Misfire p0304 with some weird symptoms!

[ollie2012]

Hey all. Some interesting stuff has happened to my 2003 bonnie sle. Please bear with me, as i want to give you all as much info as possible.
Last night i felt the car shudder as i was accelerating off a stop light (not launching or anything, just starting off normally). As i applied the throttle, it continued to shudder and refused to give me power. My SES light flashed for about 10 seconds. I stopped the car and started driving again to see what was going on and it continued to shudder right when i hit about 1600 RPMs. I ran a diagnostic and it threw code "P0304 misfire in #4". I checked it out in reverse and the car has no trouble going up its RPM band in reverse. I revved it up in neutral and there were no problems, it only stopped when it hit the limiter at ~3900. Then i started to drive, put it in neutral and revved it up to bypass the limiter. It went through the band just fine with no problems.
I changed the plugs and wires this morning. No change in the problem, p0304 kept up on the scan. So i tried to clear the code. Surprisingly, after i cleared it, the car drove fine for about two minutes of starting and stopping. then it came back. when it came back, i stopped and started accelerating real slow and smooth and it went up and went into 2nd just fine. I stopped again and started accelerating at about half throttle and sure enough at ~1600-1800 RPMs it started to shutter again and wouldn't let up.
It seems as though the engine only misfires when the car is in gear or in drive (i tried it locked in 1st, same thing), but NOT in REVERSE. Can an engine misfire, or engine sensor somehow affect the transmission behavior? In all my auto maintenance experience i have never run into a problem like this. All the possible solutions just do not make sense with how the transmission is behaving.
I could only think of something like the misfire is only happening at a certain throttle position or something like that, but then it would throw a different code along with the misfire. There were no p0300 codes or transmissions codes in the scan. PLEASE HELP! I am completely lost. Thanks!

[justdave]

Get a "real" fuel pressure gauge, attach it carefully (no leaks allowed! under-hood) and check the pressures you get AT the fuel rail. Key "on", engine "off", you should see a bit less than the spec for your specific year&engine. Try that engine "off" test multiple times, allowing 10 seconds between. Record your readings. Then, IF you are SURE there are no fuel leaks/dribbles/weeps from the fuel pressure gauge connection...start the engine and see what you get. If your model's fuel pressure regulator (usually somewhere on the fuel rail) has a vacuum hose connected to it's top, then your pressure regulator 'should' put out slightly more pressure AFTER the engine is running, than your earlier readings. And when you 'blip' the throttle, you should see the pressure drop temporarily 3 to 6 PSI on the gauge (as the injectors are spraying fuel faster than the regulator can respond).
*If your fuel pump is not supplying enough pressure, the engine will do the 'shudders'.
You mentioned nothing about your LIM and UIM plastic intake manifolds situation. See the Techinfo and other posts about lower intake gaskets failing at around 50K miles and up, from Dex-Cool eating them; you may have coolant spraying into No.4, or a cute little vacuum leak from a section of soggy gasket moving around and leaking only at a certain level of intake vacuum. [And let us not forget the infamous UIM, that has the EGR heatpipe routed right through it, touching the plastic from the Factory, and turning it to 'ash' in that area; and guess what runs on the other side: coolant.]
I would next hook up a "real" vacuum gauge (i.e.- 0-to-30 inches should be a full 270 degrees around a large 3" or 4" dial; AVOID the combination vacuum/fuel-pressure garbage gauges...their fuel-pressure side reads only up to 10PSI anyway...useless for modern fuel injected cars). You should be seeing something like 18-to-20 inches of Hg on the dial, at idle. If not, you have a vacuum leak. When you blip the throttle, the gauge should head instantly toward zero.
If you can route a 3 or 4 foot long hose around the hood seals without pinching the hose, try mounting the vacuum gauge temporarily up near the wiper so that you can see the dial while driving
(duct tape or what-have-you). You will soon get the hang of what correct readings should be, as you accelerate and decelerate. (Deceleration puts the needle toward the "30" end of the dial.)
Watch for fluttering of the needle (not normal), or an unusual drop in vacuum as you drive the car under all conditions/speeds/levels of acceleration. [And keep your eyes on the road! Don't cause problems for other drivers. Safety first!]
And for squirrely causes of weird problems, check your engine mounts; if your engine is moving too much under acceleration/deceleration, hoses and wiring will get stretched, hard-to-find cracks in hoses and the infamous GM black nylon lines will occur intermittently, and wiring harness opens can occur. Many of the engine mounts get soggy and completely useless when the usual oil-weeps (from old engine seals and gaskets) collect on the 'rubber' elastomer over the years.

