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1999 996 won't start when hot (cranks)

A few days ago my car started not starting when hot. It seems somewhat random when it will happen. I think it is heat related because it starts fine after sitting and today it was much hotter outside and I had to wait over an hour before it would start.

From reading the forums I thought it sounded like the fuel pump. I put a gauge on it when it wouldn't start last Friday and jumpered the fuel pump relay and it was only making 15 psi. I ordered a new fuel pump and installed it yesterday. Put the 5 gallons of gas back in and drove to station and filled tank full last night and everything seemed fine.

Today the problem was back. I put the pressure gauge on and there was no pressure. Cranked the engine and no pressure. Jumpered the relay and went up to about 60 psi. Cranked engine and still no start. Let cool for an hour or so and started.

Now I am at a loss where to look. This is the first problem that I have had with this car so not real familiar with what to look for.

Thanks in advance for any help.
Old 04-30-2013, 02:17 PM
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Try a new coolant sensor. That fixed mine with the same problem.
Old 04-30-2013, 04:18 PM
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Impulse sensor on the flywheel going bad.
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Old 04-30-2013, 04:38 PM
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Same thing happened to my '99. It took a year to get really bad, new starter and never happened again. The starter gets hot and after years it dies.
Only a problem with a hot engine, trying to restart after a short 5 min stop and would slowly crank and not start until it cooled off. Is this what is happening?
Old 04-30-2013, 10:33 PM
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Quote:
Originally Posted by white99c2 View Post
Same thing happened to my '99. It took a year to get really bad, new starter and never happened again. The starter gets hot and after years it dies.
Only a problem with a hot engine, trying to restart after a short 5 min stop and would slowly crank and not start until it cooled off. Is this what is happening?
It is usually longer trips, some have been over 30 minutes when it happened. Some shorter but not the first one of the day so the engine probably hadn't cooled down fully.
It sounds like it is spinning the engine as fast as normal. Did yours sound weak?

Last edited by whatnot; 05-01-2013 at 07:17 AM..
Old 05-01-2013, 07:08 AM
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Is there any way to test the impulse sensor and the coolant sensor or should I just replace them?
Old 05-01-2013, 07:09 AM
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Yes mine was spinning very slowly until it cooled off.
Old 05-01-2013, 07:41 AM
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I didn't have a lot of time to play with it today but I drove around for a while and parked it and let it sit. It wouldn't start when I tried.
While I was driving around I had torque running on my phone and was watching the coolant temperature and nothing out of the ordinary happened with the readings. When I can back to try starting it the coolant was reading 127 degrees.

Looking through the other sensors I found that the IAT is reading -40 degrees. I didn't look at it earlier when the car was running so I don't know if Torque can read it properly. I will see tomorrow if it will start.

Does anyone uses Torque for Android and know if it can read the IAT properly?

Would a bad reading for the intake temp keep the engine from starting?
Old 05-01-2013, 07:30 PM
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It looks like Torque just wasn't working right. This morning it showed -40 on the IAT but then after I started the engine went to 50 degrees.
Old 05-02-2013, 07:59 AM
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Is this the correct part number for the Impulse sensor?
986-606-112-02-M14
Porsche 996 Ignition - Page 2
Old 05-02-2013, 08:08 AM
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yes, I'm showing 986-606-112-01 in the pet, the -02 is the updated part
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Old 05-02-2013, 03:46 PM
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I replaced the impulse sensor yesterday and I think it fixed it. It was getting worse and I had to wait over 3 hours yesterday before it would start. Today after changing the sensor it started every time.

I had to give up trying to get the connector back in the place it was mounted. Even with a lift I don't see how anyone could possibly get at that location.
Is it just the cabriolet models that are impossible to get at?

Next job is to replace the fuel filter. That looks like it will be just as fun.
Old 05-10-2013, 07:42 PM
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It is a pain on a coupe too. You should try to mount it correctly instead of having the connector flopping around. I am use to working blind so it could be very difficult if you don't do that type of work all the time. Remove the wheel and try again.
Old 05-11-2013, 04:02 PM
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I probably spend over an hour trying. I don't think I can possibly get it back no matter how long I keep at it. I zip tied it to the other wires it was near so it couldn't flop around.

I am 99% sure the problem is gone now. Drove to several places today and no trouble starting what so ever. It also seems to idle smoother.
Old 05-11-2013, 04:39 PM
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I think I may be having the same issue. I've attached a video.



Did the impulse sensor fix this issue for good?
This only happens after its parked at work all day, in the desert...
Old 07-07-2017, 04:38 PM
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Yes, never had a problem with starting since.
Old 07-07-2017, 08:46 PM
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Two different issues. One didn't start when the engine was hot the other has a hard time starting when it is hot out.
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Old 07-13-2017, 05:54 PM
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Mine wouldn't start and replaced the impulse sender but ended up being the fuel pump relay. Clue was wether or not the tachometer shows engine speed while cranking. If the impulse sender is bad then the tacho won't show anything (since the tacho gets it's signal from that sensor).

The fuel pump relay is a dead easy fix. Not that the impulse sensor is hard or anything, but the relay is reach under the dash level easy.
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2002 996 - arctic silver - PSS9, H&R sways,X51 oil pan, console delete, AASCO liteweight flywheel, gbox detent, RS motor mounts, 997 shifter. Great car.
past: another 2002 996 and a 1978 SC with-webers-cams-etc.
Old 07-15-2017, 08:31 AM
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Fuel "tank" pressure sensor?
https://rennlist.com/forums/996-forum/931081-fuel-pressure-sensor-replacement-guide-with-pics.html
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Old 07-22-2017, 03:32 PM
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