Rb25det issues, :( help

[RYanR31]

recently transplanted an RB25det into my R31 silhouette, been runnning great up until recently it has started to boosted up to 10psi, which i think is what is causing my flat spot from 4 grand to 6 grand, here is what happens:

say im in second, from 2.5 grand i flatten it, it runs great and pulls hard until it hits about 4ish grand and the boost goes towards 10psi, it doesnt pull as nearly as hard as what it should, but once it hits 6ish grand the boost goes back to 7ish psi and pulls very nicely.

why would the stock boost solinoid be allowing the boost to go higher, im assuming this is causing the flat spot as its detonating and the ecu retards timing and dumps heaps of fuel, it backfires alot off the throttle.

it also missfires alot on cruisinging throttle and it seems as tho i need have my foot a little big more on the throttle to keep the same speed.

[Craved]

recently transplanted an RB25det into my R31 silhouette, been runnning great up until recently it has started to boosted up to 10psi, which i think is what is causing my flat spot from 4 grand to 6 grand, here is what happens:

say im in second, from 2.5 grand i flatten it, it runs great and pulls hard until it hits about 4ish grand and the boost goes towards 10psi, it doesnt pull as nearly as hard as what it should, but once it hits 6ish grand the boost goes back to 7ish psi and pulls very nicely.

similar issue to what i used to have in mine, depends on how defined that flat spot is! what exhaust have you got in the silhouette? my flat spot at 11psi went away after split dump/front pipe and pod filter...

why would the stock boost solinoid be allowing the boost to go higher, im assuming this is causing the flat spot as its detonating and the ecu retards timing and dumps heaps of fuel, it backfires alot off the throttle.

it also missfires alot on cruisinging throttle and it seems as tho i need have my foot a little big more on the throttle to keep the same speed.  

check the tps position inregards to the accelerator thing and the ecu seems to cope ok with 11psi-15psi fine til boost cut comes in, so it shouldn't doing anything too much to it til then!

[RYanR31]

hmm...well the flat spot has turned into a constant missfire from 4.5ishgrand onwards on boost, if i was to slowly accelerate to over 4.5 without boost its ok but its only when its under boost. this morning on the way to work, if i was to flatten it, it would totally cut out if i flattened it. it also had a very destinct missfire on idle, hasnt done it since i turned her off after that.....wierd shit, doesnt like boost above 4ish grand, wierd, any thoughts?

[Craved]

i had similar thing as your issue once.. dont know if it's the same but do you have a boost guage??? i had the worst misfire on boost after coming back from the track once... turns out the pod had snapped off the air flow meter and i was pushing 16psi before it cut out..

just a thought!! make sure its not over boosting!

[RYanR31]

it was going to about 10psi with stock boost solinoid all this week when it was a flat spot from 4grand , but tonight, after the flatspot turned into the missfire, its just gone to 8 then back to 7 straight away. its real wierd, ive had a missfire on idle and cruising throttle for a lil while, its a real bad missfire when after 4.5k on boost. it behaves wierd

[TurkeySlapper]

Hey, hate to hijack but you know, desperate times call for desperate measures.... anyway im doing the exact same conversion myself, RB25 into R31, I'm havin troubles figurin out how to wire up the fuel pump? Its ready to start and run (in theory) but i just cannot get the pump to start! Previously had NICS RB20DET, hooking up the fuel relay wires to the 25 loom just does nothing not happy Jan

[RYanR31]

dunno, i didnt do the wiring.

anyways, back to missfire...nissan wanted $205 for rb25 coils each ROFL!

[JezR31]

would your spark plug gap have anything to do with your missfire? im 90% sure it is.

0.9 is ok for stock boost but for 11-12psi they need to be gapped to 0.8

other things that you could check:

- fuel pump

- afm

- cas

[RYanR31]

hmm...well i put brand new copper gapped to .8 in the other day, but no different.

[rb25]

crapped out AFM? borrow one and test if you can....

[RYanR31]

hmm... could be but im swaying towards a spark problem, im not sure why the boost would go up with the stock boost solinoid.

[deano_82]

how you goin, you fixed anything yet?

[Sydneykid]

recently transplanted an RB25det into my R31 silhouette, been runnning great up until recently it has started to boosted up to 10psi, which i think is what is causing my flat spot from 4 grand to 6 grand, here is what happens:

say im in second, from 2.5 grand i flatten it, it runs great and pulls hard until it hits about 4ish grand and the boost goes towards 10psi, it doesnt pull as nearly as hard as what it should, but once it hits 6ish grand the boost goes back to 7ish psi and pulls very nicely.

why would the stock boost solinoid be allowing the boost to go higher, im assuming this is causing the flat spot as its detonating and the ecu retards timing and dumps heaps of fuel, it backfires alot off the throttle.

it also missfires alot on cruisinging throttle and it seems as tho i need have my foot a little big more on the throttle to keep the same speed.  

