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Car Revs Drop And Come Back Up


SKYTYM
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Hey Guys

Heres the story after 10 mins of driving and i come to a set of lights feels like my car has a heart beat or a pulse the revs go down then back up again. keeps on doing this. Feels like its going to just die on me do u know what is probally the cause of this???? i was thinking the fuel pump??? has any of you guys had this problem its 96 r33 srs2 N/A. and if so is it expensive to fix.

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Get the fuel pump tested, you can test for fuel pressure and flow. Sounds more like you have a 'clean' problem to me, dirty plugs or sensor [o2,tps etc] somewhere.

Don't replace the fuel pump without testing the whole fuel system. Cost, sub $200, probably what ever a service is worth.

Fuel pump, being an N/A, a Walbro will do it easy, around $250 fitted, but get it checked first.

And get your fuel filter cleaned/replaced, could have dirt/water in it.

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yeh i changed all of my spark plugs and replaced the fuel filter which was dirty as and still having the same problem? so im guessing would it be just the fuel pump or coil packs? and whats the o2,tps as you quoted????

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atmo bov?

if u have a standard ecu try ressetting it (unplug battery and pump the brakes or turn stuff on and off to drain all the residual power from the wires) then plug battery bak in

easy thing to do and doesnt cost anything

if not start looking elsewhere

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Hey mate, I think I'm having exactly the problem you are. I posted about it, it's only like the 10th post down lol... still haven't solved it though. There's a link to my youtube video of it as well, have a listen and let me know if the noise is the same.

Happens when my car is idle, warmed up.

My mechanic has told me my AAC valve needs replacing so I'm waiting for someone else to verify that this could be causing this noise? If so, then it could very well be your problem too.

CLICK HERE LINK OMG CLICK CLICK <== Link to my post, containing youtube link.

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yes, most lumpy idles are caused by either the AAC or IAC. mostly the IAC (idle air control). this will happen when you put your foot on the clutch and it idles like a drag car. do a search on here, it has been covered many times before. it is reasonably easy to do. all you need is some carby cleaner and (from memory) a 12mm spanner. the IAC is situated on the back of the plenum near the firewall.

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if the afm is dodgy it would affect the car throughout the rev range. i would be starting with the IAC as it is well documented to cause lumpy idle. there would probably be hundreds of threads on sau about it.

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my car did this.

i had a power FC and it happened when i turned the air con on, and it happened randomly - not all the time.

could be completely unrelated to your problem, but when i turned the air on - revs dropped to about 300 and then it kicked itself back up to 1000, then dropped back down - kicked itself back up - etc.

was a power FC setting problem.

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the AAC (auxilary air control) can also cause hunting when the aircon is turned on (or the steering is turned). while the IAC controls the idle speed, the AAC alters the idle when an accessory is turned on.

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Check: IAC or AAC ( by that I mean clean it out) couple of bolts and some time with petrol or carby cleaner.

AFM. Clean with electrical cleaning spray. I think around $10 so just spray it into the AFM, no need to remove.

If you have a BOV other than standard or GTR then remove it.

Drive the car and see if it does it when you back off in gear then put it in neutral or if you are cruising along and suddenly push the clutch in. Won't be the coils as they would be acting up under boost before they played up at idle.

Always start with the simplest things first.

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Ok can someone explain the difference between the IAC and AAC? By the difference, I really mean where both are located? I thought I'd had a go at removing the AAC from the back of the plenum... is that the right one? So where abouts is the IAC?

I always (stupidly) thought they were the same thing..... d'oh!

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IAC and AAC are located very close to each other. if i remember correctly, the IAC is located above the AAC. IAC = easy to get to. AAC = not so easy to get to. i will have a flick through my workshop manual and see if it has any pics

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IAC and AAC are located very close to each other. if i remember correctly, the IAC is located above the AAC. IAC = easy to get to. AAC = not so easy to get to. i will have a flick through my workshop manual and see if it has any pics

Could the AAC cause boost spike?? If it was failing?? I am working on an R33 at the moment and it seems to idle rough on and off. Get it idling smooth then 10 minutes later its idling like balls.....along with the rough idle I also have intermittent idle hunting after boost. Comes down to almost stall and then picks up again. Does it about 2 times then idle sets again. So couldthe AAC be a possibility?? when I say boost spike too I mean I get alot of lag then all of a sudden a major boost. Very hard to control and very unpredictable.

