1999 GMC Suburban C1500 96000 MILES, BAD VIBRATION

OLEPRO

MEMBER
1999 CHEVROLET TRUCK

WHILE UNDER POWER AT 70 MPH OR GREATER (GOING UP A HILL) AND IN LOCK-UP THE VEHICLE STARTS TO VIBRATE AND THE INTENSITY GETS SO GREAT I FEEL LIKE THE CAR IS GOING TO EXPLODE. AS I LET UP ON THE ACCELERATOR THE SHAKING STOPS. OR IF I DOWNSHIFT INTO PASSING GEAR IT STOPS, BUT GOES RIGHT BACK INTO LOCK-UP AND THE VIBRATION STARTS AGAIN. IF I TAP THE BRAKE TO DISCONTINUE LOCK-UP IT STOPS SHAKING, BUT SHORTLY GOES RIGHT BACK INTO LOCK-UP AND SHAKES AGAIN. WHEN I FINALLY SLOW DOWN TO 55 OR 60 AND NOT UNDER GREAT POWER EVERYTHING IS GREAT. THE PLUG-IN ANALYZER SAID MULITPLE MISFIRES UNDER" CHECK ENGINE SOON". ALL PLUGS, WIRES AND POINTS AND CONDENSER WERE REPLACED ALL TO NO AVAIL. THE TRANSMISSION WORKS PERFECTLY OTHER WISE. I FEEL LIKE ITS A DEFECTIVE SPEED SENSOR, BUT I CANT GET THE CHEVY PEOPLE OR THE TRANSMISSION FOLKS TO AGREE. THEY DONT KNOW! IF THEY PUT A $3000 TRANSMISSION IN AND THAT DOES NOT FIX THE PROBLEM, THEY DONT WANT TO BE OUT $3000. WHAT TO DO?

2 Replies

AACPLUS

MEMBER
60 POSTS

I can't imagine the difficulty involving replacing points and condenser on a 1999 vehicle since it should have electronic ignition, but if the engine was replaced with an older engine and drivetrain, I could see it being done, if it is leaving the country and the DOT gives a waiver to eliminate the 1999 emissions due to leaded fuel at the new destination.
If it is in the trans or transfer case, have a reputable repair outlet diagnose the problem properly.
I have seen such with propeller shaft joints, tailshaft bushings, loose pinion, and many other possibilities including bad torque converter or bad/loose flexplate, restricted exhaust, etc.
If you have misfire codes, it could be a distributor problem, (or bad coil pack if distributorless), even mounts, trans input shaft bearings or an eccentric shaft that walked, bad/arcing ignition wires, bad plug, coil pack, emission component problems, even a coil that trips out when hot or at higher speeds.
A misfire would eliminate most of these other senarios.
The list is too broad without some specific code info or hands on. Find a shop you trust and hope you don't end up financing someones vacation this summer.
Good Luck.