Solution to hesitation/stutter at acceleration and at cruise. No code.

Symptoms: Fuel efficiency decrease. Motor idles normally. Runs fine cold (open loop). When hot (closed loop) it will start to stutter/hesitate when feathering the throttle at light acceleration and on highway (cruise). Engine still pulls at WOT. CPU not throwing a code.

Troubleshooting: Ok, so not many leads to go on. Many people will go for the bad opti and wiring. If you can start your car no problem on super damp mornings, then no. Bad fuel pump?! Idles fine and still runs like a demon at wot. Not the source. Bad O2? I'm not getting a code. So probably not. Bad compression? Does not smoke, idles fine and still beats the pants off of a civic. My guess is that's it's fine. Next in line is a vacuum leak. But oddly enough, It passes.

Answer: It is a vacuum problem. It I due to a defective EGR valve at the diaphragm. The problem is that the EGR is only solicited in a closed loop environment, and under partial throttle. Not at idle and not at WOT. That is why it passed the vacuum test as it was done on idle. So, as soon as the EGR kicks in, it creates a top end vacuum leak (lean fuel mixture) and throwers the CPU out of wack, as it's trying to compensate by dumping more fuel until the leak gets so bad that the CPU can no longer compensate. Making the problem get worse and worse.

Conclusion: This problem is one that could throw the best mechanic for a loop, and can get quite expensive and frustrating to fix. A super simple test that ever body can do, in a matter of minutes. Is to disconnect the EGR vacuum hose. Though the openings, stick your thumbs in and push the diaphragm back. Then put your finger on the vacuum opening releasing the diaphragm. If your EGR valve remains still. It passes the vacuum test. If your diaphragm reverts to its original position (closed). Change you EGR. Your car will thank you for it. So will your back pocket.

Good info. Another qualifier for the EGR to operate is the vehicle must be in gear. Another way to test the EGR is to put the car up against the wall (or lift the rear wheels) put it in gear and then check it that way.

i don't think you can tell if an EGR valve is good by the method described by Woody Wood Beaver because there is a bleed orifice open between the vacuum port side of the diaphragm and the bottom end of the valve. At least that is the way the EGR valve is made on my '92 OCC.  I tested it as Woody described and it would not hold a vacuum, so I took the valve apart and discovered the orifice through the center of the pintle that prevented it from holding vacuum.  The OCC service manual also shows this orifice through the valve.  So the valve I took apart may have been OK.  If anyone knows something different I would be interested in hearing about it.

There are two types of EGR valves, and our cars have a positive pressure activated valve. The EGR valve requires that the exhaust has pressure in it to allow the valve to open. The small port/orifice is probably there to close an internal valve allowing the vacuum to actuate the EGR valve.

I have spent months working on a very similar issue in my 95 RMW w/ 165k.  Allowed a mechanic to throw parts at it for a little while before saying screw it, I'm going to diagnose properly and fix it myself.

Symptoms were very similar to what you mentioned above, the only exception being that idle was not exempt from issues.  Only happened when it was warm.   Sometimes it would hesitate or stall at idle (different than what you described), sometimes feathering the gas, sometimes at cruise, never at WOT.  So seemed to be that open loop was fine and closed loop was not.

I replaced EGR, replaced MAF sensor, MAP sensor, checked vacuum.  

Finally being frustrated and fed up, I bought a USB cable (http://www.aldlcable.com/) and installed Datamaster on my laptop, got it hooked up everyday on my way to work, and waited for it to happen - of course it took 2 weeks before the problem came back when I had it hooked up, but I finally caught it in the act.  

Fascinating result: it was the Engine Coolant Temperature sensor.  The sensor would show 200+ degrees one minute, then 25 degrees the next, and then intermittently (but not rapidly) would flip back.  As a result, the engine (in closed loop at 200 degrees) was jumping into open loop, and then back again.  When it did, the computer was apparently giving bad inputs to the air/fuel mixture, telling the EGR to open, then close, and the car - obviously not a cold engine - was reacting badly.

I replaced the ECT with a Delco Professional this past weekend - not a terrible job except for removing the knock sensors (hint: remove the starter!) - and the problem has gone away.

Apparently there are a number of different things that can cause this kind of hesitation issue, and the lesson I've taken away for sure is that the best way to deal with it is to hook up the car to a computer and do a proper diagnosis.  Wish I'd have done that months ago.

I went to TTS Power Systems site and they are no longer selling the Datamaster software for GM  cars.  They used to allow free use for 20 times but I can't find the software since they took it down.  You must have gotten yours just before they pulled it.

There is an Android app that works via Bluetooth I might try.  The OBD1 connector to bluetooth device is $75.  I would also have to get an Android tablet since all I have now is an iPad and a Macbook.  

My car sure acts like it is an ignition problem but I am a newbie to TBI systems.

Sorry to hear that!

I downloaded it in April. I just looked and called them (I still have 4 free uses left) and they won't take my money for an authorization code. Bummer - it worked great! It's a total shame to have them just stop selling it.

So much for Datamaster, there are other OBD1 software programs.
This is the one I intend to try, and its free. [You must be registered and logged in to see this link.]