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Started engine, heard a light pop, stopped, won't restart

 
Old 12-09-2017, 09:20 PM
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Default Started engine, heard a light pop, stopped, won't restart

I posted this on the 968 forum as well, but I tend to have better luck with technical issues on this forum, so I'm posting it here as well (sorry if this is a breach of internet etiquette). I fired up my 92 968's engine tonight, and it started normally, and ran fine for a few seconds. There, I heard a faint "pop", like a small misfire, and the engine abruptly shut off, and I can't restart it. It cranks fine, but won't fire. Fearing the worst (a broken timing belt), I removed the TB cover, and both belts are perfect. Suspecting lack of spark, I removed spark plug No. 1, inserted it into its wire boot, touched the thread to an engine ground, and had my wife crank the engine. There is a perfect spark, so the coil and ECU can be ruled out.

Given how abruptly the engine died, it doesn't seem like a fuel delivery problem. In other words, it didn't sputter and start running rough as if it was starved for fuel - it just died. Also, it has a brand new Bosch fuel pump, which has demonstrated to work perfectly, at least the last time I drove it, which was a week ago. I haven't done a blink test yet, but I don't see anything in the list of sensors it tests that could explain the inability to start the engine. Any thoughts on what to check next? Thanks.
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Old 12-09-2017, 09:24 PM
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Sounds like you lost a fuse or your DME relay.
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Old 12-09-2017, 10:16 PM
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Originally Posted by bw993 View Post
Sounds like you lost a fuse or your DME relay.
Maybe, but losing the DME relay would kill the fuel delivery, which would have created a more gradual, choking, sputtering halt to my engine running, I would think. And I can't think of a fuse that would cause this - do you have a specific one in mind? Thanks.
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Old 12-09-2017, 10:22 PM
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Originally Posted by Cloud9...68 View Post
Maybe, but losing the DME relay would kill the fuel delivery, which would have created a more gradual, choking, sputtering halt to my engine running, I would think. And I can't think of a fuse that would cause this - do you have a specific one in mind? Thanks.
Need 3 things to make the car run.
1) You have spark
2) Timing belt is intact
3) Do you have fuel pressure?
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Old 12-09-2017, 11:48 PM
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I tried jumpering the DME relay's connections, which I had done successfully many times while troubleshooting my recent fuel leak, and nothing. So it looks like my new fuel pump may be bad. I'll call Paragon Monday morning. So that pop I heard may have been a red herring. Strange that a fuel delivery problem would cause the engine to die so suddenly, though.
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Old 12-10-2017, 02:22 AM
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Originally Posted by Cloud9...68 View Post
I tried jumpering the DME relay's connections, which I had done successfully many times while troubleshooting my recent fuel leak, and nothing. So it looks like my new fuel pump may be bad. I'll call Paragon Monday morning. So that pop I heard may have been a red herring. Strange that a fuel delivery problem would cause the engine to die so suddenly, though.
No gas, no bang.

A couple of months back I was at the track and the car would not start. I was scratching my head when I looked at the fuel pressure gauge and saw a great big goose egg. Filled up the tank and she fired right off. Just saying it maybe that easy.

