justdave wrote:
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