da9guy

i have a 91 jeep 4.0 today i was driving when i slowed down for a light i noticed the entire jeep was shaking i looked over at the tach and the engine is idleing extremely extremely low and shakes violently even when i give it some gas it continues to shake badly up until around 2500 rpm it backfires a tiny bit and the exhuast smells horrible. the thing idles at like 50 rpm i mean it barely barely turns enough revs not to stall out. it doesnt smoke much i dont think its a head gasket maybe a sensor or something? please help guys im totally lost here.

XJ.89.Limited

TPS maybe? mine idles low, but not that low, the lowest mine goes is 350 when its really cold out.

da9guy

no man i mean this thing idles so low and it like knocks and pings and chokes. ive seen some strange stuff before but this one is really crazy

Cherockee

I think maybe you need to check the obvious first, like spark plug wires and spark plugs, cap and rotor, then maybe fuel pressure and air filter. Maybe even check the compression on all the cylinders if you can. These are simple things that won't cost money. Do you have a CEL on? After that you may want to test the TPS and CPS.

da9guy

no cel light plugs and wires are all looking good. ive heard to unplug tps and cps so i did both and left them over night plugged them back in and it seams worse now. i disconected the battery also to reset the computer but that didnt help either. it just came out of nowhere one second it was fine the next it was broken. i dont understand it

Turbo X_J

Is the bulb burned out / unplugged?

sthon

Just because they look good doesn't mean they are. Test your plugs and wires or just replace them. Same with the dist cap and rotor. It's a cheap way to start and if they haven't been replaced recently it's a perfect excuse to do it.

I bought a 96 forest service XJ that had the same symptoms. Limped it home running on 4 cylinders and replaced the plug wires and plugs and it ran like a champ.

tjwalker

In this order............

1. Get fresh tuneup hardware in there. Don't want to be chasing your tail for $60 worth of parts that you need anyways. Plugs, wires, cap, rotor, fuel filter.

2. Test your fuel pressure. With a gauge; no shortcuts. You should have 31 psi at idle. Then disconnect the vacuum line leading to the fuel pressure regulator and the pressure should jump to approximately 39 psi. Be sure that the vacuum line to the FPR is pulling good vacuum and that it is not wet with fuel, which would indicate a ruptured fuel pressure regulator. Many big box parts stores will rent you this gauge inexpensively.

3. Test your throttle position sensor.

da9guy

ok i did number one. then tested my fuel pressure i have around 40 psi at around 1000 rpm. cant get a test for psi at idle because it wont idle. how do i go about testing the tps i have rented a multimeter but am unsure what to do with it.

tjwalker

The throttle position sensor is connected to the throttle shaft on the throttle body. It sends throttle valve angle information to the PCM. The PCM uses this information to determine how much fuel the engine needs. The TPS is really just a simple potentiometer with one end connected to 5 volts from the PCM and the other to ground. A third wire is connected to the PCM. As you move the accelerator pedal with your foot, the output of the TPS changes. At a closed throttle position, the output of the TPS is low, about a half a volt. As the throttle valve opens, the output increases so that, at wide open throttle, the output voltage should be above 3.9 volts. Testing can be performed with an electrical meter. Analog meter is best. You are looking for a smooth sweep of voltage throughout the entire throttle band. While slowly opening and closing the throttle, take note to the movement of the voltmeter needle. There should be a direct relationship between the needle motion to the motion of the throttle. If at anytime the needle moves abruptly or inconsistently with the movement of the throttle, the TPS is bad

You should have 5 volts going into the TPS. At idle, TPS output voltage must be greater than 200 millivolts. At wide open throttle (WOT), TPS output voltage must be less than 4.8 volts.. The best is to use an analog meter (not digital) to see if the transition from idle to WOT is smooth with no dead spots. With your meter set for volts, put the black probe on a good ground like your negative battery terminal. With the key on, engine not running, test with the red probe of your meter (install a paper clip into the back of the plug of the TPS) to see which wire has the 5 volts. One of the other wires should show .26V (or so). The other wire will be the ground and should show no voltage. Move the throttle and look for smooth meter response up to the 4.49 at WOT.

