Very bad running problem (engine)

[Tally_XJ]

1989 Cherokee 4x4
4.0 liter
AW4 tranny

My CPS (crank position) went out on me again. Seems it happens once a year.

Usual symptoms. Running great then all of a sudden shuts off and wont crank again. No spark, no injector pulse. Let it sit a while, cranked up and then shut right back off ater a minute. Same no spark and no injector pulse.

Replaced the sensor. Jeep fired right up and idles perfectly and doesnt shut off.

However, now it will not take the throttle. Anything off idle it stutters, bucks, spits and busts up really bad. I cant get it to throttle up above 1500 RPM and even then its busting up bad. It returns to a smooth as a sewing machine idle and never shuts off, just wont take the throttle at all.

No smoke out the pipe. No cat so its not exhaust related. Has never done this before.

Had it towed to my work and there I did the following:
- checked fuel pressure (steady at 38psi)
- replaced spark plugs (Champion copper plus)
- Checked plug wires
- Checked connections at CPS and TPS
- Checked MAP connections and vacuum lines

 

[jeepdude10000]

try unpluging the TPS and see how it works, even try unpluging the battery, wait 20 min and plug back in and try it.

 

[old_man]

My bet is the TPS. Start it and let it idle. Grab the tps and rotate the little arm. The engine should sputter and pretty much die due to too much fuel.

The other test is to back probe the connector and watch the voltage ramp up as you rotate the sensor arm. It should be smooth with no jerks or dips.

 

[langer1]

That should be 31# with the vacuum connected, 38# disconnected sounds like you have a line off.

Look at the MAP sensor line, very easy to knock off and will cause those exact symptoms

 

[XJING]

If none of the above works, a classic off idle problem happens when the EGR is not working correctly, check the vacuum from the solinoid, and make sure the little rod on the bell shaped piece on the manifold is going up and down. Here a a diagnostic for the renix sensors.
good luck
http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

 

[53guy]

that fuel pressure looks a bit low. might be agrivating the symptoms, but I'm not convinced that it is causing them. It should be closer to 45 psi if my memory serves me correctly.

 

[Tally_XJ]

Ok, here is some info for some of the suggestions already made:

- EGR valve is long gone Got rid of that two years ago.

- Checked all connections to the MAP just in case I did accidentally unplug it. Was all connected properly. I will still check the line for cracks tomorrow.

- While the Jeep was idleing, I rotated the wiper arm on the TPS. No results. Made no difference in the way the Jeep ran. It continued to idle smoothly.

 

[langer1]

that fuel pressure looks a bit low. might be agrivating the symptoms, but I'm not convinced that it is causing them. It should be closer to 45 psi if my memory serves me correctly.

31# pounds with vacuum connected and 39# with vacuum disconnected is what the FSM calls for in 1989

 

[53guy]

ahhh...ok. I see.

 

[Tally_XJ]

On this same note, could it be that the CPS I just installed could be trash aswell? Would it cause this problem?

If not, then what the heck would cause two sensor failures at the same time?

 

[8Mud]

If I remember correctly (getting old) the last time I rotated the TPS arm with the motor idleing it reved up pretty darned high. Just the TPS arm not the throttle arm.
Some of the sensors are interelated, the TPS and the MAP share some wiring, I've never traced it out completely, but it sure looks like some of the wiring is in series. Easy to see on a schematic.
When one sensor shorts, it can steal the voltage from another sensor, some share a low voltage power supply.
I pulled the connections from my ECU and the MAP sensor, the TPS, the CPS, the O2 sesnor and others, jumped them at the connector at the ECU and measured the ohms, line loss due to resisitance at the sensor connector, harness connectors and grounds. I worked a lot of bugs out of my old Renix this way. The biggest improvement was making sure the grounds were good. Added a ground from the battery to the front clip and from the dipstick holder to the firewall. One ground ring on the dipstick holder, not sure if it was the TCU ground or the ECU ground wasn't soldered from the factory and was pretty darned corroded. The ground wires at the dipstick holder were all pretty well covered in oil, which can't be good.
I cleaned all my connectors and grounds. Cleaned the connectors well, scrubed them out with a very small brush and a quality contact cleaner. Took awhile, but well worth the effort.
CPS test is 200 ohms plus or minus 75 (cold). It wouldn't really take much resistance in a connector someplace to push the resistance to the outer limits of the envelope.
I trashed one CPS with a high pressure cleaner and another with hot oil from my rear main seal. I've a feeling they aren't sealed up all that well.

 

[Tally_XJ]

Just before I turned in last night, I printed all the sensor test procedures from that link provided.

I have a feeling it is gonna be the TPS since it made no difference when I oved its wiper arm while idleing.

However, I can now check the sensor with a meter and hopefully determine something.

Thanks for the responses!

 

[Tally_XJ]

UPDATE:

Still running bad off idle. Still wont take throttle at all without breaking up.

I tested the following:

- TPS performed perfectly according to test procedures.

- MAP tested perfectly according to test procedures

- Checked vacuum line from throttle body to MAP sensor

- Crank sensor tested correctly (new unit)

- Fuel pressure is 32psi with regulator vacuum / 40psi without vacuum

- Checked cap and rotor

- Checked other misc. vacuum lines and harness connectors

- Shook wiring harness close to the ECU connector (no change)




How can I test the ignition coil? I would like to test it if possible.

 

[jeepdude10000]

http://www.clarks-garage.com/shop-manual/ign-04.htm