KenMathisHD

Howdy guys! She's a Base 94 with a completely stock LT1, 150k on the motor. I've been trying to diagnose what feels like a misfire for a little while, does the whole twitching while running, a little vibration that my 92 350 SBC did when it had a miss. I'm still doing research around the forum and some other sites, and have come armed with some information, but also questions.

Changed all the wires and checked spark at each one, looked healthy each time.

All plugs replaced, if I can find where I put the old plugs I'll post pictures of them to see if they can help narrow down the running conditions in each cylinder (I know I have them somewhere, they're all in their own boxes labeled by cylinder number, just got to find them).

I've ohmed each injector, and they all read 13 ohms (this was in 40-degree weather, don't know if that makes a difference). I figured since they all read the same value it would rule out the solenoids (yes, I know this doesn't necessarily rule out the injectors themselves, just the solenoids. I don't yet know how to check the function and spray of the injectors and baskets aside from pulling them apart)

Opti is 8k miles old, and the connector for this has been replaced. Opti still looks ok, though don't have a scope and not sure how to check it without pulling it off (btw, how DO you check an opti with a scope? i.e. where do you hook up what connectors, and anyone know any shop in SA that might be able to scope it, or know a good place to get a scope?

Ignition coil was replaced at the same time the opti was replaced, though it's a duralast part so it's not necessarily ruled out. Where do you hook up on this to check coil resistance, and what else can you do to check a coil?

Both 02 sensors have been replaced, but no difference. Also have the old sensors, so will post pics once I locate them too.

I just got in my laptop scanner cable last night, and played around with it this morning. It took a bit of head scratching, but I finally got it hooked up and reading through the OBD port. I've got data recordings (through TunerProRT, .xdl files) at idle, driving, and startup then driving in case they help - just need to figure out how to upload them either here or on another site and link there. Last recording should have captured getting the engine up to 5k as well, figured a miss would show it's head more at the top RPM range than at idle(hopefully). Ended up also finding out that an already seeping right side valve cover gasket doesn't like it when you increase the engine speed to right below redline - will be replacing that this weekend (as a side note, if anyone has tips on that or on the best gasket to use for that, I've never replaced valve gaskets before on anything and would be very grateful for any advice).

Next thing I need to do is figure out how to use the darn scanner function of TunerProRT so I can clear all the old codes and look at only the current ones. No service engine/check engine lights present, but you never know what code might pop up. Can TunerProRT even do that, or am I wasting my time on that?

So I'm not sure where else to go from here. I know there's probably a few different ways, but I'm not sure what to look at next or what to test. Any ideas or suggestions are quite welcome, stay tuned for incoming photos and data files (somehow).

KenMathisHD

Idle .xdl
https://www.dropbox.com/s/3br8yh5ky5...0Idle.xdl?dl=0

Drive .xdl
https://www.dropbox.com/s/vfmljjq1ve...0loop.xdl?dl=0

Start and drive (should have 5k rev too)
https://www.dropbox.com/s/hram55iecq...0loop.xdl?dl=0

Let me know if these links don't lead y'all to the files, and I can try to come up with another way to post them. I've also got the .bin, .xdf and .adx files I can upload if needed

Still looking for the plugs and sensors though, so pictures will be up soon hopefully

ls777z

Maybe do a compression check?
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pacoW

I have a '96 with 150K that had a miss also but '96 is OBD 2 and was throwing code P0330. Turned out to be the opti. I'm kind of old school, I could hear the random miss at idle so I just hooked up an inductive timing light to all the plug wires and watched the "pulse" (light). 2-6-8 would fire okay at normal idle rate which seemed like 2-3 flashes/sec for a couple seconds then it would flash like RPM's were 3000 or above. That tells me major ignition problem. So I did plugs, wires, ICM, coil, rechecking after each step and no better. Replaced the opti (which turned out to be orig) with one from SAC city and it runs great. Having a scan tool for the laptop would have been nice but I've diagnosed lot's of ignition problems on different cars with a simple timing light.

KenMathisHD

Compression completely slipped my mind, I'll grab the tool from autozone and check it out. Really hoping that's not what it is though

KenMathisHD

Holy crud, you can check that with a timing light? I figured since it was LT1 that would be out the window. Hoping it's not the opti again (initially replaced due to a mitsubishi sensor that had a problem with heat) cause I really don't want to try and get the hub off and on again. But, at least it's still under warranty from Summit, so maybe I'll catch a break with that?

pacoW

all you're doing is looking at the spark (flashes) on each cylinder, at idle should be even for for all cylinders...2-3 flashes per sec. If you don't get a normal "rhythm" on all 8 cylinders there's an ignition problem somewhere. Plenty of times this method fails as the idle spark is fine. If you have a miss under a light or heavy load only timing light is useless. But an ignition (spark) problem at idle usually will be with you all the way up the tach.

