Weak Spark
 

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

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?

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.

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.

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.

http://forums.pelicanparts.com/porsche-928-technical-forum/607208-most-common-928-issues-fixes-3.html#post6585490

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?

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

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.

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

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. http://forums.pelicanparts.com/uploa...1330405998.jpg
http://forums.pelicanparts.com/uploa...1330406047.jpg
http://forums.pelicanparts.com/uploa...1330406065.jpg
This is my first Porsche. I have always loved Porsches and bought the car for the purpose of learning how to work on it.

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.

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.

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?

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?

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

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?

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

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.

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.

Ljet brains rarely go bad, happens, but rare.

12v directly to the coil might have fried it, I would check resistance on it, maybe just replace.

What can be measured easily with a meter to verify the green wire signal is getting where it needs to go? Fuel pump continues to run maybe?

After being unsuccessful with the timing light I took a spark plug out, hooked it up to a ground, and tested for spark. Though weak and faint, there is spark! I am under the assumption that if there is any spark at all, the ECU can be ruled out of the possible problems. If the ECU was bad, would weak spark still be possible? If so, is there a way to test the ECU to make sure it is working properly? Also, a friend who has been helping me with this project stopped by a little earlier to hear the cranking difference and thinks the car is finally cranking the way/ at the speed it should be. Any thoughts? Thanks for the replies. And I just updated my profile, I'm in the philly area.

Are you sure your timing light works?

Have you checked your plug and coil to dist wire resistance?

I am sure that the timing light works, I tested it on my other car. As for the coil, I will try another coil this coming weekend in hopes of that being the problem.

Coil on my 84 Euro measured ok, car ran bleh, bought a used one from 928Intl, ran much less bleh, on to next problem. ;)

So the car does not run, right?

I recommend buying a "noid" tester. It's a light bulb apparatus that plugs into a fuel injector connector. If your ECU is commanding the injectors to pulse, then the light will pulse rapidly. You can see it in plain daylight. Verifying that your injectors are pulsing or not will save you LOADS of time. You say you have spark, you say it's weak, but it's there, so next up is verifying injector pulse. If you do have injector pulse, time to verify fuel pressure/flow.

There's not much else to it. Save for one thing: cold start injector. If the injector is not plugged in, or has a broken wire, the car will never light off. At least my 82 would never light off with the cold start injector disconnected, even after warm.

The cold start injector operates off of battery power.

I bought one of these, brilliant design. Stick it to your windshield and watch while you crank.

http://compare.ebay.com/like/270553691729?var=lv&ltyp=AllFixedPriceItemTypes&va r=sbar

Thank you for the help! I was definitely running out of ideas. The car has not run in seven years. I bought the car in hopes of learn how to work on porsches and sure enough, this has proven itself to be a challenge, though fun. I will try swapping out the coil and seeing what happens but I will definitely look into a noid tester and report back with my findings.

Noid light is handy and cheap, 928 uses Bosch injectors with same connector as older Fords.

Usually you get so you notice the clicking sound the injectors make, but there are several diagnostic tools that target injectors because they are troublemakers.

With spark its a simple system, start at the plugs and move back up the chain until you find where good signal goes to bad output. Used to be you could hook a meter to the primary side of the coil (low voltage) and see whats going on, but I haven't a clue if that is still done.

When this is all sorted out should make a nice addition to Leo's common fixes thread. ;)

I wouldn't bother replacing the coil. You've already proven you have spark.

Buy a couple of cans of starting fluid, spray a BUTTLOAD into the intake, crank and see if it will start. If it starts, and idles, but finally stalls after the fluid is all used up, then you have a fuel delivery issue. Replace filter, verify pressure, keep going up the chain. Use the noid, too.

Spray was already tried, no luck, so ign needs sorting. Used coil is like $20, pretty cheap diagnostic.

He said he had (weak) spark, and he said he sprayed "gum cutter', not starting fluid.

This is where you need to check. Maleficio is right. You have no fuel .Check pressure at the injector rail.Should be 38psi. If no pressure go back to tank ,tank fuel filterscreen, check valve ,fuel filter,lines etc. If pressure then pull the injectors ,wash with acetone.I had this problem from gummed fuel 15 yrs old. Purchase seals and injector screens and rebuild them. Go to the wreckers and get a set of 4 electrical connectors that fit.I think old jeeps and vws have them. I got 4 with clips and about 10"leads for 10.00. Make up a test rig and with a battery charger or old battery to power open the injectors.With a compressor apply 30psi to acetone or paint thinners filled line and watch each injector spray.Once your satisfied reassemble.Acetone will clean the whole sytem fast and not hurt anything if not left more than a day or two.Paint thinners works too but slower maybe 3 or 4 days to clean tank. You may have to remove the tank. If so check the procedures on the forum as we have all done it with varying experiences.