95 gtst lapse in acceleration at high rpm???

[SlidingSideways]

Hey all
I just bought my first skyline this week, a 95 gtst bone stock.
when i accelerate hard as i come through 2nd, and third i notice that as the rpms come up, it accelerates as it should but a few spots in my rpm range it seems to stop accelerating for a moment, and then gets back on it (usually at 4500 and again at 5500).. like its bogging,, or skipping a bit.. just a little shudder. if im accelerating through 2nd and 3rd then it happes usually twice through the accleration

yesterday i was on the highway in 5th going about 110kmh, i dropped it into 4th and punched it up to whatever speed, but when i did that, it was worse, it hapened 5 or 6 times through 4th coming through 4500rmp and up, around 120, 130, 140, skipping..

sorry im super new to both turbos AND forums, so i dont know how or where to ask this question,

the car is in great shape, idles nice, it's absoutely spotless, and has ben VERY well maintained, and i dont think it's ever been beaten up on (till i got it this week) ha ha. anyways any thoughts on this? is it a fuel issue? i'm running 94 octane in it..

what do i need to do to smooth my acceleration out and keep 'em steady?

[h4X6]

could be a fuel issue... i think the first thing to do is change the fuel filter, do the same tests... i could go on and on about what the issue is, but start with replacing that and see what happens

[SlidingSideways]

will do i'll try it today and see how i go

[nimblestix]

Quick first few things any new skyline owner should do:

Clean maf(s), check plugs condition/gap,
Replace fuel filter & oil+filter and while you have the coil packs off check the resistance on them. Looking for .9 ohms.

Google search "how to test rb25 coil packs"

Sounds to me like a dirty maf but you won't know until you clean it. You can find detailed diy's on gtrc or Google for most of the basic and some complicated maintenance.

Good luck, let us know how it goes.

Sent from my Galaxy Nexus using Tapatalk 2

[evolution23]

your description of the problem seems somewhat vague but sounds to me like its misfiring at higher rpms? My misfire at 5000+ rpms was caused by 5 of 6 coilpacks being cracked (not sure which ones were actually bad of those 5) but that cleared up the misfire a bit, other half of the issue was my fuel pump was on its way out! max pressure was 40psi so under full boost at wot the pump just couldnt supply enough fuel.

to check on the coilpacks if you pull them out and look just below the writing on them on the bottom is where 5 of mine had cracks. they are hair line cracks so can be hard to see if you arent sure what to look for. And for the pump a simple fuel pressure test can eliminate that! should be 35psi with vacuum. 43 w/o vacuum.

but as nimblestix said, its always a good idea todo your own little tune up after buying a new vehicle. goodluck!

[Rblove26]

more then likely the maf, a tune up wouldn't hurt it anyway, but i had pretty much this exact same issue, changed fuel filter, plugs, cleaned the maf, replaced all vacuum lines, etc... i re-soldered my maf and it helped for a little bit but eventually came back. finally borrowed a buddy of mines maf that was not having these issues and car woke right back up.
if you can borrow one would be a great way to be sure like i did.
hope this helps cuz i chased this problems all summer 2 years ago,

[SlidingSideways]

I replaced the fuel filter, checked and cleaned all my coilpacks (i got 1.6 ohms on 5 of them and 1.9 on the 6th) no cracks, and they were clean... i took out and checked all my plugs, (iridium plugs) and theyre all good, its had a recent oil and filter change with a fram filter and royal purple stuff...

so i took it on the highway, was at 110, dropped it into 4th and ran it up to 150 160 ish and still it seems like its coughing.. or just having temporary soft pots through the power range...
it sort of feels like it could be a lack of fuel supply to the engine...

so fuel pump? is there anything else it could be?
am i just expecting too much out of the stock rb25?
any suggestions on a fuel pump that wont break the budget?
how do i test my existing fuel pump?

-help-

[Skym]

Make sure your sparkplugs are gapped down from stock 1.1mm to 0.8mm and are the correct heatrange when up boost level. NGK PFR6A is stock, NGK PFR7A is for intake, exhaust, etc. If you don't regap them, can damage coilpacks.

Fuelpump = check base fuelpressure at idle and on dyno if possible. Dyno is the best way to monitor engine, etc and trace problems.

usually at 4500 and again at 5500

Sounds like the VVT switchover which is normal or boost control solenoid changing boost level. If engine stops accelerating, then can be knock maps as ECU pulls ignition timing to 0 on knock ignition map, knock boost map drops boost level, etc.

VVT switchover is shown here on a SR20DET dyno graph (scroll down until see "making a good use of VVT") with drop in power in midrange and same problem on RB25DET -

http://www.tomei-p.co.jp/_2003web-catalogue/e064-065_camshaft.html

VVT range for RB25DET cams is mentioned here (1050rpm-4500rpm for R33 RB25DET) -

http://www.tomei-p.co.jp/_2003web-catalogue/e090_camshaft-specs.html

Sometimes the VVT solenoid can playup or settings are incorrect or like mentioned in link, need aftermarket camshaft's.

