Replaced AOS now CPS is bad - no start

2003 996 c2. Replaced the AOS due to coolant leak, ran great for 3 days after. Was driving fine, started missing, wouldn't rev above 4500, check engine light came on. Shut off to pull codes, was hard to restart. Shut off again, was harder to restart. No smoke, no funny noises, oil & coolant look clean. Currently will not start, just cranks. The CPS connector got quite a bit of coolant on it when the AOS sprung a leak, is throwing only P0335 and P0336 codes for crankshaft position sensor. Will have to test the sensor and troubleshoot with a spare. Odd thing is, the battery is getting a little tired after some testing, and the motor seems to turn over faster but sounds different now, like there's less compression? Hope the flywheel or something more expensive isn't bad. Will update tomorrow evening when I get time to work on it.

Checked your fuel?

The codes P0355 and 0356 don't exist for your car. I assume you meant P0335 and P0336? If true, those are the error codes for no crankshaft position signal, which means either an open circuit between the CPS or the CPS itself is bad. I would unplug the CPS and check if you get about 1k ohm between the two pins on the sensor side . If not, your sensor is definitely bad. If the reading is good, I would check continuity of the wires between the sensor connector and the DME connector. Does the tach needle bounce for about 1/4" from 0 when cranking?

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Oops you're right, P0335 and P0336. Yes the needle bounces just like that during cranking. Should have time and a new sensor tonight to test with. Thanks

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Since you got coolant onto the cps connector, it would be a good idea to thoroughly clean the inside using contact cleaner and make sure everything is dry. Sounds like a bad sensor to me.

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I'm hoping if I dry it out really well it will work, but for a $100 sensor will probably just swap it out while I'm under the car. One of the other 996s in my local PCA just had to do his, very close to my mileage. Thanks for the advise. Honestly at this point I'm more concerned on why it sounds different cranking over than it always has, having nightmares about separated flywheel or lost compression or some other crazy unlikeness haha.

Good luck with the repair! There was another poster who had a similar problem and he posted a video here. The cranking sound was like what you described. It sounded like no compression and even he admitted it sounded very strange. Later on he fixed the isssue (forgot what was wrong) and the engine was fine...but that weird cranking sound was just scary. I wonder if it's due to lack of oil on the cylinder walls after a prolonged cranking.

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Good memory, found it here https://rennlist.com/forums/996-foru...ease-help.html sounds exactly like mine. I remember hearing the fuel pump prime just after it failed, but haven't heard it the last couple of attempts starting. I did hear the air pump running last time I checked it. More things to check, thanks.

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Ah, there was no apparant culprit identified. No wonder I didn't remember what the fix was Regarding fuel priming, I think the 1-2s priming does not happen consistently even on a healthy engine. There's some sort of timer controlling the priming if you try to crank back to back too quickly, the priming may not reactivate. But if you wait for some time, dunno how long but maybe in minutes, you should always hear the priming.

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A healthy engine sounds kind of weird cranking with no start. It almost seems like it has little or no compression. The reason is we only hear the engine cranking for a short time before it starts and it sounds strange if cranks for any length of time. The high compression requires a bit of effort to overcome which is why the engine cranks a bit slowly from the outset but then the pressure in the cylinder is such that it gives back quite a bit of this effort and the engine seems to spin with little or no resistance.

Tested the CPS at .954 ohms on the 1k scale between two pins, and infinite between the center and other side, and outer pins. Can hear the fuel pump prime with the key on. Tach needle bounces with ignition on, even before cranking. Bad news is the little (what used to be?) 5mm socket cap screw or whatever holding the CPS in is totally rounded out. Pounded a larger torx into it, nope. Will have to get a left hand bit or something tomorrow.

A reading of 0.954 on the 1k ohm scale means 0.954 kilo ohm, which is within spec. It's strange the tach needle bounces even before cranking. Do you have a way to verify sparks? Also checked fuse C1 to C4 and E1? Do you have a mechanc's stethoscope?

I checked C4 and noted it was supposed to be 5 amp and was a 7.5. Checked every fuse in the driver's footwell, continuity on each one. I do have a stethoscope I can borrow. I do have extra coil packs I can rig up to check spark, and a spark plug tester somewhere.

Actually since the DME complained about the CPS, maybe it's good to continue troubleshooting in that direction. Since you replaced the AOS and the CPS wires and connectors are in the vicinity, can you check if the wires and connectors are ok?

Had a few free minutes so I checked all the connectors and harnesses I could reach, all were connected firmly and didn't see or feel any abraded sheathing. I can hear what I assume? are the injectors clicking with the ignition on, but not cranking. Is that normal? I had to unplug the air pump to hear more clearly- loud very rapid clicking on both sides of the motor around injector area. Clicking stopped when I unplugged the air pump, just sporadically clicked for a second or two but when I plugged the air pump back in clicking like crazy.

Old one out (fastener holding it in was bent wtf?) new CPS is too short. DAMN YOU ROCK AUTO! Cleaned the shavings stuck to the magnet, stuck it back in and still no start. Plugged an OBD2 reader into it, and ignition on engine stopped reads 200-300ish RPM while needle bounces. That seems odd? Still doing very odd rapid clicking and air pump cycles on and off like it has a short almost. Getting another CPS saturday to try.

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That is indeed very strange. The CPS is of the magnetic type meaning it does not need external power so it should not generate any signal when the flywheel is not turning. The clicking of the air pump is also very odd. You can just disconnect the air pump for now so you can hear other sound more clearly.

With the air pump disconnected, something is clicking rapidly on both sides of the engine. I'll have to get a stethescope to verify it's the injectors. Does it for a few seconds then stops. What would be giving the tach the signal to read 300rpms, and cause the injectors to cycle with the engine off?

