Running rough/misfiring- no SES light

**Matto4785** · 08-26-2011, 01:31 PM

My TA has started to gradually run rough at idle and misfire under load. I can pull a P0300 code, but the SES light won't turn on for it. It seems cylinders 4,5,6 are misfiring the most, though all do occasionally it looks according the the scan tool I used. The only other codes pulled are for the AIR system. Plugs and wires were replaced within the past 2 years. TPS sensor within the past 3-4 years, fuel pump within the past year. I cleaned the MAF and throttle body. Besides pulling the plugs and looking at them, is there any other sensor or area I should look? I'm at a loss here since it refuses to throw any other code and it won't turn the light on. The problem is gradually getting worse. The car is a 01 WS6, auto, with AC and about 161k on it. It doesn't do it constantly, but its more often than not at this point.

Thanks for any insight or suggestions.

***SMWS6TA*** · 08-26-2011, 01:52 PM

We will need some more info. Any mods? I'm going to take a shotgun approach to possible problems that are most common:

Bad gas? Was the fuel filter/regulator changed when the pump was change? Can you check the fuel pressure at the rails? Should read around 50 psi at idle.

Another thing that can cause misfires is the boots that are on the plugs. Some guys here have removed them and have no issues since. Check for arching.

Have you check for vacuum leaks? OEM PCV is usually the mail culprit when you have leaks.

You've stated that you've clean the MAF, a year ago, it is recommended to be done at least once a month depending on how you drive and if you have filters that are oil based like K&N.

Only other sensors are:

knock sensors - Have you ever replaced your KS? That much mileage they are way over due.

IAC (Idle Air Control) - located on the throttle body. It could have gone bad and doesn't throw a code. I however don't think this is the issue though. Otherwise you would have issues at idle instead of under load.

others will chime in here.

**Knight** · 08-26-2011, 02:06 PM

Agree with Scott. I'd do a general tune up first and take care of the common items - ie. cleaning the MAF, plugs and wires, ect. Misfiring will only happen for a couple of reasons, especially multiple cylinders. I'd be highly suspect of wires.

**Matto4785** · 08-26-2011, 02:34 PM

The MAF I cleaned 2 days ago. As far as mods go, just 3.73 gears, a catback, and a lid. Nothing else.
As far as the pump, I honestly don't remember if I changed the in line filter. I know the regulator hasn't been changed. Nothing on the rail has ever been touched. The engine itself hasn't been touched ever except for plugs and wires.

I didn't know the knock sensors had to be replaced.

I'm going to check the plugs and wires and replace the in line fuel filter this weekend.

Thanks for the suggestions. Keep the coming if you think of any more.

***SMWS6TA*** · 08-26-2011, 03:10 PM

Depending on the brand the KS should be changed around the 60-70k mileage mark. They can go past it but that is just the recommended manufacturer intervals.

And to be honest I didn't change mine till around 130k when I did my LS6 VCT mod.

**cammed goat** · 08-26-2011, 05:06 PM

Keep us posted. If the fuel filter has never been replaced, I would think that is the culprit.

**SS201** · 08-26-2011, 06:12 PM

Hey Matto it seems to me that you might be having the same problem I had. Car was almost giving a sputtering sound when I gassed it hard, just like a misfire. Well it was a vacuum problem I had. if I were you I would check the PCV hose. Its located on the driver side by the firewall almost. It was almost off on my car it it was giving me hell for a long time. It would give me a lean bank 2 code when checked. I created a thread at the time I had the problem here on LS1. Here is the link if you are interested in looking at. Look at page 3 close to the end of the thread.
http://www.ls1.com/forums/f6/sputter...96/index3.html

**Ain't Right** · 08-27-2011, 10:23 AM

Check the AIR valves on both sides coming up from the exhaust manifolds. Mine were plugged up and when I changed them for cheap it made a major improvement. Also at the back of the engine there is a rubber connection for the pcv lines were they meet from both sides then come to the front that melted causing a major leak. Found it when changing knock sensors.

**Matto4785** · 09-17-2011, 11:17 AM

My apologies for not updating this in 2 weeks, some things popped up and I needed to deal with that crap first before I got around to the car. Thankfully its been decent weather out for the most part so I've been able to use my bike to get around. I went through and checked a few things. Fuel filter is replaced, it needed it badly, but it didnt solve the problem. I checked through the sensor readings and the TPS looked bad on the scope. Checked the voltage and it was jumping all over the place. Replaced that and that seemed to get rid of the P0300 code, but I got a P0304 in return. Swapped coils with #1(it was easier for me to swap coils then to check the plug on #4, I got big hands.) P0300 came back oddly enough. Started pulling injector wires, no change noticable change when #4 was pulled. Went to swap the coils back and then this happens:

2011-09-17 14.27.20.jpg

Close up of broken wire end:
2011-09-17 14.34.47.jpg

It appears to me that this wire has been broken in the boot for some time. I didnt really tug on the wires much, this kinda just popped out relatively easy.

So now for new wires and plugs, since I'll already be down there.

BTW, what anyone know what the cam sensor signal is supposed to look like on a scope? Checked mine and I get the notch pattern like I should, but the flat top of the "peaks" arent flat, they kinda bulge up on some and depress down on others. This sound normal?