Thread: Need some help with S20DET, stutter-rev issues...when warm

So I have a weird miss issue that only happens when the engine is warm. The car idles and maintains an idle at 900rpm without trouble.

When I first start the car, it will rev just fine. Like simply revving from 1.5-2k or 2.5-3k, however, once the engine is warm in operating temperature, it seems to miss or stutter when you blip the throttle. The issue gets worse as the engine gets warmer. And it is reflected in poor driveability during hard acceleration. when trying to accelerate it will just load up real heavy then finally snap up into power. Feels almost like a 2 stroke in the way the power comes on. Then when you shift into the next gear, it totally falls out of the power band and has to spool back up from nothing and is very laggy on power. (it only does this when warm, when tested with a cold engine (before reaching op temp), it snaps into power immediately upon shifting into next gear and lights up tires)

I have rebuilt SR with 1k miles on it, JWT ECU, Splitfire ignition system, 740cc injectors, Z32 MAF, GTX2867R, S3 cams, NGK BKR7E plugs gapped to .025, and regular bolt on stuff.

I have cleaned the MAF, cleaned the IACV, and checked for vac leaks.
Plugs looked good, all a light brown/bronze/tan color, all equal across all 4 cylinders.
All the gaskets are quality OEM, all on the engine are less than 1200 miles old. All were attached using brand new hardware, locking nuts with locking tabs. Intake gasket is brand new as well.
Base timing is set to 15*, set just a couple weeks ago.
TPS reads .5v when closed and 4.1v when WOT
I replaced the Coolant Temp Sensor with a brand new OEM Facory Nissan part, and still the exact same thing. Revs great when cold, then begins to stutter and miss once it reaches op temp.
Checked codes, only code I get is the 55 "All Clear" Code.
I also checked BOV, BOV is in good shape.
I back probed the ECU at the CTS signal and confirmed that I am getting the appropriate reading there as well....1.2V.
Also the problem is the same with the O2 sensor plugged in, and unplugged.
Replaced o2 sensor with brand new OEM unit, same problem.
Swapped ignitors with a known good ignitor, and then tested my ignitor on another engine...same problems persist.

Whats next?

Not sure if its related, but the engine also doesnt want to maintain a regular idle when the car is rolling. It seems weird to say, but if the car is rolling (not even using the brake), just rolling to a stop light in neutral, the car struggles to idle at like 500rpm. Once the car comes to a complete stop, the idle jumps up to 900rpm and runs perfect.

Any ideas? Or how to test?

I haven't had it on a dyno, or have any real way to data log it, but I am trying to get on a dyno soon to watch it.

Alright, here's a video that gives a pretty decent idea of whats going on. First 3 clips are of the engine cold, then a quick 2nd gear pull, then 3 clips of the stutter/hesitation issue after driving it for 5 min and doing a couple pulls. But if you notice, boost doesnt come on until like 4k and power doesnt come in until 4.5 or higher, which seems way laggy to me.



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Fuel pressure?
AFRs?
Boost leaks ?
Vac leaks ?

No boost/vac leaks, checked a few times.

AFR at idle is 14.5-15.2ish, pretty regular. When I stab the throttle to rev, it drops to 10.x, but that makes sense to me.

Fuel pressure is at 44ish, roughly 38ish at idle with vac line on IIRC.

Ditch the JWT and get a nismotronic. It sounds like your problem is tune related.

Fuel or spark will be the main culprits (based of temp for instance, the boots on the coils could be separating or something along those lines).

However, I am leaning to a spark/electrical issue if the A/F checks out (although10:1 on dip in seems to be on the rich side on tip in IMHO).

I would check / replace the coil packs.

Maybe something up with the tune? Can you get your hands on a consult scanner and check the sensors/ecu?

Ok

I have cleaned the MAF, cleaned the IACV, and checked for vac leaks.
Plugs looked good, all a light brown/bronze/tan color, all equal across all 4 cylinders.
All the gaskets are quality OEM, all on the engine are less than 1200 miles old. All were attached using brand new hardware, locking nuts with locking tabs. Intake gasket is brand new as well.
Base timing is set to 15*, set just a couple weeks ago.
TPS reads .5v when closed and 4.1v when WOT
I replaced the Coolant Temp Sensor with a brand new OEM Facory Nissan part, and still the exact same thing. Revs great when cold, then begins to stutter and miss once it reaches op temp.
Checked codes, only code I get is the 55 "All Clear" Code.
I also checked BOV, BOV is in good shape.
I back probed the ECU at the CTS signal and confirmed that I am getting the appropriate reading there as well....1.2V.
Also the problem is the same with the O2 sensor plugged in, and unplugged.
Replaced o2 sensor with brand new OEM unit, same problem.
Swapped ignitors with a known good ignitor, and then tested my ignitor on another engine...same problems persist.

Whats next?


I have a consult-port datalogger Nissan Datascan now. I can send logs? Anyone up for trying to help me figure it out?

What is the maf ground resistance?

What is the maf voltage doing when the car is hesitating?

What are your spark plug gaps at?

What is the vehicle speed seen as in the ECU? I had a similar issue when I rewired my factory gauge cluster. The Vss wire to the ECU was connected incorrectly

With the engine warm, check resistance across the injector pins. Should be around 11-12 ohms. I had an injector go weird on me, was at 45+ ohms when tested. I would randomly run on 3 cylinders.

Another test to do is lift the fuel rail with the injectors in place and prime the fuel pump. See if you're leaking fuel past the o-rings. Be careful when doing this on a warm engine, can be dangerous.

Remove the coils from the head and inspect for cracks in the plastic body, or splits down by the rubber boots. May be a bad coil.