Need help with a GM 2.4L - not in a Z

[Guest bluex_v1]

This is sorta Z related: I can't pull my donor (and current transportation) car's engine until this is worked out and we have at least one reliable vehicle and we aren't going to sell this car to buy a z32 2+2 for a turbo swap either until its running right again. Its my fiancee's '96 Sunfire with a 2.4L DOHC engine and manual transmission, about 90k miles. I've searched the j-body.org site, but being what the sunfire sales demographics are, there aren't too many people on that board with real-world experience..including me.

This has just developed over the course of a few weeks in a stock commuter car that isn't driven hard. It idles nice and smooth..it is just when accelerating quickly from between 1k-3.5k RPM it stutters. Once it climbs above 3.5k RPM it accelerates pretty smoothly. The stutter seems a little worse before the car gets up to operating temp, but is still very much noticible even after driving around for an hour. It actually drives just fine through the entire range as long as you are only giving it about half throttle. It is not backfiring or smoking and the exhaust smells normal to me. It is not illuminating the check engine light, which does work at startup. I ran a bottle of expensive chevron injector cleaner in a tank of premium. I replaced the plugs last night and it had no effect..the plugs themselves I think were overdue for a change, but they didn't seem to have any abnormal wear or fouling. One thing I thought about while doing that though is it seems to have an awful lot of pressure when taking off the oil fill cap (ie trying to top off the oil when idling results in oil blown around everywhere but down the fill hole), so I'm wondering if the oil/air seperator thing (glorified PCV device) is clogged up. Unfortunately the haynes book basically says to take it to a mechanic to check this and doesn't even show pics of what the thing looks like. I don't really see how that could cause this problem anyway though. I will replace the fuel filter tonight, but I'd be really suprised if that was the cause since it doesn't have a problem at wide open throttle at higher RPM. So I think I'm down to really clogged injectors (which I kind of doubt since its drivability just took a nose-dive instead of progressively getting worse) or some peice of emissions control equip (which if it is that, I'll be pissed that it didn't throw a check-engine error). Aside from that it really all feels like a timing problem, but what can you do on a distributorless ignition system eh?

I can't see how this could be related but, probably 5 months ago I replaced the water pump. As with many things on this engine, this relatively simple maintenance problem was a huge pain in the @$$. It required jacking up the engine and pulling off the timing chain because the water pump is driven by it. After putting it back together, I noticed it made sort of a muted rattling/sliding noise at idle that went away as the RPM got above 1200...sounded as if the timing chain had slack in it (Now I can't be sure that it didn't make this sound before I touched it or if I was just super sensitive to the noise since I had never done this before). So I pulled it back apart and checked the oil pressure actuated tensioner shoe, which seemed fine but I replaced it anyway, however this had no effect. I closed it back up and it seemed to drive fine and I just assumed it to be relatively normal.

Aside from any most-welcome advice that any of you want to give, I have a couple questions..

If this were even a minor valve timing problem, wouldn't there be a performance loss at all RPM and probably backfiring?

Shouldn't OBDII checks warn me if there were any components malfunctioning to this degree?

How do I check and/or clean injectors to rule this out?

Thanks for taking the time to read all this!