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Stumbling Bonnie

My wife’s '97 Bonneville has recently begun acting up. It especially seems to not like the cold weather. It starts hard then the motor will fluctuate and hesitate and die. It usually stays running the second time. Then when I put it into gear pushing the throttle doesn’t do anything. It seems to stumble and falter. Sometimes, if I pump the gas it works other times if I move the pedal up and down slowly I can find the “sweet spot” and it will accelerate.

I’m not sure if it’s related or not, but sometimes going down the highway on long trips it will seem like the transmission is slipping and the car lurches or slows down. It’s really weird. Then if you shut it off, it won’t turn back on for 15 minutes or so.

Any help would be greatly appreciated. We don’t have too much money so we have to limp this baby along as long as possible.

Check or have the car checked for vaccuum leaks. I’m no expert on the subject but you have two problems that seem unrelated except that both of your symptoms can be explained by a vacuum leak.

is the check engine light on? if it is have the codes read and post back on here. if not then you should take it to a good local repair shop and since this has obd11 it could be scanned and have the onboard system monitored. that should be able to tell you the problem

Is There Some Significance To Your Listing “EGR” And “Catalytic” In The TAGS Area ?
Have There Been Problems With These Items ?
Do You Have More Information ?

How many miles on Bonnie ?

Bonnie May Be Keeping Secrets From You !
As Jimgray suggested, get the OBD2 (On Board Diagnostics) DTCs (Diagnostic Trouble Codes) read and post them right here. Many (Most ?) large “chain” auto parts like Advance, Autozone, etcetera, offer this service free of charge, “We don’t have too much money . . ..” They will come out to your car in their parking lot with their reader, plug it in and give you any codes that are recorded if any exist. Many (Most ?) of these stores are open all day Sunday, too.

There are several mechanics who frequent this site and can often make a pattern of seemingly random codes and offer advice. Without the information, It’s just Wild Axx Guessing that goes on by lots of well meaning people.

Write the codes down as you get them. They will look like PO133, PO420, etcetera. Put 'em up.

CSA, Proud Bonneville Owner / Operator

Yes the check engine light is on. I’ll take it in tomorrow to get the codes.
It’s done this before and the only code was the “multiple/random misfire” code. We’ll see what we come up with tomorrow. As for the EGR and Cat, those are just two things I thought it might be so I added them to the tags to get more searches. That’s it.
It has 165,000 miles on it. I put new plug wires on less than 10,000 miles ago and I change the oil and filters religiously.

I’ll just emphasize, as CSA mentioned, that all you need from the auto parts folks is the actual, specific codes in that “P1234” format. Don’t let the auto parts store characters try to tell you what is wrong or that you should replace some part or another. The codes are just starting points for figuring out problems. So just get them, post them, and then people can help.

I just got back from the Auto parts store. The codes were:
lean mixture and multiple/random misfires
I know you told me not to listen to the guy but he had his spiel anyways. He said that since it’s supercharged (I forgot to tell you that) it could be the intake manifold gasket. Also it seems to him like it could be a (as mentioned before) vacuum issue.

That’s the skinny.

Well I’ve heard a lot worse from auto parts store characters. At least he didn’t try to sell you an O2 sensor - which is often what happens with something like the P0171. (The upstream sensor is actually what reports the lean mix).

I’d ignore the P0300 for now and assume that it just comes from the lean condition. I’m pretty sure that car uses a MAF sensor, and you should inspect and clean it (MAF sensor cleaner). Then check for vacuum leaks and do pay special attention to the intake manifold. Finding vacuum leaks can be hard. I like to use an unlit propane/butane torch with a hose rigged to the end - fish propane all around the intake tract, manifold and vacuum connections. If you hit on a leak the idle should surge. The serious & professional way would be a smoke test.

These cars are all 6 cyl right? (I don’t think a 4 ever went into a Bonneville). Anyway - if its a 6 the lean report is for bank 1 - the side with the #1 cylinder on it. Focus a lot of hunting attention on that side’s intake manifold.

When was the last time you replaced the spark plugs and spark plug wires on your Supercharged Pontiac? In my experience, these engines are quite hard on spark plug wires, and usually need a new set every year.

This is based off of 5 years of ownership of a Supercharged Buick Park Avenue, with the 3800 V-6 engine, just like yours. Every time my car started misfiring under acceleration, I would replace the spark plug wires, and the problem would go away for about a year.


Ok, I did a visual check of the vacuum lines. I didn’t hear anything but I saw a vacuum tube that had a cracked fitting on it. The tube comes out of the throttle body and goes to the purge solenoid.
I found this out from the GM parts guy who couldn’t find the fitting for me. I asked what to do and he told me to duct tape it. I Gorilla taped it so we’ll see how that works.
I also noted that I was a little low on transmission fluid. I added some and now I’m above the “don’t overfill” line. Is that a big deal?

Here’s the latest.

I went in and there were new codes. PO 171,131,135.

The 131 and 135 led me to believe that it was a bad oxygen sensor. I put a new one in and I also sprayed the MAF sensor with MAF sensor cleaner.

The car seemed to run better for a little while, but now it’s back to the same thing. The check engine light came back on, too.
So I took it in and scanned it again and now the only code is PO171.

Should I buy a new MAF sensor?
What other problem could it be?

Any help,

I think its time to do a thorough check for vacuum/intake leaks. There are multiple DIY methods, none of which are as reliable as a professional “smoke test.” My favored method is to feed unlit propane or butane around vacuum connections and the intake manifold. (If it hits a leak point it gets sucked in & the idle will respond). Others will just spray something like carb cleaner around potential leak points.

There should be a vacuum schematic under the hood. You can use it to make sure you get all of the vacuum system. Any actual vacuum line or fitting can be checked by pulling it & actually applying vacuum to it to see if it holds. (Use your mouth if you have to but there are also hand held vacuum pumps).

Pay special attention to the main intake snorkel. Invisible slits can hide in there and only intermittently affect how the car runs. So pull that off and bend & pull & flex while checking in all of the curves/joints/nooks & crannies/etc. Assume that any leak would be trying to hide from you.

~ just FYI I don’t know squat about supercharger systems, so someone who does might have something intelligent to say related to that.