924Board.org

[AppleBit]

Ok, we are going to discuss CIS... I have questions and I am sure Scott- you might know the answers (you know all the answers) Cauz you work at Bosch, and Bosch knows all, tells all, and saves gas at the same time

My car has run into a small misfire these past few days. I haven't had much time to even discuss it let alone try to diagnose the problem but I took the time tonight as this is nothing tramitic, but it does raise concern as the misfire is only below 2000 rpm and when you are on the throttle to accelerate up to speed in any given gear.

This problem results more after the car has warmed up and I have tried to diagnose ignition as a possible suspect but I have come back to the conclusion that there is a fuel issue that is possibly causing my small issues.

Now I have owned many many many 924's and seen many many many German vehicles with CIS but my 1978 Porsche 924 has the most unique setup I have ever seen. Think of a normal CIS system, but there is an extra 'injector-looking device' on the side of the fuel distributor. This looks like something that could actually by-pass the warm-up regulator as the fuel lines go. I have seen this on a few other 924's as well, but none had an electronic device on there (with two wires connecting to God knows where)...

The second interesting device is another connection that goes right onto the outside of the fuel distributor box and looks like it could be a sensor for the air sensor plate. I could be wrong on this. Now I do not have any clue as to what these two devices are- but I do know this:

My fuel pump does not run until I crank the engine over. I disconnected the - what looks like sensor for the air sensor plate- and my fuel pump now runs before I start the car (after I turn the key to the -On- position. I reconnected this electrical connection and my fuel pump does not work again until I am cranking over the engine. I am not exactly sure as to what this is or the reason for this and the other electronic device on that 'injector' that I discribed first.

Any help would be recommended. Recommended, why? Cauz I'll beat ya up if ya don't tell me.

Thanks to all responses in advance.

- Jon McCullough


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Classic British Sports Car Restoration v6 + v8 Engine Conversion Swaps

Porsche 924 Wide Body LS1 Corvette 500 Horsepower Engine

[924RACR]

OK, after putting all possible ign problems to rest, you check the fuel stuff... I'd suspect a fuel delivery problem, like a slight loss of pressure somewhere... I assume you've already changed the fuel filter? Check all banjo bolts for leaks/wetness.

The thingy on the side of the fuel dizzy I believe is a hot-start fix solenoid... probably not the source of the problem.

However, sounds like you also need to check out the function of the fuel pump relay, it might have died... try jumpering it, make sure the fuel pump still runs... if so, see if the engine will run...

Have to admit, sounds like you should borrow/buy gauges and test the pressures...

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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[dpw928]

That thing on the side of the fuel distributor sounds like the frequency valve used on 80 and later 924 CIS systems. It is controlled by the O2 sensor box. If faulty it could close and increase control pressure which would cause it to run extremely lean. Do you have an O2 sensor installed? Air pump? You may want to check the numbers on yur engine to see if you've been upgraded.

Dennis

[AppleBit]

heh, nope... All 78 stuff...

_________________
Classic British Sports Car Restoration v6 + v8 Engine Conversion Swaps

Porsche 924 Wide Body LS1 Corvette 500 Horsepower Engine

[dpw928]

Jon,

Have you tried a good fuel injector cleaner? Could be that one of your injectors is plugged or stuck open. Lean or overly rich mixtures can cause backfires.

Dennis

[AppleBit]

I did check the fuel pressure, that's what the problem is. New warm up regulator here I come!

- Jon


_________________
Classic British Sports Car Restoration v6 + v8 Engine Conversion Swaps

Porsche 924 Wide Body LS1 Corvette 500 Horsepower Engine

[John Brown]

The "injector" looking device is otherwise called a 'frequency' valve. Controls control pressure in response to the O2 sensor. Later cares also had an additional 'frequency' valve which reduced control pressure for warm start.

Early cars did not use the fuel pump relay for safety cut-off. Instead used the switch you found on the flapper valve.

This is all available in Haynes, or the $15 Bosche CIS book.