Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Crazy 2005 Pontiac Gran Prix Problem

I think this Pontiac is possessed by a demon that is determined to drive me insane. It is a friend’s car, and she is a disabled single mother on a limited income, so taking it to a professional is problematic. The car has the classic 3.8 liter v-6, with over 200,000 miles. She was hoping to take it on a 2000 mile trip this summer, so I said I would do simple tuneup maintenance to hopefully prevent breakdowns. The first part was plugs, plug wires, and a serpentine belt. Of course to change the back side plugs I needed to remove the catalytic converter. When I replaced it, the second O2 sensor plug and wire are attached to a heat shield that has one rivet and a metal tab that slides under some body stiffener. It must have come loose and started flopping around and the plug became detached. The car started running rough and check engine light was flashing, which threw the code of H02 sensor. When I could look at it again I found the loose heat shield and the disconnected plug, so I reconnected the plug, secured the connection with a zip tie, and added a self-tapping screw to the heat shield to prevent it coming loose again. Then I cleared the codes and told her to take a test drive. She did and said it was still running terrible and when I.checked the codes it said misfire cylinder 6. I cleared the code and drove the car myself. It was definitely running very badly and missing. When I checked the code it now said random multiple cylinder misfires. Does anyone have any idea why it never threw the misfire code until after the O2 sensor was reconnected? If it was misfiring shouldn’t it have showed up in the codes all along, along with the O2 sensor? If it was misfiring before I would have thought the plug wires were bad, but why wouldn’t that code show up immediately? Any ideas would be helpful.

I’m really not trying to be snarky or condescending here, but if you removed a catalytic converter to replace spark plugs on this car you’re in over your head and don’t have the mechanical acumen to continue working on this problem.

There’s no way a loose electrical plug for the downstream O2 sensor could cause or be related to a cylinder misfire on that car. But driving it with a misfire and a flashing check engine light could cause damage to that catalytic converter you removed earlier.

If this problem wasn’t there before check your plugs and wires for proper installation and operation.

Well for not trying you sure do a good job of being snarky. How would you change the plugs on the rear bank of cylinders? Perhaps remove the top front motor mounts and pivot the entire engine forward? I thought that was more problematic than removing four nuts and unplugging the sensor. You still didn’t answer the question. Why would it not show a code for misfire until after the plug was replaced and the heat shield replaced securely? Your second sentence is just restating my point. But feel free to feel superior. Typical NPR listener.


Yes, you are supposed to tilt the motor forward, and you would use something like this tool

Don’t focus too much on the P0300 random misfire code, versus the more specific P0306 misfire #6

If you hook up a scanner and look at live data, you would have seen that #6 was the one with the real problem

I also tend to think something went wrong during your ignition tune-up

here’s some possibilities that come to mind . . .

You dropped a plug

The electrode was gapped wrong . . . really wrong

bad new plug wire

plug wire not plugged in correctly, either at the plug or at the coil pack

Here’s something else I’ve seen on some cars . . . the coil tower is so corroded, that you’re not able to get a proper connection

And lastly . . . was the engine running like garbage BEFORE you did the ignition tune-up?

I had a similar problem after doing a tuneup on my Honda CRV. Everything was fine for a while, then multiple misfire code. Turned out the aftermarket plug wires didn’t seat fully in the distributor cap. I threw the aftermarket wires away and bought a set of OEM wires. No more misfires.

Um, hate to mention it but asemaster is one of the best sources on this board. Agree with the options listed so far, and agree to use OEM wires. I had a similar thing happen to me when I changed wires. Didn’t have one on tight and got some wires crossed a little. I had it towed and my guy only charged me $80 to fix it. Unfortunately even running it for only 10 minutes or so ruined a $700 cat.

Welcome aboard!

That’s a Grand Prix in the little photo at left. Nice cars! Hey, I didn’t feel that @asemaster was being snarky. You just made a little extra work for yourself, that’s all. Live and learn. Cars and their owners, like the ones you are working on, can’t often afford other than DIY.

You’ll get good the best car advice here. Please stick around. We’ve got pros here and some not so pros, like me.

My point was that by doing a bunch of unneeded and unrelated work you increase the number of things that go wrong and cause problems, and don’t make the original work any easier. You now have a chance of loose connections at O2 sensor, exhaust leaks, and heat shield problems.