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The clicking sound could be the normal throttle body plate calibration. The injectors should not click before cranking plus they should not be that loud. Something is going on in your cps circuit giving wrong signals.

Just a wild idea, is the engine wire harness ground wire ok. It�s close to AOS and might be damaged or forgotten to put back, if removed while working with AOS

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Thanks for the suggestion. I'll take a look at that next time I get a chance to look at it. Didn't remove any grounds, but did drop the engine a few inches to get some clearance it won't hurt to check it.

one of my grounds actually bolts with the CPS sensor bracket.

Replaced CPS and still no start. Tach stopped doing the mystery bounce thing for the most part. Key on- Fuel pump primes, SAI pump comes on for a second, looks like tach comes up a couple hundred RPM then goes to zero immediately when the pump turns off. Cranks strong, no start. I remember the SAI pump staying on when key-on and during the first few minutes of running? Seems odd it's shutting off so soon. Didn't see any grounds around the CPS but it is dark and raining out lol. Found my old coils and plugs to test spark after sunrise.

3 things necessary for an engine to run: air, fuel, spark. Air is a no brainer and it seems you have fuel. Central to the ignition system is the CPS which you have replaced. Good idea to check for spark. If you have spark, reset the DME/ECU by removing the negative battery terminal, wait ten minutes and reconnect.

Disconnected the negative from the battery, charged it a few hours, reconnected and no start. Checked with a good coil, no spark. Checked the main ground by the passenger rear wheel, cannot see any signs of corrosion. Disconnected and reconnected, no change. Smells gassy while cranking. Need to run down why no spark now.

Did you check to see if you had 12V at the coil? Unplug the coil and check the pins on the connector plug with the ignition on. Are you getting anything from the OBDII port? In other words, is it live? Also was thinking maybe a bad relay?

More thought - the CPS is the common denominator for spark/no spark. Since you got the original error code relative to the CPS, are you absolutely sure it is the right replacement one and the connection is 100%? If the DME/ECU cannot determine where the crankshaft is, it will not fire. http://repairpal.com/obd-ii-code-P0336

I didn't check the coil connector for 12v, will go do that now. First CPS was wrong, for a 993? so I ordered by the bosch# on the old one 0261210248 and new one looks identical, with porsche part # 986.606.112.04. The reach and connector are identical as far as I can tell, went right in no problem. Hooked up obd2 to it again and have another pending P0336, seems very odd the rpm reads between 280-380rpm with key on but not cranking. Must have a short or corrosion somewhere in the wiring?

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I agree. My guess is there's something going on with the wiring of the cps sensor to the DME. Maybe coolant got into the harness and shorting things out.

checked coil connector, has battery voltage of 12.5ish with ignition on, drops to 11.8ish when the SAI pump decides to come on. Where does that harness go? Any easy way to check the harness down the line without dropping the motor or tearing the whole harness out?

I think what's happening is the DME is getting some random signal from the cps sensor wiring, which the DME interprets as some arbitrary rpm hence the rpm you're seeing during key on when the engine is not even turning. Due to the random signal, the DME can't get synchronized signal between the cps and the cam position sensor so it doesn't know when to fire the coils. Without at least lowering the engine, that part of the harness will be very difficult to get to. There are a few more things you can check. - Unplug the cps and do a key ON engine OFF and see if tach is still showing some rpm. My guess is it will still show rpm, which will prove the sensor is not the problem. Also, you can substitute the sensor with a 1k ohm resistor to convince yourself you see rpm even without the cps input. - Unplug the cps and the DME and check the continuity of the 3 wires between them. See this thread - If you have an oscilloscope, you can back probe the DME pins and check for the proper cps waveform. See the thread here .

You're right. Unplugged CPS and key-on and tach was still bouncing.

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Good. I think the next test should be continuity between the cps plug and the dme plug (with the battery disconnected and the cps and dme unplugged so the 3 wires are completely isolated). Also check for short between those 3 wires and the chassis.

Please make sure you read the whole thread in the second link I posted in post #31. It has all the diagnostic steps and also definitely check the x59 connectors (big round black connectors at the very right side of the engine bay) for corrosion and spray the pins with Deoxit. Also spray the cps connector. Has to be Deoxit and not just any contact cleaner.

Got a free 5 minutes to check the round black connectors, top one had a significant amount of some light brownish oily? substance inside, maybe some kind of contact cleanser or something? Wiped out as much as I could, checked bottom one and it was clean and dry. No change. No time for anything else edit to add- only checked wal mart and autozone, didn't see any of that cleanser on the shelves

You need radioshack https://www.radioshack.com/products/...ant=5717849605 Also, you may want to spray a generic contact cleaner to clean up the brownish oily substance first before you apply Deoxit. Deoxit will bubble when it contacts bare metal so you want the metal to be at least free of oil. Also, Deoxit will leave an oily substance which is a protectant so don't be alarmed.

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Ah HA! There is still one radio shack open around here, will hit them up tomorrow. Thanks again

Btw, the cps sensor wires (all 3 wires) go through the bottom big round connector (x59/2) so you could probably focus more on the bottom one.

Got it fixed. A shielded wire from the CPS got pinched? slightly during the AOS install, was grounding out. Fixed the wire and it's running normally again. If anyone needs a tested working 100k mile CPS let me know haha.

Nice job! Ahsai def. gets the assist!

Yes indeed. Good one, especially for sticking with it.

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+ 996 HUGE thanks to Ahsai for all the help for sure

Glad you got it sorted! Once again, the DME 7.8 is correct on its own diagnosis

Awesome. Glad you are back on the road

Great job to both of you , and glad you are running again.......