What is going on with my 1999 buick century!?

For the love of god…I hope someone here has some fresh ideas.

1999 buick century custom
3100 motor
200k+ miles.

It all Started 2months ago… once every few weeks my car would hesitate during acceleration (leaving a light for example) but just once.
Then started it up one day and it was idling rough…also stalled out twice.

dropped it off at a shop after hours so he could check it out in the A.M.
he calls n says its not acting up at all.

Get the car back and drive it 2 more weeks…then it happens again. Rough idle and hesitating…take it directly to the shop but its acting normal again by the time I get there (starting to feel like im crazy)

So I take things into my own hands… I change air filter, clean MAF and throttle body. Change the plugs and wires.

start it up and it seemed to run better than it ever has… for about 5mins.
I took it for a test drive and it started sputtering and bucking and eventually died to the point of undrivable.

Got towed home.
NOW it starts dang near everytime but runs EXTREMELY rough for a few seconds before eventually dying. It will run a bit longer if I unplug the MAF (45seconds maybe) but runs like fecal matter the whole time

I have swapped out the MAF, EGR valve and IAC valve with parts from a 97 century (which runs fine)
And nothing seemed to change the condition.

Changed fuel filter
checked fuel pressure with key on (41psi)
Also replaced the timing chain and gears (thinking it slipped) but it didnt make a difference.

I pulled codes but all it had was a P0102 (MAF low input)
But I think that might be from it being unplugged so many times?

need ideas! Im losing my mind.
thanks in advance!

Start with a compression check. Post the numbers.
Then hook a vacuum gage up to look for leaks and/or valve problems.
Do you still have the old spark plugs? How’d they look? Can you post photos?
Is this a distributor-based ignition system? Did you change the cap and rotor? Did you check the distributor shaft for play up & down and side-to-side?

If you’re still searching, test the fuel pump.
If at that point you still haven’t found the cause, you may need to take it to a different shop, one that can hook up a scope to check the operation and stability of the ignition system.

Is a 97 MAF compatible with a 99? If you aren’t sure, I’m thinking your MAF is broke. And needs replacing w/a new one. I’m a little suspicious of the O2 sensor too, since you say it tends to run ok for a little while, but then conks out. That might be b/c when the engine is cold the O2 sensor reading isn’t used by the ECM, but when it warms up, then the O2 sensor reading is used. A bad O2 sensor would result in too lean or too rich mixture. Another idea is there’s something wrong w/the exhaust system. Ask a shop to check for pre-exhaust back pressure.

It’s always good on drivability problems like this that seem to start for no reason to carefully check the entire vacuum system for leaky lines, connections, or leaking diaphragms in vacuum operated devices, esp the brake booster. If all else fail, seek out a shop that has a real time scan tool. You can then check the MAF, O2 sensor signals in real time, as well as fuel trim. Best of luck.

Good advice so far. I would be checking your fuel pressure while its running and while its having a problem to see if its in spec. A bad fuel pump can be hit and miss. Not good enough to check pressure while its not running especially since you have an intermittent issue.

A bad MAF can act just like a fuel starvation issue. I’ve had them pretty much shut me down as well as simply cause a rich condition, and one was a brand new NAPA. The only way I really found it was by swapping the one on my 86 Riv with the same part number from my 86 Park Ave. Sometimes if they are really causing a stumble you can whack it with a screwdriver to see if that changes the condition. If so its bad. Never had any luck just cleaning them.

I dont have the old plugs…they were worn but nothing crazy or out of the ordinary.
No distributer. I was also thinking vacuum leak but I dont know exactly how to find one. I read something about spraying carb cleaner around the hoses and listen for changes in idle (its hard to do that when the engine doesn’t run long)

And George, I believe the 97 and 99 MAFs are the same…same motor, looks exactly the same…installs the same and a replacement part number from autozone is the same. As for the O2 sensor. .it wont run “OK” anymore at all…itll run but very terribly…is it even possible for an O2 sensor cause that?

Its more than just a “rough” idle…its jumping and loping and rocking around like its extremely out of time or something.

Oh yea…the oil light illuminates while its running aswell… forgot to mention that.


here’s an idea . . . unplug the MAF sensor and start the engine

That will cause the PCM to use default values

If the engine runs much better with the MAF unplugged, you may have found your problem

As for the oil light . . . do you mean the red light that comes on when you have low oil pressure?

I would expect that to come on if the engine can’t stay idling properly

Can I assume the red oil light was NOT coming on a few weeks ago, when it idled okay?

Is the engine oil level correct?

Maybe the mechanics will disagree but if you are replacing the MAF, I wouldn’t go to Autozone. OEM would be best.

If the oil light is illuminating at idle, and the idle speed isn’t low, that’s a really good sign that the engine may be just plain worn out. And yes, that would cause your symptoms. Check the oil pressure using a T fitting and a gage (you can get a kit at any parts store).

Oil pressure is created by the pump forcing the oil through the small spaces between the crankshaft surfaces and their corresponding sleeve bearing son the connecting rods and block, and on the camshafts in the same manner. If the spaces wear too big, the pump can no longer maintain the pressure, sort of like trying to keep a balloon inflated that has a hole in it.

Post the results of the oil pressure test as well as the aforementioned compression test.

Check the intake hose hose etc. between the MAF and the throttle body for cracks or other leaks.

