Porsche Enthusiasts Club Forum

I wonder if I might possibly request the benefit of the hive-mind, ahead of a long multimeter session?

My 944 has developed an intermittent misfire, sometimes when cold, and clears as gets warm. Other times when warm and either remains apparent, clears itself or only clears when I restart the car. Sometimes will be driving just fine and when I accelerate the misfire begins and can take some time to clear.

As a starting point I have changed plugs, leads, rotor and cap. I also did a compression check and all cylinders showed similar compression in the zone @ 12bar.

Changing those things didn't change the problem.

I also changed the fuel-filter for a new one, and investigated the air box (which held a little bit of oil beneath the (reasonably clean) filter btw), checked the vacuum pipes and air direction pipes and boots... all looked OK. The throttle butterfly was a little dirty so I cleaned that with carb cleaner. I also pulled the injectors and noticed that a couple of them showed deterioration in the seals, so I have ordered a new seal kit from the OPC.

My speculations are:

1) Injectors... could be a problem with one or more injectors
2) Fuel system... could be a problem with dirt between tank and delivery
3) Sensors... seems like some crank / reference sensors have caused other owners to experience problems, and temperature sensors too, misdirecting either the ignition or fuel system as a consequence.
4) Other injection system issue... that's where the massive multimeter session comes in

So I would fall on my knees and beg the TIPEC community for any insight or experience that you're willing to share. Thank you

Sounds to me like it could be a problem with the airflow meter track being worn.

Another candidate could be the crank sensors, sometimes they can be intermittant, have a look at the cable insulation on the leads from the crank sensors to the intake manifold where they join the main loom, if the outer casing is cracked it is often a sign they need to be replaced, as once that happens moisture gets into them and plays havock as the sensor signal is very weak at the best of times at lower rpm.

It could be the coolant temperature sensor, there are two, one for the instruments and another for the engine management, if the engine management one is up the shoot it can cause all kinds of issues, usually hard starting or misses when cold, and less than ideal running when warm.

Another issue could be the engine and battery earth strap.. When in poor condition you can find that several sensors will not have a good enough grounded reference 0v to give the engine management a true reading of their resistance to an assumed baseline ground (0v) and can cause all kinds of issues.

An intact earth strap is not a good earth strap.. Put a multi meter on the engine and another on the battery earth terminal and crank the engine with the starter, if you see more than 0.5v as a difference, the earth strap is toast on the battery or engine is bad.. they might look fine, but the individual strands may be insulated with oxide and the individual strands may have oxidised inside the crimps or have breaks along their length, which can result in a good low resistance, but little currant carrying ability... Just remember you have 4 plugs all trying to discharge massive amounts of kv back to earth via the earth straps, whereas a multimeter on an ohms test is only passing mili amps.

Jon, that's outstanding... I would never have even thought about any shenanigans with the earth strap. I shall have a good check of that. As regards the other sensors and senders, if those fail to check out then are these parts that can be supplied by JMG?

For the AFM track, I guess I need to pop the black plastic lid off and have a look at the condition of the wiper? I'll do that first and see what it looks like, I'm assuming that if there's visible wear then it's a replacement rather than a clean up with scotchbrite?

If that looks OK (and actually as a matter of course) I'll check the earth strap and if I'm still not seeing any obvious visual clues of damage to the sensor leads what would be your sequence of process of elimination, is this a fair sequence?

1) Management temp sensor
2) Crank speed sensor
3) Crank reference sensor

Many thanks, a really great help and one I sincerely appreciate!

If the wiper track in the airflow meter looks worn with some grooves in it, manipulate the wiper arm so it wipes in a new arc on virgin material.

With the temp sensor, test the resistance between the two pins on it with the engine stone cold, and post the results.. it is the one at the front of the head, next to number one intake runner with a square connector on it.

Once you have done this, run the car up to temp, and stop the engine and test it again and post the results here.

With the crank sensors, they have obviously not completely failed, so a resistance check might not help you, but check the condition of the fly leads on both of them, if it looks like the insulation has gone hard, they are probably on the way out and need replacement in any case, and may cure your issue.

The next thing worth doing, test the resistance between the two wire terminals on the coil, then between each of those wires and the centre electrode which normally has the king lead on it. Post the results here and I will take a look at them, next to the terminals there are numbers on the top of the coil, list your results like this "terminal 15 to king lead = 2 ohms" or something like that.

Yes we can supply any Porsche part, normally from stock and TIPEC club members, with a valid membership number get a 10% discount

Before popping the lid off of your AFM, I would suggest you test the actual voltage output if you have access to a multimeter and power supply - this will then tell you for sure whether the track is worn.

