OBDII Code P0300 '01 Century

ledzilla

Full Member
200
14
18
Buick Ownership
2001 Century Custom
So, a couple weeks back, starting the car to head home from work, it was running really rough unexpectedly. Get it home, hook up the scanner, and was getting P0300. So, idle is rough, light acceleration is really rough. Picking up speed and running at higher RPMs it runs more smoothly. And it also seems to be chugging gas. I thought at first it might be the fuel pressure regulator, but after finally being able to replace it, no change has occurred. The EGR, plugs, wires, coils, ICM were all replaced in 2016. So maybe it's a vacuum issue? I mean, it seems fairly similar to when I accidentally cracked the plenum and caused a vacuum leak, just nowhere near as severe (I broke off a fairly large chunk from the corner, and it would only start and stay running with a lot of open throttle). But there isn't a whole lot of anything that seems to be connected to vacuum, and it all looks ok at a glance, so not sure what needs to be checked next. I was thinking I would test the vacuum off the test port, but I don't know if my work with older carbed motors is clouding my judgement, since it's running so rough I'm not sure it would be making good vacuum in the first place.
 

2007LucerneCXL

Full Member
2,406
725
113
Illinois
Buick Ownership
2007 Lucerne CXL. and 1995 Lesabre currently, past 1973 Riviera, 1968 Riviera
As most of the components have been replaced as a option maybe make sure they are still working properly. Depending on where the replacement parts came from they may have gone bad, maybe - maybe not.

Here's a site that might help on the code and testing which could point you in the right direction. The code is one of those could be anywhere types and it doesn't really help in getting to the cause.
Part 1 -GM 3.1L, 3.4L Index of Articles
 

hitshane

Buick Newbie
39
9
8
Saint Petersburg, FL
Buick Ownership
2005 Century Custom
That code is more of a misfire code but it could mean you're running lean. Check and make sure your coils and wires are properly connected to your spark plugs. You could try getting some MAF cleaner and cleaning your mass airflow sensor but do not touch it. On my Century not all of the vacuum ports were used one of them just had a block off and that block off would actually dry out and crack about once a year needing to be replaced causing a vacuum leak.

Did you possibly get some bad gas? Never fill up while the fuel truck is at the gas station and only buy from top-tier fuel providers like Shell, Texaco, or Chevron. TOP TIER™ Gasoline Brands

Fuel pressure regulators are often faulty especially the off brand names. I got three of them broke in a row from the auto parts store before I got one that worked don't go cheap. But, a faulty fuel regulator usually results in a rich bank code because fuel gets sucked through the diaphragm into the intake vacuum line.

Grab a can of carb/brake cleaner and start spraying around the vacuum lines and plenum if you hear an RPM increase that area has a vacuum leak.

It takes three things to make an engine run. Compression, ignition, and fuel. Diagnose the easiest things first. You could go to the auto parts store in rent a fuel pressure meter, you could rent a cylinder compression tester, if you don't have one you can probably rent a meter to check voltage and resistance across your coils. You could also grab a scan tool that allows you to watch fuel trim real time or offers logging to get a larger picture of what's going on.

Easiest solution is that you got bad gas try to burn it off and go fill up somewhere else from a brand name station. Physically check every inch of your vacuum lines with your fingers they tend to crack on the bottom where you can't see it. I also secure them on the barbs with zip ties.

One final statement most of the fuel at the gas stations is enriched with ethanol which has a shelf life of 120 days at which point it phase separates due to moisture in the air that it readily absorbs. These little Globs that look like glue or plastic balls wreak havoc on your fuel system. It happens with old gas or a fuel station that has water in its tank. On top of that older vehicles are not made for ethanol enriched gas and it eats the rubber and silicone diaphragms like in the fuel pressure regulator. After I rebuilt my entire Buick Century engine I only used ethanol-free gasoline, sometimes it's called Recreation/boat gas. It's a little more expensive but after putting so many hours and so much money into a used vehicle I didn't want to have to do those things again so I stuck with ethanol free the rest of the time I had that vehicle. Newer vehicles have Teflon lined fuel lines and different materials used in their gaskets to prevent corrosion from ethanol alcohol.

Top tier fuel providers already put engine additives and cleaners into the fuel. You could try adding a STP octane booster to momentarily get rid of the rough running while you cycle through all of that bad gas.
 
