Continuing to try to fix the Buick...
 

My struggles with my baby continue...

The car:

1994 Buick Century, 3.1L V6, Distributorless ignition, common-rail injection, 1-coil/2-cylinders waste-spark system. 50,750 miles.

The symptoms:

Rough idle. The car diles fine for the first few minutes of running, but then settles out into a weaker idle and occasionally shudders. I had it die at idle once last year when I put from P into D.

Smell of unburnt gas. After a highway drive, I usually smell unburnt gas. I've also smelled it once or twice when I've come off of a long 40-45 MPH hour run, but that's quite atypical. The smell seems to come into the interior with the climate control -- when I turn off the blower, the smell goes away; when I turn it back on, the smell comes back.

Bad gas mileage. Like 20 MPG bad. I drive mostly highway or 40 MPH back roads -- to show that it's not a lead foot somehow killing the mileage or something, I've been averaging 38-39 MPG (beating EPA highway by 2-3 MPG :D) in our Corolla all summer under similar driving conditions.

What I've done so far:

A cylinder balance test (shorting out one plug at a time to see if one cylinder is misfiring). All cylinders experienced a similar RPM drop.

Test all coils and wires for proper resistance.

Inspect spark plugs (replaced ~750 miles ago). All in good shape. There was oil on the threads of two of the plugs, but it looked to me like it had been leaking in rather than out, because there the most oil on the area right above the threads.

Clean MAF sensor, confirm that the car loses it's mind when you unplug it so it must be doing something :silly:

Disassemble & clean & test EGR and EGR passage. All clear.

What I'm looking at doing next:

Check that the EGR and MAF are receiving appropriate inputs & making appropriate outputs, as necessary.

Check compression, just because the oil on the two plugs is bugging me.

Go through fuel system. Reseal injectors and make sure that the injectors aren't leaking & have good spray patterns. Check everything else (fuel pressure, fuel pressure regulator, etc.) according to FSM (which I got for $30 on eBay, thank you SO MUCH for telling me to do so, TwitchKitty!)

I was thinking of doing something along the lines of what this guy did to play with the injectors. What do you guys think?

http://forums.pelicanparts.com/porsche-924-944-968-technical-forum/483171-how-do-you-pulse-injectors-during-cleaning.html#post4755411

One obvious thing is new air filter? And clean the mouse nest out of there :) Question when "The car diles " does it have to use 10 digits? :D

Replace the o2 sensors.

I'll check the air filter, yeah.

The car has to use 11 digits. It's very confusing.
Wouldn't a bad O2 throw a code? I wouldn't want to spend $200 on two new sensors unless I have to. I'm thinking I wouldn't want to replace them unless I've exhausted all of my cheaper options.

The FSM (I think it was the FSM?) did suggest pulling the O2 sensor to make sure that it's not dirty and not working right.

EDIT: Whoa, you can get a Denso, NGK, or Bosch O2 for this car at NAPA for as little as $20. I may look at that after he fuel system has been confirmed OK, then.

Why would the same brand have two O2 sensors, both meant to go upstream of the cat, that vary widely in price?

What is the warm-up RPM? wondering if the warm up idle speed is masking the problem. kinda sounds like a fuel leak and a vacuum leak. Kinda like the idea of replacing the O2 sensor. Still, the fuel smell sounds like a small leak under the hood. Let it run for awhile in the driveway and see if there are any drips or you can spot any leaks. (fire hose at the ready!). Take a real good look at the vacuum lines/fittings and injector seals.

I too think it is warm up related - there's probably a connection to a (possibly separate) temperature sender that then goes off the the ECU which then does some jiggery pokery to adjust the thingumy thang thing...

The car doesn't have tach, so I have no idea what the warm-up RPM is. Interesting point about it masking problems, though. I've idled it in the driveway for a while already and haven't noticed any leaks, and plan on resealing the injectors when I get at them (have seals on hand). The injectors are under the upper intake manifold, so I'll do those when I do the intake manifold gasket (maybe tomorrow).

Are you suggesting that there is some sensor that is sending bad info to the ECU and screwing up the warm-up idle? I'm not so good with all the technical lingo you packed into your post there :).

I should add that, at idle when semi-cold and in gear at a stop, you can definitely feel nonstandard vibrations and jolts through the wheel and brake pedal.

Well it is er all about that there um electrickery innit?

Don't feel bad about not knowing the technical lingo - you'll get there in the end.

(All I can say is that my M102 works that way)

Now steady on there old chap there are only good good good vibrations...

...if you're going to get all technical get your bloody terminology right - don't start making stuff up!

Unplug the vacuum line going into the fuel pressure regulator, see if its wet with the fuel. Seen more than a couple fail on 60*s.

I think you're overlooking the easiest fix.

Dye your hair grey and drive around 15 mph under the speed limit with your left blinker on for the entire day. As Buick intended.

i would place my bet on o2 sensors. a sensor can often be "lazy". you have to remember, this is an obd1 car. obd 1 is a whole lot dumber. does the coolant temp read properly on the dash?

if the temp is not reading correctly, it could be failing to go into closed loop mode. hence the raw fuel smell. you really are working blindly without a scan tool that has live data mode.