troubleshooting S2 RB25... LOTS of info inside

[RPS13-RB25]

i've finally gotten around to finishing my car, and there are some problems that i just can't seem to work out. i'll start with the info.

Car: 1989 Nissan 240Sx (S13)Engine: SeriesII R33 RB25DETTuning: None - Stock RB25 ECU

Mods/supporting mods: -GReddy intake Manifold-Q45 throttle body/TPS-stock RB25 MAFS-KOYO rad with modified Z32 hoses-FMIC with 2.25" hotside and 3" cold side piping-McKinney 3" downpipe and 3" exhaust all the way back.-Nismo fuel pressure regulator.. FP gauge in engine bay

Other Info:-No BOV at the moment. I have a GReddy RS but i haven't mounted it yet

-obviously no emissions garbage is hooked up, but also the heater core lines are looped and there's no AC or Power steering yet

-Nismo FPR is set properly at 43psi w/vacuum unhooked... 38psi w/vacuum at idle.

-timing was set at the fourth/middle mark (15deg BTDC?) following the info found here on Nico

-TPS should read 4.xx volts closed, 0.45-0.55 at WOT... it's at 4.28 closed and 0.46 WOT. i verified this at the ECU as well, the ECU gets the identical signal. wiring is good then.

-no boost controller yet, vacuum is hooked to the wastegate. i've been told by a local nissan shop that it's fine to do so.

-boost gauge pins at 7-8 psi and doesn't move during acceleration

-MAFS has been cleaned with MAFS cleaner

Problem:-The car runs and drives fine for the most part. i can give it partial throttle just fine. as soon as i give 3/4 to WOT, the car tries to give me whiplash. it spits and sputters and cuts out.

- temporary fix... it sounded like a TPS calibration problem. i went to the TPS and adjusted it.

now the car would be fine at full throttle and it felt like all the power was there, but i still had to "sneak up" on WOT. sending the pedal to the floor quickly would make it do the identical thing as before.

- another fix... adjusted the TPS some more and now it was fine at WOT, but it would get higher in the RPM and it would fall on its face. it feels like the power curve just flattened out. maybe a timing issue.. but timing hasn't moved since it was set.

now... i moved on to other things and disconnected the battery to do some soldering work and install the turbo timer. i only touched the 3 ignition wires and ebrake wire needed for the timer install. i hooked the battery back up and now it's back to the same problem i started with. it sputtered and spit at anyhting more than 1/2 throttle. the TPS hadn't moved and neither had timing.

i've adjusted the TPS to read anywhere from 0.43v up to 0.65v and at different times, different readings have worked perfectly and then regressed into the same problem as before. it ALWAYS goes instantly back to running like crap after disconnecting the battery. i know that can't be right.

needless to say... i'm freakin' lost. does anyone have ANY ideas as to what could possibly be wrong? this is getting very frustrating.

[mello88]

i would guess coils failing under load, do you happen to have a spare set to test with? Also check your grounding for the engine and ignition system... Pull the plugs out and see how they look.

Good luck

[RPS13-RB25]

i don't have a spare set of coils. i plan on pulling the plugs next to see what they look like. is there something i should be looking for? some tell-tale signs of something wrong?

the engine grounding thing should be okay. i actually forgot to add enough grounds, but a few days ago i added one extra ground to the battery terminal in the back of the car, one extra ground to the alternator, one from each side of the block, and one to the intake manifold. i went overboard i know, but i wanted to be sure.

the grounding for the ignition system should also be okay, but i'll double check. i guess you mean all of the signal wires through the harness to the ECU, right?

[240z4u]

You should be reading .46 or so with throttle CLOSED and 4+ throttle OPEN. Not sure if that was a typo, but it sounds like your tps wires are backwards.

Evan

[l0nestar]

You should be reading .46 or so with throttle CLOSED and 4+ throttle OPEN. Not sure if that was a typo, but it sounds like your tps wires are backwards.

Evan

Would it even run with the TPS backwards?

[l0nestar]

Verify your grounds, make sure you used star washers with them.

Check your plugs, should be white to tip, not dark or discolored.

Do you have a wideband?

[RPS13-RB25]

well that's one problem. no wideband. i'm pulling the plugs later tonight when i get home so i'll check for the whiteness and hope i don't have dark/discolored.

and no, it wasn't a typo. i read 4.xx with the throttle closed ( at idle) and 0.46 with it at WOT. if i did reverse the wires, would it still idle? my car idles perfectly and the throttle response is fine until 3/4 throttle like i described above.

[eh?]

As already said, you wired the tps backwards. IT will still idle, it will just idle high. It will even idle with the tps disconnected.

[RPS13-RB25]

okay well tonight i pulled the plugs and they look alright. nothing strange.

i ran out of time to work on the car before i could try switching the wires, but just for the hell of it i tried disconnecting the TPS while the car was running. no change in idle. i tried revving the car a bit, expecting the car to bog down and die or something. nope, nothing. it actually revs perfectly and goes to full throttle just fine. i took it down the road and it drives much better, but just dies high in the RPM. again, this is with the TPS unplugged.

in the morning i'll try switching the power/ground and see what happens. thanks for the replies fellas.