I’ll bet my bottom dollar one of your ignition coils is becoming intermittant. I’ve had a lot of experience with these failing on GM V6’s as they get up around 12+ years. I’d change all three.

wow lots of replys! im glad I found this place.

@Bing - sorry I missed your original comment… I wasn’t ignoring your input. I will go rent the fuel pressure tester again and see If I can get a quick reading of fuel pressure in the few seconds its running. Fuel pump was one of my original thoughts aswell. I just don’t want to go replacing pricey components all willy nilly…ya know?

@db4690 - I have unplugged the MAF and started it…it doesnt seem to run BETTER… only LONGER. runs extremely rough for 5seconds before dying with it plugged in and runs extremely rough with it unplugged for 30-45seconds before dying.
also its not the “change oil soon light” its the Oil pressure light… and correct…i have never seen it illuminate til now. oil is at proper lever… and have never had a problem with leaks or burning oil or anything like that previous to now.

@“the same mountainbike” - I will see If I can “borrow” an oil pressure kit from the store… aswell as a compression test. ill post the numbers once I test.

@circuitsmith - I have taken it off several times and looked it over… it was EXTREMELY filthy… my hands were solid black after handling it… ill take it off one more time and give it a thurough inspection and cleaning.

@“MY 2 CENTS” - this has also crossed my mind… but just like the fuel pump I didn’t want to replace expensive components all willy nilly not knowing for sure if that’s the problem… is there a good way to test them? maybe move them around and see if it runs differently?

thanks for all the suggestions guys. I had a thought lastnight too…in order to change the back plugs I unhooked the engine from the motor mounts and ratched it forward with a strap… when I went to let it back down the ratchet malfunctioned and release the tension before I fully pushed the release button…so the motor rocked backwards a lot faster than I intended… I didn’t hear any wild noises…snaps or cracks… I lined the mounts back up and started it up…sounded GREAT… then died 5mins later. I wonder if that violent rocking knocked some dirt around or put some wires where they could get hot.

Oh oh. When you put in the timing chain, did you pack the oil pump with Vaseline to prime it? If not, it may not be pumping oil.

The other thing is that I’m pretty sure the oil pressure sending unit has a low oil pressure switch with it so that if oil pressure is lost, the fuel pump is cut off. There should be a fuel pump test connector laying there (think its blue) under the hood on the drivers side. Hook a test light to that and ground for a while to see if there is interruption in the power to the pump. Also if you are having fuel pressure problems and losing the power to the pump, there is also a fuel pump relay that could be cutting out.

Good plan though to verify oil pressure, verify power to the pump when its shutting down, and verify good fuel pressure when it shuts down. Sure you ain’t got a coil problem? Can you hook a timing light or tester to see if you are losing spark? If its that bad now, maybe it wouldn’t be a problem for the shop to find it before throwing more parts at it.

Others will disagree with me but you don’t have to buy the most expensive coils. I’ve had excellent luck with United Ignition Wire coils which are available from rockauto.com for about $16 a pop.
Switching coils around would just move the misfire to different cylinders.

@Bing Just to clarify, the oil pump on the 3100 is mounted inside the engine, not like the 3800 where it is part of the timing chain cover. Good thought though.

Thanks. That’s good to hear.

A thought . . .

You have 41psi koeo, which is okay. According to my source, you should have 41-47

But do you have good volume?

Well didn’t get to do a whole lot with it today…never made it to get the gauges I need.
however I did visually check all the vacuum hoses for cracks or holes

I also took all(3) the coils off and check them. They all checked out fine.
primary being 0.5ohms (in spec)
secondary 5,800ohms (in spec)

I also found that a heat shield came loose in the rear…and there’s a wiring harness that runs inches above that heat shield around into the coil packs. I cant physically inspect it for damage but I can see both ends of it so im going to check it with my multimeter tomorrow.
wondering if the jolt from the engine rocking back after the plug change knocked that shield loose and it allowed some heat from the exhaust to get that harness n melted some wires together or something.

@bing - yea the oil pump wasnt visible when I took off the cover. The timing chain didnt fix the issue but need to be changed anyway…there was a spot where you could see it was scraping the cover.

@db4690 - good volume? as innnnnnnn?? I did do it the old school way disconnecting the fuel line at the rail…had a tube over the line running down into a 5 gal bucket… had my gf turn the key on to see if gas came out…then again to double check. There was a total of around 0.5" of gas in the bucket from 2 shots (if thats what you mean by “volume”) I honestly dont know how much SHOULD come out.

thanks again everyone!

Good resistance readings on the coils doesn’t necessarily mean they’re good. When operating voltage is applied to the primary the high voltage occuring on the secondary may still result in arcing - especially since it’s intermittant. When you check it with an ohmeter you’re applying a much lower voltage.

Fuel pressure regulators on these things have been known to leak intermittently causing starting, idling problems. It actually leaks fuel into the engine via the vacuum hose that goes to it if it’s bad. When that happens it runs way too rich. Remove vacuum hose to reg and check for the presence of fuel.

@“MY 2 CENTS” - true…maybe I shouldnt have said “they check out fine”…more so, “nothing indicated they were bad” haha whats a better way to check them? Can I put em back on the car and check the secondary with a voltmeter?

@“pete peters” - thanks, I have been pulling the vacuum hose off the regulator once or twice a day while looking for the problem…its dry everytime.