To follow up, we had a Kiggly Street Steel spring crack and break with break in miles on the car. I have reached out to Kiggly for their opinion.Valve is bent, Head is unknown (haven't pulled yet, just bore-scope). Valve guide is probably cracked, piston is ok.Lucky it fell in the stroke...

Problem possibly solved.Took off the valve cover and noticed a rocker seemed out of place.Valve spring had broken.

New harness installed.No change :(

Update: Found wires leading to coil connector had split under the loom, someone had tied in an aftermarket tach at some point.Replaced wire, no change.Ordered new harness.

Update. Redid ecu ground, replace capacitor. Added 2 more grounds.No change.

Small update: The wire to that capacitor is frayed. Will try a new one.Also going to check the main ECU ground for corrosion at the pin.

Found multiple posts that state it is just there to filter electrical noise from the radio.Then I found one single post, that is having the same issue I am, due to this capacitor.I will check it and update.

I did check this, if you look a few posts up, I had the same idea LOL. Makes sense. But voltage was fine ~13.5I have not. Not even sure if it’s there honestly. Your talking the little silver capacitor on the intake mani right?

I'll keep updating. We have also changed the gap to .018 just in case. No results.I personally believe something is causing weak spark, and it would have to be wiring at this point.Everything in the fuel system has been upgraded, everything in the spark system has been changed. Every sensor...

I did forget to update that one, yes, load tested pcm to cmp, ckp and map with no significant resistance.

CAM and MAP sensor have been changed.

Coils, PTU, Wires. Yep.

Here is a log from last night. Cruising at 5500+, it is breaking up this entire time. TPS at 33ish. No boost, 13.7-14.5 afr. Light load. If it WOT it's breaking up, if it's 1/4 throttle, its breaking up. The only factor is it has to be 5500 rpm or above. Any gear, any load, any AFR, any timing.

No, you can’t. That’s how speed density works. You are telling the engine what the volumetric efficiency is. You want another log? Smooth fuel, rough fuel, lean, rich, doesn’t matter. My tune was all over the place from me trying to do SOMETHING that stops it. Current tune on the car runs great...

Swapped injectors. No dice.Ran some data logs, lights dim at idle. Bat voltage fine though at WOT. Thought it may correlate. Oh well.

Negative. Car ran fine on SD rom before being built. Maf should have no effect. But at this point I may try.I have my money on electrical gremlin.

I do, we have an identical set in my Evo running fine. Will swap this week and give it a shot. I don’t even care what it is at this point, just need to find it lol.

Injectors were installed in February. Not too much run time on them since. Have thought about swapping them and checking. It’s the only thing in the fuel system we haven’t touched since then. Our fuel pressure did shoot up after the new pump/rail. But adjusted it back down. Has always been 42-43...

Update: Changed out the pump, fuel rail, feed line and filter.No change.

Update: Changed ROM's, no difference. Have not had a chance to pull the pump/test wiring yet.

I have not, we may try that this weekend.

Annnddd no change. :(

Small update: We have tried a Used PTU, but we are trying a new OEM PTU and new OEM Coils tonight. Just to rule out a spark issue.

ProjectBroke submitted a new DSM Profile :98 GSXRead more about this vehicle here...

It's like you were reading my mind.

This is a log of one of the more rich maps, I took out some part throttle and idle to shorten the file.Anything over 5k is breaking up.If you notice the load going up and down, that's really the only indicator that something is happening.

I am currently logging AFR TPS RPM LOAD BOOST Advance and Inj pulse width.Would anyone recommend logging something else?

I will get one from my laptop. It hasn't been first on my list of things to get uploaded.The only time I have to reply, I don't have access to it. I will attempt to upload one tonight.

I realize the above statement seems argumentative. I appreciate any insight I am given! I have just tried a lot of solutions already lol.

No, I thought you were referring to the actual narrowband sensor, I have a wideband, logged and read.I stated in the opening post what I was varying the AFR at, that's actual AFR, not a map value.AFR reads 13.x+ and break up stops. Any richer and it breaks up. Closed loop cruising is...

I'm at .020, these issues aren't boost related. Rather RPM. It still breaks up with little load.WOT shouldn't be referencing the front O2. Would there be another reason it's related?

Small update: Changed out the wires. No difference.

Tuned on/off doesn’t make a difference. Just the Tephra launch control was active. Which actually didn’t work until we wired up the clutch switch yesterday. Made no difference. Swapping a coil and wires today.

If I run 13.x out past 5k it pulls great. Any richer and break up.it will break up free revving with ANY AFR. You can just hear it like it misses for a split second over 5k. Has AEM FPR and a rewired 255. If it was a volume issue, why would it occur with no load?I appreciate the insight.

Should note, this ecu was installed before SD was implemented. No break up on MAF. A lot has changed since that time, car was built. So it could be an SD issue. May not be.

Update: TPS readjusted (small change), BPR7ES @ .020 installed, tried different firing orders. Tried new Omni 4 BarNo difference.You can free rev and hold this thing at 5k + and just listen to it break up every second or 2. AFR stays steady. Pulls are horrible.Really hurting my brain here.

Evoscan percentage is not indicative of voltage.TPS is adjusted per manual.

What is insane about it?

BUMP, tried BPR7ES at .020, no change.Should also note, this happened on 93 before as well. So its not the fueling.

It was poor prep, damn near shattered my T-case at Indy due to wheel hop, spun on a soft launch at Norwalk. This is a 400whp car.

Hi all,Currently messing with my brother's 98 GSX. We are running an EVO 8 ecu with 9653 Rom. We have an issue where hitting 5k+ will start to breakup/miss. On or off boost, doesn't matter. I can vary the fuel from 11.1-12.4 with no change.I have noticed one thing that helps, going lean...

Well, a little later than I had hoped.I threw a decent amount of mid-range timing at it, and leaned out the spool.Seems to have worked! Ran 280-290whp consistently on 18lbs. So boost is the next step.Looks like I was just overthinking, timing map was too conservative to get anything out of...

I appreciate it.I was not able to try my new map last night, I did verify base timing again.I will update tonight/ tomorrow with results.

I'm going to mess with the timing maps and see what happens for now.I'll also grab my timing light and verify base timing isn't off.

I'm finding that out, but a discrepancy of this magnitude is leading me to dig elsewhere. Maybe I'm overthinking it.

Timing is set through the ecu. You can't modify base timing on a newer 2G without a 1G CAS IIRC anyhow.Timing light follows values input into the timing maps.

93.It's a starting point for a vehicle I had no experience tuning, and i realized I grabbed the wrong map.If you notice on my log, I'm hitting 8 degrees tapering to 2-3 then back up to 8, the load just isn't there.Do these really take that much more timing than an Evo? I made 350whp on a...

Yes, map sensor is in the stock location. I had 0 knock, but I have induced some part throttle 2-3 counts while messing with the map. I have attached the first timing map, I don't have access to the current one, but it doesn't differ by much.

1). Any boost, vac, or exhaust leaks? No Boost leak tested 5psi above your max boost with no leaks? Yes2). Verified mechanical timing? Yes3). Verified base timing? Yes4). Ignition system COP or Stock Coil: STOCK Wire brand and Age: Spark Plug brand, type and Gap: NGK-.0265). Motor...