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Old 04-05-2023, 06:50 AM   #1
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'99 Boxster Tiptronic not firing after engine overhaul

Hi,

my 1999 Boxster 2.5 l Tiptronic does not start (fire) after engine overhaul.

The starter cranks the engine ok, but it does does not fire. I went through the list of usual issues to look out for including the following:

- installed new CPS (reads 880 Ohms) seated correctly
- installed new ignition switch
- installed new fuel pump relay, which clicks when the key is turned (however, pump is not coming on then)
- Fuel pump engages and pumps fuel when relay is jumpered (but not when key is turned)
- MAF connector and ignition coil connectors have +12 V when ignition is on
- All fuses (next to pedal) are good

Here is what I observe:

- the engine cranks, but doesn't fire
- The rpm needle does not move when the starter cranks the engine
- No smell of fuel at the exhaust after cranking

Is there a specific relay that powers the injectors and prompts the ignition sparks that could be blown?

Thanks for any thoughts!

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Old 04-05-2023, 08:21 AM   #2
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If the DME is not seeing the engine turning signal from the CPS, it will shut off the fuel and ignition.
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Old 04-05-2023, 09:18 AM   #3
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If the DME is not seeing the engine turning signal from the CPS, it will shut off the fuel and ignition.
Been awhile since I have been down this road.

There is a relay in the back left side of the trunk which I think sources the gnd for the fuel pump relay and the ignition coils.

Any chance the immobilizer got wet? similar symptoms.
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Old 04-05-2023, 09:52 AM   #4
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Thanks for the quick feedback!

CPS: Though it's brand new (Pelican) and no sign of not working (tests alright), I will get another new one to be absolutely sure it's not that.

Immobilizer: Car was in garage for past 3 years so can exclude that it got wet. Also: would the immobilizer not prevent the engine from cranking in the first place?

Will check the relays in the back.
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Old 04-05-2023, 10:45 AM   #5
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Thanks for the quick feedback!

CPS: Though it's brand new (Pelican) and no sign of not working (tests alright), I will get another new one to be absolutely sure it's not that.

Immobilizer: Car was in garage for past 3 years so can exclude that it got wet. Also: would the immobilizer not prevent the engine from cranking in the first place?

Will check the relays in the back.
Long before aimlessly throwing parts at it, why not check and see if the DME is getting a signal from the CPS? Trace the CPS wires to the connector at the DME and look for a low voltage signal on them at the DME end when the engine is turning over; if you have a signal, it isn't the CPS.
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Old 04-05-2023, 12:23 PM   #6
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Long before aimlessly throwing parts at it, why not check and see if the DME is getting a signal from the CPS? Trace the CPS wires to the connector at the DME and look for a low voltage signal on them at the DME end when the engine is turning over; if you have a signal, it isn't the CPS.
Thanks! Is there a chart for the pins in the DME plug? Or is it best to go by the color coded cables from the CPS?
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Old 04-05-2023, 12:39 PM   #7
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Thanks! Is there a chart for the pins in the DME plug? Or is it best to go by the color coded cables from the CPS?
Service manuals will have a complete wiring diagram.
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Old 04-05-2023, 02:15 PM   #8
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If you have Durametric or other Porsche specific OBDII scanner you should be able to view RPM while cranking (expect around 75-100 RPM) that would tell you if the DME is receiving a signal from the CPS or not.

Last edited by blue62; 04-05-2023 at 02:19 PM.
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Old 04-05-2023, 02:27 PM   #9
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Thanks! Is there a chart for the pins in the DME plug? Or is it best to go by the color coded cables from the CPS?
Maybe pin 78..?
I have a link to this in my folder of DME stuff, Says it's DME 5.2.2: https://www.renntech.org/topic/28597-porsche-996-dme-pinouts-and-connectors/
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Old 04-07-2023, 03:47 PM   #10
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Thanks. I checked the relays in the trunk per Bentley manual and they all seem ok.

I also measured the voltage directly from the CPS connector when cranking the engine and it clearly produces a signal. So the CPS seems to be fine.

I am however unclear how to check if that signal reaches the DM, because when I disconnect the DME connector to check for the signal on the respective pins the car will not start at all. I guess need to jumper the starter for that test?
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Old 04-07-2023, 04:07 PM   #11
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Thanks. I checked the relays in the trunk per Bentley manual and they all seem ok.

I also measured the voltage directly from the CPS connector when cranking the engine and it clearly produces a signal. So the CPS seems to be fine.

I am however unclear how to check if that signal reaches the DM, because when I disconnect the DME connector to check for the signal on the respective pins the car will not start at all. I guess need to jumper the starter for that test?
Use a Multi Meter to check continuity and for short to ground on the respective pins
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Old 04-07-2023, 05:17 PM   #12
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Thanks. I checked the relays in the trunk per Bentley manual and they all seem ok.

I also measured the voltage directly from the CPS connector when cranking the engine and it clearly produces a signal. So the CPS seems to be fine.

I am however unclear how to check if that signal reaches the DM, because when I disconnect the DME connector to check for the signal on the respective pins the car will not start at all. I guess need to jumper the starter for that test?
No, the CPS is a Hall Effect sensor, it only has to turn over to generate a signal, it does not need to run.
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Old 04-07-2023, 05:43 PM   #13
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No, the CPS is a Hall Effect sensor, it only has to turn over to generate a signal, it does not need to run.
Understood, but how do i get the flywheel to turn with the DME (and hence the starter) disconnected so i have access to the DME connector pins?
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Old 04-08-2023, 07:36 AM   #14
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Understood, but how do i get the flywheel to turn with the DME (and hence the starter) disconnected so i have access to the DME connector pins?
The starter should still turn over with the DME unplugged, but if not, reconnect it and back probe the pins on the DME harness plug.
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Old 04-08-2023, 09:38 AM   #15
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Many thanks for the good advice !

I checked continuity for the CPS wires on pin 20 and 78 on the DME connector and there was none. I then realized that the connector for the camshaft position sensor has the same profile as the CPS connector -- and sure enough they were swapped. With the right connector on each sensor the engine started up rightaway!

For now its only runs on cylinders 1/2/3 - but i guess I should revisit all connectors on that side as well and then take it from there. Thanks again!

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