Will do sir. I should be able to do this at lunch, shouldn't take too long.

 

OK, did the test. First off, I am using NGK TR55 plugs.

So, I hooked up the pressure tester, and at an idle it was reading 50 PSI, and the short term trims on both side were even, and right around 0.00 to -1.5. I thought that to be normal. Pulled the vacuum line, and the pressure jumped to 59 PSI, and the short term trims didn't move much, just to the positive side of 0.00 to around +1.50

Seem pretty normal eh? I was going to put the fuel filter on this afternoon.

 

OK, changed the fuel filter for good measure, and no change. Still getting misfires on all cylinders, but, it seems that most are coming from cyclinder 1. Also, another P0300 code. Looking for ideas now guys.

 

I restored the factory PCM programming, and loaded the canned 87 octane tune. Drove it around for a bit. What I noticed is, the throttle response and power was noticeably better. (imagine that) what did not go away, however, was the misfires. The tune might suck, but that is doesn't seem to be causing the misfires. I also noticed that, and reconfirmed, that most of the misfires are happening at idle, or at light throttle. During excelleration, i'm not seeing any misfires on any cylinders.

 

All this input, and no feedback? really?

 

I left the canned 87 octane tune in, and the truck drives great. I'm still showing misfires on the predator, but i'm not feeling them during driving. Filled up with some fresh fuel. What gives?

 

Just a quick update, but I was going down the road today, at cruise speed, and lightly pressed the throttle to gently increase speed (not to exit overdrive), and it started stumbling. Also, let the truck warm up a bit, then sprayed around the manifold and such with carb cleaner to see if the idle jumped, notta. I can't believe you guys are stumped. Toss me a bone or something.

 

Check your fuel trims again. Its hard to believe the truck runs bad, but the fuel trims show no adjustment

 

We've all tossed you something! quit crying! its free info. Bring it to the dealer and ill charge you a 110.00 diagnostic fee. Your all over the place trying to diagnose this thing with 50 different inputs. check this check that blah blah blah. Apparently so far you've checked the MAF, fuel pressure and for any vacuum leaks. I for one haven't messed with a predator, but IMO it doesn't have the capabilities you need. If the truck started stumbling when the converter was locked and you were cruising that would mean the truck was under a load, which to me would indicate something above that has already been mentioned in previous post like a coil breaking down internally. carbon tracked spark plug which is highly common to notice under a load. MAF acting goofy etc... hell even take a damn fuel sample. the possibilities are endless when it comes to driveability. Maybe try to determine if its temperature related. if the intake is leaking vacuum for example it would probably be more noticeable on a cold start and as it warms up it sometimes clears up, just things ive seen in the past. if your intake gaskets havent been changed. it probably wouldnt be a bad idea. up until 04 the LS motors had alot of problems with the intake gaskets collapsing. the new design, not so much. Best thing you can do is try to duplicate the concern and try to get it where you can recreate it and then diagnose from there.

 

Sorry as mentioned this is free help and it's the first time I have had a chance to check in on this.

Fuel trims seem ok but Intake gaskets is a possibility.

How old are the plugs??? How long after they were changed did this issue start???

You said you checked the the plug wires??? How did you check them and what brand are they???

Are you haveing any over heating issues or loss of coolant???

How about smoking or loss of oil???


Aside from that you may need an in depth diagnostic check.