Stuttering at 40-45 mph

[MAD King]

Hello,

one problem fixed, another knocking on my door.

My 1998 Lumina 3.1 runs great with its 170 000 mls on it except when I go to 40 mph the engine starts shaking and I can feel it on the gas pedal. When I go slowly over 45 up to 50 mph it disappears. That is a weird thing. What can it be?

Thank you

No error code and no engine light on

[Blue Bowtie]

With no error codes reporting, and no noises accompanying the symptom, it might be a minor misfire or a mechanical imbalance problem.

An injector flow balance or possibly an EGR issue could be responsible for any apparent misfire, but the PCM would likely eventually set an error code for a particular cylinder(s) or a general misfire error (PO300). Any weak ignition would likely manifest itself under load more than at cruise, and any mixture problem due to a vacuum leak would probably be more noticeable at idle and lower RPM. Again, most of the time these would eventually set an error code if driven in that mode long enough. Less likely is a wiped out cam lobe since that would probably also be apparent under acceleration.

A mechanical imbalance could be an accessory, torque converter, or something else in the driveline. However, accessories rarely exhibit imbalance that doesn't also create a noise.

A reasonably good way to try to identify whether the problem is a mechanical balance issue would be to reach the speed at which the symptom appears then place the trans in neutral and coast. If the problem still occurs with the engine idling at 45 MPH it could be a wheel, half-axle, or something else in the driveline. You might also try manually selecting 3rd range instead of OD to determine if it is an engine/transmission RPM or vehicle speed related problem.

[kevinb70]

lower motor mounts, can be seen directly behind the front wheels