'76 930: CIS plunger must be halfway down for injectors to sing.Where to start tests?
 

Hi there,

I'm reviving my fourth long-stored 3.0 930...each one has had a unique fuel delivery issue. :D

This one has got a new fuel filter and fuel accumulator. Fuel pressure reads normal pre-fuel distributor. When the CIS plunger is depressed slightly the fuel pump kicks on, sounds normal for about two seconds, then acts like it is straining severely. The fuel injectors will not start singing until the plunger is depressed halfway down.

Any ideas where to start?

Thanks.

^^^

Cole had a problem similar to that and it turned out to be that the fuel return spout on the fuel tank was plugged with shart . . . that could cause the "pumps straining" sound you describe, an also have an effect on the plunger.

Do you have an injector out so you can see and hear it spraying?
Mine don't start squealing until holding the air flow sensor plate down around halfway and a lot of fuel is spraying into a container then. I don't think it ever goes down that far while driving unless there's some kind of trick after market control pressure mechanism going on at full throttle.

you could be dumping a lot of fuel into the engine.

do the fuel flow test from the FD to the WUR. don't hold me to it, I think it is like 220ml for a minute,

also do the flow test OUT of the FD for the return back to the tank. its like a 30 sec test. 500ml??
check the spec for your car.

check system pressure between FD and WUR. check control pressures too.

you can also check for both pumps running.
pull one relay at a time. don't run either pump by itself for long

I took the fuel guage sending unit out and shone a light into the fuel tank while the fuel pump was running. I can see fuel returning into the tank (tank is half full), however I also see air bubbles constantly coming up. Is that normal or indicative of a problem?

My other 3.0 930's injectors started shreiking when the plunger was less than 1/4 down. Good idea JFairman, I will pull an injector and check.

My next step was to start investigating the WUR pressures as T77911S suggests. Can a faulty or misajusted WUR cause restriction enough for the fuel pump to strain like it does?

This '76 still has the original (1976 only) single rear pump.

Thanks for the help and ideas!

One more thing. Just now, after 30 minutes from when I observed the fuel tank bubbles, I tried pushed the plunger halfway down immediately then held it there. The fuel pump sounded normal again for three seconds with no injector singing, then started gradually straining harder as it always does. The singing started ramping up to it's loudest exactly as the pump started straining harder. This ramping up was over about 5 seconds.

With this symptom and the bubbles, I'm thinking perhaps the fuel tank intake filter might be partly clogged. If the fuel pump can't pull enough fuel from the tank, will it send air bubbles through to the return?

no it should not.
the FD (don't know the 75-76 CIS) should have its own return line.
if the WUR side is restricted the CP's will be too hi.

I did the same as you, looked in the tank with pump running. I don't remember air bubbles.
if there is a pump problem the tests I said above will show it.
I could blow thru my return line back to the tank. it was hard due to fuel resistance. (I suspected a pinched return line in the engine bay where the line runs under a holder. mine looked pinched).

if you take the WUR out, check the input screen and make sure it is clean. (not to do with your problem but a good check).

Possibly. Cavitation in the pump.
With all the suggestions, you will crack it. You should be able to break open the return line near the FD. Check the return flow. Then perhaps try blowing on the line back to the tank. It should be an open line, no resistance.
And check the fuel delivery rate - as suggested above - that will tell you if it is having trouble pulling fuel - and you may see the bubbles on that side if there is a cavitation issue.
Alan

Checking your system pressure when the pump loads up would be worthwhile too.
Alan

To me it sounds like your pump is starting to fail.I know when my pump started to fail it got really noisy.

Sent from my SM-N910T using Tapatalk

Oh man, Rawknee's comedy gold there! :)

Sorry, I have nothing..... :)

JustTheTIPpy, I think I'm in luv! :eek:

You need a CIS fuel pressure gauge with the ball valve in the line for checking system pressure. With out one of those it's all a guess and going around in circles.

???

!!!

:eek:

Yer such a freak, Alan!!!

Doing that while injectors are squealing without removing the injectors and putting them in containers to collect sprayed fuel definitely dumps a lot of fuel into the intake manifold and combustion chambers where it will drain down past the rings and dilute the motor oil.

If you did that for more than a few seconds while the injectors were screwed into the injector blocks you should change your oil and filter or else engine wear will be increased.

And in addition with what Jim said, you better remove the plugs before your turn the engine over to evacuate all the fuel in the combustion chambers. If you don't, there is a chance you will hurt the engine...

yes, hydro lock.
can bend a rod, pull a stud, ?
or you may get lucky and nothing other than it wont crank.

Thanks guys...yes I know to remove the plugs and crank over to remove excess fuel before trying to start it. Didn't think about the fuel leaking past the rings into the oil while just sitting in there...good to know.

So...I fixed the fuel pump problem. With a hammer. :D When I looked under the steering rack cover with a flashlight on the night I got it in my garage, I did not see a front fuel pump. So I was working under the assumption that it still had the single rear pump. This is a very original car down to the 3LDZ turbo and smog pump with only 41,000 documented miles on it.

When I got it up on the lift and removed the cover, a front pump was staring at me! Doh!!! A light rap on the housing with a hammer and she started pumping again. Rear fuel pump no longer strains and the injectors now sing at the same point as all my other 3.0 930s did.

Now when I turn it over it runs for just 2-3 seconds, likely due to me priming beforehand by quickly pushing the plunger down and releasing. Some of my other 930s needed this priming when starting cold. But after that first 2-3 second run I crank it again and it doesn't even try to fire. Time to get out my Bosch CIS book and pressure gauge and start all the pressure tests and WUR adjustments, right?