xxKennyDxx

Hey guys,

Well where do I start. This problem has been bothering me for the last 2 weeks, and has downed the Formula since.

I had a friend of mine who has a shop install my TSP headers, my Poly MM's, as well as spark plugs (NGKTR55) and wires (MSD). I also got my computer swapped by Bryan at PCM4LESS and loaded it as soon as install was completed. Bryan has tuned my previous LT1 Caprice, never had an issue. I contacted him and he would try to load the tune on my ORIGINAL PCM (not the one I swapped in) to see if it was a computer issue-- but the thing is the car DRIVES GOOD, no idle issues, nothing....she purrs like a kitten. I thought maybe I needed a CASE relearn due to a PCM swap, but that did not fix the problem either.

At first start-up, nothing was wrong-- aside from a exhaust leak that was fixed up (loose header bolt). I drove for a day or so, and no problems-- she ran GREAT. Upon about, I'd say the second day, I was driving around and all the sudden I looked at my dash and my SES light was blinking repeatedly-- then went to solid.

I went to Autozone and ran the code, it came up p0300 (Multiple random misfire). There were no "1", "2", or anything indicated which cylinder this was coming from.

The first time, we looked at it again and it happened to be a number 8 spark plug that was cracked. One of the MSD wires wasn't completely on that number 8 cylinder, so we thought that this was causing the miss when we replaced the number 8 plug (What a pain in the ***, and I have small hands!) Swapped it out for a new one, drove it for a day--- SES came back flashing-- this time more and more. I drove it immediately home (I was doing a test run a mile down the road, back and forth to see if I could trigger this). After some problems people had with MSD wires, I thought it was maybe the wires themselves--

So, the next weekend I dropped my car off and put the stock wires back on. My friend had a buddy who has HP Tuners to check the car to see what was going on- He could find nothing out of the ordinary and the tune looked good from what I was told. So, stock wires back on, spark plugs re-checked for any problems (They were all fine).....take it home. The next day, after about 20 miles-- SES light p0300 again, and this time flashing a little less than the last time. So, I parked the car...and haven't driven it since Monday.

The way this misfire happens is interesting. I am in low RPM (1500-2300), in 3rd or 4th gear usually...the latest time this has happened is when I was going DOWN a bridge by my house. I get a little nervous when this light comes on, because it never had prior to my header install.

When I took it back to Autozone to get the code, the tech there mentioned a Crankshaft Position Sensor, and showed me some values that were going up and down (I forget which value it was), and he mentioned to look into it-- I haven't but upon reading on the forums-- It is a problem that can happen.

The coil packs are connected (two clicks) to the plug wires, and are secured in place (I know one guy had the issue of not connecting them which resulting in a misfire, just thought I would state that that wasn't the case)

I have done lots of searching for this code on here, so I guess I know what three areas this could be: Fuel related, electrical, or mechanical.
-----I thought "Ok, you know what....not sure if I put 87 grade gas for my 93 tune two fills ups ago--(I am 99 percent sure I DID...but I get confused sometimes because I fill up both my truck and car and might not have been paying attention at the time)" So, I have read some bad gas could cause a misfire-- I will up with a full tank of Shell 93. Next, maybe my MAF sensor is dirty or on the way out-- I will try to clean it, or disconnect it like some people have mentioned and see what happens.

Anyway, if you can understand where I am coming from-- I do not want to take the "throw parts at it and burn my wallet" approach. Since so many things can cause a p0300 code, I am looking for advice on where to go from here. I realize 90 percent of the time it is spark plugs/wires....but this does not seem to be the case. These are the only things I added, aside from the headers themselves...so you can see my confusion. I ordered the o2 extensions, so I re-used my stock o2's which never gave me a code or a problem prior to this install. Just going down the list of possibilities...

One other thing-- do these engines seem to run a lot "hotter" with the headers installed? I'm not talking what the dummy gauge is saying (It never goes past 200, I also put a new 160 T-stat in upon header install)-- but when I open my hood...it seems the car really is HOT and stays hot for up to an hour or so after I drive (And I'm only making 15-20 minute drives). Part of me probably just notices it now because "I"m there", but another part of me begins to think maybe something is wrong.

