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Old 11-19-2017, 09:46 AM   #1
doc ugly
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Default Replaced CPS and Went from Intermittent Start to No Start at All

1999 SL1, Manual, 320k miles. Car was doing the intermittent start when hot thing that is indicative of a failing Crank Position Sensor (CPS). Put it on the lift at work and replaced the CPS with a new one. I made no other changes or adjustments. Everything else on the car works great. Tried to start the car and it cranks freely but will not start.

Got another Crank Position Sensor from the parts store and installed it. Same result, cranks fine, won't start. Replaced the CPS with original one just to double check and still the same result--cranks but no start.

Used the little mechanics camera to see around the starter to verify what my fingers where telling me. The CPS was fully seated and the electrical connector was properly seated. I made no other changes to the vehicle so I assume I must have messed something up in the vicinity of the CPS.

My plan is to pull the starter to get it out of the way and take a better look and check all the wiring since I can only assume I must have yanked a wire off something? Anybody have any suggestions?

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Old 11-19-2017, 12:37 PM   #2
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Default Re: Replaced CPS and Went from Intermittent Start to No Start at All

Instead of assuming three dead or faulty crank sensors and developing tunnel vision, try another approach. If you have a multimeter, measure resistance of all three sensors. Resistance should be between 700-1200 ohms. There are two more tests for these sensors but they're not needed (vibration and heat tests) as it's not likely for all three to fail at once. Fully seating each sensor ensures the gap is set correctly. Not seating it creates a larger gap that fails to allow the sensor to detect the machined teeth on the crankshaft and acts like another failed sensor. It can be faulty wiring to the crank sensor or the problem is somewhere other than the crank sensor and its wiring.

At 320k miles, your engine is well used. The best way for an indirect test of any crank sensor is simple. Try either a spark or injector test. A spark test is easier. If you have spark to all four plugs, the crank sensor if fine. Injector tests are easy too but plugs need to be removed - after several failed starting attempts, every cylinder should be reeking of fuel, injectors operate only if the crank sensor operates. Spark only occurs if the crank sensor operates.

The PCM cannot operate the EFI system unless it has crank sensor signals. When the engine turns over, the crank sensor outputs a precise pulse train of signals that allows the PCM to operate; fuel pump, ignition system for spark and pulse injectors. No crank sensor output = no fuel pump, no spark and no injector pulses, a dead PCM.

Check for loss of compression and/or timing chain issues. Compression shouldn't drop below 185 psi but many engines still run with 150 psi in each cylinder.

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Old 11-19-2017, 07:48 PM   #3
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Default Re: Replaced CPS and Went from Intermittent Start to No Start at All

Thanks for the info. When I went back and tried to start the car the following morning it started right up. Although when I turned it off it went back to turning over but no start. For whatever reason it sputtered to life with a little shot of starting fluid. It seemed to run fine after that and started fine the next several times.

Now I won't get to fiddle with it until next weekend when I will have time to put your troubleshooting tips to work. Thank you much for your advice. I'll report back with my progress.

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Old 11-20-2017, 04:03 PM   #4
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Default Re: Replaced CPS and Went from Intermittent Start to No Start at All

After reading your reply, perhaps age related issues are more likely a key with this mystery intermittent starting problem. In addition to checking compression, was the fuel filter ever replaced? (every 100k miles) Fuel pumps have a finite life so a fuel pressure check is needed, several times. A worn out fuel pump can exhibit several symptoms without rhyme or reason. A sudden loss or intermittent pump may create this intermittent starting problem and mimic a faulty crank sensor.

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