Flat spot when cold

[bruce32]

My son has a 280 ZX with a RB20DET in and it starts and idles perfectly,but as soon as you try to rev it it flat spots,but whenit is released it revs ,we have tried all we know AFM,TPS, air leaks,but it has us stuffed .After about 2-3 mins of idle its perfect for the day, any ideas?

[sweetr33]

why do u want to rev a cold car anyhow?? If its fine after 3 mins idling then let it idle for 3 mins before going anywhere. Boom problem solved

[tntdan]

As I know it, when then engine is cold, the ecu tries to stop the car from making any boost. It is not overly noticable and can easily be overcome when the revs rise. It seems to be nissans way of protecting cold turbos.

[Cubes]

Mine does exactly the same thing. I've seen another R32 RB20DET that also does the same thing.

If you drive it with that flat spot it tends to bring boost up really easy when cruising on light throttle, lots of smoke (black/grey) will be seen in the rear-vision mirror if you give it a little squirt in first or second.

Mine has done it for the last 60,000km's. Nothing to worry about, just warm it up for 3-5mins before driving it. It will also hit boost cut ALOT easier when cold.

When cold and idling if I attempt to raise the rev's slight by pressing the accelerator really lightly the rev's will die and cause the car to start running rough until I either press the accelerator slightly more or take my foot off.

I've also tried everything, there is a possibility of injectors?!?

There is no direct way the R32 ECU can reduce boost when cold as the RB20DET uses the standard plum from infront of the turbo to the wastegate with no solanoids inbetween. The only way be it intended or not is to bring the boost cut in a little earlier. I'm running upwards of 1bar of boost on the stock ecu so i'd say that is my problem.

[bruce32]

Mates thanks for the feedback,looks like its reasonably common, although its frustrating when youre trying everything though and nothing seems to improve it,the thing goes like a scalded cat when warm, so it seems he will just have to warm it up first ,just like the first rotaries.

[blind_elk]

Is this while being driven, or just free-revving?

If the former, I'd check the ignition timing.

[Kinks]

My car does the same thing. If you drive it off gently when cold you get to about 3k RPM and are trying to just go a bit faster it seems to really die in the arse. If you put your foot down a bit or let it struggle there for a few seconds it'll get through the dead spot and come good again. It really doesn't feel nice to drive when cold though! I usually try and short shift just before 3k - it has another not-perfect spot around 2.5k but the warmup period is only about 3-4 mins so i'll deal with it if it's a common fault with the ECU's tuning and not something that can be simply fixed. Contemplated cleaning the AFM but had my doubts it would make a difference.

When warm, I can sometimes feel the same dead spot but it is far reduced. Most of the time I only notice it when it's cold.

[Cubes]

I also notice a slight dead spot in the accelerator when warm and idling around in first on and off the accelerator (looking for a car park in shopping centres) Hardly noticible, its so so damn small.

My old VL 5 speed NA 3ltr had it pretty bad, normal street driving you wouldn't notice it, in car parks looking for a park it was a real pain in the arse.

Come to think of it so did my VS V8 5speed. But not when cold.

Has anyone thought of the O2 sensor having problems with its preheat?

I threw my car in diag mode with the o2 sensor flashing on the dash.

As the rich lean rich lean started to flash the flat spot went away.

Also.. Nothing related as the flat spots always been there BUT i've started to loose approx 70km's to a tank of juice. I am barely scraping in on 400km's to a tank since the last 15,000km's. Previously I would hit 470km's to a tank then if I did a little bit of 100km/h cruising I could easily break the 500km to a tank.

[xRHETTx]

If it only happens when the car is cold, its probably the ecu trying to stop the turbos making boost whilst the oil is still cold. My car does it, until the oil pressure gauge starts falling the car makes no boost at all, which is a good thing and i dont know why anyone would want to change it.

[paulr33]

I've noticed this on my r33. When its cold and you first drive it its like its an N/A for about a minute or two, then you feel the boost come on as normal

[Mase]

same with my rb20det onevia. It's good to know it's a general problem and nothing serious

[bruce32]

Today we tried an aftermarket computer on it off our mates car and it was driveable straight away , no flat spot ,he never gives his car stick when cold,but it shows that MR. NISSAN didnt want the car driven till warm,so its back on with the standard one and wait 3 MINS,thanks for the feedback.

[WazR32GTSt]

contray to previous posts, i HAD a serious flat spot when cold in my car also...

did an ECU diagnostics test and it came up with the code for knock sensor. so i had a look at the knock sensors and one of them was unplugged! so i put it back in, and presto - no more flat spot! worth a try as it only takes 5 sec to see if the plug is in

(note: plugs located just below intake manifold. one is in between runners 1-2 and otehr 4-5. they are green plugs with a white clip that has to go into it)

good luck guys,

Warren.

[Cubes]

If it only happens when the car is cold, its probably the ecu trying to stop the turbos making boost whilst the oil is still cold. My car does it, until the oil pressure gauge starts falling the car makes no boost at all, which is a good thing and i dont know why anyone would want to change it.

When cruising and the flat spot is apparent I can hear the turbo spooling & the boost guage is definately reading 1psi or so. It doesn't stop the car making boost. If anything it for some reason makes the car make boost easier and earlier. It feels like the car is sputtering without the missing the car starts to make boost then all of a sudden it comes out of the flat spot and off you go with a bit of fuel smoke behind you.

Upon cruising say 60km/h you watch the boost guage when you feel the flat spot and it will be making boost not vacuum as it should when cruising.

I remember when the car didn't used to do it.. If anything the car used to feel punchier when cold.