Determining how many cylinders are firing - how?

[Pauwl]

Knocking off the bits of recommended changes I need to make, I'm considering replacing the distributor caps, rotors & spark plug wires.

I'm not sure if this is a must-have for the moment, but the comment from the Porsche garage was they weren't sure if the engine was firing on all 8.

2 questions:

1) How can I determine for sure if the engine is firing on all 8?

2) Any recommendations for plugs & wires when I do replace? Both options for good & cheap (good enough for now) would be appreciated.

thanks,

Pauwl

[Danglerb]

Not sure if this is a good way, but this is what I did.

Used a piece of rubber tubing with one end held up to my ear and the other next to each injector to listen for the clicking.

Used an inductive timing light with a clip on probe to check each of the sparkplug wires and main wires from the coils.

Used a long set of sparkplug wire pliers to disconnect each wire one at a time and listened for a difference in the sound of the motor.

Pull the plugs and inspect the color of the insulator near the tip and the gap for any sign of fouling or differences. A nice tan is best I think, white is lean, darker is rich, dripping oil not so good either.

If you pull the plugs you might as well change them, so have a new set gapped and ready to put back in, and you might as well perform a compression test while the plugs are out.

************* Keep the stock brand and type of plugs and wires. To save a bit many places offer the Beru wires without the ends that go to the plugs and rarely wear out.

Before replacing wires unless you find a problem, I would focus on replacing all the old rubber hoses starting with fuel injection and continuing through all the vacuum etc. lines.

[Pauwl]

Thanks. Russ has already suggested I replace the vacuum hoses. I've read silicon ones are the way to go, so will look into those.

Any pointers to which hoses are the all the vacuum ones? I fixed a vacuum leak on pointers from Russ once I understood the one's coming out of the intake area.

But which others need to be done, and what sizes do I need? (a pointer to RTFM is fine as well of course)?

Pauwl

[Danglerb]

I bought this one from Carl.
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=200167625246

What they suggest is to take pictures, and ONE hose at a time take them off, measure and cut the replacement hose and put it back where the one you took off was, then take off another hose etc..

Fuel lines need the special hose for high pressure fuel injection, silicone doesn't work.

[rhjames]

Hi Pauwl,

I too, have been diagnosing a misfire (or non firing) cylinder issue with my Dad's 928.

Today I removed, one by one, the spark cable to see which cylinder is not firing.

(I suggest you do this on a cold engine)

Spark plugs are an inexpensive place to start.

The rest of the ignition system is easy to check as well.

Next I would replace the fuel filter (the next most inexpensive thing to check/replace)---have you replaced it recently?

With your CIS injection, you may also have bad injectors (this is what's wrong with Dad's car---and my Euro as well! Between his car and mine, we only had 2 injectors that were functioning properly......), or other fuel system problems.

You can check the injection system this way:

1. remove and replace the plug cable, from each cylinder one at a time. If the idle remains the same, the cylinder is firing. if the engine runs worse, you've found a bad one.

2. once you have determined which cylinder(s) is bad, you can then remove the injector, and move it to another cylinder that was firing OK. Remove the plug cable from the cylinder you just moved the injector to, and if the engine runs rough or the same, you know that the injector is bad or good.

3. Repeat this process for every cylinder you find that is bad.

This is a slow and simple test, if you don't have a fuel injection shop to just take your injectors in for a check up......

Another way to check them is:

1. remove the injector(s)

2. using spray can brake cleaner (with the little tube that fits on the spray nozzle) put the tube into the fuel injector while you hold it upside down.

3. spray the brake cleaner into the injector, and look at the spray pattern. It should be "cone" shaped (and a fine mist of spray), and the injector should "buzzz" while you're doing this. If the spray doesn't come out, or just a few streams appear, or it just dribbles out----you've got a bad injector!!! Note: I was very surprised at the minimal pressure at which the injectors "popped" allowing fuel to come out the discharge end.

The last time I checked, injectors were about $44 each at 928 Specialists here in the US. You can get more info at their website. (If you replace one, I suggest you replace them all.)

Once you have determined that the injectors are OK (or not), then you can move to the warm up regulator, the auxilary air valve, and then the fuel distributor.


I suggest you do these things in this order, as they progress in cost ($$$$) as you go along.


One other thing:

This is very important to mechanical fuel injection systems with individual injector lines.....

at the each end of the fuel lines, the flared end has a hole for the fuel to pass through. these holes are supposed to be a certain size, but due to tightening of the line clamp (sleeve), these openings can become damaged or closed off. (remember, your lines are over 20 years old and there is no telling how many times they have been loosened and tightened over the lifespan of the car......)

