Idle Hiccups Fixed, But Now Misfiring Under Load

Doctor X · 06-01-2025, 07:23 PM

Hey guys,

Really could use some help diagnosing what’s going on here.

Originally, the car was having these hiccups/misfires at idle — nothing violent, but definitely noticeable and worrisome. Normal driving felt fine otherwise.

I replaced the fuel tank breather valve and it actually helped a lot — the random idle hiccups stopped.

That said, the car still doesn’t feel buttery smooth like it used to. Idle sits around 640–690 RPM and fluctuates slightly — median is around 650 — but it still bounces up and down sometimes for no obvious reason.

The main issue (and what’s got me most worried) happened today.

I went out to try and datalog the car.

First pull — something went wrong, the log didn’t sync. Tried again and during that second pull, the car misfired hard.

Like full-on backfire/misfire sound — You’ll see it in the datalog I posted.

- Bootmod3 Stage 2+ E30 Engine Tune (I'm running probably e40+ because of how bad the 91 is in california)
- XHP Stage 3
- DP
- BMS Intake
- TU Fuel Pump

https://www.bootmod3.net/log?id=683c...ad912f9fe1013a

At this point, I’m thinking it might be one of the following:

Spark plugs — maybe not gapped properly or they were over/under torqued
Bad gas possibly messing something up, too much ethanol? too little?
LPFP or HPFP issues? I’m not great at reading datalogs, but feels like something is off or maybe crashing under load

I’m really trying to avoid just throwing parts at the car, but I’m stumped and frustrated. If anyone has experienced something similar or can offer input after checking the logs, I’d really appreciate it.

I just want to figure this out before I end up putting the car on the market out of pure frustration lol.

Thanks in advance!

06-01-2025, 07:23 PM	#1
Doctor X Second Lieutenant 111 Rep 270 Posts Drives: B58 Join Date: Dec 2022 Location: Los Angeles iTrader: (0)	Idle Hiccups Fixed, But Now Misfiring Under Load – Could It Be Related? Hey guys, Really could use some help diagnosing what’s going on here. Originally, the car was having these hiccups/misfires at idle — nothing violent, but definitely noticeable and worrisome. Normal driving felt fine otherwise. I replaced the fuel tank breather valve and it actually helped a lot — the random idle hiccups stopped. That said, the car still doesn’t feel buttery smooth like it used to. Idle sits around 640–690 RPM and fluctuates slightly — median is around 650 — but it still bounces up and down sometimes for no obvious reason. The main issue (and what’s got me most worried) happened today. I went out to try and datalog the car. First pull — something went wrong, the log didn’t sync. Tried again and during that second pull, the car misfired hard. Like full-on backfire/misfire sound — You’ll see it in the datalog I posted. - Bootmod3 Stage 2+ E30 Engine Tune (I'm running probably e40+ because of how bad the 91 is in california) - XHP Stage 3 - DP - BMS Intake - TU Fuel Pump https://www.bootmod3.net/log?id=683c...ad912f9fe1013a At this point, I’m thinking it might be one of the following: Spark plugs — maybe not gapped properly or they were over/under torqued Bad gas possibly messing something up, too much ethanol? too little? LPFP or HPFP issues? I’m not great at reading datalogs, but feels like something is off or maybe crashing under load I’m really trying to avoid just throwing parts at the car, but I’m stumped and frustrated. If anyone has experienced something similar or can offer input after checking the logs, I’d really appreciate it. I just want to figure this out before I end up putting the car on the market out of pure frustration lol. Thanks in advance!
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