Anyway, sorry for the novel guys-- I can't stand looking at this car as being a paperweight sitting in my driveway-- after 10 years of wanting to do this and buy one of these cars. Pretty disappointing, the car was fine before install--- she RUNS PERFECT now and pulls like a BEAST, but the code gets me very nervous (As in, I don't want to blow my engine so I won't risk driving it --LOL) I just thank myself that I have a daily driver in my truck, otherwise I would be S.O.L. My friend who installed this has been very open to fixing it and it's not like he's charging me anything to bring it back and re-do things. I am just trying to get an idea of what I can do and get some knowledge of this issue to figure out what the hell is going on. Process of elimination, and who better to ask than the FBODY COMMUNITY?!

Thanks for any input,
Kenny

SVTconfused

My first question... Is the missfire noticeable... while driving... as in can you feel it?

Or is it just triggering the code?

xxKennyDxx

I could not "feel" it happening the first few times. When I triggered it going up a bridge, I definitely felt something wrong, and the car does seem to dog itself after the code is triggered, and when it was flashing for like 20 seconds when I went up a bridge to drop the car off-- I just limped it over to the shop in fear of damaging anything. It's hard to describe...the car still pulls like a beast (But I don't drive it hard after code gets set, safe than sorry due to me not knowing what's up)

I am going to go to Autozone again and erase the code, it runs great when the code is erased-- if that makes any sense to you? I will see if I trigger it again driving around my block

xxKennyDxx

Ok, I cleared the code. Drove for about....I don't know, 2 miles and the SES began flashing-- but did not go "solid". During that, I did not notice anything or hear anything (rolled the windows down, turned off the music).

Then, I wanted to see if any performance was lost so I gave it a little gas, no problem she pulls hard.

Drove a few more miles, and got my camera phone ready to take the video as it came up again but this time stayed solid.



This is usually when it happens. It flashes on and on, I gave it a little gas to see if anything performance-wise was being hindered-- It wasn't. Sorry for the bad quality, and it's sideways but I didn't have anything else to record it on lol. This time I took it to 3k RPM before letting off and putting back into neutral.

What the hell, lol....I just drove right home after that happened. When this happens, it doesn't idle any different-- very steady and no shaky jumping of the RPMs. Sounds so good.....can't drive it though. What irony lol

xxKennyDxx

Any ideas?


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Bacon

Hard to nail down when you installed a lot of parts at once. The cheapest route is going to be the most work...put original parts on one at a time to see if it corrects the issue. Otherwise its a guessing game. If you dont have yours anymore, find a stock PCM and install that - might run like crap without the tune, but at least it might eliminate the misfire.

I had a multiple misfire like yours come out of nowhere on my 02 tahoe once -was running great and then started flashing while going downhill in 4th gear on the hwy. Ran fine so I drove it home. It was less than a month old (bought it used) and it turned out to be a bad intake gasket that was covered under warranty. Not sure how that causes a misfire, but they replaced it and I never had another problem in 3 years.

hardrock21

iTrader: (6)

In my experience w/ a P0300, don't look to specifics, i.e. plugs/wires/etc...Instead look at what would randomly affect cylinder firing. Things like Vacuum leaks, cracked intake/leaking gasket, fueling issues, Camshaft/crankshaft position sensors, etc.

Moreover from that, I ran into an issue just the other day with a cracked piston. The piston was completely dead, 0 compression, the fuel trims thought it was so rich from the unburned fuel hitting the cylinder that it was pulling 30%+ fuel on that bank causing the rest of the cylinders to lean out landing me w/ a p0300. Not saying that is your problem, just giving you one for instance on a specific cylinder that was causing the problem. Hope that helps.

Paul

hardrock21

iTrader: (6)

Oh, and one more thing...The flashing SES means that the ECM is detecting a misfire that is bad enough to damage the cat, if your car still has one. The flashing SES can really cause problems, my advice is not to drive it much, if any, and definitely don't get on it until the issue is resolved.

Paul

xxKennyDxx

I don't know how to do any of this stuff, that's why I had my mechanic friend take care of it. I clean windows, that's about it guys....I wouldn't know where to begin.

