light load sputter / stumble? s4 t2
 

I am getting some pretty bad stumble / sputtering / hesitation when I press the gas pedal. It seems to happen mainly when I press it in only a little. If I take it to WOT it takes off. If I hold it at light throttle and don't move it it stumbles, but then recovers and revs normal. I set my TPS, and it is still doing it. I even unplugged it to see if that would make a difference and it didn't.

I removed my thermowax, because it was busted (all the wax came out) and it was clogged in the holes.

I also have no major vac leaks, as I have tested for that by pressuring the intake and spraying soapy water all over the engine.

Any ideas? This is getting very annoying.

Thanks
~Tweak

oh and it's not the fuel filter, I changed that a week ago.

I also have my fuel pump powered right off the battery, with a switch and a fuse. Cause I just found out that the person replaced the fuel pump switch thing under the dash. I am going to replace that.. could that be the problem? O.o

do you have a 13B-RE controlled by an s5 ecu?

No, this is just an normal FC engine. This is not my RE.

I'd add more grounds. One from the batt to the chassis, one from the UIM to the chassis...there's a few grounding threads out there with more locations.

This seems to be a pretty common problem with these cars and there are multiple possible causes. Does it occur at specific engine speeds or vaccum/boost levels?

right now, I just rechecked all my vac leaks for a 4th time, and found 2 minor ones. One right at the turbo intake, I had a crack in the plastic, and one going from the turbo to the stock TMIC. I fixed those, but the stumble still happends. The car runs much better though. I also changed my TPS, cause it was being weird. It would go all the way up to 10k resistance. Changed that out with another one and that is back on spec. STILL the stumble persists, but I have narrowed it down.

The problem happends ONLY when I push in the throttle a certain amount. If I push it lets say 4 mm nothing happends, if I push it between 6 and 12 mm, it stumbles. ( it runs on one rotor). So if I rev it to 3K, and then get it bettween the 6 and 12 mm range, it will stumble or run on 1 rotor for a sec and then recover. This helps because now I know it isn't just when getting off idle.

Maybe this will help lead to something? I unplug the TPS and the problem is still identical. I wouldn't THINK it was fuel filter, because I changed it with this new engine install, AND I can reproduce it at any RPM.

Any new ideas?

Thanks!
~Tweak

ok, so I took it for a medium size drive, about 40 mins. I noticed a LOT of backfiring when I would let off, or when I would go from past the problem area, and back into the problem area. I think I am getting too much fuel and not enough air.

I am thinking that, because the TPS doesn't seem to change anything if I change the settings and or disconnect it, I think it has to be a conflict between the single throttle plates and the dual throttle plates.

I plated with an NA throttle plate for a good hour or so, looking at it, trying things, and I noticed that the single throttle plate opens, first, and then after you go a little further it connects to the duals and opens them. Do you think this could be some of my problem?

ok, starting to get really pissed at this car.

Any help or ideas, would be awesome.

Here are the facts.
1. No vac leaks, tested 8 or 9 times now doing different ways of testing it.
2. Timing is set perfectly.
3. The thermal wax has been removed
4. Idle is perfect.
5. The stumble does not happen when the secondary valves are opening.
6. The sumble does not happen when I press the pedal a little bit and the rpms go up.
7. The stumble does not happen when I press the pedal a little less than half way or more.
8. The stumble can be recreated at ANY RPM and any speed when pressing the gas the correct amount where the stumble happends.
9. It seems as though it is running on 1 rotor when it stumbles.
10. I have checked, set, changed, and even tried unplugging the TPS.
11. I am about to go crazy.

The stock ECU cuts fuel to one rotor on deaccelerations, what is the trigger that tells it that it is deaccelerating?

Any other ideas?

Thank you.
~Tweak

Faulty/clogged/leaking injectors?
I cured my 3800 RPM hesitation, and I get some low rpm (2000-3000) coughing before it clears up. I'm at the point where I am looking at an injector issue also.

it is not rpm specific. It is throttle specific. I can be at 1000 rpm and cause the spudder by pressing the throttle lightly, I can be at 4000 and cause it to spudder at light throttle, I can be at 7000 rpm, and cause it to spudder at light throttle.
But thanks for trying.

