924Board.org

[Ralf]

Hey everyone

I have a 924 2.0 (EU), naturally aspirated, from 1983. The car had big problems not wanting to start (wrote about this here a few months ago). By now, after an overhaul of the fuel distributor and adjusting the WUR, the car is starting and running, but still doesn't run as expected.

Symptoms
The main problem is that the car doesn't pick up properly when accelerating. The car jerks, hesitates, and is slow to accelerate. In addition, the engine idles poorly, both cold and warm.

Observations
I've tried to adjust the warm-up regulator, but I can't get the correct control pressure for both a warm and cold engine.
- Warm engine: Control pressure is approximately 3.0 bar, which is too low (specification: 3.4 - 3.8 bar). I can't adjust it higher.
- Cold engine: Control pressure at 15 degrees Celsius is 2.8 bar, which is too high (specification: 1.2 - 1.5 bar). I can't adjust it lower.

Possible causes
I've performed a series of checks to rule out other causes (see long list below), and I suspect two problems:

1. Cold engine: Control pressure is too high --> I guess faulty warm-up regulator?

2. Warm engine: Control pressure is approximately correct (a bit too low), but the engine still doesn't run well and barely accelerates. What is causing this, if it's not the control pressure?

Any good advice would be welcome. Thanks a lot.


-----------------------------

What I have tried so far (fuel, ignition, air and compression):

** FUEL **
Tank. Starting my investigation from the tank. It’s filled with fresh gas.

Fuel pump. The car apparently has two fuel pumps, connected one after the other (‘in series’). No idea if this is the original design, but I took them off the car, tested them by pumping fuel from a bucket and then installed them back into the car.

Accumulator. The accumulator was leaking a bit from the screw at the bottom so I replaced it with a new one.

Fuel lines. I used compressed air to ensure both fuel lines (supply and return) are not clogged.

Fuel filter. Replaced it with a new one.

Fuel distributor. Disassembled it, overhauled and cleaned it and installed it back on the car. Mixture setting set correctly.

Air flow sensor. Operates freely when I move the value manually (with the air filter removed).

Control pressure. 2.8 bar with cold engine, 15 deg Celsius (=too high) and 3.0 bar with warm engine (=too low).

Warm up regulator: Disassembled it, cleaned it, tried to set the control pressure correctly and installed it back on the car.

Auxiliary air value. Disassembled it and tested it. It’s open by default. To test it, I applied 12v directly from the battery. Result: It closes after a few minutes. Installed it back on the car. Seems ok.

Cold start valve. Removed it from the car and when 12v is applied, it sprays fuel.

Secondary start valve. This is the electrically operated valve mounted on the fuel distributor. It clicks when applying 12v on it; seems to work.

Injectors. Removed them from the engine, cleaned them using a hose full of fuel and compressed air. Visually checked that they do not start leak when a bit of air pressure is applied. Secondly, I checked that they start spraying when enough air pressure is applied, to make sure they are not clogged. Installed them back into the car.

** AIR **
Air intake: Air intake path is open, no blockages.

Vacuum: No obvious vacuum leaks or lose vacuum hoses. Checked with brake cleaner when engine is idling.

** IGNITION **
Hall sensor. When measuring with an oscilloscope, the Hall sensor gives a waveform as expected (looks like a cotangent graph).

Transistor ignition unit. Has been checked by an expert. Hard for me to measure, but I trust his judgement.

Ignition coil. Is new, NGK U1065.

Distributor. New rotor and new cap.

Timing. Adjusted when idling. Vacuum operation is difficult to check.

Ignition cables. Are new.

Spark plugs. Are new, I used NGK BP7ES. When I remove one and crank the engine, I see the spark.

** COMPRESSION **
Compression reads approx. 1200 kpa, same on all 4 cylinders.

[fiskolja]

Thorough troubleshooting, impressive!

Following the K-jetronic troubleshooting chart all your listed problems point to a vacuum leak. I'd do a new vacuum test using smoke.

What's your system pressure at?
_________________
924 NA 1977

[morghen]

Hello Ralph.

The root cause is most likely the control pressure being wrong, your fuel distributor rebuild, as well as the mixture setting.

The kjet system can NOT be successfully restored and set unless you have the correct tools, test bench and knowledge.


Instead of spending time and money trying to restore kjet, i propose the EFI kit which looks very similar to the original kjet, but has a modern system hidden inside it.
The cost of restoring a kjet (successfully or un-successfully) is similar and comparable to the cost of the EFI kit.
With the EFI kit you can do the whole thing in a weekend by yourself, with kjet...well you cant do everything yourself and it can take months of frustration and not enjoying the car.

If you want an EFI kit, you are welcome to contact me.
_________________
Supercharger and EFI kits
https://www.the924.com

[Ralf]

Thanks fiskolja! Will check again for vacuum leaks. So far I’ve been using brake cleaner with engine idling. I haven’t used smoke before.

System pressure is 4.5 bar (specification: 4.5 - 5.2 bar) so that is just within range.

And I’ll probably be looking at a WUR rebuild to get the cold control pressure within the specified range.

@morghen: An EFI conversion might be an option indeed, if the above steps don’t solve the problem. I’m a sucker for originality, but an EFI does makes sense. Homologation for EU is another concern; I need to keep the car road legal.

[morghen]

The kit keeps the engine bay looking close to original, everything modern is as hidden away as possible. When i made this kit, i had the respect for originality in the primary "must have" things. The kit has a fuel rail that looks 99% identical in appearance and size as the original fuel distributor.
There are quite a few cars with TUV in Germany with this kit that also have the historic plate, so it is 100% road legal within EU.
_________________
Supercharger and EFI kits
https://www.the924.com

[Raize]

The middle ground before EFI conversion is to install a wideband sensor and gauge. I have found mine immensely helpful.

It could also be a timing issue of course. A timing light that lets you dial in advance is useful because you can then just paint the flywheel "0" mark and use the timing light for diagnostics such as checking the vacuum advance effect.

Of all the money I have spent on the 924, these two things were the least wasteful.