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Air in fuel system

6K views 50 replies 6 participants last post by  Ralph 
#1 ·
Hi, searched but couldn't find a similar post. I'll describe my problem. 05 Scenic 1.5dci.
First the "check injectors" message came on and has been on constantly. No problem's for a few days so assumed it was a dodgy sensor but next morning wouldn't start. (had no trouble starting before). Wore battery down trying then charged it and used jump leads from van aswell to try next day. Not even catching. I replaced the fuel filter and attempted to bleed it by pumping the primer with ignition on but took 10 mins before fuel started flowing through. Amazingly it started on 2nd button press. Continued fine that day so I thought problem solved but next morning same thing except was catching and started with jump leads on 25th button press. Replaced primer bulb and turns out the return valve was faulty so again thought problem solved! Next morning same thing.
I'm struggling as I don't know how the fuel system works so I'm just guessing here. Any suggestions? Also are there any links to a description of the fuel system layout? Really appreciate any help thanks
 
#2 ·
Small leak in fuel feed system letting air in and fuel run back.

Basically, fuel in the pipes, filter, primer, all the way to HP pump is held there by vacuum.
Small leak means air gets in, fuel drains back to tank.
Result, hard starting (maybe no starting) once car has been left overnight but starts and runs okay when only left a couple of hours.

One favourite place is fuel filter connections if push in type.
O ring seals get hard with age and "leak"
Even new filters can have wrong sized seals.
Touch of grease around joints can effect a temporary seal and prove the point.

Or could be anywhere else in the lines but look at that first.
 
#3 ·
Thanks, I'll check them. If they're under vacuum with the engine off, what's the best way to see if they're leaking? If they are faulty are replacements easy to come by or is it better to push the pipe straight onto the hose tail on filter?
Thanks :)
 
#4 ·
Thanks, I'll check them. If they're under vacuum with the engine off, what's the best way to see if they're leaking?
As far as I know, Mk1 eyeball and some thought.
Sometimes get a small drip if a pipe has split or rubbed but the system never sees pressure so any leak goes in, not out.
Be logical and start with Joints (like at filter), soft pipes, anywhere where could be rubbing

If anyone knows a short cut, I'd like to learn it as well.
 
#5 ·
I've greased the hosetails with vaseline (4 on filter and 2 on pump) managed to get it started with difficulty. I've noticed there were air bubbles coming down the return pipe from pump. Every two or three seconds a cloud of very fine bubbles shot through. I took it on a good run but still doing it when I got back.
I'll see if its any better tomorrow morning but i assume that means air is still getting in?
 
#7 ·
Right, I've replaced all the 8mm fuel hose in the engine bay from the new priming bulb to the new filter, and the two lines running from the filter to the pump. Removed all the quick release connectors and put hose directly on with hose clips. Everything except the end of the hose to the pump because the bleed valve and connector are one piece (like to know where to get replacements)
Still hasn't fixed the problem!!! There's fine air bubbles shooting down the return pipe in waves every couple of second while it's running. Air seems to be getting in still.
Any suggestions on where to try next?
 
#8 ·
Can you see air only on idle, get some one to rev your car to 2,000rpm, then look
Air in the clear pipe on idle is normal, fuel pump not needing a lot of fuel, and leak off pipes going back into the filter (return)
 
#9 ·
I'll get it started tomorrow and have a look. Thanks. Getting very frustrated with this. I'm probably gonna do some damage eventually spending 10 mins starting it each morning off van jump leads. My Mechanic doesn't have time to see me and all the others round here are really bad/unskilled/dishonest.
 
#10 ·
When you go to start it, have you tried priming it first.
If your injectors are leaking back slightly, this also will give the same issue..
If after priming the bulb(goes hard) it kicks in easier, buy a bottle of millers injector cleaner, its about 15£, no its not cheap, but it works really well.
work out how much fuel in your tank, to how much injector cleaner you need to use out of the bottle.
The correct amount is enough, no need for any more than instructions ask...
Give the car a really hard drive, foot down before it hits the red .
As you are driving it (make sure no-one is behind you) dip your clutch, and accelerate hard, lift off your clutch slowly, when the revs have dropped back down...

