It feels like fuel cut...

I haven't done anything but drive the car and change fluids on it in almost 8Y. I'm approaching 82K. Mods are in sig.

Already changed plugs. Already changed IAT sensor. (I change them every year because the meth sprays right onto them and kills them). I've changed it 10x. Every time I have this problem that's exactly what it is. I've changed it, then when it didn't fix my issue I changed it again just to be sure I didn't get a bad one.

I changed the front O2 and the car is a lot less sputtery everywhere else. So that definitely needed done.

The misfire is only on a cold'ish car, typically between 2-3k rpm. But it's not a stutter, it's fuel cut - head in the windshield bucking, embarrassing, can't safely pull into traffic without fear of death.

Car is on an 8Y old flash before speed density. (Ecu controlled boost, alt maps etc) I had everything about set to swap out of the maft-pro and to factory ecu speed density and then just didn't. It hasn't been touched, and the car ran every day without issue for years and years. There just wasn't a reason to fool with it.

I dug all my old stuff off of dead laptops and fired up evoscan to try to see what was going on. IPW falls flat when it does this. If idle is 1.5ms and I free rev it, it will dip to 1 or 1.2 or 0 for a split second. It is very much pulling fuel.

My fuel trims we're about 0 LO, -10 MID. Cars on 850's, and a 255HP. I replaced the feed line with 6AN line and installed a fuel lab FPR last year because my hard line rotted out. I am running a couple extra #'s of fuel pressure to match the overrun of the factory FPR with the 255HP. (Where the car was tuned at)

Car runs 29#'s of boost on the 30R and has for almost a decade.

I thought maybe ignition coils but it doesn't feel like blow out/break up. The wideband goes full lean when it stutters. Which matches what I saw with the injector pulse width nose diving. (I wasn't sold at first, thinking maybe it was swinging lean to compensate for the unburnt fuel).

The maft-pro no longer displays anything on the screen, it hasn't for years. So desoldered that one, and I put a new screen in it. Nada. I know it's working because there's no MAF in my car. But perhaps it's starting to puke.

The JDM map sensor is still logging and showing boost vacuum. I thought maybe it was cutting out.
It feels like I blew a hose off for a second. (On SD it just goes blaaaaah and your head goes straight into the windshield). But it makes no sense because on a hot car. It runs and drives without any issues. So after eyeballing all my hoses and verifying there are indeed still connected...

I haven't an effing clue.

Cam sensor? Crank sensor? Coolant temp sensor perhaps? Doesn't that (if not present/reading) stop the car? Mayve it's sluggish... Can't remember?

I'm getting TPS steady through the bucking as far as I can tell. I thought maybe TPS.

Can the IACV puking cause it to pull fuel? I'm able to log steps and such. But?

All emission components and systems are disabled...

What system/ sensor kills fuel when it goes bad? I'm really rusty because I have had almost zero problems with this car for almost a decade. Need some fresh eyes.





​​​

 

Compression test

 

I have exactly the same symptoms but between 1 to 2k RPM. Omni 4 bar replaced, o2 sensor replaced, no boost leaks, injectors cleaned just last year.

I want to try a different IACV and coolant temp sensor I guess. Please let us know if you cross anything else off your troubleshooting list.

 

I checked my EGR block off plate today. It seemed sealed. I cranked it down with a good helping of RTV just to be sure. My intake manifold is filthy and it almost looked like it was blasting out around the plate. It was worth a try. That is something that could heat up and seal if it was just barely leaking too.

Again, the car has zero issues after it warms up. No stutters or cutting out at all.

Next is going to be the JDM map. And maybe the cam angle sensor. It appears to be seeping oil out all around it. I already have a valve cover gasket waiting. But my valve cover needs repainted, so it's probably going to get pulled and painted. And then I'll pull the cam sensor and drain it while I have everything off.

I'm in a pretty nasty pickle with the maft-pro screen being out. Is it the box, the map, a wire... Something else? I can't exactly pull the maft-pro and start relearning all this crap if something else is screwed. I'll never get the car on SD with the ecu if I'm fighting dead / dying sensors. And they don't even make the pro any longer so it's not like I could just replace it. Or honestly I probably would. It just worked. I gave FT the JDM sensor range, they sent me an update for the pro and that was that. 8 years of daily driven speed density. Zero fiddling required.

ETA: it wasn't the EGR plate. Same issue this AM going to work.

Made me wonder about shaft seals or like the BISS O-ring etc. It's been a long time since I replaced all those seals.

 

keep us posted. I have a very similar issue that's come up recently.
Feels like fuel cutting, 2-3krpm, only occurs when engine is cold. Violent bucking followed by unresponsive throttle. Can't even get the car started afterwards unless I'm manually holding the TB plate open...
my first thoughts were TPS or IACV. I've replaced the front O2. No leaking into manifold, plugs are fine..
My tuner is going to look at it this week..

 

^ that's exactly what happens. Except mine is not as severe in some ways. I have no issue starting it, it will idle a little rough when cold. But you can push the throttle and it literally goes flat. RPM falls. It will jerk your head into the windshield it bucks so hard if the car is moving (loaded). But it will do it on a free rev in neutral too. In fact that's exactly how I determine the car has warmed up enough to drive. I do a quick pop rev to like 4k. If I only get a little jitter around 2.5-3k I pull out. If it falls back to 2k instead. I sit there... in a roasting hot car... practicing for my afterlife.


