Stock ports, stock intake, stock ECU full RB true duals header back to RB mufflers. Running 87 octane and 1oz of idemitsu per gallon of fuel. no OMP

The other day I encountered something strange while on my way back home. The car seemed to stumble just a bit around the 7500rpm mark. I immediately let off throttle and attempted to replicate the issue which did occur again around the same RPM. This was in 2nd gear.

Any thoughts or idea's? Engine has approx 72000 miles.

Thanks, -M

 

Check your spark plugs.

 

If the car runs smooth & fine otherwise under load and varied RPMs, my first guess would be the ignition system is breaking down at high RPMs where it would be stressed the most. Try a fresh set of spark plugs & plug wires first and see if that improves things.

One other thing to add - I'm pretty sure the stock FC ECU has a rev limiter feature that cuts fuel/spark when the limit is reached. Shouldn't happen until 8K RPMs or so, but perhaps your tach is inaccurate (reading low), and you were just hitting the rev limit when it stumbled?

 

if I remember right the buzzer will still come on when its supposed to

OP- have you ever heard the buzzer?

 

The plugs are new with approx 100 miles on them. I did not change the wires however and NEED to. I have only ever heard the coolant buzzer and it's associated light when installing a new radiator. I have never heard the shift buzzer like I did when I owned an 86 GXL.

Is the coolant / Oil buzzer the same buzzer for shifting? Thank you,
-M

 

very possible i'm hitting rev limit and the tach is incorrect. I've never heard a shift buzzer in this car since i've owned it since NOV 17'. My 86 GXL had the buzzer, my assumption is this car has a disabled buzzer. The coolant / Oil buzzer does work because i've heard it during a radiator change / bleed.

 

whats the redline on a na s5 anyhow? any reason to even be revving that high? cant still be making any power?

 

The physical gauge shows 8k as the beginning of the redline. I am unsure what the ECU programming will allow before cut. I'd have to agree, there is probably not much more power up at that RPM.

 

8k. On mine the buzzer goes off at 8k and the rev limiter kicks in at 8500rpm. Maybe the OP's buzzer is disabled like he suspects. When I had a stock port power noticeably fell off around 7500. To the point of not really being any reason to rev any higher. I had the RB collected header pre silencer system with HKS sport cat back at the time.

 

Thanks for the info guys. Much appreciated

-M

 

Replace your fuel filter too.

 

Good call ! I have yet to do that. Thank you

 

Din't forget the oft overlooked/ignored in-tank fuel sock.

 

the buzzers are different.

if you haven't checked the wires yet, these are good candidates. new they will be in the 7k Ohm range, and the upper limit is 16k Ohms. the higher the resistance the less spark energy you have, so at 16k Ohms if it isn't misfiring at high rpm, its about to.

another place to look is to check for codes, i've seen TPS codes only come on at WOT, and it will cause a hesitation/cut, CEL may or may not do anything

it is NLA from Mazda, use Denso number 952-0043 please

 

Thanks very much for the information!

 

Fuel cut is somewhere around 8300RPM I think. It could also be a weak fuel pump.

 

My 91 NA reliably revs to 8700 with no fuel cut (it does run out of breath and stop accelerating at that point on stock ports however). My understanding is the factory ECU cuts fuel at 9000, which I can't speak to, since I can't get there. My shift warning buzzer sounds at about 78-7900rpm, which I understand is about right, it's supposed to sound before you hit the theoretical redline, since it's easy to blow past.
Besides the plugs and wires, check the engine grounds, since poor grounds can also cause weak spark/ignition breakup.