Power loss around 4500rpm (Z32 TT )

[Australian ZX]

My fully imported 91 Twin turboed Zed (modified in Japan prior to importing) has suddenly developed a massive flatspot.

At full throttle the engine starts to struggle ( as if you've lifted ya foot) around the 4500rpm mark. If you lift ya foot back to around the 3/4 mark the engine gets stuck straight back into it and powers on......

On a couple of occasions there has been a quiet backfire during this 'flatspot', and yes whan I say quiet it can 'just' be heard !!

The engine is still reving out but only if you stay away from the full throttle position, this however has meant a slight loss in the acceleration/power the car had a few weeks back....... I've changed nothing, fiddled with nothing, in fact havent even had the bonnet open prior to the sudden apperance of this problem.

I own 2 ZX's ( both of the same year ) so I've been busy swaping bits from one to the other..... here's the list of what Ive done

All electrical connectors cleaned and checked for tightness = slight improvement.TPS (throttle position sensor) swapped = no differanceMAP ( or AFM .... air flow meter ) swapped = no differanceCAS ( Crank Angle sensor ) swapped = no differanceECU = swapped & again no differance

turbos..... posiably the wastegates arent working thus creating an overboost situation where the puter detects a fault and cuts fuel.... ( yes I'm running high boost so there is the posibility that they are carboned up due to the fact that they rarely open )

end result ...... wastegates are fine

fuel pressure is next & Its perfect ......

Coil paks and sparkplugs = swapped and new plugs fitted, still no differance

the cats are collapsing and blocking off the exhaust.... nope !

I'm running out of ideas fast....... HELP

[evildky]

I'd suspect a fuel pressure issue, it's easy to test rpessure at idle while running is a different story, I had a similar problem, it kept coming lower in the power band before I figured otu it was the fuel pump, if you can get a remote fuel pressure gauge and verify fuel pressure under those conditions you might find it drops, at least worth looking into

[Australian ZX]

I hadnt considered fuel pressure as an issue

Reason being that fuel leaks have always been a problem !, Constant small leakages around the filter, and adjacent hoses probebaly made me 'belive' pressure was OK......

I'll do a pressure test even if its just to cross that one off of the list !

[Australian ZX]

Fuel pressure results...... mutter mutter mutter

After sitting for 24hrs presure was @ zero psi

Key to 'ON' position reading jumps to 40psi

start engine, reading jumps to 45psi, then drops to 38psi & then settles to 40psi again.... ( all in 5seconds )

( warm engine up to 'normal operating temp' )

rev engine to 5000rpm, presure reading jumps to 52psi but settles back to 43psi fairly quicklylet engine idle again .... presure drops to 40psi

Drive car........psi reading maintains an average of 43-45 psi during all throttle positions

park car and idle down..... reading @ 40psi

engine stops ( turbo timer ) psi jumps to 55psi then settles down to 43 psi.......Still stable @ 43psi for 1 hour after shutdown.

4 hours after test the presure has dropped to 30psi

end result ......

Its not a fuel related issue

NEXT IDEA ????

[mtcookson]

While driving did you try taking it past 4,500 rpm so that there was a load? Fuel demands will be higher when there's a load compared to revving in neutral for instance.

I'm actually having a very similar problem but on a Z31 Turbo. Still haven't figured it out yet myself but if I do I'll definitely let you know as I bet its something similar.

I've been thinking boost leak but at the same time that doesn't make sense since it only does it at full throttle so I'm still lost.

[TT2+2]

Have someone follow behind you while you drive and see if it starts shooting black smoke when you hit the flat spot, if so, it may be running rich like mine is currently. i talked to a local z guy and he said check for a boost leak so i'm doing that today or tomorrow, i'll let you know the results

[Australian ZX]

I did do a fuel presure test on the road and the presure was stable during that test drive..... 43 - 45psi

There's no excesive black exhaust smoke so its not over rich in the mixture.

There are others suffering the same symptons in other forums too and no-one seems to have found an answer yet !

[sweetdreamZ]

my friends '93 TT did the exact same thing it ended up being the boost control module after ripping all the lines out and putting it back together it ended up beeing the BCM was improperly installed with the overhaul it just underwent. so check up on that