He_162

'88 GTU S4 N/A 5 speed

I've been having an issue that slowly got worse. My car ran fine after I got a new fuel pump and filter, but soon after I got an issue where it would stutter or hesitate at about 3800 rpms and I re-grounded my MAP sensor wire?*
(The one under intake at back top of rotor housing that causes the hesitation problem)

Did not change anything.

Then I checked the wires going to secondary injectors for shorts, nothing. Pulled injectors, cleaned them, filters in the injectors were not in the best shape and I put the injectors back in. Car still had an issue where it would hesitate but slowly it started not wanting to go past 4k at all at WOT. It's gotten to the point that it will just hit a brick wall at WOT @4k rpms. Will go past if I bring it down to about 30% throttle or less.

Things I've done to try and solve the issue:
-Pulled and cleaned secondary injectors and wires
-Adjusted my new TPS and tested for deadspots (still good, no deadspots) ---- Although I can't seem to get it right at 0.999v or 1.000v it sits at like 1,016 and when I push in the throttle from the engine bay and release it it sometimes goes to 0.996 and back up to 1.016 or so and sits around there. Can't get it perfectly on 0.99 or 1.000v. I can try to set it using Ohms later today to see if I can get it to exactly 1 ohm or closer to it. (Yes, I know, unplug sensor before using Ohms method).
-Checked ALL known grounds that I've found.
-Water Thermo Sensor CEL is back? Haven't been able to test to see if that's causing problem.
-Discovered my o2 sensor is literally wired up to a random ground, not connected to engine or harness? I don't know where it's supposed to connect to. I have open headers, no emissions or air pump and no exhaust.
-Happens with 5/6 ports wired open or wired shut. (They don't open normally and it has run fine without them but I plan on getting an exhaust soon so that they open up proper).
-Ran injector cleaner through fuel tank and ran no premix w/ OMP and disconnected OMP and ran the extra premix in tank and has made no difference. (Although the first time I ran it through it solved the issue for about a day? May have been a random blip / coincidence).

Today I pulled secondary injectors and pulled the filters out which were in bad shape. They were torn and orange-ish in color. I'm thinking they were bad but they show good spray pattern with or without filters and spray same even amount of fuel? I reinstalled them without the filters because pieces of the filters were coming off and I didn't want them to get stuck in the injectors, once I had finished cleaning them for about two hours of spraying carb cleaner through them and tapping the connectors with battery voltage to open them up I reinstalled.

Ran car until warm and then took it out and gave it a go, hard wall at 4k rpms on WOT. The problem got worse, but more predictable? Any gear now it will hit a hard wall at 4k rpm whereas before it would get past 4k in first with just a minor hesitation and then stutter a bit and sort of hit a wall but keep accelerating and cutting in and out in second and then it would be a solid wall in third. Now it's a solid wall at 4k in any gear.

Read through every forum post I could find on the issue and found no solution so far but haven't tried the following:
-Haven't measured voltages anywhere or checked ECU
-Injectors are the cheap $50 ones from autozone, same brand and everything. Not sure if they just aren't working right?

Akaviri

iTrader: (1)

Your lack of proper actuation for the auxiliary ports is likely the cause. Unless you get a different exhaust or rig up an air pump or some sort of controlled pressure source you'lll be suffering. As for the O2 sensor it should connect to a thick grey shielded cable that runs to the ECU, passenger side engine harness. If you can hook that up right it might help. The ground for the ECU is what you want to clean up it's under the intake manifold.

He_162

Can I run a ground from ECU to somewhere else to avoid having to do that?

Car ran fine not long ago and suddenly had this issue. Ran fine without exhaust, was even doing donuts / drifting with it. Acts the same with aux ports tied closed or open.

He_162

I ran through the wiring once again and fixed some corroded areas in a couple of spliced spots. One of them in the harness below the injectors plugs. Today I started the car and ran it, idled a bit smoother and started easier. When I got it warmed up and took it for a spin it had no issue passing the 3800 rpm mark on WOT and is running a lot more smoothly. I'm not sure if something else may have caused this though because it's ran "fine" on occasion and not had the issue and then it would pop up. Hopefully this solved it for now but I'll be back if it didn't.

He_162

Problem came back and is less consistent now. Ensured plugs for secondary injectors were tight and went over the harness leading to them again and found no issues. Going to get my adjustable FPR installed so I can monitor fuel pressure under load and also going to rewire fuel pump to ensure no possible shorts there. Probably wiring to a switch on my center console. Thinking about getting an aluminum panel with switches on it there so I can just rewire a bunch of things and avoid those old gremlins.

He_162

Inconsistency is still the same but now it will also stutter noticeable in neutral.
(Clutch in, and whether it is in gear or not it will stutter when revved in neutral).

The car seems to have a better time and doesn't stutter as much when cold. Does it more often when warm.

Rewired fuel pump, no shorts found in the harness to plugin below driverside footwell. Not sure what's going on.

Akaviri

iTrader: (1)

What kind of injectors are you running? Make sure they are correct impedance and not some junk brand. I reread your post and the fact that you have an open header is not ideal. The lack of exhaust velocity makes for a more difficult idle, and the stock ECU probably cant compensate enough without a stable feedback from the oxygen sensor.

Not to mention whatever you've deleted from the 'emissions' you still have a BAC valve and the throttle body still has the idle cam and thermo wax? These all work together to stay stabilize idle

CryoSignal

iTrader: (1)

Have you flow tested your injectors? 3800rpm is when the secondarys injectors kick on I think, maybe you've got goofy secondary injectors?

He_162

I not only regrounded and sanded all grounds in my car, but I ran a single wire through every ground coming off the ECU and grounded to the chassis as well. Then I went through and ran new ground wires to various sensors such as MAP and whatnot.

The issue went away completely. It was entirely electrical.

My idle is almost perfectly stable at 750 - 800 rpms once warm, and cold idle at 900 - 1000 is also very stable after thermowax warms up enough to hold idle.
I learned that my harness had no wire whatsoever for the o2 sensor? It's like the previous owners cut it out or deleted it somehow. The o2 sensor was literally zip tied to a ground wire in the back of the engine bay, not connected to anything.

My inconsistent wall must have been a drastic series of bad grounds and wires leading to the various fuel and air related sensors on the harness in the passenger side of the engine bay.

(Now the car not only idles almost perfectly, no searching or bouncy-ness outside of the vibration caused by the lack of exhaust). Pulls hard and strong to redline but port actuators were stuck, wd40 got them loose, they don't come on all the way but seem to want to engage at 3800 or so RPM's. Will be getting an exhaust on the car soon to resolve that issue and then my FC will be running 100% healthy.