: Flashing SES/P0300

10-13-09, 08:32 PM
I thought this was due to my headgasket failure on my 02 sts but even after getting the head gasket fixedit still has a random misfire... I replaced the spark plugs during the repair with Autolite double platinums... which seem to work just as good.

It's a very slight misfire normally occuring at heavy acceleration or idle... but my RPM's do not jump at idle...

Question is, how hard is it to get in there and figure it out? what should i check and how? Thanks!

10-13-09, 09:17 PM
A flashing SES light means severe misfire detected. I'd suspect the Autolite's.

00 Deville
10-13-09, 09:23 PM
N*'s don't like any plugs but the A/C Delco 41-987 they come pre-gapped to .050 and they're not cheap expect to pay $6-8 each.

10-13-09, 09:41 PM
Yeah, that ^^^^.

Pull the Autolites and, while you're in there, check each of the plug boots for cleanliness and proper inner spring tension. While you have the coil cassettes off, remove each ICM from the cassette body - there are 2 clip tangs which hold them in - and check for connector pin corrosion. Smear a tad of dielectric ignition grease on the silicone seal for the ICM and plug it back in. Put a tad of the grease in the lower tip of each plug boot. Check the wiring harness connectors to the ICM in each cassette. Torque the plugs to 13 ft/lb cold. If you're very careful, use a tiny dab of anti-seize on the very tips of the plug threads (this from a tech at Dart Heads).

Check the PCV orifice (no valve for you) vacuum pipe and fresh air feeds for integrity. Check the throttlebody vacuum connectors also. Any cracked plastic pipes can be patched with proper sized vacuum hose from the Help ! blister packs at a parts store.

10-14-09, 08:36 PM
Ok, i get it... A/C Delcos are good plugs... but! i had the misfire before i got the autolites... plus they seem to work fine because sometimes the SES light goes away completely... but will return... not much different that the A/C Delcos. I DID have a head gasket failure before... not sure if that would cause a misfire...

So i'm curious if there are any other common points of failure within the spark plug system. Such as Coil packs? and how would i check that it?

I have no general knowledge of cars... it's just this caddy is making a tech out of me due to numerous things that go wrong (headgasket, radiator, plugs, p/s pressure hose... etc) and i have to learn it because most techs don't know much about these cars since they usually go to the dealership or the junkyard . In other words... how easy is it to get a step by step on doing this stuff...

10-14-09, 09:06 PM
Remove the engine cover and start the engine. Use a piece of vacuum tubing stuck in your ear or a modified stethoscope to listen for vacuum leaks. Spray a shot of choke cleaner or starter fluid down at the screw clamp base at the connector plenum between the intake manifold and throttlebody casting. Any idle changes in speed ?? Vacuum leak. I said that before..............

10-14-09, 09:27 PM
Ooo, forgot about the plenum. Follow Subs advice.

Yes a HG can cause a misfire, but not after it has been repaired.