exactly what the problem is. Months of no car and then they find that one coupling that is torn or has a broken clamp and is leaking......

Good luck!

[karupt93240sx]

i've gone over all the couplings and clamps and everything is as it should be. Im making a big move on monday across country and the car is getting trailored. Once im home and settled in im taking it to my brothers shop for some tlc. once there im sure we can figure out the problem. Have not had time to mess with it the last few days because of packing. I'll check back in late next week. thanks for the imput guy's much appreciated.

[karupt93240sx]

update: changed knock sensor... the plastic housing was cracked in 3 places. still no results. car falls on its face at 4000 rpm in every gear. has a rough idle at start up but cleans up after 3-4 seconds and idles normle. car runs fine till 4k then bogs till i shift or let of the throttle.

[karupt93240sx]

the starting problem i solved by replacing the igniter.

[karupt93240sx]

ok so i finaly figured out what was going on. the vss (vehicle speed sensor) on the trans sends a signal to the dash cluster for your speedo and then the cluster sends a signal back to the ecu so the ecu know's the vehicle speed. my ecu is tuned and had a launch controle wich was set for 3800 rpm. checked the continuety on both vss wires and figured out i was not getting the signal from the cluster to the ecu. figured the wire was grounded out or something in the harnes so i ran another direct line to the ecu and figured out that the wire was fine that my dash cluster wasnt putting out the speed signal and so my car would never come out of launch controle because my ecu wasnt getting the speed signal from the cluster. so i had my ecu guy retune me some chips to plug into the ecu with out the launch controle and whallah.

[karupt93240sx]

also had my ecu guy tune in the new n62 300zx maf