Please help a noob - 924 cold running issues

Before I tested the fuel pressure and new adjustable WUR, I eliminated as many variables and unknowns as possible.

I drained the tank and added a gallon of fresh fuel.

Then changed the oil, oil filter, plugs, plug leads, distributor cap, rotor arm, air filter and fuel filter.

Whilst I had the plugs out, I did a compression test which showed them all to be well within spec.

So, with the service complete, I removed the fuel pump relay and bridged terminals 30 and 87 to a switch so I could remotely operate the fuel pump without having to run the motor.





I then fitted the pressure gauge and valve between the fuel distributor and the WUR.





The WUR is buried deep down the back of the engine, mounted to the rear of the inlet manifold, hence my desire to be able to adjust it without having to remove it each time. I've got removal and refit down to about 25 mins total. It's not a sport I intend to take up professionally.



First thing to do was to test the overall system pressure by closing the valve in the test line and running the fuel pump which showed 4.7 bar which is within the range given by the k-jet bible of 4.5 - 5.2 bar.





I let the system stand for a 30 mins over lunch and it didn't drop to below 1.8 bar showing a healthy pass on the leak down test.

Next was to test the cold pressure by running the pump and seeing what the WUR was regulating to. First though I had to see what the ambient temperature was...

About 16 degrees according to the house thermometer...



But to make sure, I double checked with a proper pyrometer to confirm the results...



Which according the the K-Jet bible under the 924 section Shows I should have a cold pressure of 1.35 bar.



If you recall, I'd see the post well below where I anticipated it would need to be to give me the chance to gradually draw it out on the adjustment nut, hence the first test showed what I expected - pressure below the spec required...



This is where the effort of making the unit adjustable paid off, as I was able to simply wind the nut down which effectively pull sup the insert to adjust the pressure to my desired 1.35 bar.



Which resulted in the desired 1.35 bar.



Next it was time to connect the heating coil, start the motor and check the pressure when at operating temperature.

Plenty warm enough (fan on, thermostat open, temp gauge central)...



Resulting in a warm control pressure of 3.5 bar which is within the 3.4 - 3.8 bar of where the K-Jet bible says it should be...





So far we know...

All the service items have been replaced.

We have fresh fuel.

We have good compression.

We see no leaks at the injectors.

The cost start valve/injector works as it fires straight away from cold.

The aux air valve works and has been tested hot and colt as well as resistance over the heating coil.

The system pressure is within spec.

There are no leaks on the leak down test.

The cold and warm pressures are within spec and prove the WUR is working.

It idles when warm at 1,000 rpm on the nail so the idle speed is correctly set.

So how does it run from cold?

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Crap.

There's no fast idle and feels like it's idling too slow with lots of vibration as if it's laboring and when I try to rev the engine, it stumbles in a kind of "B-B-B-B-B-vroooom" like it's missing slightly and hesitant.

Exactly like is was before and exactly like there is insufficient fast idle or enrichment.

As far as I can see, I've done everything possible to ensure it should run in a 'factory' manner, yet nothing has changed, which to me suggests the issue lies elsewhere.

But where?

Thoughts appreciated, keeping in mind that when warm, it starts, runs, drives and accelerates perfectly, so whatever is wrong, is not impacting the warm running.

 

Sorry if I missed it but did you check the AAV has 12v running to it?

 

I've not checked the power to it, but have checked it opens and closes with temperature changes. Open after a spell in the fridge and closed under a hair dryer. I compared this to a spare unit and it behaved identically.

Just so I can cement my understanding of the AAV - as I understand it, it starts off in it's intrinsic cold state in the open (or partially open depending on temp) and the heating coil is there (along with a rise in engine temp) to close it as temp increases?

As my problem exist from the moment the engine first fires, that would be a while before the AAV could effect any change as it would take a short amount of time for it to close up due to the coil or engine temp.

Therefore, as my problem exists straight away before the AAV can do anything, that would suggest that the coil, or power to it, is not a relevant factor, otherwise it would run correctly straight away then if the unit and or coil, pr power to it was defective, then it simply wouldn't close, hence the running would deteriorate as the car warmed up, rather than improve as it does now.

If I'm mistaken about what the AAV does and how it works, please correct me and maybe I can find the cause, but from what I can see, when cold, it's open and it runs badly instantly, hence I can surely eliminate the AAV from the equation?

 

I've not yet checked the injector spray patterns - how do I do this?

Can I inquire why poor injector spray patters would only affect cold running and not warm running?

When I try to rev it up, I can hear it missing and stuttering - I'm convinced it's running too lean when cold, yet the fuel pressure is exactly as per the text book...

As I say, it runs absolutely perfectly when warm.

 

I think I might have found the problem...

I just sprayed a load of brake/carb cleaner around each injector and on one of them there was a change in engine note and on another such a change that it nearly cut out.

I guess that'd cause my cold running issue?

New set of injector seals I guess.

 

On the VW K-Jets the AAV is heated up by the 12v supply - I'm assuming the Porsche system is the same - is there not a 2-pin plug on it? But you're right, if there was no 12v it would be stuck open leading to too high an idle - the opposite of your problem.

Hopefully you've solved the problem now

 

Update.

I fitted new injector seals, seats and new injectors which smoothed out the idle a little but it still idles way too slowly and coughs and splutters when trying to rev it up, getting better and better as it warms up.

In desperation I tried to adjust the mixture to see if it improved one way or the other - it didn't. Either way I turned it it got worse and coughed more and spluttered more. At one point the engine coughed quite heavily and the allen key popped out the hole so I lost my point of reference of where it was and now can;t get it back to running as well (poorly) as it was. I don;t have access to an exhaust analyser, but it proved the point that it wasn't poor mixture causing the problem.

So far this is what I've done/replaced/checked...

New plugs
New leads
New rotor arm
New dizzy cap
Compression test
Timing checked
WUR rebuilt and pressure tested
AAV tested and working
All inlet joints tested for air leaks
New crank case ventilation hose
Fresh fuel
Fuel pump tested
Fuel delivery measured and correct
New injectors
New injector seals
New injector boots
New idle control needle valve o-ring
New fuel filter
Injection system pressure drop down tested
Cold system pressure tested
Warm system pressure tested
Donor AAV tried in desparation - no difference
Donor WUR tried in desperation - no difference

I'm all ears as to what to check next?

Just to recap - runs fine when warm. Starts instantly hot or cold, but idles too slowly at about 600 rpm when cold and stumbles, coughs and hesitates when trying to rev it up, sounding like an old banger. All perfect when warm. Absolutely nothing I have done has improved matters other than new injectors smoothing out the idle.

 

Have a look / inquire on ... https://www.porscheclubgb.com/forum/tt.aspx?forumid=16

 

The air cone? What am I looking for and how would I adjust it and to what spec?

Would that being out of adjustment only effect cold running?

 

The crank case vent hose is new and the vacuum hoses are to the brake servo are all attached, present and correct.

How do I access the screw to adjust the air cone rest position?

I've found the idle mixture 3mm allen head screw - where's the cone rest position screw?