I don’t design or build the cars, I just fix them. On that car the rear plugs are accessed from above, by simply reaching over the back of the engine. Some guys pull the alternator and lay it aside but I don’t.

Since you have a misfire you didn’t have before, my guess is you cracked a spark plug by working at such an awkward angle from below or don’t have a plug wire installed properly.

Did you double check your spark plug wire placement? Are the appropriate wires connected to the correct coil tower and spark plug?

Asemaster I don’t choose the cars my needy friends at church need fixed, I just try to help. I have plenty of experience with mechanical jobs, including engine rebuilds and swaps, timing belts, head gaskets, et al., but GM cars are not my favorite to work on. I would rather work on an Asian import or my Jeep, but when a needy mom gets a free car I do what I can to help. Professional service is not an option in this case. The only information I could find at first when researching the job was to remove the dogbone and then drive into the garage and slam on the brakes to move the engine forward. I didn’t like the sound of this for obvious reasons, as it sounds like the start of a Cartalk call. The information about the tool above and to concentrate on the cylinder 6 misfire are very promising. I already found and ordered the tool on Amazon for $41.00 with Prime I’ll have it by Thursday. Then I will pull the plug on 6 and replace it. If it still is a problem I’ll try a different set of wires. I still find it odd that the misfire code was not appearing before the O2 sensor was fixed. Perhaps the programming just ascribes any misfires to the O2 sensor problem. Again, I’ll take a Subaru, Honda, Jeep, or even my girlfriend’s Suzuki over an 11 year old 200,000 mile Pontiac any day, but when someone gives a car to someone in a difficult situation, you do what you can to help.

The 02 sensor seems to be central to the issue. If you want to try and get lucky, you might try a reset on the brains. Now I was always use a memory saver, but I have a mechanic friend who works at a dealer, a few weeks ago replaced plugs and a leaking pressure regulator, car was running fine till I needed defrost that kicks in the ac. Every 4 0r 5 seconds 300 rpm surge for a few seconds then drop back down to normal. Called him and he said reset the brains, now I like to do this with ac and fan off, then life was good.

Hope it works, if not level2


Just being curious . . .

What brand plugs and wires did you use?

What condition is the coil tower in . . . the coil that fires #6 cylinder?

I’m kind of leaning towards the possibility of a new plug getting the porcelain cracked during the installation and especially if the plug was grunted down.

Another concern, which may be none of my business, is the idea of a disabled single mother taking an aged and apparently problematic 12 year old Pontiac with 200k miles on it on a long trip.
What happens if that misfire or any other issue crops up when she’s a 1000 miles out?

You refer to your church group so what about this proposal. How about the congregation passing the hat and paying for a car rental? Far less of a chance for a breakdown, cold air, and more than likely better fuel economy depending upon the type of rental car.
Just a thought anyway.

One idea not mentioned above, the O2 sensor malfunction damaged the cat. And the damaged cat is causing the misfires. If the engine can’t freely eliminate the exhaust gasses, it won’t run correctly. It’s like the old potato in the tail pipe situation.

Since you say you know how to remove the cat, try that as an experiment. If it runs ok w/out the cat, you know what to do next.

Although Pontiacs and GM cars aren’t amongst your favorite, don’t underestimate the venerable GM 3.8L that’s in that 05 Grand Prix!

They’re as close to a bulletproof engine as one can get. I own 3 cars with them. I bought them for the engine reputation (and their excellent transmissions). One has nearly 300k miles and is a strong runner and uses no fluids, and never been wrenched on.

By the way, Grand Prix owners are usually very happy with their cars and many owned more than one. I bought my Grand Prix by accidentally seeing it on Craig’s list last fall. I didn’t even need another car, but I was impressed and have not been at all sorry or disappointed! I love driving that little thing.


Although Pontiacs and GM cars aren’t amongst your favorite, don’t underestimate the venerable GM 3.8L that’s in that 05 Grand Prix!

I agree. There’s a reason that that 2005 Grand Prix is using an engine that dates from the 1960’s. That 3.8 is in my opinion one of the 2 best V-6 engines ever produced in an American car. It is virtually bulletproof and in all cases very easy to work on. Routine maintenance is simple and even in the event of a major repair no special tools or procedures are required.

There have been some issues with the plastic upper intake manifolds but the block and heads are among the most durable and simple out there.

Yeah I’d agree with that. Had three of them I think if I count right. 86 Park Ave 120K, 85 Riviera 350K, 89 Riviera 530K.