I have the stock PCM, I highly doubt it's a tune issue though....don't really know what to do anymore. Car was running fine before installation, if anything it should be running better than before due to new plugs and wires and a better tune.

Of course it's a p0300 and not something easier to diagnose like a p0301, telling me what cylinder is giving me a problem.

Ridiculous man, absolutely ridiculous.

Thanks for the replies though, I will talk to my friend and see what he can do about this in a week or two whenever he can look at it.

5 Liter Eater

iTrader: (5)

You need to hook it up to a scanner and log misfire history on each cylinder. This will tell you where to look, or, if it's misfiring all over, it could be a crank pickup or reluctor wheel problem. If its isolated to one cylinder check that plug, swap coil packs and wires left to right and see if it follows. If so coil or wire, if not swap injectors around and see if it moves. If it's still not moving then a leakdown test may be in order.

_JB_

iTrader: (33)

I say its the tune. The reason it pulls fine at WOT is because the computer isn't trying to trim the a/f ratio. Or I think its from the new O2 location from the headers. It maybe be close to the original spot, but it won't be 100%. What happens if you go under the hood and hold the throttle in one spot for 10 seconds? Will thedumengine suddenly shake for a split second? If so that's a misfire.

LilJayV10

iTrader: (39)

This^^

xxKennyDxx

Well, a little update:

Yesterday my friend changed out the NGKtr55's and MSDs

I now have A/C Delco Iridium, and AC Delco Plug Wires. MY MSD wires are for sale, if anybody wants them less than 50 miles on em

After 8 hours of taking off and installing, checking every electrical connection, looking around everything......

She still misfires. Or let me be real with you, "SHE PRETENDS TO MISFIRE". I had two other people ride with me in the car, they can not hear a miss, they can not feel a missfire, nothing.

The damn light will flash, I lose absolutely NO performance, nothing. She idles better than you could ever ask for. It's to the point now where I am going to send my Stock computer into PCM4LESS and have them load another tune on it because I am retracing my steps and the only other thing I did beside the headers was tune the car.

It's also at the point where I think the computer is detecting false misfires-- Electrical tape on the dash to cover the SES Light or pull the bulb lol... LOL


One thing I will look into, when my friend changed the plugs-- it was either some residue from the threads in the engine from the old spark plugs- or coolant. This whole issue could be a leaking intake gasket, or a blown head gasket. Im going to drive the **** out of it today and go back tomorrow to my friend's shop and see what the plugs look like. But this issue is completely mind bending, I could understand if I was losing performance and the car was acting funny-- IT'S NOT, she drives like an absolutely beast and has no signs of misfires:: Aside from the flashing SES, which now will NOT store itself and go "solid" on the dash...and will not come up with anything other than p0300

_JB_

iTrader: (33)

What happens with rusted knock sensors? Would that affect their readings?

roninsonic

iTrader: (3)

Have you tried cleaning the MAF? That's a quick, easy fix that causes a lot of misfire issues

lt1pwr1

iTrader: (1)

PLEASE get a scan of your o2 sensors and make sure they are switching correctly. Are your headers ceramic coated? Installing coated headers can trash out your o2 sensors. And a bad o2 sensor can cause a multiple misfire po300.

If they are bad stick with Denso replacements. Don't do Bosch.

Also, you can try spraying some carb cleaner around the intake and see if the idle changes. Do this when engine isn't hot. If it stumbles or picks up idle then you have a vacuum leak there.

LilJayV10

iTrader: (39)

Has the crank sensor been replaced or do you know if the one you have is good? The reason I ask is the PCM uses the crank sensor along with a crazy math formula to detect a miss fire. I am wondering when the sensor gets hot it starts breaking down and causing this.

I know that a knock sensitivity can be adjusted but even if it was getting false knock it wouldn't throw a miss fire code.

SVTconfused

im leaning towards the knock sensors picking up the "ting" from the headers...


but im not a LS expert

blknSS

Its the wires, i just went threw this with my car. All wires were plugged in. It was only thing i changed. Threw a po300. Changed back to stock wires and all it well. Throw the stock wires back on.

blknSS

Nevermind, you did i guess, sorry.