Are you using the stock fuel rail and regulator?
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Old 12-10-2017, 04:10 AM
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Fuel pump fuse would be the one I would suspect if it was a fuse. A bad DME relay would result in no spark if the 1st stage failed. If the 2nd stage failed, no power to fuel pump.
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Old 12-10-2017, 09:41 AM
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Mine just stopped with no sputtering when he DME relay went failed at a red light.
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Old 12-10-2017, 10:36 AM
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Originally Posted by odonnell View Post
Fuel pump fuse would be the one I would suspect if it was a fuse. A bad DME relay would result in no spark if the 1st stage failed. If the 2nd stage failed, no power to fuel pump.
I checked the fuel pump fuse, and it's good. I took the DME relay out and jumpered its connectors in the fuse box, which normally engages the pump, and it didn't, so the only thing left is the pump itself, unless there is a loose connector somewhere between the battery, the relay, and the pump. I'll apply 12V directly to the pump to confirm. It's just unusual for a brand new pump to fail after working for a week, and the suddenness of the way the engine stopped made me suspect spark or something mechanical (like a broken timing belt), but the evidence points otherwise. I'll test the DME relay as well, but that can't be the cause of my current problem, because the pump won't run with the relay removed and its connectors jumpered. Thanks for all the responses - as I've said many times, this is the best forum for technical help that I've found in the 4-cylinder transaxle Porsche world.
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Old 12-10-2017, 12:21 PM
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Well it is often the last thing you "fixed" that causes the problem. I'd probably pull a spark plug and crank the motor, [carefully] looking and smelling for fuel in that cylinder. You'll know it's there if it is. Assuming no fuel, then I'd check the fuel pressure to see if (a) you are having a problem pumping fuel up to the rail with pressure (e.g., fuel pump, plumbing, regulator, etc.), or (b) you engine management system is having a problem firing the injectors (e.g., frayed injector connector wires a common problem, DME injector drivers can fry out internally, etc.). At least on a 951, a single injector connector short will short out ALL of the connectors and cause and instant-off scenario.

https://rennlist.com/forums/944-turb...onnectors.html
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Old 12-10-2017, 12:28 PM
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I had this happen to me once on my old 944 NA. The engine back fired (pop) and it blew apart one of the couplers on the intake. After reconnecting, it started right up.
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Old 12-10-2017, 09:26 PM
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The "pop" tends to steer me away from fuel delivery, unless it's an intake pop due to lean mixture.

As it was recommended, check for fuel pressure and for injectors firing.

I would check the rotor under the cap. If the screw holding the rotor came out, the rotor may be pointing the wrong way, directing spark to the wrong cylinder. This could explain the "pop".

A big vacuum leak could cause all sort of issues, don't rule it out. As a quick test, unplug the MAF, the engine will run in limp mode and you'll get a CEL.

If you had/have fuel all along, there is a good chance the spark plugs are soaked and will not fire. I would have a new set handy.

Start by eliminating possibilities:
- fuel (pressure and injectors pulse)
- ignition (spark and to correct cylinder, plugs)
- air (eliminate possible vacuum leaks)
- compression

The problem/solution is probably a simple one. Just a bit time consuming to find.
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Old 12-10-2017, 11:51 PM
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OK, this is getting weird(er). As I said in posts 5 and 9, when I jumpered the DME relays connectors in the fuse box a couple of days ago, the fuel pump failed to turn on, which told me in no uncertain terms that my new pump has gone bad. But just to rule everything else out, I removed the panel the pump is attached to, exposing the connectors. I removed the (+) connector from the pump, and with the battery disconnected, applied 12V from the battery directly to the pump via jumper cables and alligator connectors. To my surprise, the pump turned on. Also, I could hear fuel gurgling under the engine compartment, telling me the fuel rail was empty, and running the pump was filling it back up, as the gurgling stopped after a few seconds. So I disconnected the jumper cables and alligator connectors, reconnected the (+) connector to the pump, reconnected the battery, removed the DME relay, and jumpered the DME relay connectors again. This time, the pump turned on!

This is bizarre. How could the pump not be getting a signal a couple of nights ago, preventing it from running, and tonight, it's all working? It's possible the DME relay has a problem (I still need to test it), but this wouldn't explain why the pump wasn't running with the DME relay connectors jumpered a couple of days ago, but is working again tonight. Any suggestions as to what to try next? Thanks.
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Old 12-11-2017, 12:19 AM
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How did you jumper the DME relay -- which pins?
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Old 12-11-2017, 01:25 AM
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30, 87, and 87b - the same ones I've jumpered multiple times. And to cap off the utter bizarreness of this issue, I put the DME relay back in, cranked the key, and it started right up, and ran flawlessly for about 15 minutes before I turned it off. I didn't do anything to fix it, and it's acting as though nothing had happened. Which is great, except I'll never know when the problem will happen again, and leave me stranded.
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