Throttle Problems

[cantfindmybutt]

When driving, either slow or fast speeds, my 1998 Jeep Grand Cherokee frequently will throttle up (especially if I hit bumps in the road or low spots) and then return to normal. I can see the RPM's actually go up and then fall back slowly. At highway speeds this is a very quick phenomena. The RPM's spike up and return almost instantaneoulsy and the car just shudders. To add to the issue, at stops (traffic lights, etc.) the car actually seems to idel down almost to the point of stalling and then returns back to normal idle. The throttle body has been cleaned according to my trusty mecahnic.

[dcx96]

Change the TPS

[edwardmilk]

I just had a problem with my 94 G Cherokee idle speed. My IAC had a bad coil, one opens and one closes to control the idle. It is a very basic piece and performs a very basic function, but only when your foot is off the gas pedal and it is idling. It doesn't do anything when cruising above 1000rpm. Mine was frozen in the wide open position and it idled at about 1500 rpm. I replaced it and all is right in the world. It will take off to a very fast idle after you replace it, but let it run for a little while or take it for a drive and it recalibrates. This valve has been failing off and on for a few years so I was never sure what caused the problem. I took it apart and one of the coils looked to have overheated and was discolored.

Early symptoms are stalling after getting off of the hiway, but for only long hauls, stalling at lights but restarts just fine, extremely high revving when I come to a stop after getting off of the hiway. I am sure the magnetic coils had bad wiring and it would break connection once in a while, but reconnect again. I broke the original valve monkeying around with it. It seems a little delicate and I twisted the plunger and shouldn't have. I think OEM parts are that type and replacements are different. They can be rotate or have to be in order to retract the plunger before installation or it will be damaged.

It turns with only light pressure, don't crank on it, it may be the other type that can't be rotated, if so just push hard and it will go in. You will feel something rotating on the inside as you push on it. It feels like one of those old tops with the pump handle to make it rotate, yes I am "older".

[edwardmilk]

I still have issues with the idle. It seems to want to quit but only after the car has been sitting for a while. The morning is the most common time for it to stall, but it will be fine for the rest of the day if I am just tooling around town. If I leave it sitting, like during work, it will stall again once I go into to idle mode. I can still be doing 60 mph, but once I take my foot off the gas pedal the car will stall sometimes and lately it would restart itself seconds later as I am coasting. I am suspecting two parts at this point and both have to deal with idle speed. The first is the TPS, it is more than likely a sweeper arm style potentiometer and can wear down over time and the other is the CPS that is inside the distributor, it has a wire cable connected to it and is just under the distributor cap. It is a hall effect device that uses magnetic field disturbance detection to "see" the rotor when it is in the #1 position. This helps to determine fuel injection timing and is important for a smooth idle.

My car will stall sometimes even when I have my foot on the gas and have the rpm up while I am in nuetral. I do this in the morning to keep it from stalling while I am at a light. It works, I think, most of the time, but like I said the car will shut down fast, no stumbling, it just quits. I am thinking that the TPS is not getting a good signal to the computer and it cuts out fuel because it doesn't know when to inject the fuel at the correct time. I am just guessing and may end up spending $40 for nothing. I think that I am still ahead of the game, the shop already have 2 of my limbs so I have to keep the other 2 or I can't drive, legally anyway.

PS These icons are still cute to me. I just started using them. I am betting some of you are so over them by now.