Engine shaky/skips a beat/chugs along

[bob k]

I have a 1997 Cadillac Seville. Roughly 80,000 miles on it.

I have this problem that's been on and off for a while now. I took it to the dealer, but the mechanics didn't solve the problem, all they did was change the spark plugs.

The problem is, that the engine "skips a beat". The problem starts for the first time when you first turn on the engine, not while driving, and it continues this engine shake. The RPM appears lower (dipping really close to 500 rpm at times). When in high RPM, the shake goes away, but still present when its low. The average MPG is lower by 4 mpg. When I go up an incline slowly, you can feel the car chug along, chug-chug-chug-chug. You can hear it when its in neutral. Its slight, but you can hear it and the engine shake. The Service Engine Soon light sometimes stays on, sometimes it blinks.

Usually this problem goes away after 3 or 4 days, however this has been going along for a full week. The idiot mechanics only changed the plugs and didn't do anything else. Any ideas as to the cause and the solution to this problem? I hope I have been descriptive enough.

[mastertech]

Have you or the mechanics pulled any codes? Sounds like you could have an issue with a damaged catalytic converter. If the ses light is blinking, that means there is a misfire that is potentially damaging your converter. Try to post any codes you have pulled, and we'll help you figure it out. Good Luck!

[bob k]

Where can I pull these codes? If I have to go to the dealer, they will charge me 100 bucks for diagnostic.

I got a copy of the invoice this morning, this is what they did:
-changed plug and wires
-replaced fuel pressure regulator.

Thats it. And apparently that didn't fix the problem. At the time they pulled a misfire code P0300 #7 Cylinder.

I talked to a different mechanic, not specific to Caddy's, but he tells me it could be the ECM. By the looks of it, that may be the case and im ready to replace it unless you guys think its something else. A 115 dollar replacement of the ECM sounds better than spending 700 dollars for a repair that didn't fix the problem.

[bbozsik]

To pull the codes, on the climate control center, hold in the "off" and "warmer" buttons simultaneously for a few seconds. The display will then list each code, or lack thereof. Be ready to write them down if needed.

If cylinder 7 was misfiring and they changed the plugs, maybe you have a bad coil pack?

There are a few guys on here that know how to check it out. Do a search for coil pack.

Personally, I don't see how the ECM would cause the intermittent problem you are describing... then again, I'm no expert.

Good luck.