Are you on speed density? What map sensor / IAT sensor?
How long? How old are your sensors?


The no start issue from my reading is cam/crank sensor dying. I wonder if our issue is an intermittent signal or it's slowly dying? The cam is easy enough. It's right up front.

It rained hard 2 days ago and the car was an absolute mess for twice as long when I left work. Not sure how that factors in.

 

its progressively gotten worse.
At first it was a hole at 3k with mild buck during cold drives. I'd just let out, then stab the throttle and i'd be good. No problem after warmup.
Then it was also a start issue w/unresponsive throttle. I had to hold the TB open and it would start up and idle. Again no problem after warmup
Now i'm getting the violent bucking issue as well.
Which leads to believe it's a sensor.
Yes i'm on SD & omni 4bar. I don't know how old the map is, I've only had the car for 1yr.

I def don't deem the car safe to drive as of now. If I push the clutch in while it's bucking (no load) it falls flat. It stays alive if I manage to keep it above 3.5k .

 

I unplugged the cam sensor today and hit the ground with a wire brush to clean the surfaces there. Removing the sensor didn't reveal a ton of oil inside even though it's seeping all around the valve cover. Cleaned and put it back. Unplugging it while the car was running changed nothing. I didn't expect it to, IIRC it's only for startup?

Unplugging my IAT took the car out of closed loop and had only a slight change in idle. Unplugging the maft-pro...

And it was at 10-1 AF and idling at 400rpm.

So the pro works, at least sort of. And it's just the screen that's dead. I have a screen coming. The guy who made them will still fix them even though they are now extinct. (He's nice too)

Something else is going on. Which leaves MAP. Crank sensor. Or perhaps something else.

 

+1

 

After asking around this seems to be next up on my checklist.
The car stays alive if I manage to get it past 4k. Now I believe my car is tuned on a closed loop <4k, >4k is open loop. That means my O2s are doing all the work <4k, so they're prob on their way out. When this happens the injectors open and dump fuel, which would explain why I need to open the TB to get it start up after it dies.
The only problem I have with this scenario is that I changed my front O2s <3k miles ago...

 

Mine is not tuned that way.

For the guys saying compression test. Please tell me what I'm looking for inside the motor that is shutting off my injectors? I've blown up plenty of 4g63's. Pushed heads off, shattered pistons, and melted valves.

This is different. Its like someone kinked the garden hose for a second. The AF goes full lean. The IPW drops like a pot hole in the map.

What are you expecting to find in the engine? I've driven cars with a dead hole and coolant in the cylinder before. It doesn't do what this is doing.

I did plugs and there were no signs of oil or coolant on them either. It was normal.

 

Compression test was a suggestion. Usually when diagnosing a driveability issue compression is always on the things to test.

Have you substituted the omni 4 bar with good known unit?

 

So it was a generic statement. I thought maybe you were after something specific that I was not thinking of.

I am on a mini-battery that has 1.5Y on it. I highly doubt it will make it through a compression test consistently for all 4 cylinders. The car is 100ft from my door. So running a charger down is a huge pitfa.

I am not running an omni 4 bar. I am running the original JDM map sensor. Which I guess is now the Evo X MAP. It was brand new back when these were not $200.

I would absolutely love to borrow a (10) MAP to try a cold start.

The meth could have very well killed it like it kills the IAT's. It's been quite a few years (8) and has not been touched, despite many, many IAT's dying.


I need that screen so I can see at the maft-pro if the MAP is going flat when it cuts fuel. The pro is feeding the ecu airflow data, so logging airflow at the car may be wonky. That would be a good way to tell if it's the MAP or something else. IIRC I was still seeing MAP data when I did a quick log. (It's been a while, and firing it all back up was mildly frustrating) And the MAP is TAPPED, and loggable by the ECU with an edit in the rom. I'll recheck that in the next day or two.

I haven't fooled with any of this in a long long time, but I promise that when I did I was smarter than I sound now.



What sensor's can shut off fuel if they do not work? Crank sensor? TPS? (I was seeing TPSv) What about the IAC?

 

Is your ECU maybe going into limp mode? Valet?

 

That's a really good thought.

My other thought was the rom went bad. I haven't touched the car since I got it where it needed to be 8 years ago. Except... when my fuel line rotted out and I switched the line. It wouldn't move because the fuel pressure dropped a lot. I think I changing the scaling to see if giving it fuel would make it idle. I decided against it, and bought an AFPR and just turned the FP back up. And flashed it back.

Now. I dug my hard drive out of the dead laptop, and moved everything from the old laptop and used that ECUflash because the XML / defs would have been there for the ROM. Perhaps something got effed when I changed it.

There's a new and much updated version of ECUflash on the current laptop, but I didn't dig through it and set it up yet with the XML etc. I also do not remember where I left off in the sequence of Tephra roms, so I have folders of rom's I was working on 8Y ago and I just do not remember where I left off. I think it was the one right before the speed density patch, but I have those on file too as I had started getting them ready for when I pulled the MAFT-PRO. And then stopped giving a ****.

So I looked and looked for the original of what was flashed for the car recently and I'm just not sure. I don't trust myself to remember all the tables to start from scratch. I'm foggy, and I will probably miss something.


I will check and see if valet mode / map switching is on in the ROM. I know mine was set to TPS because I never wired in a switch / LED. But my car is not cranky and starts without any gas pedal.