Weak Spark

sbrinz · sbrinz's Garage

Hey Everyone,
I am new to the site but have heard great things about the tech forums. I have an 84 928 with weak spark. The resistance across the coil matches the 928 manual along with the voltage. The resistance along each of the spark plug lines also matches. I have recently replaced the rotor in the distributor thinking it could be shorting out from old age.. still no change and I am stuck. When using a timing gun to test for spark, I was getting a very random and weak signal a week ago. Now I am not getting any signal at all. The car has not run in about seven years. I purchased it recently in hopes of getting it started. Any thoughts on what to do would be appreciated. Thanks in advance.

Spencer

szyzygy · szyzygy's Garage

if you tested the spark plug lines, why both with the distributor? that's where it comes from, right? where is it weak exactly?

You have spark. how about air and fuel?

sbrinz · sbrinz's Garage

There is no signal from the timing gun at any point including the line from the coil to the distributor. There is air and, I am assuming, fuel when turning it over. I tried spraying gum cutter in through the air intake to make sure there was at least an attempt at a start and still no luck. The spark plugs were also replaced when I purchased the car.

sbrinz · sbrinz's Garage

There is no signal from the timing gun at any point including the line from the coil to the distributor. There is air and, I am assuming, fuel when turning it over. I tried spraying gum cutter in through the air intake to make sure there was at least an attempt at a start and still no luck. The spark plugs were also replaced when I purchased the car. Thanks for the help.

Danglerb · 02-26-2012, 07:48 PM

No spark instead of weak, sounds like the Green wire, sends the mag pulses from the distributor to the ignition box.

MPDano · MPDano's Garage

Quote:

Originally Posted by Danglerb

No spark instead of weak, sounds like the Green wire, sends the mag pulses from the distributor to the ignition box.

I've posted some pics of my Green Wire replacement on the "Common Fixes" Forum.

**MOST Common 928 Issues AND Fixes**

Mrmerlin · 02-27-2012, 06:26 AM

is this a Euro?
if so then it has LH EZF MAF no green wire
OP please add more info to your post as its lacking detail
what is this car Auto or 5 speed,
USA or Euro ,
what was the last thing you fixed?

MPDano · MPDano's Garage

^^Ditto^^ Plus post pics of your engine bay. Heck, post interior and exterior as well. We like pics.

Danglerb · 02-27-2012, 09:41 AM

If its a Euro, then crank position sensor maybe, there is also a relay to check.

Weak makes me think poor connection at some point.

Mrmerlin · 02-27-2012, 12:23 PM

if its a Euro try swapping the coil wires to the other distributor,
if they are swapped then you wont get any spark.
Also verify that the distributor rotors are lined up to the hash marks at TDC,
you may need a new distributor belt

sbrinz · sbrinz's Garage

The car is USA and is a 5 speed manual with 45,000 miles on the odometer. The last thing I attempted was checking the coil by running a 12 gauge line from the battery to the positive of the coil. What I have not been able to understand is why, if the coil is okay and is getting the full twelve volts, am i not getting any signal of spark with the timing gun? Thanks for all of the comments so far. Here are some pictures of the car.

This is my first Porsche. I have always loved Porsches and bought the car for the purpose of learning how to work on it.

MPDano · MPDano's Garage

Wow, looks like your Front Spoiler is intact. Almost unheard of especially when the interior looks like that. Well done. What did you pay? You for sure have an L-Jet there.

Look at the Common Fixes thread and disconnect the battery and get into all the grounds, 14 pin. Clean the CE Panel Relays and Fuses. Your gonna have to do that stuff anyways, if that doesn't work replace that Green Wire. We'll get you there.

Maleficio · Maleficio's Garage

You should verify your grounds before you do anything else. The most important ground is also the most difficult ground to locate. It's the computer's main ground that allows it to pulse the injectors.

Location is on passenger side valve cover, above the "O" in Porsche. It shares the mounting point with the supplemental air valve. You can't see the ground because of the valve. Other grounds will affect performance, but this ground will prevent your car from running.

Mine was bad. The screw hole was stripped out, so I couldn't apply any torque to the screw head, so I re-located the computer ground to the other screw hole on the forward end of the valve cover. Problem solved.

sbrinz · sbrinz's Garage

I hear the air flow valve opening when turning over the engine. I replaced the fuel pump, fuel pump relay, and the missing bridge to disarm the alarm system. I am assuming I am getting fuel. Like I said earlier, I tried spraying gum cutter in through the air intake and there was no attempt at starting. I was happy with the price I payed for the car as well, $1500. I also went through and cleaned the grounds I could see, six of the seven listed in a diagram. I did not know about the computer main ground. I will have to clean that. Is there a way to test the green wire to confirm that it works or does not work?

MPDano · MPDano's Garage

I think in the WSM there are readings that you use one those electrical testers with. I never tested mine and since it was a known issue AND 30 years old, it was one of those no-brainer purchases. Someone with electrical experience can chime in on this one, Alan?

Mrmerlin · 02-28-2012, 04:43 PM

from the looks of that car you need to buy a new green wire and install it,
then report back .

dont try jumping any wires with power so you dont damage anything else put in a new green wire first

Danglerb · 02-28-2012, 07:16 PM

Seems like some extra wires running around, maybe an aftermarket alarm still buried in the wiring?

Where did those red air filter straps come from?

sbrinz · sbrinz's Garage

I'll order a new green wire and report back, thanks. And the red air filter straps came with the car.

sbrinz · sbrinz's Garage

Hey everyone,
I have purchased and installed a new green wire for the car. I also spent some time and cleaned each ground along with the 14- pin connector. I have also replaced the ground strap from the battery to the body of the car along with the ground strap from the engine to the body. There is significant improvement in the speed in which it turns over. It seems like the engine is still not turning over fast enough, though. And lastly, when I test for spark with a timing gun, there is no sign of spark, no signal from any of the spark plug lines or the distributer line from the coil. Any thoughts? Thanks for the help.

MPDano · MPDano's Garage

I wonder if your L-Jet is on the brink. Put your location in your Profile. Maybe someone near you can swap out Brains. Or there is a place that can verify that your L-Jet Brain is good, just not sure of the name of that place.