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Could the AAC cause boost spike?? If it was failing?? I am working on an R33 at the moment and it seems to idle rough on and off. Get it idling smooth then 10 minutes later its idling like balls.....along with the rough idle I also have intermittent idle hunting after boost. Comes down to almost stall and then picks up again. Does it about 2 times then idle sets again. So couldthe AAC be a possibility?? when I say boost spike too I mean I get alot of lag then all of a sudden a major boost. Very hard to control and very unpredictable.

the AAC shouldn't affect boost at all. it only lets in a tiny amount of air. it could be causing the hunting idle after boost but there could be other causes too. when i first got my 33 it was pretty much stock, but if you gave it the beans then lifted off it would nearly stall. it was caused by worn spark plugs. if you have an atmo bov this can be made worse. i only had the stock bov though, so it isn't just atmo bovs that can make this happen if the plugs are crappy. what happens is that when you lift off and the bov opens, with an atmo bov all the air in the cooler piping is released out into the engine bay and this sucks a bit more air through the afm so the ecu puts more fuel in, making it too rich and the spark plugs aren't able to ignite all the fuel. with the stock bov a similar thing happens although it doesn't suck as much air through the afm, but you get some of the air that is being recirculated passing back out the afm, and since it doesn't know which way the air is travelling it dumps in extra fuel also. the fact that your idle drops after lifting off then comes good again makes me think that your problem is like mine and there is a very good chance that new plugs should fix it. and if you haven't replaced your plugs in a while it wouldn't hurt doing it anyway since it will only cost you about $30 for a set of plugs. the part number you want is BCPR6ES. don't bother with iridiums. i was getting similar life from copper plugs as from iridiums with a $100 price difference.

as for the lag, don't really know about that one. that could be caused by a lot of things. could be to do with the wastegate or the boost controller.

as for the AAC valve location, i looked it up again (it's been about 3 years since i had my 33 so i'm a bit rusty on locations of things). the AAC is attached to the side of the IAC. if you look at the back of the plenum you will see a round cylinder about 1 inch wide pointing towards the passenger side with a plug on it, and just behind it what looks like a really long nut with a plug in the end of it. the round cylinder is the AAC solenoid. the other is the IAC. you remove the whole unit with 3 bolts (i think they are 12mm, maybe 13mm) and 1 or 2 of them are located under the unit. then disconnect the plugs and hoses. the AAC i think is held on by 2 small phillips head screws. give the whole unit a good spray with carby/throttle body cleaner. then stick it back together and install the unit back in the car. after doing this it is best to readjust the idle which you need to lock the car into base idle mode. this can be done without a consult but i can't remember how. has something to do with bridging 2 wires in the consult plug.

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Too late for the irridiums. Replaced with platinum plugs gapped at .6mm and new Yellow Jacket coils. AFM has been cleaned and a million other things have been tried.

I have a feeling that the lagging, idling, and boost spike are all to do with the same thing. I am doubtful that there is more than one issue causing these issues. The ECU is a Garage Saurus complete replacement plugin. Unfortunately no one here in Perth knows anything about the Garage Saurus gear. As in no body knows how to access the ecu to re map. I have a feeling thats what it needs.....

Thanks for your help anyway. I will have a look at a few more things. Do you know of anyone anywhere in australia that I could call to pick brains?? I have called most performance places here in Perth.....they all say the same thing, get a Power FC.....but I am not confident on spend $1500 on a Power FC until I know that the turbo or motor aren't shagged. Low compression can cause rough base idle.....yet to do a compression test. Pffft.....stupid R33!! Goes hard but this is frustrating!! :huh:

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atmo bov?

if u have a standard ecu try ressetting it (unplug battery and pump the brakes or turn stuff on and off to drain all the residual power from the wires) then plug battery bak in

easy thing to do and doesnt cost anything

if not start looking elsewhere

I'll have to give that a go, mine has been doing it and it has an atmos + plumb back bov.

My car is known to die though while driving but only when braking quickly after a boot.

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