Rokas

Hey guys. My new 96 has a random misfire, code p0300, and while I haven't done anything but change the coil pack yet, I want to get some advice before I proceed.

First of all, am I correct that this isn't a huge problem? I know it could be tainted fuel, bad spark plugs or wires, or even a bad battery.

About the battery. The last owner has 2 screws in between the negative terminal and the connector - I suppose it's too big to tighten so they used a couple screws. Can't this code be caused potentially by that? If the car doesn't get enough constant power than I believe that CAN be the reason. Am I right?

Will an ignition coil from an SC300 (93 I think) work for the LS400? That's something else it could be, yeah?

No, I haven't looked at the spark plugs but if I take them out and see that they're shot, I'm obviously going to. I think my problem is probably one of the two: the battery connection (it's the negative terminal) or bad spark plugs.

When I first got the car, before I ever disconnected and reconnected the battery, the check engine light was actually flashing - something I heard is common for this code and this leads me to think maybe it IS the battery terminal.

Did I miss anything? In order I'm going to do battery connector, spark plugs, spark plug wires, and distributor cap. How hard are the rotors to change?

ANY ideas are appreciated. I'm going to work on it after work so I need some suggestions. The car runs beautiful except for, well, misfiring somewhat. It'll drive smooth if I drive slow but I need the problem fixed anyway. I can't drive like that.

Anyone had this problem? What was it?

By the way, changing the coil pack helped when it comes to starting the car and it seems to misfire less but the problem is still there.

Thanks.

PS. No vacuum leak.

Maybe fuel filter?

Edit: My negative battery terminal was held on by two screws jammed inside it. When I got them out and fixed the terminal so it's tight, the problem went away.

Yamae

Believe it or not, the code P0300 is often caused by the failing ECU among pre VVT engine models.
I'd check the ripple level or the ESR values of QAS capacitors ASAP.
https://www.clublexus.com/forums/ls-...-by-yamae.html

Rokas

Thanks Yamae. I didn't know that and I'm seriously glad that wasn't my issue.

My issue was a bad battery terminal. It was loose so the previous owner jammed two screws in to hold it. I took them out and used some pliers to bend it back, I tightened it, and no more random misfire.

Bad battery or connection = p0300. For me it was.

For anyone else, I did change the coil pack too which was probably good but hey, new coil pack.

The other causes can be a restriction in fuel flow like a clogged filter. It can happen if your gas is tainted. Also it could be a bad coil pack, bad ignition coil, bad spark plugs, wires, and/or distributor rotor caps or maybe the rotors itself. That's all I can think of. Maybe it will help someone.

But check for continuity of flow of electricity first.

Yamae

If things were just as you have found, you'd have no more problem.
But I wonder why you were able to start the engine with the battery not well connected.
Things are not always simple and I worry about the root cause of your problem is still hiding. You might think that the battery disconnection revived the ECU temporarily. If so, you'd have the problem again.

Rokas

The battery was connected but it didn't have constant flow because the terminal wasn't connected right.

Anyway, I have a habit of speaking too soon but that did help some more. It doesn't buck anymore which is what it was doing before the battery thing. Now it just shakes a little if I'm accelerating "quickly." No problems accelerating slow. Well I'm getting the spark plugs and I'll go from there.

Is there a way to test the wires?

My pops has one of those things you plug into the OBDII and Bluetooth it to your phone. Then it tells you info on a bunch of things. I'm gonna see if I can borrow that. Can somebody tell me what to look for? I still have hope it's the spark plug system but if it's the ECU, so be it. It would greatly suck though.

Quick rundown: Changed Coil Pack - car went from losing 2-3 cylinders at a time sometimes to maybe 1-2. This is when it would buck (is that the correct word?) Fixed battery connection: No more bucking. Now the car just shakes when I accelerate depending on the throttle. Baby butt smooth cruising.

After the plugs, if that doesn't fix it and I can't diagnose it then I might take it to a shop. I hate doing that but I can't throw parts at it all day this time.

Again, thanks Yamae and anyone else. Very helpful.

timmy0tool

iTrader: (7)

when I diagnose a misfire, I always replace the plugs first just to get them out of the way. cheap and easy!
70% of the time that fixes it!

Rokas

I got NGK Iridium IX by recommendation.

On another forum, people were saying that the '96 needs a 0.8mm gap. Is this true? I'll check the manual, it should say, but I would've never realized this. That could mean the gap on my old 95 could be wrong.

And yeah, I have a good feeling it's the plugs. Drove the car around today and it did real well but it does shake a little. Should eliminate that.

Rokas

Hey guys, earlier I asked about the gap. I have a 1996 and my manual says the gap should be 1.1mm or 0.044in. They also have to be either Denso or NGK brand - it literally says this.

I'm waiting to take my wife to work and then I'm going to change the spark plugs and pray I have no more random misfire. The car does fine but I don't like having anything wrong with my LS400 if I'm driving it... And like I said, it shakes when I'm accelerating. I'm hoping to see old spark plugs and a belt that looks like it has a month of life... I need a couple weeks to change it. Bills hitting me all at once.

Rokas

Will I damage my engine driving it with a bad spark plug wire?

I figured out the problem... I changed out the spark plugs which were due for a change and found the problem in the process. One of the connectors of one of the distributor wires is broken. The wire is literally ripped and has some free metal that makes some sort of contact when I shove it in.

Now, the car accelerates fine and doesn't shake while driving anymore. Now it shakes when standing still. I think and HOPE I have an old spark plug wire from the old car but if not, until I get one, is it SAFE to drive the car? I don't like it but I might have to very soon.