[Cloud9...68]

I did a search on this problem, thinking it had been covered earlier, but I couldn't find anything quite like it.



It's just like I've described it in the subject line: It never has the slightest problem starting from cold (one crank typically does it), but after driving it around enough to let it thoroughly warm up, and stopping for a few minutes, makes starting the car a fairly tricky operation. I have to time the turning and releasing of the key and the stabbing of the throttle JUST RIGHT for it to catch on the first try. If I miss, I have to crank the engine over and over, stabbing at the throttle just as I release the key, sometimes a half dozen times (all the time praying I have enough charge in my battery to make it), before it will start.



I did a starter clean-up a couple of years ago, and that seemed to help, but it didn't completely cure the problem. It's hard to tell if the problem is caused by the starter prematurely disengaging, or from the engine simply not "catching," or some combination of the two.



My motivation for getting this fixed is that my battery is close to four years old, and when its time comes, I'd like to replace it with a lightweight dry cell battery, and I'm afraid that, while having plenty of cold cranking amps for the central Texas climate, one of these little batteries wouldn't be able to handle this little drama that I periodically have to go through during hot starts. Thanks.

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[968workaholic]

Do you have the new DME relay? If yes, does the car start if you crack the throttle a tiny bit while cranking, or do you just give it a poke at the end of cranking?

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[Cloud9...68]

[quote name='968workaholic' post='43463' date='Nov 11 2007, 06:59 PM']Do you have the new DME relay? If yes, does the car start if you crack the throttle a tiny bit while cranking, or do you just give it a poke at the end of cranking?[/quote]

I forgot to mention that yes, I do have a new DME relay. It's actually a few years old, but it didn't have any impact on this problem.



I haven't tried cracking the throttle a little while cranking; I crank it with my foot completely off the throttle. When the car is cold, it starts right up without my having to poke the throttle at all. But when it's hot, cranking the engine with my foot off the throttle doesn't work reliably. It sometimes just doesn't want to fire, and this is where poking the throttle at just the right moment can "save" it.



I think the problem is compounded by the fact that the starter seems to disengage after a few cranks. Maybe if my starter were healthier, it would start on its own after a couple more cranks. But I think it's more than just the starter, because as I said, it starts on the first crank when it's cold. So it's a combination of for some reason needing more cranks to fire when it's hot, and the starter not giving it the opportunity to do so. Do other people's cars need more cranks to fire when hot than when cold? Thanks.

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[94SilverCab]

Honestly, that is classic DME relay symptom, try replacing, cleaning the contacts. Sometimes it helps to plug and pull it a few times.



Regards,



Jay

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[968workaholic]

Try this, while you are letting the car sit after it has warmed up, take out the DME relay. Make a jumper wire using two spade type connectors and a small piece of wire. Now right before you try and start the car, jump terminals 30 and 87B, they are labeled on the fuse box. Then try to start the car. If it lights up right away, then it is most likely the relay. If not, try moving the wire around inside of the connections to see if anything is loose and try again. It could also be one of the connections that you are not using that is loose. Try having a friend listening to the right rear of the car for the fuel pump to come on when you are doing this testing. Maybe someone you know has a 968 that you could borrow a know good relay out of, so that you don't have to buy one if that is not the issue.

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[unknown4u]

You forget one wire to connect.........the one from the idle stabilser and the o2 sensor.

Otherwise it is still hard to start.

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[flash]

actually, to me it sounds like one of the vacuum hoses has popped off - some of them are very hard to see, and cleaning up could have dislodged one of them - unlike other cars with disconnected vacuum lines, this one will seem to run fairly normally, except for odd things like this

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[kwikt]

Good point Flash, maybe someone here has a vaccuum diagram.



Kevin

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[Cloud9...68]

As usual, good suggestions, much appreciated. Do either the DME relay or loose hose connection theories fit with the difficulty in starting only happening when the engine is hot? Also, it doesn't have the problem every time; sometimes it fires up just fine when it's hot. It's just some sort of marginality, I think.



In any event, I'll at least remove the DME relay, clean the contacts, and re-install. I've recently made it a habit to spray any electrical connectors I remove with the CRC spray made for this purpose, but I didn't do it back when I replaced the DME relay. And I'll look around for loose vacuum lines, but I went through this pretty thoroughly a couple of years ago when I had a severe hesitation problem at WOT (which turned out to be bad plug wires). I need to add that I've had this hot start problem for years, and I just thought it would be a good idea to finally start troubleshooting it.

