1976 930 CIS issues (fuel pumps strain, air bubbles in fuel tank from return line)

[jkarolyi]

I’m helping a friend who inherited a long stored (6 years), very original ‘76 930.

I did the usual storage revival steps and tests: Drained fuel, new engine bay fuel filter and accumulator, had CIS Flowtech rebuild the FD and WUR (took them nine months and about 30 phone calls!). I have the StarTools Bosch CIS injection test kit and checked system, cold control, warm control, and residual pressures. Checked flow rate to and from FD. Everything in perfect spec.

The car will start and run fine for a few seconds if I prime the cylinders by briefly holding down the plunger. The injectors start to sing if the plunger is halfway down.

When the FD lead is disconnected the front and rear fuel pumps (it's been updated from the original single pump) sound normal at first but both quickly sound like they're straining. I checked the return line into the fuel tank and could see fuel returning but I also saw random air bubbles appear from the return tube every few seconds.

I disconnected the return line in the engine compartment and blew into it, and I was able to hear air bubbles coming from the front, so the return line appears to be free of obstruction.

For the life of me I can't figure out how air bubbles could get into the return line. Any ideas for this or why the pumps might strain even though the return seems clear?

[T77911S]

you have done all the correct testing. including blowing thru the return line. I did that on mine too.

there is a strainer in the gas tank that can get clogged. but, you did pass the FD/FP flow tests correct?
if you are able to pass this test the car should be able at least start and run.
I would put the air bubbles issue to the side right now and I would look into the FP circuit.

1 remove the AFM connector and turn the key to verify the pumps run.

2 if the pumps run, try pushing down on the sensor plate when cranking to make nonpermanent mixture adjustments to keep it running.



this has me a little concerned but it may be nothing:

"The injectors start to sing if the plunger is halfway down."

you should be getting fuel as soon as you press the sensor plate. (I don't remember if they make noise at this point so this issue could be nothing). you can remove a fuel line to an injector and put it in a glass to check. no fuel with sensor plate closed. if you can get an injector into a glass this would be better but don't bend the lines.

IF you suspect the mixture is too lean a baseline setting is to adjust the mixture to point where fuel starts to come out then back it off until it stops. (do this ONLY if you don't know what the mixture setting which I assume you don't since you did all this work and it wont run)

I also assume you have replaced plugs, cap and rotor.

[BoxsterGT]

If you having hose issues I can help, but I suspect its more to do with the Control System/Fuel Dist.

What are the actual fuel pressures at the FD? Are you certain you are getting sufficient fuel volume?

Are you observing the air bubbles in the gas tank by removing the Fuel Level Sender and looking in the Tank? The more info you can describe the sooner this board can help you.

When Tony (boyt911sc) comes across this thread he will pop in and offer advice. He is one of the most knowledgeable CIS members on this board.

Len

[JFairman]

From past experience CIS Flowtech does not use ethanol tolerant rubber parts in their fuel head rebuilds. Therefore they may not be the best place to send your CIS stuff if you're using E10 gas with 10% ethanol mixed in. E15 gas may be in the future.

Rebuild the stuff yourself, it's not difficult if you have a bench vise and use ethanol tolerant Welcome to salvox! - salvox fuel head rebuild kits.
They're not expensive and come with ethanol tolerant rubber parts.

I never tried it but I wonder if AFR would be rich enough so you could run E85 in CIS when ethanol tolerant rubber parts are used in the fuel head and you remove the 12 volt heater element plug from the control pressure regulator or "WUR" so control pressure stays low in cold start mode while the motor is at operating temperature.

Don't know where your air bubbles are coming from.

[T77911S]

sounds like the FD was just rebuilt, no time for gas issues, plus its a 76. wouldn't that be a cast iron FD? yours has the rubber membrane in it, the cast iron has a metal shim. I think the cast iron only has 8 or 9 rubber Orings in it.

