Rough idle and shakes

TNB13

MEMBER
2004 JEEP GRAND CHEROKEE
4.7L
V8
4WD
201,000 MILES

The vehicle was running fine. When I started it up in the morning it had a rough idle and was shaking. Shakes around fifty five mph and hesitates a little. But drives fine in town. Check engine light flashes usually when going up small hills. It is giving off a code: misfiring on all cylinders. I replaced all plugs and the TPS. Other than this problem the vehicle has no issues at all. Any help would be amazing. Thank you.

1 Reply

STEVE W.

MECHANIC
13,574 POSTS

Misfire on all cylinders is a common symptom of a vacuum leak, low fuel pressure or plugged exhaust (and increasingly bad timing chains). If it was a single cylinder misfire then you would look at the plugs, coils, valves.

First thing I would do is to look and listen for a vacuum leak. A hose off or cracked will lean out the mixture and you get random misfires.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Using carburetor cleaner or other spray is best on a cold engine and be careful where you spray it. You can also use water and listen for it to be drawn in, or use a smoke machine to fill the system with smoke and look for a leak.

Next would be to test the fuel pressure, the engine requires more fuel under load. A bad connection in the wiring to the pump or a failing pump can cause lower fuel pressure and that will also cause a lean condition that will show up under load.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Be sure to test it under the conditions that cause the light to flash.

Plugged exhaust will cause this issue because the spent exhaust cannot get out of the system. This one can be hard to find unless you do a back pressure test on the exhaust.