I was trying to say that ISTILL have the sputter. Like you are describing.

If you jab the throttle quickly from idle, it spits and sputters right?

no. cause it will go past the dead spot that makes it hesitate.

To clerify, it only runs on one rotor at a certian spot in the throttle. That is it.

If I am at idle and WOT it, it's fine.
If I am at 3,458 rpm and WOT, its fine.
If I am at 2,125 rpm and I 1/2 throttle it, it is fine.
If I am at 1,743 rpm and 1/8th throttle it, it is fine.
If I am at 2,564 rpm and I give it the somewhere between 1/4 and 1/3 throttle, it will run on 1 rotor.
If I am at 5,432 rpm and I give it the somewhere between 1/4 and 1/3 throttle, it will run on 1 rotor.

It does not matter what RPM I am at, it only depends on how much throttle I am giving it if it runs on 1 rotor.

I guess tomorrow I will change ECU and AFM, see if that helps.

i had the exact same symptoms.

went to beckstrom automotive which used to be pretty much an all-rotary shop. they had the same people working there from when they were rotary-only.

they asked me when i experienced the problem.
at first the stumbling was intermittent, i couldnt see the pattern..
they couldnt do anything unless they could hear it for themselves.

i had to wait it out about a month until i could reproduce it on command. by then it was just happening all the time, so eventually one day my engine flooded.

my freind tonda came down from cotati to look at the car for me. he looked at it for about 45 minutes and said its probably injectors. i called napa auto parts to price replacement injectors. tony at napa referred me to funkspectrum.

funkspectrum located a friend with a set of injectors and i paid them to come down to put them in for me (since my car was handicrapped).

ever since then, no more stuttering stumbling.

i forget if it was in the primary or secondary powerband that it would stumble...

i want to say primary.

any way, its worth looking into.

yeah I guess I will change em out tomorrow just to make sure. O.o.

Ok, tried a different ECU and AFM, that didn't seem to do anything. Going to change primary injectors tonight.

I did change my primary injector, and nothing changed. :/

I guess. ... I will change my fuel pump or something I donno.

Any gurus that have some ideas? I'm fresh out.

Have you had this issue across a couple tanks of gas? It could be a bad tank that doesnt like the advance at a certain load, or mid-range where the engine would regularly be leanest. If the voltage to your fuel pump is lower than it should be (bad grounds, old corroded wires etc) then you may be getting less pressure and therefore the leanest spots would be suffering. Either that or possibly your gas cap is not allowing a little air in to counter the pressure drop as the tank gets lower. They're possibilities anyway

thank you for thinking outside the box, but I would think that those kind of things would not be repeatable exact throttle position problems.

Thank you though. I am going to change fuel pump and I found a slightly air leaking OMP injector. So I will change that tomorrow also. I really doubt that is the problem though. (the omp injector)

Do you know what your fuel rail pressure is? Is the fuel pump stock? Its also possible that you're overpowering the stock FPR so that it physically cant flow enough fuel to bring the pressure down to where it should be at lower loads (where the rail pressure would be fairly low)? Is the vac line thats hooked up to the FPR going to the correct nipple and giving the correct manifold pressure (actual vacuum signal, not one of the air bleeds like the ones used for the oil injectors etc) ?

I do not have a fuel pressure gauge, so I am unable to test that, but I am thinking that is going to be a future purchase really soon if I don't find the problem. I am going to check the pump, and switch it out with a stock s4 t4 one tomorrow just to eliminate that idea.

I do not have the rats nest, so I have the FPR going to the nipple in the middle of the LIM, like this picture I have attached.

The weird thing is I ran this whole setup all last year, same manifold and everything, just this time I took the thermowax off. After I change the fuel pump and OMP injectors little leak, I am going to put the blown thermowax unit back on.

Again thank you for the ideas. :)

I only have 3 nipples on the back of my throttle body, I am missing the one on the very top. I put the primary injector bleed to the highest one I have, and the rest I have capped off. All except the brake booster on the back.

It doesnt hurt to run the air bleeds for the secondary injectors and oil injectors, IIRC the point is to just help their sprays atomize.

My guess would be to double-check the FPR line then, and try unplugging it from the FPR when the engine's running to make sure that theres a vacuum there.