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.

Try a new coolant sensor. That fixed mine with the same problem.

Impulse sensor on the flywheel going bad.

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?

Quotes source post: 7416597

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?

Is there any way to test the impulse sensor and the coolant sensor or should I just replace them?

Yes mine was spinning very slowly until it cooled off.

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?

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.

Is this the correct part number for the Impulse sensor? 986-606-112-02-M14 Porsche 996 Ignition - Page 2

yes, I'm showing 986-606-112-01 in the pet, the -02 is the updated part

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.

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.

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.

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...

Yes, never had a problem with starting since.

Two different issues. One didn't start when the engine was hot the other has a hard time starting when it is hot out.

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.

Fuel "tank" pressure sensor? https://rennlist.com/forums/996-forum/931081-fuel-pressure-sensor-replacement-guide-with-pics.html

Bringing back this thread - as I am having a starting problem while hot. Car starts fine when cold - I have been topping up the battery, but it is a new battery and I don't think it is the problem. My car is a 2002 C4S (with a 2005 engine). Yesterday after stalling out at a light (purely my fault), it would not start up. It cranked but very very slowly and then slowed down even more as I tried again in a few minutes. I called for a jump - but then after 30 minutes, I was able to get it to start. Cranking was a bit slow but fast enough to start up. I am tempted to change the Starter, but have also heard that wiring can be less conductive when hot, or it could be one of the many above suggestions, fuel pump relay, impulse sensor, etc. Any suggestions on how to narrow it down without buying a lot of parts???

Yes, check your crank position sensor; they are simple Hall Effect units which tend to fail when warm or hot, but still work when cold.

But... if the main symptom is that cranking gets really slow and so the car won't start is the main symptom, does this point to the Crank Sensor? About 30 minutes later upon cooling, the cranking speed gets faster and the car starts... If it happens again, I will record the audio....

Sounds more like you have a weak battery issue. When the CPS fails when warm, the DME does not see the engine turning over and shuts off the fuel and spark, so the sensor will stop the car from starting; but will not cause it to turn over slowly, that is either a battery or primary battery cable resistance issue.

Pretty new battery - I am leaning towards cable resistance issue OR Starter.... Will try to clean up the ground and check out the battery cables. More to come.

Run a voltage drop test across the primary cables, that will tell the story.

Do you also have battery drain issues? I’m assuming so since you said the battery is new. If so, the positive cable in the rear corrodes at the connection to the starter. It can cause a whole number of issues. You can change it yourself, but you will need all of your ratchet extensions to reach the nut on the starter. And the alternator needs to come off so you can get your arm in there to thread the cable through. https://www.pelicanparts.com/More_Info/99660701903.htm?pn=996-607-019-03-M100&gclid=CjwKCAjwloCSBhAeEiwA3hVo_UhWkpYIRlhCYIZZs-aJCvpex6Px6yb2lWdYYQ5tw2bFniiP9PrF6xoCgcYQAvD_BwE

Thanks, Van.... i have a new starter and the cable in my cart ready to go..... based on the review on the cable and starter from someone who had the same problem. I may try the cable first to see if that resolves the issue. Can I replace the cable without moving everything out of the way, e.g. plenum, air box, etc? And yes - starter lagging as the car gets hot but battery seems to be draining a bit - but lagging occurs pretty quickly when car gets hot and battery still about 14.5 volts.

@jcsjcs sorry for the delayed response. I was able to do it with just the airbox and alternator removed. You have to go in and out from the right side where the positive jump terminal box mounts. You’ll spend some time with your face pressing against the engine, but with some good lighting and some patience it can be done in a couple hours. It fixed my battery drain, slow cranking, and occasional failure to crank on hot starts.

@Van_B. and others... A friend and I completed the job on Monday - replaced both the Starter and the Cable. The Starter was from 2008 and was having minor issues. The cable was definitely corroded and the slow starting was annoying. The car starts perfectly now. It is such a huge change whether hot or cold that I can't believe it each time. If anyone is having any lagging while starting, do the work - well worth it. FYI - We were able to make the change without removing the rear plenum. We also removed the alternator and used the HOT WATER trick which saved us from having to move the AC compressor. For the HOT WATER trick, let the boot soak for 5 minutes or so in very hot water and it will work. We also likely could have saved the old boot since we were able to fish the cable out without damaging the boot - since we could bend the old cable pretty significantly while removing it. This gave us the option to use the old boot if we damaged the new one. I expect you could also cut the old wire since you are removing it anyway to try and salvage the old boot. Hot Water Trick: https://rennlist.com/forums/996-forum/870715-alternator-starter-cable-replacement-simplified-5.html#post18068358