`95 Lesabre engine misfire/shaking badly occassionally/randomly

[carolinaguy]

I've spent the last hour or so looking at this sites archives and seeing that my problem is not new, but I still haven't seen a definitive answer on any of the posts. I'm hoping one of you might know a solution that will work, and if you do, you can have the pride of saying that you solved the problem before the mechanics at Hendrick Motorsports Buick in Cary, NC. (to their credit they have not charged me for any of the diagnostics)

My 95 Lesabre randomly shudders/shakes/feels like the engine is about to jump out of the car. It comes and goes, seemingly without rhyme or reason... there is nothing in paticular that i've found to stop it or cause it, it starts missing and stops at different lengths of time and under different conditions. I've tried to narrow down what causes it, but it starts to happen in all conditions: cold and hot weather, cold and hot engine, heater on and a/c on, crusie control on and off, while running at speeds over 65mph and just after starting and driving in the parking lot. Sometimes it goes away while I'm driving (several times en route to the dealrship). Sometimes it runs perfect again when I start in the morning.

The most that they could find at Hendrick is that the second cylinder was missing when the car was shuddering. During their diagnostics, though, it started running fine again, and did for the next 2 days that they kept it and test drove it... frustrating. They were about to narrow it down between a sensor (can't remember the name) or replacing the computer, but didn't want to do either until they could verify it was the exact problem. I've seen a lot of different responses in the forum from replacing coils to torque converters to the upper plenum of the intake manifold to bad fuel injection.

Hendrick did a full tune up last month, and recently they flushed and cleaned the coolant system. It's had regular oil changes and inspections. Sorry for the long post, but i wanted to save you all having to ask clarifying questions as much as possible. If you have any ideas, I'd love to hear them so I don't take it in and have some expensive part replaced that won't help. Thanks!

[HotZ28]

Hello and welcome to AF!
Several things need to be checked, or verified when you have random misfire. First, if you have a tachometer in the IC, look at it the next time it “shudders” and see how much the tach needle fluctuates. Next, if it is shuddering that often, you should have misfire DTC’s stored, if so what and how many are showing? This process should help determine if the problem is ignition primary or secondary. Try a few “freebies” first; like removing and cleaning the bottom side of the ignition control module (ICM) and inspecting the ICM terminal for tightness & cleanliness and the main ground under the ICM. Next, check the coil(s) primary & secondary resistance to see if they are all within spec. Also check the ignition wire resistance while bending and flexing the wire. Be sure all wire ignition terminals on the plugs and coils are clean & tight. Finally, check the wiring from the ICM going under the alternator and to the PCM. This area under the alternator is especially venerable to cracking and corroded wires.

BTW, when they said “the second cylinder was missing” did it give a P0302 code? When someone refers to the “Second cylinder”, this could mean several different things. Is it the second cylinder on the front side, or the second cylinder on the back side, or is it in fact the # 2 cylinder? Was the shudder present before the recent tune up? This does sound like ignition breakdown somewhere, however, do not overlook the typical fuel system diagnostic test.

Woops, I almost forgot, be sure to have the ICM tested under load off the car. Damm this server is getting slow!