Light throttle surging..'96 3100

The car runs great but during light throttle acceleration I have a constant surge. During it's 200K tune-up I replaced plugs,wires,O2's,PCV,Air filter,fuel filter, vac. modulator,cleaned MAF and TB, IAC, TPS and cleaned the injectors an a machine. I'm getting 18-19" of vacuum at idle and Tech II shows no codes, PID's show nothing out of the ordinary. The fuel pump, sock and pulsator haven't been changed because fuel pressure and flow is fine. The pressure doesn't sugre up and down when driven while engine is surging.. It's not a huge problem, but it's sorta annoying. Any thoughts?

EGR pintle and seat clean/sealing? Does the MAP report variation during the surging episodes?

EGR has been removed, cleaned, tested and swapped with a known good one. Seat and passage were cleaned also. MAP shows a nice steady airflow. I'm kinda stumped on this one. Could the fuel pulsator hose cause this? I'm not sure since it goes up for 1 second, then down 1 second. This doesn't sound like a bad pulastor to me..

An easy method to check that is to monitor fuel pressure. I wouldn't be against duct-taping a fuel pressure gauge to the cowl for a test drive.

The car runs great but during light throttle acceleration I have a constant surge. During it's 200K tune-up I replaced plugs,wires,O2's,PCV,Air filter,fuel filter, vac. modulator,cleaned MAF and TB, IAC, TPS and cleaned the injectors an a machine. I'm getting 18-19" of vacuum at idle and Tech II shows no codes, PID's show nothing out of the ordinary. The fuel pump, sock and pulsator haven't been changed because fuel pressure and flow is fine. The pressure doesn't surge up and down when driven while engine is surging.. It's not a huge problem, but it's sorta annoying. Any thoughts?

That's exactly what I did, and tested flow also. I got some looks like, WTF is hanging out his hood?

:D

Since the surging is apparently occurring while the fuel pressure is rock steady, I think you can safely rule out the pump, pickup, pulsation damper, fuel filter, and FPR. I've had split pulsation dampers which "come and go" but a little more consistently than what you are describing.

You may want to revisting the injection. You mentioned that they were "cleaned on a machine" but was it one of thos on-the-car systems or were they pulled, ultrasonically cleaned, inlet screens replaced, and flow and pattern tested after cleaning? I've used the Motor-Vac type cleaners, then pulled the "cleaned" injectors and sent them out to Rich at www.cruzinperformance.com less than 500 miles afterward, whereupon he found over 35% flow differential and terrible spray patterns from the "cleaned" injectors. Spraying some chemical through an injector isn't the same as a thorough cleaning. He's got mounds of empirical evidence showing that. If he's too far, there are similar places on the west and east coast doing the same type of service. There is a reason they are in business - And are usually swamped with injectors. IMO, Motor-Vac is to injectors what "SeaFoam" is to a valve grind - Sometimes you get lucky, but usually you just get a lighter wallet.

Doing a resistance balbance test may help point to an injector that the windings are getting weak. It won't be a difinitive test, but if an injector is way off from the rest, it could point you in the right direction. I also agree with Blue that injector cleaners can't get through a blocked passage, so they don't always work.

That's a good point. What should the resistance be? I was thinking around 14 ohms, but I've forgot...

EDIT: Yep, they were cleaned on a MotorVac...It did smooth out the idle somewhat, but I agree...You can't put cleaner thru a stopped up screen..

I beleive it's in the 12-14 ohm range, but not sure. I ususally just compare readings between all 6 and any that deviates from the rest is the one I investigate. Of course a balance test won't put the finger on a clogged injector, so if that's your issue, other testing is required.

Yeah, thats true, but it would spot a weak injector. As soon as i get everybody else's junk fixed, I'll ohm 'em.

If you connect a diagnostic tachometer and disable the IAC after getting the engine to idle, you can perform a power balance test by unplugging the injectors one at a time. Disconnecting an injector on any cylinder which does not result in a similar or equal RPM drop as compared to the rest of the cylinders can indicate an injector problem. Those possible problems include poor flowing injectors, stuck injectors, or poor spray pattern/streaming injectors. The test is not foolproof, but can be an indication. The IAC MUST be disabled, or the ECM/PCM will simply add IAC steps to maintain target idle RPM, rendering the test useless.

Come to think of it, I did one of those with the GM Tech 2 a while back, but it was before it started surging. You may be on to something....

If you connect a diagnostic tachometer and disable the IAC after getting the engine to idle, you can perform a power balance test by unplugging the injectors one at a time. Disconnecting an injector on any cylinder which does not result in a similar or equal RPM drop as compared to the rest of the cylinders can indicate an injector problem. Those possible problems include poor flowing injectors, stuck injectors, or poor spray pattern/streaming injectors. The test is not foolproof, but can be an indication. The IAC MUST be disabled, or the ECM/PCM will simply add IAC steps to maintain target idle RPM, rendering the test useless.
Great diagnostic advice as usual Blue, to add if you are unsure of which cyl. is missing an infrared thermo will tell you this info as the exhaust manifold branch will be signifantly cooler on the cylinder that is not firing.

Thats true. Although I have manifold covers.... I've done that on race engines with headers, however...

You may be on to something....

Sure, but I try to not let that cloud my judgement. ;)

HAHA!! Hey I got to thinking, it doesn't do it cold, it's starts when the engine goes into closed loop. You think I have an oxygen sensor getting lazy? I'd check it on the Tech 2, but it doesn't refresh fast enough to really tell. Damn, I wish I still had my Fluke multimeter. At least it had a 'scope function..