Breakup in 4th gear at WOT

SupraTrbo89

Member
Sep 21, 2006
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West Chester, PA
I currently have an 89 7MGTE (JDM motor) 5spd. Mods include full 3" exhaust (turbo elbow, dp, test pipe, blitz nurspec) , blitz ss bov, intake, new iridium plugs and magnecor wires. The head was also resurfaced with a stock hg and arp head studs.

So the other day I was racing a 300ZX tt and I when I hit 4th gear at about 5k rpm my car suddenly began missing real bad. It missed 3 times in about 5 seconds before I decided to back off the throttle. When the car missed those 3 times it was a pretty pronounced jolt enough to move me in the seat. The car runs very strong through all the other gears.

Even though boost is stock the car gets pretty close to 8 psi at WOT. Is it possible I could have been hitting fuel cut on stock boost?

A while back a mouse had chewed on the black boot of one the of the plug wires. It got into the insulation a little but did not chew into the wire.

Before I just start replacing shit I was curious of the best approach to find the problem. I will probably start with new NGK blues.
 

Quin

Trans killer
Dec 5, 2006
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I would replace those wires anyway but I doubt that was it. If it was hard enough to move you around (plus only happening in a higher gear and I'm assuming this was at night) probably fuel cut.
 

IJ.

Grumpy Old Man
Mar 30, 2005
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Spark blowout, knock the gaps down a bit and try again, if you had some way of tuning timing you could add a few degrees of advance just before Tq peak (5200) and this would cure it if you had the overhead to not detonate it.

Hotter coils/more dwell is an option as well.
 

SupraTrbo89

Member
Sep 21, 2006
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West Chester, PA
Ok thanks for the input....new iridium plugs were just put in when the head was done...probably 1k miles on them. I know there has been a lot of discussion on this forum about the best plugs to use. I will start out by getting new wires and gap the plugs. Should I stick with the iridiums or get something else? Also what should I gap to?
 

Grandavi

Active Member
Sep 25, 2008
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Careful with playing with gap. NGK Coppers = good, but your not running far from stock from what I am seeing so playing around with your gap can be bad. (I cant remember stock gap settings)

I don't see anything, but I assume your running on a stock CT26 and stock injectors? Lots of different things factor into fuel cut and I had that same issue you are having in 4th with my 87 with a shimmed wastegate. It would do that, but was more like 10 psi when it stuttered. Stock boost is about 6 psi btw...
 

SupraTrbo89

Member
Sep 21, 2006
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West Chester, PA
yea stock boost from the factory is 6.8psi but since putting on the full exhaust on a cool night i see the boost gauge approach just under 8psi at WOT. The TSRM says gap to .8mm (.031") and since my car only has BPUs I think I should stick with that.
 

Grandavi

Active Member
Sep 25, 2008
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I can't see it being spark blowout on a 7m with 8 psi boost. I ran 19 psi with stock coils and wires doing just under 500 at the wheels on a oem plugs and gap with no issue.

BPU setup is Lexus AFM with 550 injectors and an upgraded ct26 (usually 57 trim) and full 3" exhaust. If you are running like that, 8 psi is quite low. Should be 10-14.

Because your getting fuel cut in fourth, my guess is your on stock AFM , injectors and turbo.
 

SupraTrbo89

Member
Sep 21, 2006
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West Chester, PA
So then I guess its not pugs and wires if you are able to run that kind of power/boost on stock wires and plugs....Im still probably going to change out the wires and plugs since the one wire was damaged but I'll be looking to move toward lex afm, 550's, walboro 255 and afpr in the near future.
 

Grandavi

Active Member
Sep 25, 2008
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Calgary, Alberta, Canada
Yeah, normally (depending on your altitude) you could go up to 11-12 psi before fuel cut.. but I found that at 11 psi, I was sputtering at about 5000 rpm with my 87 setup like yours with just a shimmed wastegate.
An electronic boost controller can help you control how the boost functions going up that high. The sputtering is just your car protecting itself from a lean situation (the ECU likes to run very rich at high RPM).

Normally you wouldn't hit fuel cut at 8 psi though, but a lot of factors can kick in for that. Im hoping you already have a 3" exhaust because its probably the one thing the MK3 really needs... best upgrade to start.

I also had a sputtering on my red 88 from when I washed the engine bay with an exposed cone filter. Got water on that and it sucked through into the AFM. Caused the car to sputter at WOT for about 2 weeks... so.. there are other possible issues. Getting everything nicely done and no exposed/cracked/broken wires is a good start.

(as a footnote.. I am not a mechanic and just relate experience that I have had through the past years with upgrades/issues...IJ is normally bang on with his advice. Not sure why he said blown spark though :) )