Last edited:

LARRY70GS

Full Member
2,065
109
63
Oakland Gardens, N.Y.
Buick Ownership
98 Riviera, 70 GS455 Stg1
Try disconnecting the MAF, and see how it runs. It will cause the CEL light to come on, but it's a cheap and easy test.
______________________________
 

ledzilla

Full Member
200
14
18
Buick Ownership
2001 Century Custom
I find it funny that my attempt to post a reply previously never actually posted. Anyway...

I know the fuel isn't old, and the last time I put in gas the fuel truck wasn't around. As far as ethanol-free gas goes, I haven't seen it for sale around me, which would be great for my carbureted vehicles. I really do suspect a vacuum leak, and I'll double check the three vacuum points that actually exist. Or maybe there are four... Frankly, I'd be pissed if it really was the pressure regulator and the replacement I have was faulty. I've been dealing with that for the starter on my old F-250. One faulty starter after another, repeatedly replaced on warranty. I finally gave up on it, and I'm just getting a different brand from elsewhere. But with the pressure regulator, at least now it won't be a nightmare swapping them out, now that I went through the trouble of using an air hammer to remove the old screw and used a nice new replacement that threaded in very smoothly. I do have a compression tester and OBDII Bluetooth adapter with the Torque Pro app on my phone, which is how I pulled that trouble code. I figure it couldn't hurt to test spark, so I grabbed a HEI spark tester yesterday, just to make sure none of the coils or the ICM failed. And I do have a couple multimeters floating around, which get used regularly. If I find spark failures, I'll certainly be busting one of them out for testing the ICM/coils. I also cleaned the MAF a little while back, and shouldn't be dirty, but it could be going bad, so yeah, I'll check that out, too.
 

LARRY70GS

Full Member
2,065
109
63
Oakland Gardens, N.Y.
Buick Ownership
98 Riviera, 70 GS455 Stg1
When my MAF went bad on my 98 Riviera, the engine would not run in closed loop. It would start and run cold for about 5 minutes, and then it went rich and would stall. I attempted to clean the MAF twice, but it did no good at all, had to replace it. The engine would run with the MAF disconnected. If you suspect a vacuum leak put a vacuum gauge on it. My 98 3800 pulls 20" of vacuum at idle fully warmed up in gear. Coils rarely go, but when they do, they effect 2 cylinders as each coil fires 2 companion cylinders. On the 3800, they are 1-4, 2-5, 3-6. I just looked, the 3.1 V-6 has the same companion cylinder pairs. OBD2 maintains a running count of misfires by cylinder number. You can view the counter with any good scan tool. Misfires put raw fuel into the catalytic converter which will damage it, so misfires that happen frequently will trigger a CEL. If you see any pair of companion cylinders misfiring, suspect that coil.
 
Last edited:

ledzilla

Full Member
200
14
18
Buick Ownership
2001 Century Custom
I got some spark testing done before I drained the battery too far. I verified visible spark from #2, and I could hear, but not see, spark from #3 and #5. Waiting for the battery to charge to re-check #4 and #6 because when I tested them I couldn't see any spark, but from where I was positioned I wouldn't have been able to hear it (wasn't thinking about listening for spark when I tested them). #1 has no spark at all, from what I can tell. No visible or audible spark.
______________________________
 

ledzilla

Full Member
200
14
18
Buick Ownership
2001 Century Custom
Ok, so confirmed no spark from #1 and #4, both on the same coil. Confirmed no spark from both towers on the coil. Confirmed power to the supply terminal on the ICM, and I seemed to confirm proper switching for that position on the ICM. So, seems like bad coil. Got a new coil, and still no spark. So not sure what the deal is.
 

ledzilla

Full Member
200
14
18
Buick Ownership
2001 Century Custom
Wanted to double check that the switching signal was good, but I can't remove the coil. The head of the mounting bolt closest to the firewall rounded off. So... I think i can just remove the other coils and the mounting bolts for the ICM mount plate, and then I can figure out how to get that bolt out.
 

hitshane

Buick Newbie
39
9
8
Saint Petersburg, FL
Buick Ownership
2005 Century Custom
When my MAF went bad on my 98 Riviera, the engine would not run in closed loop. It would start and run cold for about 5 minutes, and then it went rich and would stall. I attempted to clean the MAF twice, but it did no good at all, had to replace it. The engine would run with the MAF disconnected. If you suspect a vacuum leak put a vacuum gauge on it. My 98 3800 pulls 20" of vacuum at idle fully warmed up in gear. Coils rarely go, but when they do, they effect 2 cylinders as each coil fires 2 companion cylinders. On the 3800, they are 1-4, 2-5, 3-6. I just looked, the 3.1 V-6 has the same companion cylinder pairs. OBD2 maintains a running count of misfires by cylinder number. You can view the counter with any good scan tool. Misfires put raw fuel into the catalytic converter which will damage it, so misfires that happen frequently will trigger a CEL. If you see any pair of companion cylinders misfiring, suspect that coil.
Yes misfires dump raw fuel into the catalytic converter just like a failing fuel pressure regulator but both of those instances should also throw a rich fuel bank code.