I cant say I have ever notice air bubbles in my tank. I know I looked in there when the tank was low and watched the fuel being returned but I remember air bubbles

check power to the cd unit with the key ON. could be you are losing power and the engine is running off the function of the delayed action relay that keeps power on the CD for a few seconds after the key is turned off.

[JFairman]

Originally Posted by T77911S

sounds like the FD was just rebuilt, no time for gas issues, plus its a 76. wouldn't that be a cast iron FD? yours has the rubber membrane in it, the cast iron has a metal shim. I think the cast iron only has 8 or 9 rubber Orings in it.

I cant say I have ever notice air bubbles in my tank. I know I looked in there when the tank was low and watched the fuel being returned but I remember air bubbles

check power to the cd unit with the key ON. could be you are losing power and the engine is running off the function of the delayed action relay that keeps power on the CD for a few seconds after the key is turned off.

I was just writing a long post for the last hour with many pictures and details about these miserable fuel heads and somehow lost it. I hate it when that happens...

You've obviously never rebuilt one of these fuel heads.
I've rebuilt 4 of them.

There are 8 small fuel sealing Orings that go on little oval shaped stainless steel retainers around the fuel metering slits in the control plunger cylinder and three approximately 1" diameter Orings for the bottom half of the fuel head where the control plunger cylinder fits inside both cast iron and aluminum fuel heads and one bigger one for the bottom outside of the fuel head where it mounts on the CIS air flow meter housing. That one is an air seal Oring.
There are 8 small Orings because 930 fuel heads were originally derived from Mercedes 4.5liter V8 CIS fuel heads. Two of them on 930 fuel heads don't do anything because two of the 8 ports are disabled and non functional for the 6 cylinder 930 engine but parts of them are still there.

You can see the 12 used non ethanol tolerant Orings from CIS Flowtech that came out of my old 007 fuel head in the top left of this picture and 12 new ethanol tolerant Orings along with new copper sealing washers in the tunafish can in the top right of this picture.


You can also see what the ethanol in E10 gas has done to the non ethanol tolerant rubber diaphragm I took out of my old CIS Flowtech 007 fuel head. It's next to the new Salvox ethanol tolerant rubber diaphragm before I installed it.

You should smear a little motor oil on the Orings to lube them before pressing the two halves of the fuel head back together so they don't get rolled over or torn.

About the return line bubbles.. only thing I can think of is fuel pump cavitation can cause fuel vapor bubbles in the gas but I doubt they would remain all the way through the fuel pressure regulator in the fuel head and return line going the length of the car back up to the fuel tank. Gasoline is nasty stuff so m a y b e ...

[jkarolyi]

> you did pass the FD/FP flow tests correct?

Correct. From FD to WUR was exactly 225ml in 1 minute. Outflow back to tank was to spec also.

Other readings: System pressure 95psi, cold control pressure 20psi @ 75 degrees F. warm control pressure 50psi.

>1 remove the AFM connector and turn the key to verify the pumps run.

I did this...the pumps run and sound normal for 1 second, then sound like they are straining.

>2 if the pumps run, try pushing down on the sensor plate when cranking to make nonpermanent mixture adjustments to keep it running.

OK I tried this. The engine will run with me holding down the sensor plate about half way.

>this has me a little concerned but it may be nothing:
"The injectors start to sing if the plunger is halfway down."
you should be getting fuel as soon as you press the sensor plate. (I don't remember if they make noise at this point so this issue could be nothing).

This concerns me too as other 930s I've had started making noise even with the sensor plate barely down.

>you can remove a fuel line to an injector and put it in a glass to check. no fuel with sensor plate closed. if you can get an injector into a glass this would be better but don't bend the lines.

Good idea...Will give this a try and report back. I did have the CIS injectors flow tested by CIS Flowtech and they were found in spec.

[jkarolyi]

OK tried and no fuel came out of the fuel line until the sensor plate was more than halfway down.

