Cherokee sometimes starts...?

[Dude13450]

It'll always turn over without problem, but here recently it chooses whether it wants to start. Sometime it fires up after 2-3 seconds of cranking, sometimes I have to crank it 5-6 tomes for periods of about 1 minute each (with cool down breaks) for it to eventually start. Once it starts, it runs perfectly.

Any ideas what the problem is?

It's an '87 Cherokee Laredo XJ with a 4.0L I6 and automatic tranny.

[4.3LXJ]

The Renix computer needs to see a 300 RPM crank for three revolutions with a minimum of 10.5 volts while cranking and adequate fuel pressure before it will fire. How fast does it crank?

[Dude13450]

I don't know how fast it cranks, I will take a look at it when I get home.

[prcjeep]

perhaps CPS(Crank position sensor) is bad or has bad connection. check connections first then replace if necessary...

[Metlic53]

my 87 does the same thing...as of late i cant get it started without letting it crank for a full minute...

[Mudderoy]

The starter will slow down over time and you won't notice it is turning the engine slowly until you compare it with another 4.0 or you replace the starter. I know this first hand. :)

[Dude13450]

perhaps CPS(Crank position sensor) is bad or has bad connection. check connections first then replace if necessary...

Yeah, I need a new CPS. According to this site: http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm, the CPS is supposed to show a minimum of .5v(AC), mine's only showing .1v(AC) ... I guess it's a strong enough signal to the ECM for it to start sometimes, but not all the time.

[Dude13450]

The Renix computer needs to see a 300 RPM crank for three revolutions with a minimum of 10.5 volts while cranking and adequate fuel pressure before it will fire. How fast does it crank?

Mine is cranking at/near 300RPM. My tach has the first little tick, and then after that it has a bold little tick, the bold tick says it's 500RPM, and the tach jumps b/w the first tick (whatever RPM that is) and the bold tick (500RPM), so I guessing that is right around 300RPM.