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89 XJ Warm Engine/No Start

1989AMCXJ

NAXJA Forum User
Location
Big Bear Lake
I have read countless articles on this issue, and I have found zero solutions. Any guidance here would be gratefully appreciated. OK, Starts cold no problem, drive it up to normal operating temperature, park it for 25 minutes or more, No start, just crank...Let it sit for hours, until cold again, starts.

Engine: Renix L6 4.0 with Cam, Polished ported head, bored out throttle body to HO specs, TB spacer, pacesetter headers.

Fuel New: Fuel Pump assy and pump, hoses, FPR, 3rd Gen Bosch four hole injectors, Insulation on manifold, fuel rail and injectors. Fuel Pressure 35lbs. Air fuel gauge reads Ideal to sometimes rich condition.

Sensors: ALL NEW including CPS. (tried to purchase better quality parts, not cheapy ebay china crap.)
Also Running the "R.E.M.+ V3" (hooked directly to the ECM port) and all diagnostics are within factory specs and no error codes out of range (Pre-OBD).

Ignition: Yellow top Bat good, Bosch High output Alt. Good, Cap, Rotor, Distributer (1 year old), Plugs, wires (1 year old), Coil, Coil Module, Starter relay switch, all other relays(4)(1 month old).

Grounding: Extra #4Cu gnd from rear of engine to Firewall and from bat gnd mount on block to second location on firewall, and from bat to gnd on front of frame.
(Positive cable from bat to starter good)

CPS Testing Results:

Cold Engine
Testing: CPS plug (Wiring harness end ignition on): 3.4V
CPS Resistance: 232
CPS AC V Spike when cranking: 0

Drove XJ, all warmed up, no issues. 180F
20 minute wait, test then crank: started up no issues

Engine Warmed up
Testing:
CPS plug (Wiring harness end ignition on): 3.4V
CPS Resistance: 278-280
CPS AC V Spike when cranking: 0

20 Minute wait, crank: started up no issues 160F
25 more Minutes wait, crank: 140F no start
CPS ohms test on harness side: 9.36
5 minutes later, started right up

Drove 10 miles, parked, came back hour later, no start, than I tried open throttle, started right up...
I am losing my mind!!! :looney:
 
Can you verify if it's no spark when it won't start? Also, check it the wiring harness where the fuel rail would usuly want to wear through it.


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OK , wiring harness looks ok. This is the 3rd CPS I've replaced in six months... All diff brands...I'll re check spark after warm up soon...been a bit busy...
 
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 won’t start with the CPS disconnected.

Hooked up voltmeter, set to AC to CPS harness connector, cranked, Reading was 000
 
Ok, so with no signal coming out of the harness side, how does that reflect making a mod in the CPS side? The harness side has no signal from the ECU, so wouldn't you repair that?


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Maybe try checking for CPS signal voltage with an analog meter? I've heard the digital ones can have a tough time understanding the waves of current that these types of sensors produce.

Also, might check your IAC. No start, then starting with open throttle is a tell tale for that, from what I've experienced in the past.
 
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