Since it looks like you had bad coils and you've been dumping Fuel into your engine make sure you go get your oil changed once you fix the problem.
 

Zeroboostbuick

Well-known member
1,174
312
83
S.W. Ontario Canada
Buick Ownership
2018 Regal Sportback - 2010 Yamaha R6 (track bike) Past cars: 92 LeSabre, 98 Regal, 02 GrandPrixGT
Also a bad coil can result in the spark plugs affected getting fouled.
______________________________
 

ledzilla

Full Member
200
14
18
Buick Ownership
2001 Century Custom
Ok, so it ultimately turned out that the problem was a bad ignition control module. When I first tested the slot for the #1 and #4 coil, it seemed to show that it wasn't signalling the coil. I thought maybe I had goofed up, so I tried on a known good slot and got results indicating the slot was getting signalling. So I tried the trouble slot again and it seemed to look like it was good this time. So I (incorrectly) deduced it was a bad coil.

I went to the local NAPA and got a new coil. After installation it was the same problem. I wanted to retest the signalling in that slot, but I couldn't because somehow the head of the bolt that mounted the coil towards the rear rounded off. So, I then tested the "bad" coil in a known good slot, and ti was working correctly. So, there was my final confirmation that the problem was indeed the ICM. I ordered a new ICM and a new set of mounting screws for the coils. The ICM came Monday, and I was able to get it installed, and only used one screw on each coil, lightly secured. I had to get the Dremel out to cut a slot in the bad bolt, and used my impact driver to remove it, all so I could get the mounting bracket freed.

With the new ICM in place and the coils lightly secured, I did a spark test and I was now getting spark on #1 and #4. I reconnected the fuel pump relay and started it up. Nice and smooth. Backed it out of the garage, down the driveway, and pulled it back in. All still smooth, no hesitation. Last night the new bolts arrived, so I got everything all tightened properly with the new bolts (the originals were so rusty I no longer trusted them after that one rounded off).

So it's running all nice and good again. There was minimal travel in this condition, so I'm not too concerned about fouled plugs or oil dilution. I really only needed to get the car home from work, and all further operation after that point was moving the car around and testing fixes.
 

ledzilla

Full Member
200
14
18
Buick Ownership
2001 Century Custom
Right on.
Thanks for the detailed diagnosis. Someone should find it helpful.
The article below was actually quite helpful in providing a task list to fully test the ignition system. I'm just bugged that both the three-year-old ICM went bad outside of warranty, and that during my testing it decided to provide signalling just one last time to throw me off the diagnosis. So yeah, follow that article, it's very good. And I was lucky that my test light also had a speaker in it, really helped with testing the signalling. I highly recommend that if you're testing anything with ignition, you test everything more than once, and always validate against anything that should be working. Could have saved some trouble if I retested the signalling a couple more times, but at least I now have fresh coil mounting bolts out of it.

Also, the coil bolts are discontinued, and not a common size. I had to find them on eBay. Some people want a crazy amount of money for these bolts. They're not worth $10+ each. I had to change one order out because somehow it racked up something like $23 in shipping for six lightweight bolts.

As most of the components have been replaced as a option maybe make sure they are still working properly. Depending on where the replacement parts came from they may have gone bad, maybe - maybe not.

Here's a site that might help on the code and testing which could point you in the right direction. The code is one of those could be anywhere types and it doesn't really help in getting to the cause.
Part 1 -GM 3.1L, 3.4L Index of Articles
 

2007LucerneCXL

Full Member
2,406
725
113
Illinois
Buick Ownership
2007 Lucerne CXL. and 1995 Lesabre currently, past 1973 Riviera, 1968 Riviera
Glad it worked out. Had a new alternator I installed, about a week later heard some noises and figured the water pump was going south. Pulled the belt and spun the alternator and noticed it was making the sound. Took it back for a replacement and for giggles spun the replacement at the counter, yep same noise, got another one and spun it ahhh no noise. So while we all assume it's not a new replaced part sometimes it's just that simple, too simple at times LOL
______________________________
 
Top