SP ENGINE MISS AT 4-4,500 RPM

[blackster]

One way you could try it, is block of the return in and the leave the Blow of side in free Air and see if there is a difference

Probably not a good idea as VTA on MAF sensored car's will cause idle issues, there is an associated "double dipping" effect. More fuel in the absence of re-circuated air.

The SE plumping seems very familiar to the SP but they dont seem to have the same problem. Maybe with the addition of the boost sensor to the SE Mazda was able to do some trickery in the ECU tuning to avoid the over fueling?

OEM plumbing on the SE has its recirculated blow off a long way from the MAF sensor, so reverse back flow is not an issue. On some aftermarket short RAM CAI for the SE, there is an issue with re-circulated MAF reversion causing idle issues but not an engine miss to cause black soot.

To the op

I am not sure if an OBDII is applicable to the SP, if so it may able to isolate the misfire?

Cylinder (injector, coils, plugs)
Leak (pre or post turbo)
Tune (open/close loop?)

[Rotary]

One way you could try it, is block of the return in and the leave the Blow of side in free Air and see if there is a difference

Probably not a good idea as VTA on MAF sensored car's will cause idle issues, there is an associated "double dipping" effect. More fuel in the absence of re-circuated air.

Please explain why there would be idle issues? Isn't the Blow off valve closed for idle therefore no recirculation or lost air,
and so no effect on any other system

There will be a condition of extra fuel, because of the MAF Counted Air Mass
but this condition only happens when the BOV is open, which is closed most of the time, and mostly open when off throttle.
It does deppend a little on how the system is set up and BOV(i.e. exactly how soon Venting starts), and so may work or may not
i don't know much about the SP BOV, but on other cars iv'e seen that fit External Venting BOV's with stock ECU's and MAF's there were no recirculating air issues (but maybe the difference in Valve adjustment could make a difference or or some other ECU setup difference)

If an SP will work with an Externaly Venting BOV(as a lot of cars do) then that would solve the problem and there may be a chance what i suggested works without the extra expense. if someone has tried a venting BOV and it didn't work with an SP, then it won't

[Garry]

The SP BOV is a plumb back design so it only closes under boost conditions, ie it is open at idle.

Blackster,

When I was looking around the SE engine bay the return port on the turbo inlet pipe for the BOV seemed to be not that much further away from the MAF than it was on the SP. From memory it's about 180mm on the SP.

[blackster]

When I was looking around the SE engine bay the return port on the turbo inlet pipe for the BOV seemed to be not that much further away from the MAF than it was on the SP. From memory it's about 180mm on the SP.

Recirculated blow off is attached a long way from the MAF.

Please explain why there would be idle issues? Isn't the Blow off valve closed for idle therefore no recirculation or lost air,
and so no effect on any other system

for those of you whos cars have a MAF and an atmosphere-venting valve, you would promptly find the car would stall. The valve MUST be closed at idle, and it must be closing as the revs are dropping.

I believe brett is refering to the type aftermarket VTA used, alot of them are piston controlled by a spring. Unfortuantly spring load isnt proportional to the amount boost/vacuum at different conditions (idle, partial, full, off throttle) causing them to leak and bringing upon idle issues. I guess that also explains a degree of compressor surge, as spring/valve opening is not proportional to the amount of boost at any one given time.

FWTW, again I believe the SE ECU still thinks its getting that air, as its returned after the MAF. So in a VTA set up you get spluttering and poor throttle responce comming off/on throttle as there is less air and more fuel.

In a VTA setup ideally I would be looking for a valve that is actuated by pressure alterations, unlike spring type that are pushed open slightly under boost. That way you ensure quick valve response and complete closure during idle. However if the car is ECU/MAF dríven you may still get problems with air/fuel. Vacuum leak isnt an issue with recirculated bovs as air doesnt escape to atmosphere.

[Garry]

I had another look at my engine tonight. The BOV exit port is even closer to the MAF than I thought, only about 110mm away. So you are right as always Blackster, the port is a lot further away from the MAF on the SE.

[Gdekwant]

from what you guys ahve said it sounds like the same problem the nissan sr20 motors experience and hence why alot of people change to a MAP (manifold air pressure or something )sensor as opposed to airflow which stops the ecu from thinking theres more air than there really is.

I dont know if this is an option for an mx5 but surely its somehting to look at.

[Boags]

Manifold Absolute Pressure.

You need to change ECU to allow you to run a MAP sensor though...

[fredbareoz]

Thanks for the input lads.
To this point we have spoken to Lachlan at Chip torque in Qld about the problem, he has been pretty helpfull but as he is a bit of distance from Melbourne he is just guessing.
We are still suspecting a problem with the blow off valve at present. (Fingers crossed)
I have a customers SP in here and he has agreed to leave it with us for a few days so we can compare the two cars.
His car has done 130k and is a ripper, no jerking, excessive black smoke, goes well and hard, everything mine SHOULD be.
Will keep you informed.

Al

[bruce]

His car has done 130k and is a ripper, no jerking, excessive black smoke, goes well and hard

Stock as a rock?

[Garry]

Let us know what you find Al

[fredbareoz]

ITS FIXED.....

Bloody blow off valve... swapped over mine with the other Sp i have and it's beauuuuuuuuuutiful...
Even got some more boost.
Just waiting for my replacement unit to turn up.
*waiting*
*waiting*
*waiting*

[Garry]

Yay! Gotta love an easy fix

[NMX516]

Those Bosch BOV's really are junk... What make and model are you replacing it with Al? I've used a Turbosmart kompact BOV on my other car and it works a treat. I intend to get another one when I find an SP to put it on!

[fredbareoz]

what make? Errrrr in the interests of a quick fix, i threw another one of those dodgy Bosch units.. *gulp*
I wanted to find out more about the Porker one, but time was the factor.
I believe Garry has said somewhere else on here that he used the Porker one and was pretty happy with it. The problem is, which one?

[NMX516]

Al have a read of these couple of threads re which one:

viewtopic.php?f=81&t=34458

...and the last two pages of this thread:

viewtopic.php?f=18&t=19189&st=0&sk=t&sd=a&start=135

As you'll gather reading those, I'm a fan of the Turbosmart Kompact BOV. Adjustable, and made of metal so no cracking ala the Bosch ones.