Jim Grant's Tech Tips
Grand Cherokee, shuddering
Q: We own a ‘96 Jeep Grand Cherokee with
72K. We have had all routine maintenance performed. The vehicle is in
great working order except for the following problem. Our check engine
light keeps coming on when accelerating up hills at 55+mph. When we are
about at the crest of a hill the light starts flashing then when the vehicle
starts to down shift it starts shuddering real bad. This shuddering will
last until you back off the gas and slow down. We have had the valve springs
replaced, then had the mechanics do some work (because of the codes thrown)
and this last week a new computer was installed. Two days after the computer
was replaced the check engine light came back on under these same conditions.
What could be the problem?
ALLDATA Information System shows several Technical Service Bulletins (TSB’s)
that are related to driveability problems with check engine lights coming
on. The computer will report information to the technician that is not
valid and send him/her down the wrong diagnostic path. With the recent
replacement of the vehicle’s computer I would hope you got the latest
and greatest software update. No, your Jeep isn’t running on Windows 98.
If the computer is up to speed then the concerns move to the ignition
and fuel system. If the problem is happening as the transmission down
shifts and the engine RPM jumps higher than the norm, there may be a problem
with the ignition wires. Strange as it may seem, we’ve found that under
load and high engine RPM tired spark plug wires will introduce an immensely
confusing engine skip that will not only turn the check engine light on,
it also sends the computer into the high alert mode causing the check
engine light to blink. The blinking check engine light on most ‘96 and
up vehicles is the computer attempting to alert the driver to the pending
doom of the catalytic converter. In closing, do not ignore the fuel system
as a possible cause. A fuel pressure gauge should be installed and the
vehicle driven in a manner that causes the engine to go into convulsions.
Only then could you be sure of the abilities of the fuel pump. The fix
will likely be simple. It’s finding the cause that will drive you nutz.