Why car does not go on high apeed?

most likely bad injector tips
have them serviced and raise the idle to 750 rpms as the lower idle speed may not be raising the combustion pressure temperature to ignite badly sprayed fuel ( diesel has to be atomized not squirted )

Idling is too slow at 500 rpm.......think about at least 650 rpm to charge the battery and keep up inertia for better take-off power.
Find out how to increase rpm,whether it's a screw-stop adjustment or a timing problem. don-ohio (:^)

Vacuum leaks or fuel pressure, or ECM.
When you press the accelerator you allow more air into the engine. The same extra air can get into the engine through leaks in the Vacuum system. Sometimes using a spray bottle of water and spraying the seams of the Intake manifold on a running engine can reveal the opening. Also spray any Vacuum lines and the Brake booster. A leak will change the RPM.
A high fuel pressure can cause an increase in RPM.
ECM- has a lot to due with timing and fuel demands.
Binding Linkage. Disconnect the throttle linkage and see if the RPM drops. If it does not, shut off the engine and work the Air horn Butterfly and check for smooth operation. It is possible that a mounting screw on the Butterfly shaft has slipped and the flap is binding or dirty.

It can be issue with IAC( idle air control valve) causing the problem.

There are many other possibilities causing these idle issues. Engine idles rough.

There are few help links with solutions to troubleshoot such idle issues.

I suggest you refer the link below and follow its troubleshooting details:------

Click the link below:-----

Ford Expedition IAC valve replacing

http://technoanswers.blogspot.com/2012/12/ford-expedition-iac-valve-replacing.html

Bad miss at idle?

http://technoanswers.blogspot.com/2012/01/bad-miss-at-idle.html

-----------

Stumbling and stalling at idle after engine rebuild?

http://schematicsdiagram.blogspot.in/2012/03/stumbling-and-stalling-at-idle-after.html

------------

1999 Ford F-150 Idles rough and quits when gas pedal is pressed less?

http://technoanswers.blogspot.in/2012/01/1999-ford-f-150-idles-rough-and-quits.html

---------

Engine stumbles coming off idle when warm?

http://repairhelpcenter.blogspot.com/2012/01/engine-stumbles-coming-off-idle-when.html

------------------

Erratic idle on car?

http://schematicsdiagram.blogspot.com/2011/12/erratic-idle-on-car.html

-----------

Cold start problem and vehicle shudder around 2500 to 3500 rpms

http://repairhelpcenter.blogspot.com/2012/10/cold-start-problem-and-vehicle-shudder.html

----------

Low idle when cold out side?

http://repairhelpcenter.blogspot.com/2011/12/low-idle-when-cold-out-side.html

-----------

GMC Sonoma Stalls and idles low

http://technoanswers.blogspot.com/2012/10/gmc-sonoma-stalls-and-idles-low.html

--------

GMC Idle Air Control (IAC) Valve Replacing?

http://schematicsdiagram.blogspot.com/2011/12/gmc-idle-air-control-iac-valve.html

-------------

1996 Camry intermittent engine failure while idle?

http://repairhelpcenter.blogspot.com/2011/10/1996-camry-intermittent-engine-failure.html

------------

Engine quits at idle?

http://repairhelpcenter.blogspot.com/2011/10/engine-quits-at-idle.html

-----------

Car overheats and Air blows hot at idle?

http://howtobyme.blogspot.com/2011/11/car-overheats-and-air-blows-hot-at-idle.html

---------

High Idle Problem

http://repairhelpcenter.blogspot.com/2012/10/high-idle-problem.html

-------

While stop engine idles like cylinder missing?

http://schematicsdiagram.blogspot.com/2012/01/while-stop-engine-idles-like-cylinder.html

---------

Idle flair is noticed on car? Rough idle? car will not idle? Erratic idle speed?

http://technoanswers.blogspot.in/2012/02/idle-flair-is-noticed-on-car-rough-idle.html

--------

High rpm in neutral? Inconsistant high RPM in neutral?

http://technoanswers.blogspot.in/2012/02/high-rpm-in-neutral-inconsistant-high.html

-------

Vehicle stumbling at idle and shows code 54?

http://schematicsdiagram.blogspot.in/2012/03/vehicle-stumbling-at-idle-and-shows.html

----------

Rough idle, staling and Erratic shifting?

http://technoanswers.blogspot.in/2012/03/rough-idlestaling-and-erratic-shifting.html

-----------

While stop engine idles like cylinder missing?

http://schematicsdiagram.blogspot.com/2012/01/while-stop-engine-idles-like-cylinder.html

---------

These details will help.

Thanks.

sounds like a vaccum line or once again tps/poss timeing off

I am not familiar with this engine. But, if the alternator, PAS and AC are being driven by the same belt, then its possible that the belt is too slack, have you checked the belt tension ? That would be my first step. Hope that helps

You will find idle speed controllers in the area of the throttle plates. In order to work, air must bypass the throttle plates.

check automatic idle speed motor if equipped.

You have a vacume leak somewhere. You can try to listen for it or you can get a can of brake parts cleaner and spray it around the engine. Anywhere there is hoses or connections of any kind. When you find the leak the engine RPM's will increase. If thats not the problem it could be your EGR valve is clogged or your map sensor could be bad. Those things would usually cause a check engine light. Vacume leaks wont. Good luck

change plug and wire if its been a while most cars have 90000 miles platinum plugs in them.but the wires wear before plugs.so change plugs and wires.+clean fuel injectors .change fuel filter and air filter.if car still run rough your idling to low.have idling check .