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Unread 04-24-2011, 07:13 PM   #1
bravojmc
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1987 MJ Comanche 
 
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Location: palisade, colorado
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Hard start....(HELP)

Hi guys. Ok I've got a 1987 jeep comanche with a 4.0 auto. When I go to start my truck it cranks and crank and cranks before it starts. I've replaced spark plugs, cap, rotor, wires.... but I still does the same thing. Has anyone had this problem?

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Unread 04-24-2011, 10:47 PM   #2
tbburg
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Pretty common problem with all the XJs/MJs. Sorry, I don't have a fix for you. My MJ('88) does it when cold, but will start immediately when warmed up.
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Unread 04-26-2011, 10:11 PM   #3
Joshharris98xj
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This just happened to me it's the relay switch close to your fuel pump but don't get me wrong it's NOT the fuel pump it's the relay an it's hard to get to so try this this is what I do and it fires up every time
-turn the key on for just about 2 seconds
-repeat on and off (not yet cranking) 2 to 3 times and it will prime the fuel lines then crank it should fire up
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Unread 04-27-2011, 07:03 AM   #4
cruiser54
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Could also be a faulty/weak CPS aka crankshaft position sensor. It can be tested easily with a meter.
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Unread 05-06-2011, 04:57 AM   #5
grabber1000
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CPS does not have anything to do with starting.. only running.The quickest way to trouble shoot is first check injectord with noid light( 5$ @ Amazon) That will let you know if all signal to ECU are their, Then fuel Pressure. Work on this problem for 6 months.. Down to fuel regulater
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It's slow, heats up like a *****, probably draws more amps than a locomotive and sounds like a bucket of bolts while running but hasn't failed YET.
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Unread 05-06-2011, 05:16 AM   #6
cruiser54
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Quote:
Originally Posted by grabber1000 View Post
CPS does not have anything to do with starting.. only running.The quickest way to trouble shoot is first check injectord with noid light( 5$ @ Amazon) That will let you know if all signal to ECU are their, Then fuel Pressure. Work on this problem for 6 months.. Down to fuel regulater
Gonna have to disagree here. A weak CPS CAN cause long craning times as it generates an AC voltage signal that gets stronger the longer and faster you crank the engine over. When it finally starts, the added engine RPM helps to generate the stronger signal and it keeps running. Not saying it's your problem but wanted to straighten this out.

If you have a manual trans, worn clutch material can accumulate on the CPS and cause your exact symptoms.
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Unread 08-16-2013, 08:45 PM   #7
OldDoom
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Agree with cruiser 54. CPS fixed the hardstart problem in my 89 with a 4.0L six.
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Unread 08-16-2013, 09:11 PM   #8
cruiser54
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Quote:
Originally Posted by OldDoom View Post
Agree with cruiser 54. CPS fixed the hardstart problem in my 89 with a 4.0L six.
I know this thread is now revived from the dead, so I'll post the CPS testing procedures for Renix.

There's also 25 write-ups in the link in my signature on mostly REnix tips.



Renix CPS Testing and Adjusting



Renix CPSs have to put out a strong enough signal to the ECU so that it will provide spark.

Most tests for the CPS suggest checking it for an ohms value. This is unreliable and can cause some wasted time and aggravation in your diagnosis of a no-start issue as the CPS will test good when in fact it is bad.

The problem with the ohms test is you can have the correct amount of resistance through the CPS but it isnít generating enough voltage to trigger the ECU to provide spark.

Unplug the harness connector from the CPS. Using your voltmeter set on AC volts and probing both wires in the connector going to the CPS, crank the engine over. It wonít start with the CPS disconnected.

You should get a reading of .5 AC volts.

If you are down in the .35 AC volts range or lower on your meter reading, you can have intermittent crank/no-start conditions from your Renix Jeep. Some NEW CPSs (from the big box parts stores) have registered only .2 AC volts while reading the proper resistance!! Thatís a definite no-start condition. Best to buy your CPS from Napa or the dealer.

Sometimes on a manual transmission equipped Renix Jeep there is an accumulation of debris on the tip of the CPS. Itís worn off clutch material and since the CPS is a magnet, the metal sticks to the tip of the CPS causing a reduced voltage signal. You MAY get by with cleaning the tip of the CPS off.

A little trick for increasing the output of your CPS is to drill out the upper mounting hole to 3/8Ē from the stock 5/16Ē, or slot it so the CPS bracket rests on the bell housing when pushed down. Then, when mounting it, hold the CPS down as close to the flywheel as you can while tightening the bolts.




Revised 01-26-2013
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Quote:
Originally Posted by chris87xj View Post
If you own a Renix Jeep, you'll want to check this out. >

An index to over 25 write-ups can be found in Post #1.

Cruiser's Mostly Renix Tips
My Jeep Technical Photos

http://www.cherokeeforum.com/g/album/1725214
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Unread 08-17-2013, 03:00 PM   #9
Soldier905
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I had same issue and it turned out to be the fuel pump relay . It ran great after i replaced it
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Unread 08-17-2013, 10:32 PM   #10
cruiser54
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Quote:
Originally Posted by Soldier905 View Post
I had same issue and it turned out to be the fuel pump relay . It ran great after i replaced it
Only testing narrows it down. These fixes are not "one size fits all".
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Quote:
Originally Posted by chris87xj View Post
If you own a Renix Jeep, you'll want to check this out. >

An index to over 25 write-ups can be found in Post #1.

Cruiser's Mostly Renix Tips
My Jeep Technical Photos

http://www.cherokeeforum.com/g/album/1725214
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