Slow Rb25. Need Help Please

[RB30 Ceffy]

Okay. I’ve had this RB25 in the car for a while. I did the conversion myself but Ive done many others with success. The car runs like a champ, doesnt miss a beat and starts fine. No codes come up on the ECU and I have plugged in the Nissan consult and everything seems fine. Timing is good, TPS is good and I have even gone thro every sensor diagnosis procedure listed in the factory Nissan service manual.

Now the car sounds fine driving like a normal person but when your foot gets heavy and you want to go fast it just sounds like the exhaust is plugged up or something. I have normal power in low RPM but when I get up to 4000RPM it just feels held back. The engine sounds fine but you can tell by the exhaust note that’s is just not right.

I had a RB30 in the car before and it sorta felt like it had the same problem. I didn’t think much of it though as I was swapping in the RB25 so I could sell the car but thinking it was the same problem as the previous engine I thought it could be something wrong with the cars components like intercooler, exhaust and fuel system. So I unbolted the exhaust and took it for a rip and was still slow. I checked fuel pressure, changed the filter and upgraded to a high flow pump and still slow. Then I took the intercooler out and tested it and cleaned it out with cleaner and still slow. I have took apart the entire engine harness and pretty much rebuilt it. I did find a bad ground for the o2,TPS and AAC so fixed that but still the car is still slow.

I have done a basic tune up as well like plugs, packed the coils, fluids ect. I have also took the intake off the turbo to see if my turbo was still moving freely and seems just fine though one thing I haven’t checked was if the exhaust blades are still on the turbo but I still build boost like normal so didn’t think that was necessary. I have ign timing set right at 15 deg. Another thing that isn’t hooked up is the neutral switch to the ECU. Is this a problem??? I have done other rb26 conversions without neutral signals in race cars and they were fine with factory rom tuned ecu’s

This problem is going to be retarded, I know. Something really simple but I am running out of ideas. If you guys can think of anything I can check over please let me know. Any tips are appreciated.

Edited September 9, 2007 by RB30 Ceffy

[XKLABA]

When you dropped the exh did you drop the cat aswell ?

Does the boost stay up in the high rev range ?

I had some thing like that with the GTR the exh didn't sound right and felt flat and was losing boost turned out to be the cat.

[RB30 Ceffy]

Ya, I pulled the whole exhaust. Im also running a test pipe. Boost stays at .6 bar pretty steady up to redline. :S

[Dale FZ1]

Slow is relative.

Now you've spent quite some time and still feel it is running weak at higher rpm and load, you should really have little problem buying 30mins of dyno time. It should provide some additional insight as to whether it really is running weak, or if you need to be reassessing what is slow or not.

[RB30 Ceffy]

Well I guess I have to hope you can take my word on this one. I personally dont need dyno time to tell me its a slow car and down on HP. A factory rb20 can eat this alive right now Ive done over a duzon other rb25 conversions and this one just doesnt cut it unfortunatly. But I see where your coming from. Dyno sheet would help me see where the problem might be coming from. If all is bad I might have to consider this but I dont see how I should be able to fix this in the shop. Thanks!

[Richieb]

Fuel pump not cutting it, as in no pressure, or Coil-pack problem.

[MBS206]

Have you crushed a fuel line somewhere?

Has it been tuned?

[Guilt-Toy]

what about the diff ratio ?

what gearbox are you using?

remember rb25's in skylines had 4.11:1 and 4.39:1

in australia people make the mistake of putting the rb25 into the VL Commodore with the stock 3.45 ratio or whatever it is and wonder why its slow as hell but it will go up to 200ks an hour in third gear.

check your ratio's!!!

[Johanz]

i'm having the exact same problem. everything runs right but the power is not as what an RB25DET is supposed to provide... boost reads nearly 1 bar on the boost meter but feels like RB25DE.

if Guilt-Toy is right, then i might have to change to a stock RB25 rear diff as i'm running stock Z31 300ZXT auto rear diff, 200kmh @ nearly 6500rpm @ 5th gear...

[RB30 Ceffy]

Car is a A31 Cefiro. Diff is a factory A31 unit. 4.11:1 diff and RB25 trans. The gearing is as normal as any other factory nissan.

I have changed the factory fuel pump with a nismo unit to see if that was my problem about a month ago. Pressure is fine and all lines are normal. Changed the filter aswell. Still the same after all this.

Ignition is fine, coils are mint, 180 and 1 deg signals from CAS is good. Timing is fine.

I have tried to advance the timing 5 deg and still slow. I tried to boost the thing to 1 bar and still slow.

Edited September 10, 2007 by RB30 Ceffy

[Guilt-Toy]

i'm having the exact same problem. everything runs right but the power is not as what an RB25DET is supposed to provide... boost reads nearly 1 bar on the boost meter but feels like RB25DE.

if Guilt-Toy is right, then i might have to change to a stock RB25 rear diff as i'm running stock Z31 300ZXT auto rear diff, 200kmh @ nearly 6500rpm @ 5th gear...

That diff ratio seems short and it will make the car feel faster because it will be accelerating quickly.

Car is a A31 Cefiro. Diff is a factory A31 unit. 4.11:1 diff and RB25 trans. The gearing is as normal as any other factory nissan.

I have changed the factory fuel pump with a nismo unit to see if that was my problem about a month ago. Pressure is fine and all lines are normal. Changed the filter aswell. Still the same after all this.

Ignition is fine, coils are mint, 180 and 1 deg signals from CAS is good. Timing is fine.

I have tried to advance the timing 5 deg and still slow. I tried to boost the thing to 1 bar and still slow.

:(

Take your CAT out and check it. maybe its blocked? do you have any air leaks in the piping when on boost? maybe it only leaks on boost ?

have you put it on the dyno and checked the air fuel ratio ? i would be doing this after checking the turbo and cat

[RB30 Ceffy]

cat is fine.Dont need cat's where Im from

But ya. I guess I will dyno it and see what I can come up with. I dont have any boost leaks. All pipeing is fine and pressure tested it just a couple weeks ago to make sure. Didnt leak to 80PSI so thats good

[acsplit]

Have you had it on a dyno??

4000 rpm - 5000 rpm is boost cut territory and i have seen cars on std boost have this issue.

It will go rich on the dyno and take timing out

[RB30 Ceffy]

Im going to book a dyno run maybe later this week if I have time. I was thinking along the same line as you acsplit. It feels like it pulls timing up top. Thats what made me think I had a sensor on the fritz but Ive checked it all and nothing comes up on the consult. I also thaught that the 25 would need the factory boost control solenoid. So I tried to plug that back in and with it not in and still ran the same either way. Confuses me lots but im all out of ideas. I'll get the dyno sheet up as soon as I get er done but for now keep shooting off ideas guys. Thanks!!!!!

[Az_StrYdeR]

Is this a standard setup or has it been modified?

If it has been modified, can you put up all the changes??

[BAMR33]

+1 for boost cut, try turning down the boost.

Might also be the wastegate actuator.

[munro'sgts-t]

hey mate i know this may sound way off track but it may help, u say its higher up in the rpm coul dit possibly be the vct on the rb25 not switching over to give it more or less timing ect? im dnt no a great deal about this stuff but am slowly learning and thought that this may be your problem. hope it helps

[mickr33]

Is the problem present when the boost is turned down? This could possibly be rich and retard (R&R) as my car was doing the exact same thing and a SAFC solved the problem. Hope this help's

[Mike_]

Since we're throwing ideas out, have you tried a new air filter?

Know of a similar problem solved by cleaning/replacing the air filter.

[robots]

Any news ?