[eh?]

Who did the wiring for the tps? I'm sure I've posted the info for the Q45 tps on S2 rb25 somewhere.zerothread/283142
Modified by eh? at 4:15 PM 8/5/2008

[ninety1two40]

I had a simliar issue when my MAF got wet (heavy rain). I had to let the intake system dry for a day and it was fine. But anyway, it sounds like it might be MAF related. I'm running a powerFC and when it would cutout (nasty whiplash too!) it was running the injectors at 100% duty cycle. I could creap up past that "cut out point" but it would do it shortly after, and it only really happens when the engine is under load, I can free rev all day. I don't know if this makes a difference but I'm running my MAF in a blow-through setup. Try testing another MAF if you can.

[RPS13-RB25]

eh? - i had actually wired it up myself according to the info in that thread. earlier today i tried switching the power/ground wires and now it's 4.30 WOT and .55 closed... which is what it should be i guess. there seem to be no more TPS related issues, i can go full throttle just fine. one problem i see there however is that in order to get the proper voltage readings, i had to spin the TPS all the way to the one side. at .55v closed throttle, there's no more adjustment to bring the number lower if i should need to.

now the only remaining problem is the lack of power up high. i found a J30 at the junkyard so i snagged the tachometer. today i wired it all up and i figured out that the car idles steadily at about 750 rpm. more importantly, i found out that the power issue happens almost exactly at 5,000 rpm. it'll rev higher just fine, but it actually slows the car down. i'm stuck there.

ninety1two40 - i would love to be able to try another MAF, but it's the stock RB25 MAF, and i don't really have a way to get another one. i have a Q45 MAF sitting in the garage but that does me no good. haha

[mattleegee]

almost sounds like turbo lag

[Darius]

^^no, not turbo lag^^

The MAF you have seems to be working fine.

Look at the FSM for the TPS settings. You almost have it spot on, but it might need to be adjusted to a slightly lower voltage at closed throttle. I think it is 0.4V. As long as WOT is <5.0V, the ECU will be able to compensate.

If the engine is building boost, pulling strong and then cutting out around 5000, it sounds like it is ignition related. What are the plugs gapped at? Do you have a wideband O2 sensor to see what the AFR is when the problem occurs?

[RPS13-RB25]

haha yeah it's definitely not turbo lag. with the stock RB turbo on there i only have to think about pressing the gas pedal and it's already at 7psi.

darius.. thanks for the reply. i don't have a wideband.. which sucks. it would make things a lot easier. i'm going to go back over everything ignition related today and see if i can't spot a dumb mistake. i'm hoping the wiring is alright.

one major problem is that a local shop (not afraid to say... Import Intelligence) did my wiring and it was hacked up and only about 50% done when i got it back. they refused to fix their mistakes so i had a friend of mine finish the wiring. he had to work through all of their problems and basically start from scratch. it was hard to tell what they had already done and what they had left unwired or wired incorrectly. i'll have him look over it with me.

the other thing is that there is no more adjustment available for the TPS. like i had said above, the TPS is spun all the way over to one side until it hits the adjustment screw and there's no more adjustment to bring the number below 0.55. there's got to be something wrong there.

thanks again for the replies everybody you guys have obviously done this before and i really appreciate the input!

[ninety1two40]

Are you using the Q45 TPS or the RB25 S2 TPS?

I don't much about rb25 S1 wiring but are the two TPS's compatible?

[240z4u]

Are you using the Q45 TPS or the RB25 S2 TPS?

I don't much about rb25 S1 wiring but are the two TPS's compatible?

S2 tps won't fit a q45 tb.

Is your idle high? I am not sure if there is an idle screw that may be holding the throttle plate open a hair. Closing the throttle plate completely may give you the last hair of adjustment you want.

Evan

OR slot the bracket with a rat tail file.

[WhatsADSM]

Not sure if it has been mentioned before, but you will probably also want to boost leak check the car.

[RPS13-RB25]

i'll go ahead and do the boost leak check but ii'm wondering how it could be a problem.. wouldn't i have problems reaching boost and holding it if there was a leak? like i said earlier i hit 7psi and it stays there at all times. i mean i get to 7psi going up hills in 5th gear. the thing spools so incredibly quickly and stays nailed at 7psi until i let off.

[smxtmpr34]

i have a series 1 rb25det and i am getting the same issue, whiplash at wot and is worse as i turn the boost up... does anyone know the correct wire colors that should be connected if splicing?

[rb24t]

ihope your not running the stock 89 sohc fuel pump?

[carmo]

Maybe between the manifold and exhaust it might be time for some dyno tuning. My car did the same thing (fall on its face at higher RPM) and a trip to the dyno shop for a chipped computer solved my problem. My motor was heavily modified however. Port, polish, 20 overbore, cam, headers back and honed out manifold and throttle body. But still, maybe time for tuning. Just a thought. goodluck.