Before I sent the FD, WUR, and injectors off to Flowtech (but after cleaning the fuel system and replacing fuel filter and accumulator) I tried starting it and had this same problem. I did the pressures and volume check...all was in spec except the FD to WUR flow was only 160ml. I figured the FD was clogged so off to Flowtech.

Now with the rebuilt FD I am up to 250ml spec but still have to push down half way on the sensor plate to get any fuel to go to the injectors.

[356911930]

Even though you just had your fuell distributor rebuilt the Control Piston may be sticking. I have heard some very knowledagable contributors say never to remove any material from the piston but you may have some fuel varnish coating the Control Piston that you could VERY carefully clean up. My CIS manual says to use Crocus cloth to clean but I do not know what that is or where to get it. When you take the fuel distributor off the piston should move freely. I believe they should drop on their own weight and there should be no resistance up or down but the movement can be slow as ling as it is smooth.

Hopefully someone will add to this path of troubleshooting.

Best of luck,

Rahl

[flightlead404]

Originally Posted by JFairman

I was just writing a long post for the last hour with many pictures and details about these miserable fuel heads and somehow lost it. I hate it when that happens...

You've obviously never rebuilt one of these fuel heads.
I've rebuilt 4 of them.

There are 8 small fuel sealing Orings that go on little oval shaped stainless steel retainers around the fuel metering slits in the control plunger cylinder and three approximately 1" diameter Orings for the bottom half of the fuel head where the control plunger cylinder fits inside both cast iron and aluminum fuel heads and one bigger one for the bottom outside of the fuel head where it mounts on the CIS air flow meter housing. That one is an air seal Oring.
There are 8 small Orings because 930 fuel heads were originally derived from Mercedes 4.5liter V8 CIS fuel heads. Two of them on 930 fuel heads don't do anything because two of the 8 ports are disabled and non functional for the 6 cylinder 930 engine but parts of them are still there.

You can see the 12 used non ethanol tolerant Orings from CIS Flowtech that came out of my old 007 fuel head in the top left of this picture and 12 new ethanol tolerant Orings along with new copper sealing washers in the tunafish can in the top right of this picture.


You can also see what the ethanol in E10 gas has done to the non ethanol tolerant rubber diaphragm I took out of my old CIS Flowtech 007 fuel head. It's next to the new Salvox ethanol tolerant rubber diaphragm before I installed it.

You should smear a little motor oil on the Orings to lube them before pressing the two halves of the fuel head back together so they don't get rolled over or torn.

About the return line bubbles.. only thing I can think of is fuel pump cavitation can cause fuel vapor bubbles in the gas but I doubt they would remain all the way through the fuel pressure regulator in the fuel head and return line going the length of the car back up to the fuel tank. Gasoline is nasty stuff so m a y b e ...

Can I ask about your recommendation for lubricating with motor oil and not something like Fuel Lube, which would be my first choice. Fuel Lube is fuel resistant and will never become hard. Or do you choose motor oil specifically because the fuel will wash it away quickly?

[flightlead404]

Originally Posted by 356911930

Even though you just had your fuell distributor rebuilt the Control Piston may be sticking. I have heard some very knowledagable contributors say never to remove any material from the piston but you may have some fuel varnish coating the Control Piston that you could VERY carefully clean up. My CIS manual says to use Crocus cloth to clean but I do not know what that is or where to get it. When you take the fuel distributor off the piston should move freely. I believe they should drop on their own weight and there should be no resistance up or down but the movement can be slow as ling as it is smooth.

Hopefully someone will add to this path of troubleshooting.

Best of luck,

Rahl

You can purchase
Crocus Cloth from Grainger.

I'm not sure I'd be brave enough to use it though. There are plenty of solvents that should take off fuel varnish. Lacquer thinner or acetone has worked for me.

[356911930]

^^^

Agreed!

Rahl

[Alan L]

Originally Posted by flightlead404

Can I ask about your recommendation for lubricating with motor oil and not something like Fuel Lube, which would be my first choice. Fuel Lube is fuel resistant and will never become hard. Or do you choose motor oil specifically because the fuel will wash it away quickly?

