1999 Regal persistant P0300 code driving us nuts .. help please

[Zedster]

Our 99 Regal (3800 Series II engine) has had the "Service Engine Soon" light persistantly coming on from time-to-time over the last year. The light comes on but the car is always driving beautifully. We've had it into the garage each time it has come on and the repairs have now exceeded $1500 and the problem is not fixed. The light continues to come on. This latest occurrence we took it to a different garage. Service writer called to say they couldn't find anything wrong and they would reset the computer. Another $80's down the drain. The service writer did say that the code (P0300) logged in history was set 938 miles previous to when we brought it to him and the speed at the time the code set was 99mph. Hardly! This is a Buick folks and even though this engine has more than enough guts to do it, my wife and I drive it like most older folks who own Buicks. I just got it back from this last time at the garage four days ago. This morning, I was entering an on ramp to the local highway and I agressively accelerated to 70 mph to get up to speed to merge with traffic on the highway. Guess what!?!?!!? You betcha, the Service Engine Soon light came on. Arghhh! I understand that this P0300 code indicates a misfire, but there wasn't any noticeable engine hesitation or anything like a misfire. Part of the previous $1500+ repairs included all new tuneup parts plus coil. They've also power flushed the fuel body and injectors. Does anyone know what can be causing this condition? I understand we can choose to just drive it with the light on and not pay another charge to reset it, but that doesn't provide us with any comfort since we won't know if another different condition arises. Any assistance would greatly be appreciated.

[public]

First, I wish I had a better answer.

I would suspect the speed sensor. Due to the history you provided, the only way for it to indicate 99 mph is that it is bad. All sensors are interconnected via the PCM so an intermittent short can create all kinds of codes. Unfortunately other sensors could be corrupting the speed data. 900+ times. Somethings up. Speed sensors are usually cheap, and easy to change. In your mechanics defense these problems can be extremely difficult to fix. But $1500 buys a lot or sensors, PCMs, and wiring plugs.

If you decide to put up with it you may wish to purchase your own code reader and just check it every so often. A good Actron unit is about $150 at any parts store. It is about the size of a Palm Pilot and plugs in under the dash. It will work on all cars sold in the U.S since 1994. Plus you will sound very professional the next time you take it in for service. Good Luck.

[Ruley73]

How many miles are on the car? Does it have the supercharged engine? I'll assume at the moment that it is a plain 3800 Series II engine with a fair amount of miles on it. First thing I'd check is your antifreeze level. If it is low at all, I'd be worried about a warped intake manifold plenum. When this happens antifreeze seeps into your intake from a water jacket around your EGR valve This is a common problem with this engine and the antifreeze leaking into your engine's intake could definitely cause a misfire that's not noticable. You may have caught it in it's early stages. In a worst case scenario your engine will eventually hydrolock and warrant an engine overhaul or replacement. If you've been taking it to a GM dealer I highly doubt that they would have overlooked this. If otherwise, I wouldn't rule this out Another thing I might suspect would be a dirty fuel filter. I can't explain the 99 MPH. If your speedometer is accurate, the cruise is steady, and your transmission shifts fine then I wouldn't worry about it at all. Just to clarify...the Actron code reader that Public was referring to will read all OBD II compliant vehicles sold in the US. This would include ALL 1996-present vehicles, most 1995 import vehicles, and most GM or Ford trucks in 1995 and only a handful of 1994 models. It's only capable of retrieving and erasing generic (not manufacturer specific) codes and CANNOT read realtime data. I work at AutoZone and I use it everyday. By the way, we do code scans for free.