[ollie2012]

Get a "real" fuel pressure gauge, attach it carefully (no leaks allowed! under-hood) and check the pressures you get AT the fuel rail. Key "on", engine "off", you should see a bit less than the spec for your specific year&engine. Try that engine "off" test multiple times, allowing 10 seconds between. Record your readings. Then, IF you are SURE there are no fuel leaks/dribbles/weeps from the fuel pressure gauge connection...start the engine and see what you get. If your model's fuel pressure regulator (usually somewhere on the fuel rail) has a vacuum hose connected to it's top, then your pressure regulator 'should' put out slightly more pressure AFTER the engine is running, than your earlier readings. And when you 'blip' the throttle, you should see the pressure drop temporarily 3 to 6 PSI on the gauge (as the injectors are spraying fuel faster than the regulator can respond).
*If your fuel pump is not supplying enough pressure, the engine will do the 'shudders'.
You mentioned nothing about your LIM and UIM plastic intake manifolds situation. See the Techinfo and other posts about lower intake gaskets failing at around 50K miles and up, from Dex-Cool eating them; you may have coolant spraying into No.4, or a cute little vacuum leak from a section of soggy gasket moving around and leaking only at a certain level of intake vacuum. [And let us not forget the infamous UIM, that has the EGR heatpipe routed right through it, touching the plastic from the Factory, and turning it to 'ash' in that area; and guess what runs on the other side: coolant.]
I would next hook up a "real" vacuum gauge (i.e.- 0-to-30 inches should be a full 270 degrees around a large 3" or 4" dial; AVOID the combination vacuum/fuel-pressure garbage gauges...their fuel-pressure side reads only up to 10PSI anyway...useless for modern fuel injected cars). You should be seeing something like 18-to-20 inches of Hg on the dial, at idle. If not, you have a vacuum leak. When you blip the throttle, the gauge should head instantly toward zero.
If you can route a 3 or 4 foot long hose around the hood seals without pinching the hose, try mounting the vacuum gauge temporarily up near the wiper so that you can see the dial while driving
(duct tape or what-have-you). You will soon get the hang of what correct readings should be, as you accelerate and decelerate. (Deceleration puts the needle toward the "30" end of the dial.)
Watch for fluttering of the needle (not normal), or an unusual drop in vacuum as you drive the car under all conditions/speeds/levels of acceleration. [And keep your eyes on the road! Don't cause problems for other drivers. Safety first!]
And for squirrely causes of weird problems, check your engine mounts; if your engine is moving too much under acceleration/deceleration, hoses and wiring will get stretched, hard-to-find cracks in hoses and the infamous GM black nylon lines will occur intermittently, and wiring harness opens can occur. Many of the engine mounts get soggy and completely useless when the usual oil-weeps (from old engine seals and gaskets) collect on the 'rubber' elastomer over the years.

Thanks for all the helpful advice! I was wondering though, if it was fuel pressure, compression, or a leak somewhere, why does the car still operate just fine in neutral and in reverse? the idle is also solid. wouldn't one of the problems mentioned above cause a more consistent shudder or misfire? Also, when i cleared the ECU data, the problem DID go away for about two minutes of driving, then it started up again. This would lead me to think its a sensor problem. Any thought? Thanks

[Mechanical Mike]

Some of the symptoms sound like you might have a plugged catalytic converter. Have you heard any sounds like marbles in a tin can or whooshing from the tailpipe. You might want to try to shake the cat & see if you can hear any rattling.

There's less of a load on the engine when in reverse or neutral, hopefully you don't have a tranny problem.

You should also check the coolant level. let us know what you find.

[ollie2012]

Coolant level is good, although i did have a coolant leak last weak. Elbow cracked and was spraying fluid out, nothing found in the block or oil.

No strange noise coming from the tailpipe.

And when i had it in reverse i accelerated pretty hard through the whole band a few times just to make sure, and there wasn't an issue.

Here's something interesting. I just got back from a 20 minute drive around my town without any problems.. not a shudder, no flashing SES, nothing. I thought at first it maybe only occurred when the engine was hot, so i kept driving. Sure enough, right under 200 degrees where it always is and still no sign of this problem!! what is going on!?! i didn't clear the ECU or anything. I parked it last night after driving with the shudder, and now today all is well (here's hoping).. I know something is still up so I will check all of your suggestions, and many thanks for those I do appreciate them. I would also like to hear more suggestions you all may have, and will check them all to see if i can loacte the cause. Thanks