Do the acceleration bit a few times,,, see if this cures your issue.
Dont use the cheap injector cleaner, as that stuff is really for use to keep the injectors clean... Millers will clean the injectors, and if only just starting to leak back,,, within 100 miles
 
#12 ·
Thinking about it.. Logically, for air to get in- fuel must be getting out. There are no leaks that I can see so fuel must be going back into the tank. There's a check valve in the primer so it must be going down the return line. If I put an inline tap that blocked it at night, would that stop it? Is that pretty much what the solenoid does? So does that make it plausible the solenoid could be the culprit?.
The problem happened literally overnight making something electronic seem more likely to be at fault than a leak suddenly developing. But that wouldn't explain the bubbles while running and the constant "check injection" warning. Grr! Wish I knew more about this stuff!
 
#13 ·
Grr! Wish I knew more about this stuff! ( wish I did as well)
How much air can you see, air will get in, but fuel wont get out.
What does your primer ball feel like, first thing in the morning... is it hard, or do you have to pump it
 
#14 ·
In the morning there's alot of air. The priming bulb is soft but if I pump it for a bit it gets firmer. Still can't get it to start easily though even if I pump it for 15mins solid first. Generally takes about 15 - 25 presses of the start button before it catches properly. It splutters each time like it has a bit of fuel but doesn't start.
 
#16 ·
You seem to have covered most of the common places for air ingress.
Bear in mind that the fuel system on diesels which do not have electric lift pumps is based on suction.
As the main pump rotates it sucks fuel in to pressurise.
But is not a self priming pump, ie it cannot suck air and draw fluid up, it has to have fluid to suck.

Any leak in the system, from tank pick up to main pump can break the suction, letting air in and fuel back to tank.
Making it hard to start after resting a while.
Then, when the main pump has fluid, the suction can also pull in air.
Add the returns from the injectors, injectors themselves and the HP pump, plenty of places for air ingress and unless fortunate enough for a little fuel to drip out, difficult to find.
Long winded way of saying work through the system, anywhere there is a joint must be suspect, any worn pipe, even the pick up in the tank (often shown as the problem only happening once tank level gets below a level)
 
#18 ·
even the pick up in the tank (often shown as the problem only happening once tank level gets below a level)
worth trying. It had a fair bit of fuel in when I got it and the problem showed up after a few days use. I'm gonna put in a tank full so I can try the injector cleaned suggested so that could be a good experiment.
Is there any significance to the problem showing upso suddenly? I assume a leak or split would be more gradual. What do you think?
 
#20 ·
Yep, bought it privately, ran it about for two days then didn't use it for a couple of days (didn't have any difficulty starting after being sat unused for two days. Used it around town a couple more days with no issues at all then next morning just wouldn't start.
So pretty bad luck then.. Though I once sold an Astra van that head gasket went on two days later so swings and roundabouts I suppose.
 
#23 ·
Really would check them.
You could have had two faulty glow plugs, and another one has packed in.
only reason I mention this, if you give the car a good hammering, and over 3000rpm ,,, no issues, its unlikely an injector issue.
I.E, leak off the injectors, You have covered everything that D.D. has mentioned.
Apart from the tank side......
 
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#27 ·
Yes, easier than an astra...
But they ,, like all glow plugs are stubborn , so its undo , re-tighten , back and forth till they come out..
Dont attempt on a cold engine, get engine as hot as possible.
If it means putting 1 new one in, then restarting the engine to get it hot again, to remove another glow plug, do this.
One broken glow plug, will ruin your week..
 
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#30 ·
Filled up with fuel and injector cleaner, ran it around a bit. No difference this morning. I've clamped the pipes going from and to the tank today after I've finished using the car. Thought it might prove the point of wether fuel was draining back or not. Will see if it starts better tomorrow. If no difference then I'll start on glow plug route.
 
#31 ·
Well, It didn't help clamping the fuel lines. No better next day.
So I've been looking at the glow plugs today and they seem fine. I've tested them with a multi meter and they only have a couple of ohms resistance and they all allow the full battery voltage through when tested from the top of the plug to the +ve terminal. Ive also checked the power to the glow plugs and all the connectors become live when the engine tries to start (although they only are getting around 11v)
Everything seems normal except the plugs are getting 5.1v even with the engine off. Not sure if my meter is dodgy cause I put a low voltage bulb across and it didn't light. And the battery doesn't seem to get drained. Maybe its very low current- just a signal to check continuity. Strange though with the ignition off.

So I'm stuck!... Any other suggestions? Thanks
 
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