95 LeSabre Bucking

[64conv65hard]

I have a 95 LeSabre with 140,000 miles, 3800 engine and has been driving me nuts. This car will start bucking intermittently. Doesn't matter what speed, coasting down a hill, accelerating up a hill, stopped at a light or even just idling in park. It can go a month and not have any problem then 1 day it will start bucking. I have found that sometimes turning off the ignition will sometimes temporarily fix the problem but never permanently. When it first started I replaced all plugs and wires but that did not help. replaced the Throttle Position Sensor and that did not help. Transmission has also been replaced (different reason) and that did not help. The bucking has become more prevalent the past few months. I do have an OBD 1 & 2 diagnostic tester but the place to plug into under steering wheel is just a fake plug, no wire going to it so I have no idea how to get codes. (Check engine light is on). I have also checked for misfire on coil packs/plug wires while in idle and you can visibly see and hear the engine stumbling but all wires show they are firing. I would appreciate any advice out there. As you can see by my username, I'm into older cars that are much easier to diagnose.

[Tech II]

Well, if you did a coil output test with a spark tester on each coil terminal, and the coils checked out, I would first find out which cylinder is misfiring....this can be done by disconnecting one injector at a time, and noticing the drop off in rpm/misfire intensity.......once the cylinder misfire is found, check compression in it, and also the cylinder(s) next to it.....if ok, could be an injector problem....

If you think it is an injector, pull the rail and swap injectors, to verify that the misfire moved to a new cylinder.....

As for your ALDL connector, '95 was a transition year, and not all code readers/scan tools can provide information, like codes....

[64conv65hard]

Last time the engine was stumbling I removed each spark plug wire from the coils and still could not isolate the cylinder that could be misfiring. When removing the wires, I could tell that the cylinder would stop firing by hearing the misfire until I put the wire back on. Definitely an intermittent problem, not a continuous miss. Also, when this happens I can watch the rpm's bounce until it stops and everything smooth's out again. It used to do this on rainy days but now it's not weather related. How do I get codes?

[Blue Bowtie]

My '95 LeSabre had only a couple wires on the ALDL, but I could access it with a Snap On MT2500 scanner, and even control items like the IAC, EGR, trans solenoids, air bags, etc.

While it will be good to retrieve codes, my guess is with Tech II - Ignition coils and the ICM are suspect.

[64conv65hard]

What's ICM?

[Blue Bowtie]

Ignition Control Module. It is the switch/amplifier/interface upon which the ignition coils mount. They also receive their ground connections from it. The ICM serves as the junction point where the CMP and CKP connect to the PCM and help time the ignition primary switching. The ICM has a habit of growing corrosion on its top surface, and that interferes with ground connections and cooling. There is a large connector at the front of the ICM which is semi-exposed to the elements under the hood. All of that should be inspected, cleaned if necessary, and re-sealed with dielectric grease to keep moisture out.

[64conv65hard]

OK, Thanks. Will look into that. I work on locomotives and all the acronyms take forever to remember, now all these acronyms are going to put me over the edge.

[64conv65hard]

OK, replaced the ICM that I obtained from the local junk yard and seems to have fixed the problem, appreciate all the great information. Last question, is it possible to clean up the old ICM and make sure ground connections are good or should I just throw the old one away?

[Blue Bowtie]

It is pretty common to clean/polish and re-grease the ICM surface and re-use it. So long as all the electrical connections are in good condition and internal electronics are undamaged, that frequently repairs the problem.