[SlidingSideways]

thats quite the reply!
full of info on all sorts of expensive things! ha ha i REALLY am not ready to start chasing this problem into internals.. nor am in a position to start messing with my cams...

the sputtering is sporadic, at various high rpm ranges (i know i said 4500 and 5500) but after playing with it a bit today, it seems to happen at various points in the higher rpm range under heavy boost...

i forgot to mention that i also cleaned the MAF and i have just reset the ecu and i'll see how it goes tomorow...
i'll also try to figure out the tps voltage and set to 48~49

should my stock fuel pump always be able to keep up to the boost? or are they typically under powered? stock is 135lph so i read, if i upgrade the fuel pump am i getting into alot of other mods? (fpr, af ratios, guages to moniter everything..) i just bought the car this week, and was planing on taking it slow... so to speak... if i replace the fuelpump next with another oem one that pumps 135lph, might i have the same problem? (assuming that it is a leaning problem) suposedly a 1998 240sx pump is the same thing..

also... forgive my noob question, and its probaly covered somewhere i havent been able to find yet.. how do i test my fuel pressure... all i can find is what it is supposed to read at idle, and under boost... but i dont exactly know how to take said reading.. or where to take it from..

[cooke24]

coil packs replace with $400 yellow jackets. saving for some on mine.

[Rblove26]

like i mentioned before, try a known good maf, id lend you one but ended up upgrading it to a z32 maf and my buddy sold his car that i borrowed it off. your headed down the same path i went before, just my opinion tho...
good luck

[Skym]

SlidingSideways, my post was just to show what happens with VVT and how to get a smooth powerband (thinking it was the VVT or boost control solenoid switching over that you were feeling that felt like it slowed acceleration and how to smooth that transistion, usually with aftermarket Link G4, ViPEC, etc ECU + boost control solenoid + aftermarket cams + tune). With ECU tune, tuner should have picked up that fuelpump needed replacing, checked sparkplug gaps, etc before doing any tuning.

With a old car it's worth checking everything, as sometimes previous owner or owners fail to do proper maintenance due lack of knowledge on how to, which causes these type of problems. It's something you learn by trial and error (part of the way of how I learnt) or via someone thats knows the correct maintenance schedule for a modified car (different to factory car).

Sounds like sparkplug gap, just like my car did before it damaged 2x coilpacks. Heat range is another that can cause melting (widening sparkplug gap if heatrange is too hot on a lean engine) or fouling of tips (if heat range is too cold on a engine running rich).

What did you clean MAF with??? Should use CRC MAF cleaner. Some use carb cleaner which eats / damages MAF hotwire, etc.

When upgrade fuelpump to 255-300lph, rewire wiring to fuelpump to get 14-15?volts direct from battery (aftermarket fuelpumps need more voltage than the factory 12?volts), the FPR orifice is too small, so fuelpressure rises and get more fuel through injectors when they are open more at mid to high rpm. This requires a ECU retune to close injectors to stop engine running too rich or adjustable FPR to dial base fuelrail pressure back to 35.5psi from 40psi+. ECU tune is the best way.

Also can rewire stock fuelpump with more voltage (I think old factory wiring drops voltage to fuelpump). But if fuelpump has done over 100,000km (recommended km to change fuelpump) or previous owner didn't change fuelfilter every 20,000km or less (depending on how bad the fuel is that you run), it can damage fuelpump.

With checking fuelpressure, use a fuelpressure gauge (compound? gauge) and T into fuel hose inbetween fuelrail, fuelfilter (consult manual first, but I think need to remove fuelpump fuse in engine bay fuse box when engine is idling that should make engine run, then stall to remove all fuel in fuelines and remove fueltank cap to remove tank pressure before working on fuel system). Should be around 35.5psi at idle (It is for RB20DET, RB26DETT, but I think RB25DET is the same) and 7.1psi higher than 35.5psi before starting engine (no vacuum to FPR) or when vacuum hose to FPR is removed when engine is idling.

I think Tomei? made a FPR that can have a little fuelpressure gauge, like shown on this car -

http://www.youtube.com/watch?v=2na_gunvbcQ

Can buy manual from here, which makes working on car alot easier -

http://www.jpnz.co.nz/afawcs0131170/CATID=24/SUBID=273/page=1/products.html

[SlidingSideways]

Thanks everyone for the advice
I took apart my MAF and it looked like this

http://s1152.photobucket.com/albums/p484/slidingsideways1/?action=view&current=IMG_0033.jpg

http://s1152.photobucket.com/albums/p484/slidingsideways1/?action=view&current=IMG_0032.jpg

The circuit board was corroded in places so I bought one from the wreckers, cleaned it up (with crc sensor clean) and it fixed everything