79 SC won't fire...where to start?

[porschefool]

Hey gang,
She ran beautifully until I went to start her up this morning. Cranks fine but won't catch. Doesn't even seem like it's trying... Checked all fuses, coil wire, had to give up and take other car

Where should I start troubleshooting? I know I need to check spark, but what's the best way without hurting the CD ignition? I did get the normal CD whine with ignition on. Will arcing the coil wire to the fan frame hurt anything?

What about checking fuel delivery? I swapped the two fuel pump relays around, just for the hell of it. Nothing.

It's strange.. seems like something came loose. It's never taken more than 1/2 second to start!

I'm so glad it happened on the first sunny Seattle day in 5 months...

Thanks for any help.

[jorian]

Could be a vacuum leak. My SC had the same symptoms and I traced it to a loose pop-off valve in the airbox.

[porschefool]

No pop off installed (yet)...
Even with a vacuum leak I would think it would at least try to catch.
Thanks for the input. I'll check all the lines.

[porschefool]

I'm remembering my days in the summer heat with a balky 928 CIS check valve.. could this be an issue, even though we're talking about a dead cold engine?

[Hoffmanmotors]

Take off the air filter and push up on the air meter plate to see if it is stuck, happened to me once.

-Jeff

[Kirk911SC]

Ignition coil.............I went through 3 of them on my SC in just over a year. All of them were "Made in Brazil" and silver.

Each time the car just died while driving.

Used an MSD instead of the Bosch and no problems since.

Kirk

[porschefool]

How do I check the coil? I'm afraid of what might happen to the CD unit if I arc the coil wire to ground... I'm pretty handy with a multimeter. Any quick measurements I can take to see if it's dead?

Thanks, guys...keep it coming! I've got a nice to-do list for when I get home today (and I'll leave a little early)

[fred cook]

Check the fuel pump relay fuse in the front fuse panel. If the ends of the fuse get too oxidized it won't pass (enough) current to activate the relay. A quick check is to use a piece of wire as a jumper across the two fuse contacts with the ignition on. You should hear a slight "click" or "clunk" as the relay closes. Most of the time all that is needed is to throughly clean the fuse contacts and put in a new fuse.

Good Luck!

[porschefool]

Just got done cleaning the fuse connectors and replacing all fuses a week or wtwo ago...everything is clean/shiny. Nothing blew.
I'll take out the relays and check them with a 12v source if nothing else seems to work.

[Superman]

With the key on and the air cleaner removed, I can lift the air flow sensor plate with my finger and hear the injectors scream. I can't imagine that arcing the coil wire against the fan housing would hurt anything. That's a spark that is supposed to go to ground anyway.

[DaddyGlenn]

Let's slow down a bit. The problem could be any one of a hundred things. It does no good to guess. Lets narrow it down first.

First check to see if you have spark. Remove a plug or use an old plug and attach it to the plug wire and ground it to the chassis. Then crank it and watch for a spark. If you have spark, that is not the problem.

Then check the fuel flow. Remove an injector, while still connected to the fuel line place it in a jar (to catch the fuel) an with the power on lift the plate in the air box. You should see fuel discharge from the injector.

Try those and let us know what you find.

[porschefool]

OK. Spark is good, but no fuel flow from injector w/power on and plate raised.
Fuel pump? I'm going back out to check the relays/fuses...

[Hoffmanmotors]

With the key on try unplugging the connector on top of the plate sensor and see if the pump turns on.

My plug on the sensor plate is bad so if my connector is plugged in the pump will NOT work.


reply with what you find...

-Jeff

[porschefool]

Got it. Fuel pump fuse was just loose enough to stop the flow. Not blown, just loose. Grrrr..
I gotta tell ya, for all the beauty of Porsche's engineering, they make Lucas look like electrical geniuses.
I wish there was an aftermarket fuse block that took the modern blade fuses!
Hmmmm... note to self---

Thanks to all who contributed!

[dan79brooklyn]

My SC used to cut out going over bumps..It was the fuel pump relay.
Take it out, clean it with an eraser and carefully bend apart the tabs with a thin blade (razor). It shouldn't cause you any more problems...knock on wood.

[boyt911sc]

Porschefool,

You are going the right direction in checking your starting problem. But you haven't checked your fuel delivery pressure. Assuming that you don't have a big vaccum leak, you need fuel, compression, and ignition to start your motor.

Your fuel pressure should be high enough to open the injectors. An injection fuel gauge kit will be very helpful in diagnosing your problem. CIS is not difficult to maintain if you understand the principle involve.

TD

[fred cook]

Do a search for Fuse Blocks, ATO Fuses, or on my name (Fred Cook). You will find a huge amount of info on the subject of replacing the old style fuses with the ATO type. Send me a PM with your email address and I will send you the details of my conversion.

Good Luck,