968 Hard Starting

[patkeefe]

Hi all:
The 968 has become a bit difficult to start. It always starts, but it may take a couple of tries. It seems to run ok, but not it's usual self. I'm thinking it may be fuel filter related, or perhaps sensor feedback to the DME. I can likely troubleshoot it with the proper references, but...

I have no shop manual nor reference materials. Does anyone know what types of reference literature is available out there. I'm hoping this won't be a rerun of the 924s literature search (we learned a lot of expensive lessons on that car).

Thanks!
Pat

[AFJuvat]

Unfortunately, not much has been written about the 968. That said, much of what applied to the 944 also applies to the 968.

I could really use a bit more information about your problem. Does it do this all the time, only when hot, cold, etc.

Based upon what you did describe, I would venture to guess that your problem lies either in the fuel pump check valve - which is located on the output side of the fuel pump, and is replacable as a seperate part

Or

You have a poor connection to ground. Your main negative battery cable runs from teh negative post of the battery, through the firewall, and then ultimately bolts to the upper surface of the bellhousing. With age, the cable tends to corrode , and can cause a hard start.

AFJuvat

[patkeefe]

Thanks, AF.
I will check the ground connection this morning. The problem is generally on cold start. I will ask the GF re: hot start (she uses the car mostly). I have not noticed it myself on hot start.

On the fuel pump check valve...is that resutant a bleed back through the faulty check valve, resulting in low fuel pressure to the fuel rail?

I will guess the only source of repair manual is factory, like the 924s was?

Pat

[Jason Judd]

Sounds similar to what I experienced with my '95 968...turned out to be a bad O2 sensor...ran very rich.

Jason

[patkeefe]

Thanks Jason:
I was not ruling that out, either. I had that problem once on the 924s, and it was very similar. If I had some voltage specs, I could check it with my VOM.

Meantime, I'll clean out the tailpipe and see how black it turns in the near future.

Where are you in NY?

Pat

[mike944]

I have a similar problem with my 968. it always starts when cold, but it cranks longer than i would consider normal. - I suspect the fuel pump check valve as AF suggested, but i have not had time to troubleshoot it, since it still works sort-of ok. I also suspect GSS (gummy starter syndrome) since it cranks slowly. More about that at the end.

On a hot start, usually it catches almost instantly, but occasionally it tries to catch, and doesn't. After that happens, i have to crank it for a good 20 seconds or more, before it finally sputters and stumbles to life. This one, i can't figure out.

Otherwise, it runs normally.

Enough with the long-winded note.

My point, if it cranks slower than you think it should, it could be GSS. The fix for this, involves disassembling the starter, and cleaning the old gummy grease out the planetary gearset, and replacing it with a high-quality grease. 968.net has a good writeup cleaning procedure. http://65.61.16.109/forum/topic.asp?TOPIC_ID=9379

[patkeefe]

MIke:
I don't think it's the starter...it cranks fine. It just has a bit of abnormal start, but always starts. Sometimes, it needs to be started a couple of times, due to stall. Condition is worse when ambient temps in the AM are cooler than hotter...it's in the 90's today and yesterday, and started ok.

I just checked the tailpipe...it is running very rich. I have it on stands now, so I can get under it and check some other perhaps obvious stuff.

Thanks!
Pat

[mike944]

If it starts, and then stalls, several times, that leads me to believe it's not the fuel pump check valve, but it's still a possibility

Now that you say it's running rich, coupled with the fact that it's worse when it's cold out, points me towards a bad sensor. I would normally say, temperature sensor, but you have some peculiuar combinations that lead me away from a bad temperature sensor. First, it's running rich. that makes me think the sensor is telling the computer the engine is cold, and to richen up the maps. But......an over-rich condition should start ok in the cold, and should cause a hot-start problem instead. If the sensor is telling the computer that the engine is already warm, it won't inject enough fuel for a cold-start, but then it would be running lean, not rich. A bad fuel pressure regulator can cause all sorts of strange symptoms, that's a possibility, or some other sensor. I would think it's not the O2 sensor. When cranking, the computer is in open-loop mode (it's not using the O2 sensor input). But, depending on how the 968 DME is programmed, it's possible that a bad O2 sensor can cause the computer to start compensating toward the rich side, and MAYBE it adjusts the start maps by the same percentage as it's compensating the rest of the maps by. But....that's speculation, most engine control computers are not programmed that way.

Things I would suggest checking:
Grounds
Fuel pump check valve
engine temperature sensor,
fuel pressure regulator
Leaky injector (or injectors)
Some other sensor (MAF comes to mind, but then again, they don't usually go bad)


You've got a problem with some strange combinations of symptoms. You might have multiple problems.

Is it affected by the length of time it sits between when it was last running, and next time you try to start it?

[patkeefe]

Mike:
Thanks for taking the time and putting so much thought into the logic. I am attaching a photo of the paper towel I used to wipe the tailpipe this morning. I also wire brushed the tailpipe area so I could start out clean. I did a quick continuity check from the engine to battery and it has continuity, but I did not shake out any sensor circuits due to lack of documentation. The engine bay of the 968 is a nightmare compared to the 911. I got sidetracked drilling and tapping some 8mm holes which carried the gravel pan. PO had them wired on with baling wire.

I started it a few times today, and we took it for a ride for half an hour or so. I just wiped the tailpipe again, same blackness. It is definitely running rich, and gets loaded up pretty quickly. It also seems to be lacking power and responsiveness. Starting was a non-issue; it started in about two seconds every time I tried it today, both hot and cold.

Without documentation, I have no clue how it feeds back to the computer, but I do agree it is in open loop at start up. I might pull the plugs out tomorrow to have a look; my guess is black. Porsche probably has a spec for checking the check valve via pressure loss per uniy of time, again, no clue.

How similar is the 944 engine to the 968? Is it a 944 with a variocam and really big pistons. Are the electronics common schematically?

Maybe it just has the flu I may have to schedule a doctor visit, or send the GF to the dealer and sweet talk some photocopies of the workshop manual relevant sections.

Pat




[img]

Got sidetracked power washing and tapping new holes.

[Jason Judd]

Pat,

I'm in Westchester.

Jason

[Damian in NJ]

My old coupe and my present cab both run rich like that, I wouldn't worry-and carry a spare dme relay, these can often go on the fritz. And I'm in Millburn, btw.