My guess is you have 2 separate problems;

#1. The missfire is usually plugs or fuel, since you have changed plugs, my first suggestion would be to check the fuel filter. If the fuel flow is OK, then pull the spark plugs and look for which cylinder is missfiring. Swap plugs and coils and see if the miss is still in that cylinder. Next I would be doing a leak down test just to make sure all is well internally.

#2. The 4K to 6K rpm flattening is always a problem on RB25s' when you run 10psi with the standard ECU. Here is a post I prepared earlier :cooldance

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Hi guys, a quick, simplistic explanation of how an SAFC works might help...

As the airflow into the engine increase, the AFM records this as increased voltage that the ECU sees. What an SAFC does is sit in between the AFM and the ECU and take the voltages from the AFM and either increase or decrease them depending on what you have programmed the SAFC to do. By increasing the voltage, this tricks the ecu into pumping in more fuel, you do this when the engine is running lean. By decreasing the voltage, this tricks the ecu into pumping in less fuel, you do this when the engine is running rich.

The next bit is hard to understand unless you remember that the standard ecu does not supply fuel in direct proportion to the afm voltage ie; 4 volts is not twice as much fuel as 2 volts. Engine rpm, boost and throttle position also help the ecu determine how much fuel to add.

It has been my experience that RB25's run a little lean down low in the rpm range (Nissan do this for fuel economy and emissions) and a lot rich up high (Nissan do this to protect the engine). So I have to increase the voltage (using the SAFC) up to around 3,000 rpm and decrease the voltage over 5,000 rpm.

The real problem is in the 3,000 to 5,000 rpm range as the engine comes on boost. They really need lots of fuel very quickly in this area. This can mean that you need to increase the voltage (that the ecu sees) over 5.1 volts to get the right A/F ratios. The ecu then goes into engine protection mode, rich and retarded (sounds like a girl I knew once). Past that rpm you can start leaning it out as it runs too rich, so the voltage needs to be turned down under 5.1 volts. The ecu sees this as the engine not needing protection mode any more, no more rich and retarded.

So you get good performance up to 3,000 rpm, then sluggish from 3,000 rpm to 5,000 rpm and then good performance from 5,000 rpm and over. The SAFC may not help this, in fact as explained above, it can in fact make it worse if the tuner is not switched on to this stuff.

With bent afm voltages, the ecu (tricked by the SAFC) also fires the ignition to suite the airflow it THINKS the engine is getting. This is not a good thing as you generally end up with ignition that is too far advanced in some rpm ranges.

The poor tuner has to juggle the SAFC settings, so that the A/F ratios are OK, the ecu doesn't get into rich and retard (engine protection) mode and the ignition timing is not too far advanced so as to cause detonation. My experience (I am not a good tuner) has been that this is full of compromises, sometimes you just can't win and have to reduce the boost level a bit to get even a reasonable compromise.

Keep in mind that this explanation is very simplified to make it fit in a reasonable space, the rpm's used are rough guides only and every car is different.

Hope it helps (and makes some sense).

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[RYanR31]

thanks alot, well it wierd because when its cold it runs great without missfires until 4 grand but when its heated up it will missfire badly on idle on 5 and doesnt like anymore than normal accelerating throttle. i can rev it to 5 grand if i want to without boost but as soon as i put the foot down a bit it just missfires constantly while throttle is down. it varies with heat. i am going to sus out the coils n plugs etc. tomorrow for cracks etc. any thoughts? i know im repeating myself here but its very frustrating driving the car like this

[RYanR31]

i also think my TPS is ****ed, very sensative when adjusting it, and very sensative thottle, when i slightly press throttle and the vvt solinoid clicks its very on/off, very sensative when crawling along in 1st.

[deano_82]

yeh i can hear that also while under the bonnet if i touch the throttle there is a distinct click.

i have checked my plugs and all seem ok, but i am going to replace them anyway (even though they are iridiums) i'm just going to put some cheapies in while its running sh!t 7es or something like that?

[deano_82]

i also cleaned my coils but didnt check thoroughly for cracks

i think mine is worse when its cold coughing and spluttering then missing under load its ok if you ease the throttle on all the way to 7 but as soon as you start giving it load at around 4800

[Sydneykid]

Hi Guys, when the engine is cold the ecu supplies extra fuel (rich) to keep the engine running and running smoothly. It sounds like you have a lean engine when it warms up. This is usually a good sign of an air leak. This leans out the mixture at normal running but the richness when cold covers this up. Well, until it warms up anyway.

My first suggestion would be to have a very good close look for leaking vacuum hoses and fittings. There are lots, so take your time, a small split in the underside of a hose is hard to see if you rush.

Hope that helps

[JaFF]

SK, does the powerfc fix the 3000-5000rpm problem?

well with a decent tuner i spose?

[Sydneykid]

SK, does the powerfc fix the 3000-5000rpm problem?

well with a decent tuner i spose?

Shoot yes :aroused: , even straight out of the box (untuned) they sort this out, they have no over boost cut or rich and retard.