CFM928

1980 928S has a hot start problem. (And maybe a cold start issue, I just started it for the first time this season today.) With the pressure gauge at the curved fuel line at the WUR, I get 70 PSI with the key off, fuel pump jumpered. When I turn the pump off, the pressure drops to zero quickly. (In seconds) I can hear a ‘tapping’ at the rear of the car. I have changed the check valve, same symptoms. Bad replacement? How to test it? Am I testing for this in the right place? I need to give it a shot of starter fluid to start it, so I know it is fuel related. Another possible clue – The idle changes. Normally at 800 indicated on the dash tach, when I restarted after sitting, engine still warm, it would stall if I didn’t give it a bit of throttle. Later in the evening when it cooled down, it idled fine again, so I can’t just adjust the idle screw. I’m not a CIS expert, barely understood it back in the day, but it always was reliable. At least after I rebuilt the FD. Any clues of what to look at or test appreciated. Oh, I measures 20” of Vacuum at idle. Blipping the throttle drops it to almost zero, and it recovers to 20 in a second. Thanks in advance.

The Forgotten On

Replace the check valve in the fuel pump ($20 part). I would inspect the fuel accumulator as well to see if the diaphragm inside has failed (it will visibly leak fuel).

The accumulator also causes poor hot starts where you need to crank the car for ages.

You also may also have a damaged o-ring inside of the fuel distributor when you rebuilt it causing pressure to leak out and return fuel to the tank instead of maintaining it.

CFM928

Thanks, I'll check the fuel accumulator. That lives in the right rear wheel well, right? Is the only test for visible or smell of gas leakage?

The FD rebuild was some time ago, it ran well after that, have not messed with it lately. So looking elsewhere first.

checkmate1996

You are losing fuel pressure hence why the hot start issue. Common fail with hot start is the fuel accumulator. It's in the location you described. Visual and smell test should give it away. Also a good time to change the fuel filter.

CFM928

Thanks for the reply. Tonight I removed the cold start injector, placed it in a clear baggie, blipped the starter 5 times to run the fuel pump, then cranked for 5 seconds. The baggie was bone dry. No fuel.
I checked the fuel accumulator. It also was bone dry. (I have the style without the drain hose, feels like a screw head at the top.)
I then removed the fuel line from cyl #5, placed it in a baggie, and cranked the starter again. No fuel. Should there be, with out actually starting the engine?
Again the symptom is that if I give it a spray of starter fluid, under the air filter into the air box, it starts fine and runs OK. So wondering when the injection system actually kicks in.
Fuel filter has recently been replaced. It is by the fuel pump, under the tank, did it when I replaced the check valve.

jpitman2

The thermo-time switch should enable the cold start injector for some seconds on cold start up - check that you have +12v at this switch. The pump should run for a few seconds at switch on before cranking - can you hear this? If not, relay might need replacing. Also look for the sensor (screwed into heater water outlet) and solenoid over cylinder 3 (if fitted) - this sensor is supposed to lower control when HALF hot to assist starting; I had some hot starting issues such that if it was stopped for ~10mins from fully hot, it might catch briefly, then die, and not start at all unless it stood cooling for another 10-15mins. Testing the sensor showed that when it closed it still read 20 ohms, enough to stop the solenoid firing to lower the pressure. Replaced the sensor with one from a 944, as people who had the sensor in US wanted more than the price to ship it to me. Fixed my problem.The sensor is 92860510400.
jp 83 Euro S AT 57k

CFM928

Thanks, I found a previous similar thread that you guys worked through here: https://rennlist.com/forums/928-foru...art-issue.html ("CIS K-JET Cold Start Issue")
I will check the electrical input on the cold start valve.

checkmate1996

I would pull the fuel pump relay and build a fused, switched relay jumper; long enough that you can switch it on and off outside of the car. Then look at the injectors when you turn it on and off. Once you turn the pump on you should see fuel. If nothing, you got another problem. Also, if you don't have them yet, get the a set of CIS fuel gauges. These are invaluable in troubleshooting pressure and WUR issues.

jpitman2

You wont see fuel at the injectors with pump running unless the air plate is deflected by hand or by air flow caused by cranking. Agree on the switched relay jumper, very handy, and the gauges!
jp 83 Euro S AT 57k

CFM928

Brad and JP, thanks. I have a fused jumper, and gauges. And the Watson and Probst books - somewhere. Although like Thermodynamics and DiffEq, I barely understood it back in the day... :-/
Today I verified operation of the ThermoTime Switch, or at least that I had 9 volts when cranking at the Cold Start Injector. I removed it, verified that I could hear and feel it opening under power, passed some Sea Foam through it, and blew out with compressed air. Made a pretty spray pattern.
After having fuel lines disconnected from the WUR and Cold Start Injector, I reconnected with the gauge, cranked the starter just to watch the gauge, and it started in 7 seconds. 20 PSI, valve open. 70 PSI with valve closed, but drops immediately when shutting off.
Will there be leakage at the fuel accumulator with the screw at the top? Or does it seal the vessel even if the diaphragm fails? Do I need to remove the screw to verify it 'leaks'?
I'm a bit confused by seeing only a tiny plastic fuel filter in that compartment. It looks it connects to an oblong plastic box tucked into the top of the right rear fender, maybe connected to the fuel filler. (80 ROW, left hand drive car.) So apparently I only have 1 fuel pump and 1 filter under the tank.
Still think it is a bad (second) check valve.) Maybe will see if I can connect gauge to pump output and observe if it holds pressure.
CFM 80 Euro S at 260k

jpitman2

I thought the EARLY cars had a second pump and filter up in the RH wheel well? Can you get us a pic of the 'tiny plastic fuel filter' please? And what you have under the tank, and in the wheel well?
jp 83 Euro S AT 57k

CFM928

I drove the car for some errands today, including picking up a new fuel pump check-valve. Cold start was OK, every hot start required a spray of starter fluid under the air filter. With the covers off the under tank fuel pump and fender well fuel accumulator, I investigated the ‘clicking / pinging’ noise which occurs shortly after shutting the engine off. Not only is the sound clearly from the fuel accumulator, but I can feel the ‘pings’ when I touch it. (Sounds like a spring uncoiling, then like a basketball dribbling out till it stops bouncing.) So I believe the Fuel Accumulator works, but the fuel is going somewhere… I replaced the fuel pump check-valve. Same result. (Or at least the accumulator is clicking after shut off. Haven't tested hot start yet, but plan to drive it tomorrow.) When I had the gauges connected, I believe I saw the fuel pressure gauge drop from 70 PSI to zero quickly upon pump shutoff, with the valve closed. Doesn’t this mean it can’t be leaky injectors? Ideas? (JP Pics coming shortly. )

CFM928

Fuel Accumulator and Small Plastic Filter in RR Wheel-well. (Cramped - Didn't remove tire...)

CFM928

Mystery Tank, R R Fender Well. Looking up. Front of tank.

The Forgotten On

Small plastic thing has a check ball in in that in the case of a roll over, fuel cannot escape through the tank ventilation system. This part does connect to the charcoal canister in the front wheel well.

The tank in the rear wheel well is an expansion tank. Also part of the tank breather system.