I have rebuilt/reassembled a few FDs too. The slit O rings are difficult to reinstall without tearing them (ask me how I know). You are just trying to lube them as they slip in tightly without damage. Once in place, not an issue. I used engine oil too. You don't need any lube after fitted.
Alan

[JFairman]

Yup and the tiny trace of motor oil you wetted the Orings with will mix with the gas that will be on them when you turn on the fuel pumps.

When I rebuild them I take the control plunger and wrap it in some toilet paper and put it somewhere safe until I put it back together.

The SC fuel heads I've rebuilt didn't have the stamped steel thing on the bottom that keeps the control plunger from sliding all the way out from gravity. You have to hold it in with your fingers while handling it in the upright position or it will slide out, fall, and could easily be damaged if it lands on something hard.

WD40 or anything similar petroleum based works well to lube the control plunger. It's lubed by gasoline once it's back in use.

[T77911S]

Jfairman, no need to get defensive, I would never try to insult you, but no I have not done the aluminum heads but he does not have an aluminum head and I have done the cast iron ones.

I agree with you, ethanol is harder on the aluminum heads. much more rubber but the cast iron ahs only Orings and a metal gasket.

[T77911S]

did you do the flow test OUT of the FD back to the tank?
can you do this and just let the fuel run for a minute or longer. (until the pumps sound strained)
I have done this, I ran it into a gas can to save it.

if you think the pumps or fuel flow is an issue, check the strainer in the tank.


pushing the sensor plate half way down has me concerned.
with the pumps on, is there resistance on the sensor plate immediately when you touch it?

if larry rebuilt the head a really doubt there is anything wrong with it and I would exhaust all other possibilities before concluding this.

[mreid]

“Sat in storage for a while”. Did you drain and clean out the tank? To the points above, rust, scale, and dirt can slowly build up on the in tank strainer as the pumps run, eventually clogging the pick up and causing the pumps to strain and your problem. When you shut down the engine, this debris falls to the bottom of the tank, only to happen again when you restart. So, you start the engine and it runs fine only to slowly degrade as the crap builds up on the strainer.

[jkarolyi]

Finally able to work on this again after my fuel pressure gauge stopped working. Lang tools kindly (but slowly) sent me a new one even though it is out of warranty!

>did you do the flow test OUT of the FD back to the tank?
can you do this and just let the fuel run for a minute or longer. (until the pumps sound strained)

Did this, and flow rate is within spec.

>if you think the pumps or fuel flow is an issue, check the strainer in the tank.

I did suspect this, so I drained the tank and replaced the strainer with a new one. No change in any of the above measurements.

>pushing the sensor plate half way down has me concerned.
with the pumps on, is there resistance on the sensor plate immediately when you touch it?

Yes there is normal resistance with fuel pumps on for the full range of motion of the sensor plate. I don't think the control piston is sticking because with the fuel pumps off I can feel very slight resistance when the piston rises, then if I release the sensor plate up quickly I feel it rest again on the sensor plate lever 1 or 2 seconds later.

>if larry rebuilt the head a really doubt there is anything wrong with it and I would exhaust all other possibilities before concluding this.[/QUOTE]

Agreed. With my new fuel pressure gauge I discovered that when I turn off the fuel pumps my residual pressure immediately dropped to .5 bar, then to zero in the next few minutes.

I just replaced the fuel accumulator with a brand new Bosch before reinstalling the rebuilt FD and WUR. Unlikely that would be faulty What else could cause loss of residual pressure?

[Alan L]

Non return valves in the pumps.
But that does not explain air bubbles. So a leak somewhere would cause drop in pressure, and air bubbles. Leak at some fuel connection point - union, hose?
Alan

[infraredcalvin]

No leaks on the ground? I had a small crack in my return line that caused similar issues... check the hose In the drivers wheel well, make sure it’s not brittle.