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#1 09-14-2015 10:49:29

BuckB91XJ
Newbie
From: Oak Harbor, Wa.
Registered: 01-27-2009
Posts: 10

Hesitation after start

Group, I have a 1999 Cherokee Sport that has served me well. I've got a problem I'm trying to solve. When I first start the vehicle, I've got a definite hesitation in the accelerator pedal when pushing it down for the first time. The weird part is, once you push through it, the vehicle takes off and from there on out, runs just fine. If you shut the vehicle off and restart, the same problem occurs, but only once. I've heard that the cam position sensor may be bad and once the computer resets, the vehicle runs fine. Has anyone else had this problem? Any thoughts?


US Navy Chief Petty Officer
91 XJ Sport - 4.0 HO, AW4, NP231, HP D30 4.56 Aussie, D44 4.56 Aussie, XRS8000, Armor, 6" lift, 32" Mudders, Cold Air Intake, 2.5" FM Exhaust
87 XJ
06 KJ Sport

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09-14-2015 10:49:29

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Re: Hesitation after start

#2 09-14-2015 10:56:03

frank
Newbie
Registered: 06-04-2012
Posts: 9

Re: Hesitation after start

Me too!  I think mine is only when the engine is cold that I notice it.   But lately backing out of the short driveway, putting into Drive and the first initial acceleration I hesitate! (I don't accelerate when backing up)  But the odd thing is I have been stalling if I gas it moderately upon that initial take off!  Seems minor and just a little bit annoying but I too get that problem.   1988 4.0 w/ 228K strong!

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#3 09-14-2015 11:01:12

howlxj
Helper
Registered: 10-05-2008
Posts: 30

Re: Hesitation after start

I have a few thoughts. As I recall 99 was the first year of distributor-less ignition in the 4.0L which I am assuming you have. On these systems if the cam sensor and crank position sensor are not in sync you don't get any fuel injected. So I would not suspect the cam position sensor since it starts. Also they don't "reset". Changing one is no small task either. At least getting it set correctly isn't. You almost need an oscilloscope to get it exactly right.
Now if you are depressing the gas pedal shortly after or during crank and startup you may be experiencing low fuel pressure. One thing people do not understand is that when the ignition is turned on the fuel pump relay closes and the fuel pump runs for a few seconds. It doesn't run again and pump fuel until the computer senses the engine crank/cam is turning at 150 rpm or more. This is to build up fuel pressure to minimum levels for starting. On OBD ll systems like yours, the fuel pressure is around 49 psi. If the system is tight and holds pressure when parked and not running, the half a second you give it before cranking is enough to start up on a warm day. On a cold day the extra rich requirement for startup (simulated choke mode)it may not be enough for an instant start. Of if you are holding the gas pedal down some on start up you may have insufficient fuel pressure for acceleration for a few seconds. If everyone with an electronic fuel injected vehicle paused briefly when they turn the key on to ignition (idiot lights come on) before continuing on to crank, the world would be a better place to live.
Since your system has no external fuel filter or fuel pressure regulator, there isn't much you can do about a fuel pressure problem other than check for it. There is a test port on the fuel rail and I would suggest you check your fuel pressure and do a leak off test before you start changing every sensor it "might be". If fuel pressure is slow to reach proper levels then your only option is to replace the fuel pump. In an XJ this is about as easy as it gets. You don't have to drop the fuel tank as you do on most every other vehicle made. You can get to it on the front side of the tank rather easily. A leak off test is priming the pump bringing the fuel pressure up to normal and than with the engine shut off watch the fuel pressure. It should not leak off more then 5 psi or so in an hour. If there is a leak in any part of the fuel system the pressure will drop off quickly. Injector leak, regulator check valve leak are the more likely culprits. Assuming you don't have an obvious leak you can smell or see.

I once ran into a broken weld on the fuel sender that caused the fuel pump to run slow as this was also the ground path for the pump. The vehicle would barely run and would not accelerate at all. It had a new pump but only provided 2/3 the normal fuel pressure.
If you let it idle for 10 seconds first does it still hesitate the one time? Are there any codes stored? Are there any other symptoms?

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#4 09-14-2015 11:14:49

raul
Newbie
Registered: 10-05-2008
Posts: 11

Re: Hesitation after start

My 98 ranger also did this and it turned out to be the idle air controller.  Anyone think it could be a similar sensor?  Like the idle control?

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09-14-2015 11:14:49

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Re: Hesitation after start

#5 09-14-2015 11:59:41

tar
Helper
Registered: 09-27-2008
Posts: 32

Re: Hesitation after start

I had something similar or maybe it might have been in reverse? Well anyway it was some startup sensor that wasn't working the choke correctly, or something like that. It thought that the engine was always cold. So it started just fine but ran worse as the engine heated up, the choke was not releasing. I suppose it could work in reverse in that it's not choking at all so it runs badly until it heats up. However I would assume if you turned off the engine after it was warm and started it up again right away it would run OK if it hadn't had time to cool down.

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