Perform the test procedure again and wiggle and/or tap on the TPS while you watch the meter. If you notice any flat spots or abrupt changes in the meter readings, replace the TPS.

The TPS is sensitive to heat, moisture and vibration leading to the failure of some units. The sensor is a sealed unit and cannot be repaired only replaced. A TPS may fail gradually leading to a number of symptoms which can include one or more of the following: -

NOTE: The throttle position sensor is also DIRECTLY involved with transmission shifting characteristics! It should be verified early in the troubleshooting process, when a transmission issue is suspected!

• Poor idle control: The TPS is used by the ECU to determine if the throttle is closed and the car should be using the Idle Air Control Valve exclusively for idle control. A fault TPS sensor can confuse the ECU causing the idle to be erratic or "hunting".
• High Idle Speed: The TPS may report faulty values causing the engine idle speed to be increased above normal. This is normally found in conjunction with a slow engine return to idle speed symptom.
• Slow engine return to idle: A failing TPS can report the minimum throttle position values incorrectly which can stop the engine entering idle mode when the throttle is closed. Normally when the throttle is closed the engine fuel injectors will be deactivated until a defined engine RPM speed is reached and the engine brought smoothly to idle speed. When failing a TPS will not report the throttle closed and fueling will continue causing the engine to return to idle very slowly.
• Engine Hesitation on Throttle Application: The TPS is also used by the ECU to determine if the driver has applied the throttle quicker than the Manifold Air Pressure sensor can read. The fueling is adjusted acordingly to cope with the sudden increase in air volume, however a faulty sensor can cause the ECU to ignore this data and the engine will "hesitate" when applying the throttle. In extreme cases with the engine at idle, a sudden application of full throttle can stall the engine.
• Engine Misfire: A fault TPS can report values outside the deined acceptable range causing the ECU to incorrectly fuel the engine. This is noticable as a slight misfire and can trigger the misfire detection software and/or Malfunction Indicator Lamp (MIL) light on the dashboard. Extreme cases can cause excessing misfires resulting in one or more cylinders being shut down to prevent engine and catalytic converter damage.

da9guy

here is an interesting situation with the fuel pressure ill do my best to explain this lol. so if the jeep runs at all or even when it just cranks over i get about 40 psi and it holds but as soon as i shut the key off the fuel pressure rapidly drops back to zero within 10 seconds. now if i turn the key to on but not start just to the on position the fuel pump will prime up to about 20 psi but then instantly drop back to zero. i know that it should hold pressure for a little while but it doesnt. so i took the fuel return line off at the back of the regulator and plugged the line off with my finger. then i primed the pump up again with the line blocked off and it jumped primed up to 35 psi and held it there for a long time. so clearly the injectors are not leaking the fuel pressure out. im leaning now towards the pressure regulator nut i just wanna know is there anything past the regulator that could be letting the pressure drop. i doubt the fuel line leaks or i would see the fuel on the ground right? ive heard there is some type of check valve on the fuel pressure sending unit that could cause this. is that true?

da9guy

as for my tps results i havent gotten to do anything with it because the meter thing i rented was digital....

da9guy

just replaced the fuel pressure regulator. it holds pressure now when priming with the key in the on position much much better than it did before. however it still has the exact same problem it wont idle on its own now it just dies and it smokes white smoke out the exhaust this thing is running terrible i really need help with this one. its my daily driver and i dont know what to do. ive called 3 mechanics they are all stumped as am i. i really dont know what else to do. i hope i dont have to start getting bids from salvage yards.....

da9guy

what i dont understand is that when i turn the key to the om position i only get about 10 psi. shouldnt i be getting more like 30? also when running about 2500 rpm i get around 45 psi biut the needle of the guage bounces around like crazy. im really leaning toward a fuel issue what do you guys think

tjwalker

1. Pressure should be at about 31 to fire the engine and should maintain approximately 31 at idle. If you are not in this area, you still could have a pump issue here. Did you get a new fuel filter in there yet? That is a must.

2. You are going to have to either test or replace the throttle position sensor to rule it in or out. I buy my engine management sensors directly from Jeep.

3. You should also test your MAP sensor and be sure that the vacuum line leading to it is not cracked and is pulling good vacuum.