Have a look here: http://www.the944.com/afm.htm

Good luck, I have been through similar on my S chasing a minor stumble at idle (worse when cold, but still there when warm) and have checked most things - am now left needing to check the hall effect sensors, grounds and fuel system/pressure.

Hope you get to the bottom of it soon

OK, so a little progress made... after a bit of time sitting on a hill thinking, and a bit of help!

I managed to borrow from a kind friend an AFM, an engine management temp sender and a battery-to-chassis earth strap. All of these were known-good, as had been quite recently employed to make an almost identical same-model 944 run like the proverbial Swiss watch... problem still evident...

So, taking your advice first Jon I checked the reference / speed sensors. The shielding on the wires is undamaged, but a little brittle... so not ruling those out. A bit of fishwife knowledge from Rennlist suggested that if the tach' needle wasn't bouncing under cranking then there might be an indication of some issue with the sensors:



What do you think?

I also took the coil off and checked the resistance as follows:

15+1 (set to 200 ohm)


15 + 1 by iancleary, on Flickr

King + 1 (set to 20k ohm)


king + 1 by iancleary, on Flickr

King + 15 (set to 20k ohm)


king + 15 by iancleary, on Flickr

I think that the King + X readings should be quite a bit higher? Also the coil itself looks like it might be the 1984 original, a bit rusty and crusty and only the faintest remainder of the Bosch label is left intact...

Gleaned the following from Clark's Garage website...

Ignition Coil Resistance Check

In addition to the test above, you may elect to perform an ignition coil resistance check as confirmation of the coil's condition.

Check the ignition coil primary coil resistance by connecting an ohmmeter between the positive (Black wire) and negative (Green wire) terminals on the coil. The resistance should be 0.4 to 0.6 ohms.
Check the ignition coil secondary coil resistance by connecting an ohmmeter between the coil output terminal and the ignition coil negative terminal. The resistance should be 5000 to 7200 ohms.

On the strength of that, perhaps my coil is kippered... as I'm not achieving the 0.4 ohms across the 1+15 terminals...

you'll get a coil for about £55, might as well change it

Fixed it yet???

Well... not quite, more broken in fact.

Changed the coil and that didn't cure the misfire, so decided to dive in and replace the crank sensors... and they were seized into the bracket, which had to be broken off... and of course the broken bit fell in the bell-housing.

*deep breath*

New bracket is now here, new injector seals are here and so tomorrow I'm going to have a crack at getting it all back together and see where we are from there.

*sigh*... still misfiring... so far have:

1) Compression tested: all cylinders = @ 12 bar
2) Changed plugs, leads, cap & rotor
3) Confirmed AFM is making good contact and track is not worn (also tried spare AFM)
4) Confirmed engine management temp sensor is working, using good used part
5) Replaced crank sensors (and bracket)
6) Replaced coil
7) Replaced fuel pressure regulator
Replaced external fuel filter
9) Checked engine earth straps (less than 1 ohm resistance each across both battery to body and body to engine straps)

the symptoms are that if the car is stood then it will fire up like a champ, idle well and look peachy... but once it has been given a decent throttle opening it then is missing. So I am now looking on the fuel side, wondering whether there is some crud in the system somewhere, or whether a hitherto unknown to me filter is blocked (Broady mentioned one in the tank), or whether an injector is breaking down (now contemplating a set of used injectors)... or slinging a can of fuel system cleaner through and letting that have a crack at it... bit at a loss as traditional checks that would have usually indicated the problem haven't revealed it. The only thing I don't have equipment to check is the fuel pressure... which could be a major blind spot.

Thoughts folks? I'm really struggling...

Hi, I had a similar problem, it would just miss on opening the throttle and you get a feeling that there's more power to be had. After going through all the electricals like you have I put a bottle of forte injector cleaner through. Whilst it didn't completely cure it, it certainly was a significant improvement to the point I'm not worried about.

Cheers man, I think I might be engaging with Broady and the fuel pressure gauge, but a trip to ECP in the morning for some fuel system cleaner is also in order... I don't like not knowing the cause, and frankly I'm happy to put the hours and £'s in to sort it... but sadly I'm lacking JMG-like god powers, although learning as fast as I can, devouring forum and manual wisdom alike! Appreciate all of the suggestions

Just a thought, when I made a 4x4 sports car out of a Sierra 4x4 I too had this problem of starting fine and then running rough and the most simple fault in the world was to blame but made me replace almost all the injection setup...............the fault was a blocked fuel cap breather because the used fuel caused a vacuum to exist hence fuel starvation, left the car for a minute or two and it ran fine for a little while.
Fingers crossed you sort it.