I have a 98 Bonneville SE and the 'check motor' light comes on (doesn't blink) like clockwork any time it'* been raining for 24 hours or more. It seems some sensors are highly sensitive to moisture. The light goes off the next day when the car has 'dried out'.

The scariest time was when it rained for two days, then an ice storm came through. That light blinked on and off like mad and the car lurched like there was water in the carburator, until I decided to try a trick in the manual - add gas. I was at 3/4 full and added in some high quality gas and the blinking stopped, but the light stayed on. When the car 'dried out', the light turned off as usual. But that was bad. It was like the moisture in the engine turned into ice and clogged the carb. Jeez! Scary! :(

I adore this car and nothing else has ever gone wrong with it except this chronic moisture problem.

I wonder if there is a way to fix it permanently???

Oh, and some gas cap history. This problem occurred with the factory gas gap. That cap was stolen and I replaced it with a non-dealer cap from NAPA. So the type of cap didn't seem to matter.

You might wanna try getting fuel injection for your 98 Bonnie!
How did you add fuel???

If I'm not mistaken, your engine is a 3800 Series II normally aspirated, fuel injected motor. Bonnies haven't been carbureted for nearly two decades.

The first thing I would try to do is to check every single electrical connection under the hood to make sure it is seated correctly. Take off the plastic engine cover by turning off the oil cap counterclockwise, but instead of taking it off, grip it firmly and try to turn it counterclockwise even more, so that the plastic tube under it will twist off as well. This is how you take off the engine cover. Then, for every electrical connection, unplug it and replug it a few times. This will ensure that the connector is properly seated, and if there was any corrosion on the metal contacts, maybe it'll rub off and expose bare metal again. Don't forget all six fuel injector plugs (that was my problem), all six spark plug wires (check both ends!), and all the sensor connectors. I would suspect the ignition coils though...maybe there'* moisture getting inside?

Good luck with the issue and keep us posted on what happens.

only difference between sunshine and rain is low and high air pressure. IAT, MAF, things like that could be going south. get it scanned for codes.

Sounds like everyone is right on track here, but it'* extremely rare for water to be a problem at the top of the engine. I'd go OEM gas cap, and spray some type of light teflon based lubricant on all the contacts of every connection you can find under the hood. It'll help force moisture out and prevent more from getting in. The other thing that may be happening is that with the increased air density from moist air, your injectors may not be able to deliver the proper amount of fuel to keep the mixture correct. You may be running extremely lean. This can be caused by a fuel pump or pressure regulator.

There are a ton of things that can do this, and it may be more than one thing. You just have to whittle away at them and rule them out one by one.

WOW!!! You guys are the best !!! But why am I surprised?
Pontiac owners RULE !!!

I'll check out each of these things and report which of them
fixed the problem!

Happy New Year!!

Well, you guys you pretty much nailed it!

I got the codes checked free at Autozone and the code came up 300
(actually 300-306 for the six cylinders) so I guess I'm looking at
a coil problem or injector issue.

I'll report back when I have it looked at. If I can find someone I trust.

Thanks again!

Well, the guy who ran the scan said that while the code was 300 indicating misfire, it could be, but didn't have to be, on all six cylinders. His first choice, after my insisting that this only occurs in wet weather, was the coil. He seemed to know what he was talking about.

Remember guys that if a Ignition Module goes bad then all 3 Coils will read bad too to the ECM! You're right it IS rare for all 3 coils to go but, NOT impossible if the Module itself has taken them out of service though first.

As for the gas cap problem I would go with the GM one and apply a silicone emulsion on the cap'* rubber O-ring seal first. This will keep that seal soft & it will seal better & longer that way.