Major bucking issue

[Steller]

OK so I have 98 ranger 4.0 v6 4x4 lifted on 33'sx12.50's

I'm having huge bucking/jumping issues while in park/reverse/drive idle specifically 35-45 mph. Accelerating is perfect. I currently have 185k miles, have replaced plugs/fuel filter 4 months ago. I've read so much.. spark plugs/wires maybe coil, cam synchronizer, egr valve. I don't believe its transmission or the engine itself but something inbetween making the mixture or combustion offset.. any advice would be appreciated I've had this truck for 9 years and would like to keep her runnin.

 

[RonD]

With engine warmed up, remove the EGR vacuum hose and see if idle smooths out, and see if there is suction(vacuum) on that line, there shouldn't be.
If idle is the same, rough, put a hose on the EGR vacuum port and suck it to open EGR valve, engine should run rougher and then stall, if idle stays the same then EGR valve is stuck and partially open.

Exhaust gas fouls fuel/air mix unless engine is under load, like when accelerating, that's when the EGR valve normally opens, so acceleration would be smooth, but not perfect.
So symptoms, as described, fit EGR issue.
And with EGR valve failure you often won't get a CEL(check engine light)

If cam position sensor fails you would get a CEL, and it wouldn't run all that rough, computer would switch to 'limp mode' so run injectors as Batch fire instead of Sequential fire.

MAF sensors on the 4.0ls are VERY important, and they often need to be cleaned, read here:
http://www.therangerstation.com/tech_library/CleanMAF.html

Also the MAF sensor only works well if the air tube(plenum) from MAF to intake has no leaks, while this is not "technically" a vacuum leak, it is a "ported" vacuum leak which would allow unmetered air into the system, so check for cracks or loose seals.


Also don't discount some bad gas in the tank, water in the fuel tank will often cause those symptoms, but usually acceleration would be iffy as well.

 

[black_demon69]

no egr on 4.0l ohv at least on the 94
don't know if sohc has egr

 

[zekew64]

Actually, the '94 4.0 should still have an EGR--the EGR wasn't deleted, TMK, until OBD-II in 1996. Should be located near the throttle body, next to the firewall.