Ahmet

The problem I'm having is the car stumbling when loaded highly (usually happens when at full throttle/around or above 14psi). There's only a narrow band 02 readout installed in the car, and it dithers around lean/stoich when this happens. The car *sounds* and *feels* like it's lean but I cannot be sure.

I've tried swapping the DME and KLR as well as the alarm control module, which made no difference. My initial guess was the fuel pump, and I will try swapping that when I get around to it, for the time being I guess I won't be hitting too much boost...

I would appreciate any input.
Ahmet

Ski

Ahmet, if you have a multi meter, ohm out the injector leads with the DME harness wires 14 and 15(1-2, 14 and 3-4, 15 IIRC), check to make sure you have good continuity or if you have any resistance. There was someone a couple of months ago who was having a similar problem and he had a cracked wire at the back of the harness where we all bend it if we take the fuel rail off.

Sometimes a high speed stumble has been associated with TOO much fuel...got any black smoke behind you when this happens?

hosrom_951

I had a 'similar' problem, which accelerating (full throttle) does it accelerate to higher rpms smoothly or do you feel itr cutting out? if so, then it is the TPS

Sami951

It probably goes slightly lean somewhere... I'm sure everybody agrees the narrow band O2 isn't very accurate, but it will tell you if you're above or below the stoich ratio -> should read somewhat rich all the time you're on boost/accelerating fast.

Do you have any means of adding fuel to that spot?

Maybe you're going slightly lean, getting some knock -> DME retards ignition, and that gives the stumbling feeling..?

jim944s2

Up in the 924/931/944/951 discussion page, it seems a guy had a similar problem, it's under the heading 'everyone, accelleration problem solved' (or something similar). It's a bit cryptic, but it seems a replaced air-flow meter solved his problem.

OZ951

Hows your FPR ?, if its an adjustable FPR check that the pressure setting hasn't backed off. Is your static fuel pressure OK ? You can also do a static test of your fuel pump delivery volume. I dont have the delivery rate specs on hand but they are around.

Ahmet

I have a 3 bar FPR, and right now no way of checking it. I really don't want to throw parts at the car to figure out what's wrong but I do have a fuel pump I can try... It's certainly getting worse, also seems that when the car's cold it'll let me do a few full throttle runs, and seems to get worse if the car stays on for a while.
Ahmet

eclou

Does it always happen at the same rpm (peak torque)? I had the same issue with a healthy AFR - turned out that I was getting some pinging/pre-detonation so the following was done to prevent pre-ignition on a hot car:

1)colder plugs - the NGR BPR7ES
2)smaller plug gap - to .020
3)octane boost
4)heavy concentrated FI cleaner

between those 4 things, my car quickly stopped all the stumbling. BTW the narrow band AFR (I have the Rennbay one) is totally useless

hosrom_951

TPS?

Tom M'Guinn

Spark plug wires can cause high-load problems. How old are your wires? How is the cap and rotor, and plugs?

Ahmet

TPS/Wires are about a year old. I will check cap/rotor (which are older but no older than 2 years, however missing the dust cap). I'm running colder plugs, less boost than I have before (by at least 3 lbs) on the same octane gas (93).

Problem does NOT happen at the same rpm, and when the car's cold sometimes it doesn't happen at all for some time.
Ahmet

Danno

"when the car's cold sometimes it doesn't happen at all for some time."

Sounds like a lean condition if it doesn't happen when cold. That's because the DME gives you a tonne of extra fuel if it sees cold air from the air-temp sensor.

"The problem I'm having is the car stumbling when loaded highly (usually happens when at full throttle/around or above 14psi). "

Actually at this load-level, the fuel-mappings should be adquately rich. So a lean mixture is definitely not it. Did anything change with your car's configuration between the earlier time when it was working to now?

I suspect an ignition misfire. Probably due to the richer mixture now since you're running less boost than before. One thing that may help is to reduce spark-plug gap down to about 0.025".

Also helps to get a hotter spark by putting a condenser (capacitor) on the ignition coil. This stores up a charge from the battery as the coil's charging. Then after the coil's fired, the capacitor discharges its stored current into the coil faster than the battery can. Gives a quicker charge to the coil and makes a difference at high-RPMs when the coil doesn't have enough time to fully charge. You can pick these condensors up at any auto-parts places. Bolt the body of the condensor to the coil-clamp bolt. Then the single lead coming out of the condensor goes to the +12v supply post on the coil (larger one with black wire).

What version are your GURU chips? I think you have v8. The latest v9 are programmed with extra coil-dwell time in the upper-RPMs to get a hotter spark as well. Why are you running less boost than before?

Ahmet

It's been doing it today more erradically, including when the car was colder... I still haven't had time to check anything, but I will start w/the cap and rotor.

I'm running less boost simply because I can't build the full boost because of this runability problem. The wastegate's getting a bit tired, but another one's on the way.

I believe I do have version 8 chips at the moment. It's a bit jerky in the mid range at full and part throttle (it's been this way since the chips, and doesn't have anything to do w/the current problem), I believe either ignition or air/fuel's a bit off. Anyway, it's funny that I'm hosting a dyno day this weekend and my own car may not be there... If I can fix it by then, I'll know about the "stumble", but like I said the two aren't related and I need to fix this issue first...
Ahmet

Ahmet

Update;

It was the coil to distributor wire @ the coil. It had pretty much corroded itself to pieces, INSIDE the wire. I will be calling magnacore tomorrow... They were about two years old.
Ahmet

Ahmet

Oh, and thanks for all the input guys, I certainly do appreciate it.
Ahmet