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Hello All,

i need your help.  i have a 2.5L that I cannot make run with the MAP sensor plugged in.  I have tested the MAP sensor and it reads good.  i have tested the MAT, Engine Temp and Idle Air Control Motor as well. Replaced the TPS, O2 sensor and the ECU.  i have checked for Vacuum leaks and have come up with nothing.  it runs better with the MAP unplugged from vacuum.  I am running out of ideas.  

Any help would be greatly appreciated.

 

 

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Nelnoc...The MAP sensor provides a vacuum signal/electrical value to the PCM.  When intake manifold pressure changes the voltage signal from the MAP sensor, the PCM responds.  The signal tells the PCM what the barometric pressure is in the intake manifold.

The MAP vacuum signal should be from an intake manifold source.  (See illustrations in the PDF below.)  Verify the vacuum circuits.  You may be using another source for the MAP, like TBI ported vacuum, and this would be incorrect.  The reading should be directly from the intake manifold port.  Ported vacuum at an idle would provide a low MAP vacuum reading that would suggest the engine is under load.

Check this out with a vacuum diagram for your MPI 2.5L engine.  You may have a routing issue that is providing the PCM with an incorrect vacuum source signal that adversely affects the air/fuel ratio, idle stability and performance.  Also, be certain to check the voltage and ohms resistance readouts for the Air Temperature Sensor, which screws directly into the intake manifold.

 Here is a PDF of MAP and Air Temperature Sensor tests:   Jeep 2.5L MPI MAP and Air Temp Sensors.pdf

Let us know whether this helps...

Moses 

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The vacuum has been checked and moved between its port and another without changing the way the engine runs.  The distributor was changed to correct shaft play Along with new plug wires. 

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Nelnoc...There's no way to turn off the PCM's MAP function.  It's possible that the PCM has either a poor connection or defect.  When you add the MAP signal, the engine performance goes haywire, right? 

The MAP is also affected by the throttle position sensor (TPS) and the vehicle speed sensor (VSS).  Defects at either of these devices can impact MAP functions.  TPS  sensors do wear out.  Test the TPS and the VSS.  There is, of course, a possibility that the MAP sensor is defective or even the PCM.  Do not throw money at these devices before ruling out a TPS switch defect.

If that doesn't do it, I would pursue a connection issue.  Disconnect the 60-way connector at the PCM.  Look for debris, corrosion or any "black" coating, which is oxidation or corrosion.  Use a spray electrical cleaner and fine ScotchBrite pad, not anything abrasive (no scrapers, screw drivers or knives!), to remove oxidation and debris.  Plug the connector back into the PCM.

Moses

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The vacuum has been checked and moved between its port and another without changing the way the engine runs.  The distributor was changed to correct shaft play Along with new plug wires. 

 

Well Moses I just realized that my last message wasn’t sent.  I have replaced everything but the speed sensor the intake air temp sensor which tests ok. The crankshaft position sensor. The only code it is or has been throwing is the vacuum on the map being unhooked to get the engine to run. With vacuum it is 10 degrees out of time and won’t start warm.  with no vacuum it is at 0 degrees and starts and runs strong. We are going to check the wiring harness to see what it is doing.  Just a rundown of parts replaced. Distributor, cam shaft sensor, coolant sensor, o2 sensor,  map sensorX2, ECM,  throttle position sensor, idle air motor,

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Nelnoc...When you changed the distributor, did you restore its phase/indexing with the camshaft?  There is a distinct method for setting up the distributor on your 1995 2.5L MPI engine:

Jeep 2.5L MPI Distributor R&R.pdf

The distributor/rotor position, crankshaft position sensor and MAP must be synchronized.  Otherwise, spark timing will be off.  In your case, a timing error could be glaring when MAP applies.

Begin the process by setting #1 piston on TDC of the compression stroke.  This has both valves closed and the crank position sensor indexed properly.  Follow the attached Mopar PDF guidelines, using the "Engine Rotated After Removal" instructions.  Properly index the distributor and rotor with #1 piston at TDC.  You can index the oil pump drive shaft if necessary by rotating the drive slot with a large screwdriver.  

Moses

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Had to just take a break.  I’ve downloaded the service manual and I’ve  read both it an the pages you sent. I’m lost on what you mean by syncing the distributor and the map sensor.  If I wasn’t bald I’d pull my hair out.   Nelnocs dad. 

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Nelnoc...I'm referring to the distributor sync'ing.  The spark timing and reference for the PCM are important and related to the distributor rotor and housing position.  Spark timing is not adjustable at the distributor with these engines.  You must set the distributor as described in the PDF factory details. 

The rotor angle at crankshaft TDC (top of compression stroke) and the distributor housing in its locked-down position provide the base timing point.  (The distributor housing base should have a notch that indexes the housing with a pin position on the engine block.  Timing is not adjustable, and the distributor housing should not rotate on the block.)  Once the distributor housing and rotor have been indexed properly, the combination of the crankshaft position sensor at the flywheel and the distributor's rotor position will provide a base timing point for the PCM.  The PCM controls all spark timing from there.

Where the MAP comes into that equation is not a mechanical adjustment.  MAP depends upon the correct manifold vacuum.  If the spark timing is off, due to a bad reference position for the rotor, the manifold vacuum will be in error.  This could throw off the MAP function by providing the wrong vacuum reading.  The MAP signal to the PCM would be in error.  Sorry if my statement implied that the MAP must be somehow "adjusted".  I was referring to the vacuum created by correct spark timing at an idle.

You can read the manifold vacuum at an idle to see what is going on here.  Also, check for any kind of intake manifold or throttle body vacuum leak.  Use a low-volatility spray (WD-40 works here) around the gaskets with the engine idling, including at the throttle shaft in the throttle body.  A change in engine speed indicates a vacuum leak, which could impact both the MAP function and the oxygen sensor readings.

Moses

 

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Guess there was a little difference in vocabulary. We did time the distributor when it was installed. Rotor is settings where the book calls for it to be. Tomorrow I will have a look at the speed sensor it’s the only one that hasn’t been checked out.                                                   Just wanted to update Moses as to the end result of the battle with my Jeep. I’ve continued to work on and drive but couldn’t correct the issue with the map sensor not allowing the engine to run properly. Fast forward to the first of may   While driveing the jeep it just stopped would not start. Found no fuel at the injectors rail. Changed filter did not help. Pulled fuel pump and it wasn’t even still hooked up to fuel line the tubing had turned to jelly and the sock had  disintegrated   There was fibers smaller than hairs plugging the fuel pump. Replaced with new pump and sock. Runs like a top now. Never even thought about it starving for fuel. But it was. Again thanks to all who shared  nelnocs dad. 

Edited by Nelnoc
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Nelnoc and Dad...Worth confirming speed sensor function, though it's rare that the sensor would affect MAP performance at an engine idle.  Are you getting any MIL codes?  Troubleshooting codes would help.  You have the last year of the OBD (not OBD-II) diagnostics.  There is a plug connector for pulling codes that requires a scan tool with Jeep/Chrysler OBD-I hookup.  Is there an engine check light showing?

It's possible to have a PCM issue or even a PCM connector problem like I suggested on the November 13th reply above.  Also, though a long shot, the crankshaft position sensor could be faulting or intermittently causing trouble.  Oil from a rear main seal or even road debris can make the CPS signal faulty.  I would at least check and clean the CPS to eliminate a possibility of trouble here.

Another concern with the Jeep YJ Wrangler chassis is electrical grounds.  The electrical system is noted for engine-to-body-to-battery/alternator ground issues.  Check the grounds and grounding points as a general concern.

Moses

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