98 R34GTT 4k engine stumble

[JC71]

98 r34 gtt

Been rocking my latest setup and car running like never before.

Have put a good 200 miles on it since all the latest changes and additions.

everything is dialed in and have been driving it a bunch.

 

However, all a sudden last weekend as soon as I pushed throttle passed 4k and the engine stumbles, (slow or fast acceleration) hesitates and doesn’t go above 4100 or so, like a misfire. Everything else seems fine.

I had a “good” set of coil packs that I removed from car when I first got it when I changed them over to new coils.

I swapped out current ones for those, but no change. I also changed the plugs, no change.

Seems to rev over 4 when out of gear with no load just fine no “misfire”, but as soon as its in gear with a load of any kind, it “misfires” as soon as rpm drop back below 4 k it runs perfectly, smooth, starts, restarts and drives fine as long as I keep it below 4 k while in gear.

All readings look fine, no CEL

 

Any idea as to what could all a sudden cause an issue when pushing rpms passed 4 k?

[GTSBoy]

It's either an ignition failure or a fuel failure.

[Spearzy999]

Hi mate I am having an identical issue with my 98 R34 GTT. The engie suddenly cuts out under load at about 3-4k, black smoke out exhaust. Almost feels like traction control kicking in. No CEL but still going to check codes later today.

Car was sitting in the shed for about 5 months before hand and only idled a few times, took her out with about 1/4 tank, filled up and thats when the issues started happening. 

Threw on a spare set of coils/harness and checked plugs which was no help. Recently had fuel pump/filter changed so surely thats not the issue unless condensation in fuel, I just havent had the time to drain it

It's running a hi flow turbo, bosch 1000cc injectors, walbro 450L, Z32, Nistune etc... All about 2000km old.

[Yeetus]

Run some injector cleaner through it

I had a similar issue that plagued me for months, even tried replacing the cat. If one or more injectors are blocked up the ecu sees ping so pulls timing and makes the tune rich overall, you'll see black smoke coming out of the exhaust and like me assume it's running rich but it could only be one or two cylinders 

$10 bottle of injector cleaner and the next time I hit boost it pumped out a huge plume of black smoke, cleared the blockage and then ran great, if that's not the issue it's only a cheap thing to try haha

 

[GTSBoy]

1 minute ago, Yeetus said:

Run some injector cleaner through it

I had a similar issue that plagued me for months, even tried replacing the cat. If one or more injectors are blocked up the ecu sees ping so pulls timing and makes the tune rich overall, you'll see black smoke coming out of the exhaust and like me assume it's running rich but it could only be one or two cylinders 

$10 bottle of injector cleaner and the next time I hit boost it pumped out a huge plume of black smoke, cleared the blockage and then ran great, if that's not the issue it's only a cheap thing to try haha

 

Not a terrible idea. But if I ever had concerns that my injectors were sufficiently dirty to be causing me a problem, they would be out of the engine and onto the actual injector cleaner/testing bench.

[Yeetus]

1 minute ago, GTSBoy said:

Not a terrible idea. But if I ever had concerns that my injectors were sufficiently dirty to be causing me a problem, they would be out of the engine and onto the actual injector cleaner/testing bench.

Given what I know now - same haha, but this was 12 years ago with second hand 740cc Nismo injectors in my S15 daily, I was too busy hunting elsewhere for the issue to even look at the injectors, running through some cleaner was a happy coincidence haha

[Duncan]

Given it happened suddenly, and when you come on boost, I'd check all your inlet - hose clamps in place and no split rubber or silicone hoses. Even the hard to get to ones near the intercooler. It's possible air that went past the air flow meter is being dumped out a big hole.

[JC71]

I had double clamped all the pipes but I checked them over any way, everything tight and secure.

feels and sounds like a electrical short/failure, as it is an abrupt and aggressive stumble, that instantly comes and goes right at 4k mark...like clock work

fuel pump and filter are new

new coils and plugs

injectors where cleaned and tested 200 miles plus ago

no black smoke

cleaned MAF

 

same results, other ideas?

some other sensor?

 

Edited Wednesday at 08:28 PM by JC71

[MBS206]

You now say stumble, but you said cut out before.

Will it slow rev through the cut/stumble?

What gap were your spark plugs set to when you them in?

[JC71]

neither stumble or cut really seem to be an appropriate term....hard to explain its like a rev limiter but at 4k, but it violently shakes engine and entire vehicle as the rpms will not rise over 4k, even with slow acceleration. as soon as it hits 4k, it sounds like entire spark is lost entirely. plugs were 1.1 which I used as such, but later put in new plugs gapped down to .8 changed back after issue arose when I replaced the coils, still does it with either plug gap...damn and it was all running so good.

Edited yesterday at 12:43 PM by JC71

[JC71]

forgot to mention that when it does this it does not kill the engine, but I have not kept the pedal down long enough to let it, as I see no reason to beat up on it as its got to be something simple, small and dumb causing this.  And as soon as I let off pedal, the engine returns to its normal super smooth operation and rpms re-stabilize quickly, once below 4k. 

Edited yesterday at 05:22 PM by JC71

[MBS206]

List all of your mods.

It sounds like an aftermarket ECU hitting rev limiter. Like a valet mode, or a cold rpm limiter?