mysterious engine problem

Hi all.
First Im sorry, Im not native english speaker...
I have 1989 cherokee (AT/AC) 4.0.
It has some mysterious engine problem. When I start the engine it takes about 10-20 seconds. When it idles then one or two cylinder will fail I think, anyway it is choppy. When I’m accelerating slowly the engine fails, when I do it extra slow the engine fails completely (shuts off). When I’m driving at solid speed the engine runs, but after some time it goes choppy again. When I’m accelerating very strongly (pressing the pedal to the floor), then the engine “thinks” a moment and runs well. Acceleration is normal and the engine has the power.
So I have changed wires, fuel filters, spark plugs, MAP sensor. But the engine still has the same problem(s).
I suspect that on problem can be in fuel pump and/or vacuum lines. Also I found one resistor, in under hood (driver side).
So my question is what may cause the problems? Also does anyone have a vacuum diagram? Does anyone know how big the fuel pressure should be?
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This message was posted also to other jeep forum, but guys didn't know mush.
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So, yesterday I decided to look those vacuum lines. I closed EGR system completely (solenoid and sensor are also disconnected). It didn’t make any difference… Then I disconnected the vacuum line between vacuum fitting and air cleaner, I left the system open, and the idle speed raised and the engine runs ok (if system is open, it takes too much fuel).
My question is how the vacuum system operates? How it reacts to rpm’s?

I suspect, that the problem may be caused by fuel pump. Or does someone have other solutions?

Do you have a fuel pressure gauge available?

There is a port on the fuel rail, called a Schrader Valve. It looks just like the end of a tire valve. Screw the fuel pressure tester to it, and test the pressure with the motor running. It should be approximately 31psi. Now remove the vacuum line from the fuel pressure regulator. It will be attached to the fuel rail and is approximately the size and shape of a 35mm film can. The pressure should rise to 39 psi.

If that is okay, check the entire supply and return lines all the way to the tank for kinks,bends, or pinched spots.

To flow test your pump. Pinch off the fuel return line.Rig a hose to the Schrader Valve, and the other end into a container at least 1 liter in size. Put a jumper in the diagonostic connector that shorts D1-5(battery) to D1-6(fuel pump). A good pump will pump at least 1 liter of fuel in a minute.

Hope this helps.

kkerem said:

I have 1989 cherokee (AT/AC) 4.0.
. . . . Does anyone know how big the fuel pressure should be?

Click to expand...

Fuel Pressure for a 1989 4.0L should be 39 p.s.i. (pounds per square inch--I'm not sure whether you need me to convert that to different units, or what units they use in your country). See this webpage for more information: http://www.jeep4.0performance.4mg.com/tech_specs.html

I think you're right, it sounds like the problem is the fuel pump. I'm not sure, but I think you will find a fuel pressure test port on the fuel injection rail. I'm not sure of that because your engine is older, uses a different injection system than mine. I'm sure someone else on this forum has the same engine as you, and will be able to tell you for certain. Good luck, and welcome to the forum.

I am guessing that the problem lies elsewhere than in the fuel pressure, because it accelerates well after hesitating, but performs poorly when accelerating more slowly. Poor fuel delivery would more likely allow it to run well when treated gently, and begin to fail when driven harder.

I think the problem might be either in the wiring harness, or in a sensor, perhaps the throttle position sensor or the crank position sensor.

You should also try to make sure that the EGR valve is closing all the way. The diaphragm of the EGR valve is open to the atmosphere on the bottom, and you can reach around it with a finger, and open the valve. If the valve is good, then when you do this the engine will immediately stall. Sometimes when this valve wears, it will fail to close completely, but if you open it with your finger and let go it will snap shut.

Spark plugs and sensors are fine, as much as I have tested them. I disconnected the TPS, and nothing happened, except acceleration wasn’t that good. I suspect fuel pump because it has been once rebuild and it wasn’t solving the problem, but it went much better. Meanwhile I suspected the vacuum lines and sensors, I bought new MAP sensor, wires etc. EGR is closed, I checked the valve also.
Back to the fuel system, correct me if I’ wrong, fuel pressure regulator is “passive” device. Only “signal” is the vacuum, what tells to regulator what to do. If the fuel pressure is low, and I disconnect the vacuum line the fuel pressure rises. Fuel pressure regulator doesn’t know the real pressure, it reacts to rpm’s..

Am I correct?

I've got over 260k "miles" on my '89 and never had any trouble with the fuel regulator. I have had trouble with the individual injector connectors though. They easily hold water/moisture, corrode and become intermittent behaving much like a bad plug wire or two when that happens.

Search "Renix" and learn to qualify the handful of sensors the system requires.

XJXJ said:

I've got over 260k "miles" on my '89 and never had any trouble with the fuel regulator. I have had trouble with the individual injector connectors though. They easily hold water/moisture, corrode and become intermittent behaving much like a bad plug wire or two when that happens.

Search "Renix" and learn to qualify the handful of sensors the system requires.

Click to expand...

While on the subject of bad injectors - I found when hunting down a wiring harness problem a couple of years ago that a bad connection to an injector does not show on a vacuum gauge. Since many other problems do, including, usually, ignition problems, this can be a valuable clue. One of my most useful diagnostic tools is a vacuum gauge with a line long enough so that I can lead it out of the hood, and tape it to the side view mirror. I can then consult the gauge while driving.

I try to measure vacuum and fuel pressure. One more question about TPS, if it is disconnected (engine tps), will the ecm take the averge?
I have question about brakes also. When I press the pedal, I hear some airflow (vacuum leak or smth), what it could be. When I pump the pedal and the engine is off), it goes hard (this is normal, I know), after that I start the engine first breaking is much better then next ones...