p0304 and SES

[my98saturn]

My car is a 98 SW2 DOHC but with a 1995 DOHC engine. 123k on my car, around 102k on the engine.

Was driving back from DC last weekend, and a couple of times the SES flashed when I took my foot off the throttle.

Started the car today and the SES light was fully on; code p0304. Idle was rough, but I didn't notice any huge performance loss. Car might have been 'ticking' a little at idle.


Went to the parts store and bought a compression tester as well as an extra spark plug.

Compression test was:
60, 200+ 200+ 200+. Plugs 1-3 looked normal (gray ash color), while plug 4 might have been oil fouled, and had eroded much more.

The spark plug wells for cylinders 1 and 2 were full of oil, and I thought I could see the rtv gasket in the spark plug wells, so these were 'wet' numbers, as the oil drained down into the cylinder. Oil consumption has been very high (1 qt/500 miles) and I've seen a lot of oil on the outside and top of the engine.

I replaced #4 spark plug and cleared the SES light. I'm facing emissions in a few weeks, and wondered if there's anything I should do (e.g. seafoam) or if I should simply just give up on the car.

[fdryer]

Can we assume the 60 psi is cylinder #4, right most cylinder (looking from bumper)? If so, that's your problem with cylinder #4/P0304 misfiring. Compression too low can't support combustion. The valve cover leaking oil is a minor issue but may contribute to misfiring so try keeping oil out of plug wells for a day or so and see if misfiring goes away. A new spark plug may not help with low compression but can't hurt. If misfiring doesn't go away and your state inspection is close, your choices are few with only one real repair - new rings for the low compression and maybe a rebuild of the cylinder head if the valve(s) are burned/leaking. A very short term fix may be using thick motor oil without any guarantees of a temporary solution to pass inspection to make P0304 go away.

[my98saturn]

Here's one for you:

I reset the SES light and parked the car. 5 days later, my wife got in the car, which was hot, and turned on the AC.

No SES light, as long as the A/C is on, but the mileage averaged 27.1, and the car seemed low on power.

The SES light did come on when I had accelerated quickly up a hill and took my foot completely off the gas while descending the hill -- saw a few flashes of the SES light and then it came on a minute or two later. After I turned it off, it hasn't come on for the last 200 miles.

[fdryer]

When a cylinder doesn't come up to ideal compression and ignition doesn't occur or does but intermittently, the unburned/partially burned air/fuel mixture leaves thru the exhaust system. The O2 sensor detects this as too rich a fuel mixture while the unburned mixture fuels a hotter catalytic converter. Excessive fuel allowed to burn in the catcon eventually destroys it from cooking itself to destruction - the flashing ses light. Destroying a catcon can occur in as little as 50 miles of driving in this condition. In the meantime, as the O2 sensor is detecting a rich condition from unburned fuel, the signals sent to the pcm tells the pcm to lean out fuel mixtures for all cylinders. Now a struggle ensues between a cylinder that isn't up to compression with less fuel and misfiring altogether while the other three cylinders are running leaner. The EFI system is running incorrectly and you'll probably see a number of error codes, any one of which can be seen as failing emissions inspection. One warning; any manual reset of any error (fuses pulled or pressing reset on a reader) right before state inspection usually results in emissions inspection failure; monitors learned and updated continually while driving are erased, requiring either driving a few days or performing the dreaded 'drive cycle' to have the emissions self tests relearn emissions parameters before a 'not ready' status turns to 'ready' - only seen on readers.

If you managed to get past inspection with the ses light off, you're home free until next inspection.

[my98saturn]

Well... After 300 miles of driving with the A/C on, and having the SES light come on exactly once, I managed to get it to pass inspection

I read the description of how the car determines the 304 code, and it seems to be that the engine speed varies by 2% when that cylinder should be firing.

It struck me that under a very low load, a leaky cylinder would generate less vacuum braking and actually speed up the engine vs the other cylinders, and might be triggering the code.

Having the AC on seems to keep the load at a slightly higher level, which prevents this.

Driveability doesn't seem to be an issue.

So I guess I get to keep it for another year.