Checking the resistance is good for when there is an obvious ground fault that stays in place, but if there is one that occurs when the engine moves under load, that could also create a problem. Just throwing ideas out there.

Does the problem occur whether the engine is warm or cold? And does it only occur when you get on the throttle hard?

 

Few things -

The '94s did NOT have the updated ignition harness - I don't believe any of the US FD's did. Not sure if you've replaced it or not, as you bring it up at one point then say you replaced it in a later post.

It could be possible it's fuel cut. The stock ECU doesn't cut fuel at a "hard" PSI - it looks at a lot of factors to determine the fuel cut "zone". Since the stock ECU is speed density, it's looking at throttle position, RPM, intake air temp, and pressure to compute airflow volume through the engine. When the ECU says "I'm getting too much air volume" it cuts fuel, not by boost pressure. But, it does sound like you're relatively safe as far as boost pressure is concerned.

I assume you're using NGK 7's and 9's for plugs, right?

Dale

 

The engine can be fully warmed or cold. If I gently accelerate in gear I can get to almost any RPM, but the moment I try to accelerate it misfires. I can do it in neutral, clutch in or out, or in gear. (When I'm in gear it results in bucking due to the coupled drivetrain). If I gently rev up to, say, 5k, it will take very little additional throttle to create the bucking. If I'm at low RPM, it will pull pretty hard until it bucks, and the bucking could set in from -5inHg or 8psi. It would be nice to have a wideband but I don't. I did remove the old, hardened vinyl sheaths from the harness and wrapped them in silicone tape, so IMO the wiring looks good.

In any case, I will soon see if the injectors are the problem. I had my original set cleaned by a local shop with a new ultrasonic bench cleaner, and because they 1) didn't have a side-feed testing setup to actually test the before/after and 2) they didn't have much experience using this thing yet, I suspect they've been messed up since. The ones I'm installing were cleaned by RC on 1 Sept, so hopefully it will be all different.

If it's the main wiring harness, I'll have to think about if I will spend $700 right now since I'm in a tight spot until next year and winter is on the way.

Dave

 

The ECU initiates fuel cut when it senses excessive boost pressure for a certain amount of time; otherwise, it would initiate fuel cut during boost spikes over 12 psi.

 

New, updated ignition harness last week. You're right, my 94's harness had the ground on it.

Stock NGK 7s/9s non-platinum. I might put in plats after I get it running good again.

The boost is currently maxed at 11psi (was 10, until I added the RB cat-back in spring). The MAP sensor and TPS sensor are spot-on spec. I used my Mityvac to test the MAP through the full range. Water temp, fuel temp, and air temp sensors tested out fine, but I haven't explicitly tested their wiring.

Dave

 

Could also be a flaky connection to one of the secondary injectors - I think Turbojeff ran into that on a car once. The secondaries only come on when there's load, so that totally makes sense. Could be a broken wire or the metal conductor in the plug is pushed back or intermittently touching the injector.

Dale

 

Just a thought here: Although the TPS sensor checked out, have you taken a close look at all the wiring and connections associated with it? The fact that it takes very little throttle movement above 5K rpm, and then actually will accelerate strongly for a moment at lower rpm, leads me to think that the TPS might be the cause. Perhaps there is a bad connector somewhere that starts causing voltage fluctuation that confuses the ECU, causing it to immediately initiate the default function, which in this case might be fuel cut. At lower rpm, airflow is slow enough that maybe momentary confusion with the throttle position isn't as critical as it is at higher rpm. Even though there are no codes being shown, the fact that none are being shown at all even with the problems you're having shows that maybe that function is kaput for some reason.

The fact that you are able to create the stumble in neutral and during vacuum makes me wonder if it's the secondary injectors, as they are not functioning at that point. Also, if it was just the secondary injectors failing, you would still have some fuel coming from the primary injectors; yet you say the "stumble" is total, as if the ignition is cut at that point. Again, just throwing more theories out there.

 

I said ...on other thread as mine did this. I had a new exaust HKS Drager that was out the box clogged.only on boost did it buck. I could feel exhaust at idle but underload it bucked.
Shop looked into the can and the HKS factory did not take a plate out!
worked fine after. This was at three shops and even got new injectors fron Marvelspeed cause another shop thought my injectors were bad.
MZM in Austin Tx. Found the problem.Just another idea.

 

It seems to me that it's a control problem, like Kento said, the ecu is getting confused by a bad signal from a sensor. I'd check/ clean all the related conectors you can get to, like the tps the map sensor, cas, igniter.

 

to pursue the tps back probe the wires for the tps at the ecu connector. put the car on but not running. slowly press the gas and watch the voltages sweep. look for nice even increase. any drops or spikes in voltage mean the tps is bad. this is the right way to check the wiring and function of it. a hole in the feed could cause the problems you see

 

I finished changing injectors and it appears to be fixed.

I did a lot of wiring harness checks, installed new OMP lines, and in the end I think the injectors were the trick. Zkeller called it first, but for financial reasons (to test the cheap stuff first), I took a while to buy another set of freshly cleaned injectors from RC.

Moral of the story - FD injectors like to be a PITA. Only get yours cleaned from a reputable injector shop that can also do before/after flow tests. Otherwise you're gambling with success.

As well, without a wideband or other diagnostic equipment, it's hard to tell if the problem is ignition or fuel related in situations like this.

Dave

 

Well, to bring closure to this story, my injectors were just finished with cleaning by WitchHunter. One of the secondaries was stuck closed, and one of the primaries was leaking. I could not detect any of this when I checked for leaks and operation when the injectors were in the rail. Following the WitchHunter cleaning, they are working normally again.

Moral of the story? Don't skimp on injector cleaning. The fact that I had bucking under *boost* suggests that I was frigteningly close to blowing the motor.

Dave