KenMathisHD

Ok, so I was looking through the last .xdl file, and noticed something odd.

In open loop, before putting the car into gear, the right O2 sensor bounces within 160-180, and the left at about 15-25 (not sure what the measurements are in, these are just the numbers the computer is showing). Once the car is put into gear and the loop becomes closed, the values on both sensors go absolutely nuts, with right side varying between 100-900, and left side between 50-900. The gauge it shows for measuring these has a total range of 0-1113.22, so maybe these numbers aren't really so insane?
At idle in a closed loop, the spark advance sits at about 16-17, but gets as high as 40 when driving. Don't know if that's normal either, just seemed like it might be a little high for light driving while keeping engine speed under 2k.

Gonna grab that compression tester and an inductive timing light to see what's what, but figured I'd throw these up here.

FAUEE

Sounds like you'e replaced everything but thenicm, how old is that?

Are you sure it isn' bad fuel? It couldn' hurt to run some octane booster/fuel cleaner if you can't rule out a bad tank of fuel.

I'd also consider how old the fuel pressure regulator is. It may be leaking causing your miss?

KenMathisHD

I ended up replacing the FPR a little while ago when the car was starting hard. Vacuum hose still smells like fuel, but I’m assuming that since it’s vacuum it might take some time before that smells clears up (course that was back in October...)

ICM is probably from the factory, though I haven’t been able to find anywhere to test it. Ordering a new one wouldn’t hurt, if anything maybe I’ll just have a good one lying around somewhere?

I haven’t run any cleaner through her yet, though the miss has persisted through several dozen tanks of gas (I go through about 1 or 2 tanks a week), so I’m not sure if the bad gas may still be present? What cleaner would you recommend? I hear some people say fuel cleaner is snake oil, and others say it’s worked...

Crossed Flags Fan

Any solution or success?

KenMathisHD

Not yet, been occupied by work and school this week. Got a buddy of mine to do compression on her today cause I've had literally no time to do it myself (1 job, 2 job, school, sleep, repeat). Still waiting to hear the results, but will post back asap. Sorry this has been silent, just had a bit of a time shortage

Crossed Flags Fan

Got it. Thanks for responding, good luck and keep us posted.....

KenMathisHD

Just got back the results of the compression test. They are as follows:

Passenger Side:
Cylinder 8 - 200psi
Cylinder 6 - 200psi
Cylinder 4 - 200psi
Cylinder 2 - 192psi

Drivers Side:
Cylinder 7 - 206psi
Cylinder 5 - 200psi
Cylinder 3 - 191psi
Cylinder 1 - 196psi

They seem fairly consistent, though I'm not sure what the compression should be normally, or how far off it would have to be in order for things to get unhappy. Is this pretty normal, or am I outta spec here?

KenMathisHD

Sorry bout the long wait time with this thread, I'm itching to get this thing solved but really lacking in free time right now. It'll be a slow one, but I'll shoot up some more info here soon.

KenMathisHD

Ok, so, got a chance to check timing per plug at each cylinder with an inductive timing light

Wire to Cylinder 1 fired fine
Wire to Cylinder 3, pattern stuttered every few fires
Wire to Cylinder 5 fired fine
Wire to Cylinder 7 fired fine

Wire to cylinder 2 fired fine
Wire to cylinder 4 fired very rapidly first read, disconnected and reconnected the connector and pattern stuttered every few fires
Wire to Cylinder 6, pattern stuttered every few fires
Wire to Cylinder 8, pattern stuttered every few fires

I guess this points to a distributor issue since some cylinders are fine while others aren't? Or could it be the ICM dying a very slow death?

KenMathisHD

Or would a distributor issue not be certain until it can be scoped?

KenMathisHD

Bump

JimLentz

I know you already changed the wires, but sounds a lot like bad wires. Also could be a bad/loose rotor or bad cap. I use a scope like this with Wildcat firmware, but there are cheaper options with 2 channels.

KenMathisHD

Changing wires wouldn't hurt, I reached out to Summit to see if they'll help me out as far as the dist goes - want to see if I can get a brand new unit instead of messing with replacing cap and rotor on the current opti so I don't void warranty.

For scoping the car, where on Earth do you connect your probes? I can't find it anywhere (Admittedly, I don't yet have an FSM either), just videos of what the wave should look like when you hook it up. For your Wildcat firmware, is that something that comes with that unit, or is it something additional you need to buy for it?