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[flash]

yes, absolutely - that's what led me to this - when cold, the engine will draw air differently than when hot



there are quite a few lines to look for that are easy to dislodge - off the top of my head.....



there is a tee up near the battery that runs to a line that crosses the engine bay by the windsheild



of course the heater control valve line



a line that runs down by the intake manifold



a tee up near the fuel pressure regulator

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[ldeprat]

Sounds electrical to me. I think a vacuum leak would make the car run badly, but still start. Does the 968 have hall sensors, like the 944? My hot start problem was fixed by replacing both units on the 944 Turbo. Used to have to wiggle those wires to get the car to start again when hot.

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[flash]

oddly, no - i have had this happen to me, and the car ran relatively fine



i'm not saying that it isn't something electrical, but there are only a couple of electrical things that could do it, and they help control mixture



it really sounds to me like a mixture control problem - like the car can't tell what to do - a vacuum leak can do this, so can a throttle position sensor or a MAF



can you smell fuel at all when it's hot? have you pulled a plug to look at it yet?

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[Cloud9...68]

[quote name='flash' post='43529' date='Nov 13 2007, 07:50 AM']oddly, no - i have had this happen to me, and the car ran relatively fine



i'm not saying that it isn't something electrical, but there are only a couple of electrical things that could do it, and they help control mixture



it really sounds to me like a mixture control problem - like the car can't tell what to do - a vacuum leak can do this, so can a throttle position sensor or a MAF



can you smell fuel at all when it's hot? have you pulled a plug to look at it yet?[/quote]

No, I can't smell any fuel when it's hot (or any other time). And no, I haven't looked at a spark plug yet. The last time I took one out to look at it was about 12,000 miles ago, and they were perfect at the time. It's probably about time to replace them, anyway (probably been about 20,000 miles).



I guess I should also do a blink test, in case the hall sensors show up. Been pretty busy with other stuff this week; I'll start digging into the very long-standing problem this weekend.



One thing that supports the vacuum line theory is that back when I had the WOT hesistation, I rented a vacuum gauge, and got a pretty erratic reading. That was with bad plug wires, but I don't see how that could impact the vacuum reading. Soon after doing the vacuum gauge test, I found the hesitation was caused by the plug wires, and the car idles smoothly and runs fine otherwise, so I never went back to follow up on the erratic vacuun gauge reading. I need to do that now.



Thanks again for all the suggestions.

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[S_Cal968]

Have you ever replaced the check valve [on the output side of the fuel pump]? As with any F/I system fuel pressure is critical; so I'd suggest verifying the pressure when starting hot. And to verify the CV is working, monitor the fuel pressure once the engine is shut down; the system should hold pressure for a minimum of ~2 hours. Ideally [when new] the CV should hold pressure for several hours.



re: vac leak; most start issues are either related to ignition or fuel. It would have to be a fairly big vac leak to prevent starting. Additionally a vacuum issue may store a trouble code if the car is driven for any length of time, (however it depends how big the leak is).

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[Cloud9...68]

[quote name='S_Cal968' post='43579' date='Nov 14 2007, 08:48 PM']Have you ever replaced the check valve [on the output side of the fuel pump]? As with any F/I system fuel pressure is critical; so I'd suggest verifying the pressure when starting hot. And to verify the CV is working, monitor the fuel pressure once the engine is shut down; the system should hold pressure for a minimum of ~2 hours. Ideally [when new] the CV should hold pressure for several hours.



re: vac leak; most start issues are either related to ignition or fuel. It would have to be a fairly big vac leak to prevent starting. Additionally a vacuum issue may store a trouble code if the car is driven for any length of time, (however it depends how big the leak is).[/quote]

No, I haven't replaced the check valve on the output side of the fuel pump. Bruce Ward did a long write-up of this on 968.net, to troubleshoot a different problem (don't remember what it was). But the check valve is way back at the fuel tank, which I wouldn't think would ever get very hot, and the hard start problem seems to be totally dependent on the engine being warm. I'll have to pay closer attention, but I don't think the problem is particularly worse on a warm day. Also, if it were the check valve, wouldn't starting the car be much harder after letting it sit for a few days? I can let my car sit for a week, and it fires right up on the first crank.



Maybe I'm not understanding the theory, which I'd like to, because you're the second person who has mentioned it (the first one was in another thread I found while researching the problem). Thanks; Sorry I haven't been able to act on any of your suggestions yet, but I've got a sick wife and 10-year-old at home, making it hard to sneak into the garage to play mechanic.



