Closed loop breaking up
 

Hey guys, I have a weird issue that I've been fighting with for months. I was hoping getting a proper tune from a reputable tuner would fix the issue; but it didn't. So my issue is backwards from the norm. My can runs perfect at full boost in open loop operation. When cruising at part throttle or even idle the engine breaks up badly and doesn't want to hold rpm. Even with the car sitting in the garage It will break up if I try to hold the rpm around 3,500 or 4,000. The problem existed when I was running MAF and is still there on SD. Plugs are gapped to .022 and I've tried several sets. (currently on NGK Copper BR8ES) Tried two sets of coil packs with no improvement. Boost leaks are nonexistent. PCV has been deleted for a catch can. Everything else on the car is perfect; but this part throttle issue makes it no fun to drive as it shakes all over the place. AFR's jump around from 13.0 to 17.0. Basically, I've narrowed the issue down to being exclusive to closed loop operation. If I go WOT during the breaking up the car shakes, backfires loudly, and then boosts perfectly to redline. During a pull in all gears as long as I'm in open loop the car pulls excellently all the way to the top of 5th gear and the wideband AFR's read dead on what the fuel map shows. It's got to be some issue with closed loop operation. AFAIK the only sensor that the ECU references for closed loop is the o2 sensor and I've changed it twice with no improvement. The only thing odd about my setup is the lack of an o2 heater circuit. (It's a JDM spec IV wiring harness) This issue has persisted across two ECU's as well, so it shouldn't be ECU related. I'm about to push this car off a cliff... The rpm line looks like crap in Evoscan; but all the sensors seem to be reading the way they should. I'm at a loss here. Current setup is as follows:

2.3 stroker with around 1,500 miles since the build
Manley TT I-beams
Wiseco 1400HD 9.0-1CR pistons
Curt Brown head
GSC R2 cams
stock Evo 4 turbo with a 20G cold side (licp is dry so no oil fowling problems)
current tune is set at 22psi and the car makes 370whp
ID1700cc injectors (Car is setup for big power, but I'm holding off until I can get this issue sorted. Also, the issue persists with E85 too; so it shouldn't be an issue with the injectors being too big. The issue was there when I was on DSM 450cc injectors as well)

Anything else I can post if needed, but that's the jist of it. Any of the gurus out there have any ideas? I'll post a log here in a second and a video as well.

Here's a log from sitting in the garage. I rev the engine up to 4k rpm from idle and hold it there for a short time. Everything seems normal to me; but the entire time it was breaking up. Note the tps sensor compared to engine rpm as well. Rpm jumps around without my foot moving. Such a weird problem to have.

Can you make a video and post it here so that we can have a better idea of how the car is behaving ?
Are you willing to check mechanical stuff if you had to ? If you have checked all the sensors . Cam , crank , O2 then i think your problem is mechanical. Experienced some thing similar ,eventually we had to check the head, found bent valves and other ugly things.

Haha yeah I built the entire car including the engine; so mechanical work is no biggie. I replaced the intake manifold gasket yesterday (was hoping that would be the issue, but no dice) so I just finished doing another boost leak test and also fixed an oil leak today. Ran out of time to make a new video today; but I will tomorrow. I don't think bent valves will end up being my issue here as it's a fresh engine and it's had this issue since day one. I'll upload a quick vid I shot the other day. AFR's were a bit off in the video (gonna run a long log tomorrow and make a video to show how it reacts in boost and at the boost threshold); but you can clearly hear what it's doing. Stay tuned for the vid here in just a few minutes.

Better video uploaded below

Initially i thought this would be a loud misfire but it isnt but if it gets worst during driving under load and a.c then it is a cause of concern. Probability of a mechanical failure is going to be on the top of the list if you have checked every thing.
- While holding the throttle can you ask some to listen to the injectors, if one of them is ticking badly. May be take em out and check them for flow.

- Wiring related issue with coil packs. Usually ign coil and injector misfires are loud.

It is fairly loud; but no backfire usually. I'm actually starting to question fuel pressure. I had my afpr set to 43.5psi with the vacuum line off and when I attached the vacuum line back it would drop to 38psi. At idle just now it was at 35psi. Not sure that'd be enough to cause a problem; but maybe. I wish I had an electronic fuel pressure gauge so I could monitor it while cruising!

Here's the promised video:

I've also attached a quick log with me cruising at part throttle, boosting from part throttle, and a quick run through the gears. Only get a single count of knock each time interestingly enough!

You haven't checked the fuel pressure with a gauge?

I have checked the pressure. I just can't monitor it from inside the car while driving. It's set at 43.5psi with the vacuum line off and capped. I was thinking maybe it was an issue with the amount of vacuum it's seeing because of the big cams; but then I realized I set the base pressure without a vacuum source so it shouldn't make a difference. I'm back to thinking it's got to be an electronics issue now after running the car around town some more and monitoring the wideband. Fuel trims look perfect. It's just not consistently igniting in low load situations.

Check your maf wiring. Engine wiring for open circuits. Since you said that this is an old issue
it is time consuming but worth it.

No MAF. It's on SD now. The issue persisted when it was on MAF though. For the record the readings from the MAP look good in the log.

What does it read at idle , warmed up, idling correctly with the vac line hooked up as it is when you're driving it?

Generally it reads 38psi. I've seen it read 35psi though. That's what I was suspecting fuel pressure. Now I don't think that's what it is though.

It's supposed to be 33PSI.

It's in the manual if you want to read it.




Modified cars always have more than just one problem.

Well, technically it's variable depending on the manifold pressure. Since it's a big cam engine (GSC R2's) there isn't as much vacuum at idle and that changes the fuel pressure. That's why you set the base pressure on the afpr with the vacuum line off and both ends capped. Modified is the understatement of the century if you look at my build thread haha. I know this car inside and out and I don't think there's a bolt on it that hasn't been removed. This issue is just weird to me. I've never seen anything like it. I'm going to check my resistances on my injector wiring today and check over my cam angle sensor and wiring. The CAS and trigger plate are brand new and tight; so I'm sure that isn't the issue. The cam angle sensor is original Evo 4 from 97 though so maybe it's got an internal issue and needs replaced.