The car in question is an 89 Buick Century with a 3.3 and a 3 speed auto. The car only has 40k miles on it and it'* in pretty good shape.

Ever since I've owned the car there'* always been a very light stumble to the idle. I always chalked it up to being an older car with relatively primitive engine control/fuel injection. I've put the last 20k on the car with no issues including some very long road trips. However, on a recent long trip, it died rather suddenly and spectacularly as I was passing a tractor trailer with my foot down. It burbled once or twice before doing this but otherwise no real warning. I had installed oil, temp, and voltage gauges which were all reading normal. I pulled over and it would not restart. While sitting waiting for the tow truck I was able to start it once but it ran for about 30 seconds and quit. The local shop called me the next day to say it was running perfectly fine. They told me that fuel pressure was fine (I had told them I suspected fuel pump/filter) but that the alternator was not charging properly under load and that could cause the engine computer to shut down if the voltage dropped far enough. They replaced the alternator and I asked the shop owner to drive it around a few days to see if it would act up. He had no issues all week but as soon as we went to pick it up it drove about 30 feet in the parking lot and quit. This time the mechanic actually got to see and diagnose it while it was acting up so they replaced one of the injectors (#6 I think, far right on the front) and the camshaft sensor. I drove it several hours home with no issues. I drove it around town and out to the local city several times, maybe 1 hour round trip max. One day as I was heading home it started to stumble pretty bad again to the point that I thought I might stall. I nursed it the 30 minutes home at about 55 mph, all the while it was still running poorly as if it were missing a cylinder or something. The next day when I went to start it to hopefully diagnose it, it started very hard. I had to really crank it to get it going and it was not idling well (though it didn't die). I jumpered the diagnostic ports (with the engine off but the key in the on position) and got no code beyond the three iterations of the initial 12 "this car is in diagnostic mode" code. About the time I was considering unplugging engine components to see if I could find one that was affecting things it started running fine again. There'* no rhyme or reason as to when it runs poorly. I've had it stumble/fail at cold and operating temps as well as at idle and under load. It goes back to running fine and works great until it decides not to.

I should mention the car doesn't leak oil and is reading exactly on the full line. I change it regularly and always use synthetic so I doubt there are any issues there.

Any ideas? My current plan is to keep the car close to home and wait until it acts up again, pull over, and start unplugging things one at a time until I narrow it down to a single component. I was considering replacing the remaining five injectors but if the fault is in the wiring then I'm wasting my time and money. Thanks in advance.

I would start with fuel pump, but you already checked fuel pressure. Pull the vacuum hose off the fuel pressure regulator and smell for gas in the hose. If you smell gas, the diaphragm in the FPR has ruptured and you need a new one.

That'* a component I hadn't considered. Thanks for the advice. I'll try that tomorrow.

If you don't find leakage at the FPR, be aware that the first generation Multitec injectors are notorious for intermittent problems, i.e. shorts that shut down the engine. They should measure 12 ohms. Anything less is a partial failure already.
I'm 99% sure the 3300 never had a camshaft position sensor, as it is a multiport fuel injection system, not a sequential. The hole wasn't even bored in the 3300 timing cover for the sensor, unlike the 3800. You can see the untapped hole in my timing cover;

I'll guess it was probably a Crankshaft sensor that was installed, which requires a bit of skill or the special tool though the 1992 model.

Have you done anything with the ICM in the time that you have had the car? These could all be ICM related issues as well, and when the ICM goes, it often does not throw codes. They also like to be intermittent at times too. If the ICM is to blame, it'* likely you will not have spark when the car is not starting.

I loved the 3300 engine.....it was a bear.....had few problems......the only "thing" it was known for, was a slightly rough idle...it was the nature of the beast......it wasn't much, but it was noticeable, when compared to the idle of a 2.8/3.1......

If I remember right this engine doesn't have an EGR valve.....and as mentioned, it has multiport injection, which means 1-3-5 injectors fire at the same time, and 2-4-6 injectors fire at the same time.....so, three injectors are driven by one quad driver in the ECM.......if the quad driver senses too much current from driving the 3 injectors on one circuit, it will shut down the circuit and you will lose 3 injectors.....since it is multiport, they replaced the crank sensor, not the camshaft sensor(since the vehicle does not have one)

Now, you have replaced one injector, and only god knows if it was needed......but I have seen this in the past......when you replace one injector, in a 27 year old engine, those other injectors are jealous....chances are the flow from that new injector, is not the same as the other 5......this can cause, in some cases, idle/acceleration problems......it might be slight, or major, depending on the flow difference.......just bringing that fact up, if you notice a major difference in the idle before this all happened....

As for your current problem, I would be leaning towards two things.....fuel pump and ECM.......

You could drive the vehicle with a fuel pressure gage attached to the fuel rail and have it come out from under the hood and held in place by a wiper blade......so when driving, you can see what the pressure is when the car starts to act up.......

If the car acts up, and fuel pressure is ok, then I am thinking ECM.......those late 80'* and early 90'* ECMs had problems......one thing you can do, is known as a tap test.........the ECM is under the dash on the passenger side if I remember........drop it down, with the harnesses still connected......drive the vehicle, until normal operating temp is achieved.......then pull over in a safe area(do not shut the vehicle off), keeping your foot on the brake, and the car in drive, raise the rpm'* to 1500 and lean over and tap the top of the ECM(the top will have an access panel for the PROM), not the bottom, with your finger tips......if the car stumbles, or the rpms change, you have found your problem.......like I said, this is known as a "Tap Test", we used many years ago, because there were so many ECM problems back then.....

Unfortunately, this test is not foolproof......you can tap the ECM, and nothing may happen, and the ECM still could be at fault......

So, do as some of the guys have suggested.....check that f/p reg vac line for fuel.......could be an ICM acting up......The IAC could be a slight problem at idle, if gummed up....it may even need an idle learn procedure, but that only happens if the battery was ever disconnected, and it'* symptom is a stall when backing up/coming to a stop.....if you were to step on the gas slightly with your foot on the brake, the stall should go away.....but since your vehicle continues to run rough when you do this, I doubt it'* the IAC(Idle Air Control)....

You're right, I mixed up my terminology. Thanks for the advice.

I was considering replacing the rest of the injectors just to be safe. This might just be the nudge I needed to pre-empt that possible issue. I was reading up on someone else with a similar issue (though theirs died immediately and didn't stumble first) and replacing all the injectors solved their issue.

So I checked and the vacuum hose on the FPR smells a little like gas. It'* not real strong but I can definitely smell it. I think I'm going to replace the remaining five injectors (might buy six so they match and keep the one the garage replaced as a spare) and the FPR and put a new filter on it while I'm at it. This is the first time I've ever messed with a fuel injected vehicle'* fuel system. How does one bleed pressure out of the fuel lines after disconnecting the battery? There'* a **** on the side of the FPR and I'm wondering if that'* the release.

You can start the car, and while it is running, remove the fuel pump fuse.....the car will use up the fuel in the line.....

That black round cap on the fuel rail, is the connection for your fuel gage......remove the cap, put a rag there, and depress the schraeder valve to get rid of any residual pressure.....

Before I WOULD REPLACE INJECTORS, I would check that fuel pressure with a gage while the car is acting up(if necessary drive the vehicle with the fuel pressure gage attached), and I would also do the tap test on the ECM.....