I'm still waiting to hear from somebody else who has had this same problem. I swear I've seen this discussed before, maybe on 968.net, but I can't even do a search there, because it's been so long that I've logged in there that I've forgotten my password. Could somebody who still has a password there do me a favor and do a search? No rush, as I doubt I'll be able to look at this till the weekend (and again, I've had this little annoyance for years). Thanks.

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[flash]

at tis point i think a search for "trouble starting" is only likely to turn up all sorts of viagra ads

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[sp4149]

[quote name='Luis de Prat' post='43527' date='Nov 13 2007, 02:42 AM']Sounds electrical to me. I think a vacuum leak would make the car run badly, but still start. Does the 968 have hall sensors, like the 944? My hot start problem was fixed by replacing both units on the 944 Turbo. Used to have to wiggle those wires to get the car to start again when hot.[/quote]

Yes the 968 has two Hall sensors just like the 944, however one is up by the distributor and is a different design, The one in the rear is identical to the 944 sensors. The front hall Sensor when bad or disconnected will cause the car to run poorly, ignition is retarded. Having a rear sensor die on our 944, kept that car from starting at all.

A somewhat similar problem in our 944 was due to a fuel pump that would cut out when the car had been driven for about 45 minutes and climbing a grade or trying to accelerate hard. Generally had to wait 30-45 minutes for engine temp and fuel pressure to drop, and then the car would start again. We had been to several shops, replacing fuel filters, injectors, check valves etc... with little improvement, before replacing the fuel pump.



-sp4149

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[ldeprat]

[quote name='sp4149' post='43602' date='Nov 15 2007, 08:05 PM']A somewhat similar problem in our 944 was due to a fuel pump that would cut out when the car had been driven for about 45 minutes and climbing a grade or trying to accelerate hard. Generally had to wait 30-45 minutes for engine temp and fuel pressure to drop, and then the car would start again. We had been to several shops, replacing fuel filters, injectors, check valves etc... with little improvement, before replacing the fuel pump.



-sp4149[/quote]



Now that you mention it, difficulty starting when hot is a classic symptom of a failing fuel pump in the troubleshooting section of the 944 "Haynes Manual." I would think the pump would become louder, though?

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[Cloud9...68]

Ah, so there are some direct experiences from others who have had this problem, and it was fuel pump related. So let me see if I understand - the fuel pump sends fuel up to the engine, and the portion that is not burned in the engine is recirculated back to the tank. I imagine the check valve, which is on the output side of the pump, is there to prevent the pressurized fuel from back-streaming back into the pump. If the valve gets weak over time, it allows some fuel to dribble back into the pump, which somehow decreases the ability of the pump to pump fuel into the engine. I'm sure I've totally butchered the fine points of how the system works, but hopefully this covers the basics.



So why would this only cause trouble with starting when hot? Why wouldn't a bad check valve or fuel pump result in fuel starvation, causing the car to buck and hesitate, particularly under wide open throttle acceleration? I've taken my car to five weekend track events, without a hint of fuel delivery problems. Could someone please fill in the gaps in my understanding of the fuel delivery system, so maybe this will make sense to me? Thanks.

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[S_Cal968]

Not necessarily, electronically controlled engines have 2 modes, a) cold start b ) run/warm start. These 2 modes have different parameter requirements. The CV is there to assist starting when the engine compartment is warm.

No, the CV doesn't fail due to temp - it fails by leaking and not maintaining system pressure. BTW, I checked the WorkShop Manual and the CV is consider bad if the pressure [3.8 bar] drops .5 bar after 30 mins., [I'm not sure where I saw 2 hrs].

Also I found there are some other things that may affect hot starting, [see WorkShop Manual Troubleshooting Guide Section 24/28 pg. 8...].



If you don't have a manual here's what's listed for a "Hot Start Problem":

- Open/Closing Winding of the Idle Stabilizer

- Tank Vent Relay [this is most noticeable when the filler cap is removed and there's an in-rush of air - vacuum in the tank].

- Fuel Pressure [power to pump or faulty pump or CV on hot start]

- Faulty Fuel Injector [not holding press.]

- Intake System Leak

- Ignition System. [coil can be affected by heat, but most likely wouldn't start at all or will cut-out at high rpm]



This may be a big list, but a few tests should quickly narrow the possibilities. The fact that is does start when the throttle is bumped leads me to believe it's fuel delivery.

However as mentioned if it's an intake leak, most likely a trouble code is stored b/c it's a constant condition lasting long enough to be stored.



HTH

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