Solution:

being very careful using a slow speed drill and using gradually larger bits, you can enlarge the opening to its original size.

this is something you must have great patience with, and mostly a steady hand while drilling these lines. Breaking off a bit is a huge mistake---drilling a hole in the side wall of a line is just as bad. BE CAREFUL WHEN DOING THIS!!!!! (make sure you flush the lines after doing this to remove any debris from the line!!)

If you do this to one line, you absolutely should do all of the lines, as uniformity is the rule.

I found (on Dad's and my Euro) that our lines were in horrible shape. Some were crimped two drill sizes smaller that the rest, some were OK---you get the idea.

this procedure should also be done on the lines that go from the fuel distributor to the warm up regulator, to the pressure regulator, to the cold start injector, etc.....

This is a technique we use on diesel truck engines that use mechanical fuel injection systems when doing a full-on tune up. (Most diesel tech's don't even have a clue about this....)

when returning the lines to the engine, do not use excessive force to tighten the lines!

SAFETY NOTE-----

Do not disconnect fuel injector lines while the engine is running (allowing gas to spill onto a hot engine).

You can do this on a diesel engine (cracking a line loose to see which one is firing or not), but not with GASOLINE. You could risk setting yourself and everything else on fire----not good.

As for the vacuum line kits:

I bought mine from Carl @ 928 Motorsports (about $45 including shipping).

It's a good start, but found that there wasn't enough line to do the entire car including the lines that go into the firewall to the HVAC box.

There is another company (here in Texas) that sells a kit on ebay for $130 or so, that is supposed to have more line that Carl's. Look for it on eBay.

As Danglerb said, replace one line at a time. That way you are not searching for a schematic to find where all of the lines actually go (if you take them all off at once).

And yes, if you replace one, replace them all. If one is bad, the others are--or will be soon.

[Pauwl]

Russ,

thanks for that very extensive post, I'll get down to it...

Unfortunately you confused me on the first step

Let me run it by you again and see if I understand.
I'm to run the engine, and one-by one disconnect the PLUG WIRES, seeing if the engine runs WORSE when I've disconnected one. If so, the wire etc. are all good.
If the engine runs the same, I've found a bad one and need to fix one of the things you go on to list below.

Am I right?

I promise to not blow myself up on this one.. With winter coming and no garage (just the driveway) working close to a warm engine sounds so nice though...


Danglerb: thanks for that one - am following up with them now on shipping to France.

[rhjames]

Pauwl,

you are correct, just remove one spark plug cable at a time.

if the engine gets worse, that cylinder is firing OK.

if the engine stay the same, you've got a bad cylinder (either spark not working or fuel not getting there properly).

you're on the right track!!

---rhj

[tkelly1250]

I too have a 1979 and I just finished replacing the entire vacuum system line for line. Here's the schematic...



If you need a colored version of this, with each line separately colored, let me know.

I learned that it is also very easy to miswire your plugs! I have a number of other collector cars and on many you have to place each plug wire onto the distributor cap in the order of the firing sequence. Hence, plug #1 would go to distributor cap location #1; but, plug #7 wire might go to distributor location #2 if that cylinder fires second. Not so on 928s (unless your distributor cap is numbered in sequence (1,2,3,4,etc.)) On 928s, #1 goes to #1 location on cap, #2 to #2 and so on. Each cap is already ordered in the sequence of firing.

In checking my plugs for firing, just take one off at a time. You will hear a difference in engine tone.

Also, make sure the prior owner replaced the small auxiliary paper air filter in the front left corner of the engine compartment. Mine was filled with grease-like oil remnants that flowed through my entire oil breather system. Replaced every line as well as the regulator on the back of the engine.

Take pictures of everything on your engone before and during your disassembly. Very helpful.

Good luck.

Tim

[Pauwl]

Tim,

thanks for the great schematic. I'm in England at the moment, and will only be able to work on this in 1.5 weeks' time. I'll keep you posted on how it goes.

Rus, saw your comments on where to get the vacuum tube, and it's on order.

thanks all.


Pauwl

[jthwan22]

If your wires are in sad shape, pulling on it while engine may get you hell of a shock. Try rubber gloves and boots if you really want to try it. I like the timing light idea better.

[Danglerb]

Most of my plug wires I had to fight with to get them loose, so I would agree, start the testing with an inductive timing light that clips over the wire. Before attempting it with the engine running I would take my time and remove them once without the motor running and apply some silicone grease to the fittings.

BTW if the wires look bad, why not start with replacing them.