Air Bubble in Fuel Line Hard Starting when it sits overnight

Wayne Boudreau

Wayne Boudreau

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Memramcook , New Brunswick Canada
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2000 Jetta TDI Sedan and 2003 Jetta GLX Wagon
Well here we go again, Same as last year, when the weather cools down a bit the car gets harder to start I look at the clear fuel line and a 2 inch bubble is in the line replaced O rings on the tee put extra clamps on the hoses to the fuel filter cracked open an injector to let air out any other ideas that you may have already tackled.......:)
 
K

Knoxville

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KS
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04 Jetta BEW
Man oh man, if you get this figured out, let me know. I have the exact opposite problem. When it is cold out, my car starts great. Kinda rouch when it's below 40F outside, but it cools down nice.

Here's some ideas. Check the top of your fuel tank under the seat. If you've ever replaced the lift pump, ensure that cap is tight. Anywhere those stupid fuel lines connect to another fuel line, there are o-rings. It really sounds likes somethign rubber oriented with you. Rubber will shrink up as it cools down and that in turn is probably allowing your air in.

In the summer, my 04 PD takes 3 to 4 spins of the engine before it starts. 40F or below, first hit. I have a translucent polyurethane tubing running from the firewall (both lines), over to fuel filter and then both lines from filter to engine.

Anyways, Good Luck!

Brian
 
Wayne Boudreau

Wayne Boudreau

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Hahaha I will check the connectors on the fuel pump under the seat I may put some silicone on them just in case. That would be an easy fix and as far as your problem.Could be the temp sensor but again who knows but that could be a cheap fix if so. it seems to be something that advances your timing when cold?
 
Hobhancock

Hobhancock

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I dont know if this will help or not but it might be worth a try. I have had similar like this but with a small difference. I run biodiesel and I have had junk get caught in the sending unit in the tank. I have taken that out and took it all apart and found that on the bottom is a molded screen that comes off. I then removed the black arm with the float that leads to the small intake into the canister. The canister comes apart in two larger pieces where one slides inside the other. With that out you can see in the bottom a black piece of rubber that covers the intake. Clean all that out and see if there is positive evidence of a blockage. It was was with mine. Cleaned it all out and fuel flowed beautifully.
 
jcrews

jcrews

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My starter bailed this year. I think the brushes were worn out, but the thing was turning over really slowly when cold, about 45F and below. The car was not starting at that point unless cranked for 5+ seconds.

I think the motor was getting down to 100 RPM at times, and the battery is in good shape. The new starter has cold starts back to normal.

How's your starting engine speed?
 
Wayne Boudreau

Wayne Boudreau

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Memramcook , New Brunswick Canada
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If I leave it 4 hours it starts fine but if I leave it 8 hrs then I have to crank 6 or more turns to get it started If I check the clear fuel line there's a two inch bubble in there and during the summer that bubble wasn't there. and as well car started with less that 2 turns. I really do think it is related to the fuel I know by putting new bushings in the starter and replacing the battery it would certainly start but I feel that if there was fuel available right away it would start in a few turns with less stress on the battery and starter.
 
Wayne Boudreau

Wayne Boudreau

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Also Knoxville I found this while looking around " Fuel Temp Sensor"

"I have replaced mine on my 2005. If I remember correctly, if the engine and fuel are cold, and the resistance of the sensor is low, then it is bad. I believe the output is used by the ECU to retard injection timing therefore reducing power output. Mine never threw a code.

If you have VCDS you can also check when everything is cold, like after the car has been sitting overnight. Before starting the car at all, check the temp values for intake air, fuel and coolant. They should all read the same temperature. If not, the one that is far off from the others is likely bad.
 
jcrews

jcrews

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Streaming bubbles are the worst to find the source of.

Cracked thermostatic fitting
Bad seal with fuel filter
Cracked filter drain
Mystery hole(s) along any fuel line/connection.
There are 3 sets of plug-in connections:
Fuel tank
Main lines to tank lines
Main lines to filter lines.

That's about it.

If it's injection pump leaking (bubble forms over time), the only way to tell is to look for moisture on and around the pump. A mirror can be handy in a few places.

Leak points:
Upper housing to lower housing
Upper housing cover
Return fitting
Front seal
Pressure control valve
N108 Advance control valve (under bracket)
Advance piston cover (above water hose)
Rear (head) seal

That should be it.
 
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jcrews

jcrews

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You might also check under the timing belt cover/barrier plate for fuel moisture. It might be hard to spot because the fuel could sneak behind the inner cover.
 
deerleg

deerleg

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N E Ohio
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Jetta GLS 2002 Black
Air in fuel line reviving this thread...

jcrews said:
Streaming bubbles are the worst to find the source of.
Cracked thermostatic fitting
Bad seal with fuel filter
Cracked filter drain
Mystery hole(s) along any fuel line/connection.
There are 3 sets of plug-in connections:
Fuel tank
Main lines to tank lines
Main lines to filter lines....
Click to expand...
2 more questions for clarification:
There should be no bubbles in the clear line?
Beside hard starting could this cause power loss?
My 02 Jetta starts hard and lots of bubbles and some intermittent power loss when accelerating.
 
loganbmx4gt

loganbmx4gt

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97' Passat (1Z), 02' Golf
Hey man. Your probably not going to get a response from everyone above because the last post was made 2 years ago. But yes you shouldn't see any bubbles, maybe 1 or so every so often, but nothing as described above.

Your power loss could be a clogged intake or number of other things. Have you looked at the "diagnosing limp mode" sticky?

Also is your MIL on and do you have vagcom?
 
deerleg

deerleg

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N E Ohio
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Jetta GLS 2002 Black
loganbmx4gt said:
...yes you shouldn't see any bubbles, maybe 1 or so every so often, but nothing as described above.
Your power loss could be a clogged intake or number of other things. Have you looked at the "diagnosing limp mode" sticky?
Also is your MIL on and do you have vagcom?
Click to expand...
Yes, I've experienced that issue a couple years ago and addressed it.
The intermittent nature makes me think it could be fuel starved at times because of the amount of air I see going through the lines....just a guess so far.
I have some extra o rings for my fuel filter that I will change and may also pull and clean the intake in the tank as I do run B-20 and occasionally home brew B-100. Maybe I plugged something up.
 
TornadoRed

TornadoRed

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When your injection pump is pulling fuel with a lot of bubbles in it, that air is probably entering the fuel filter through the thermostatic-T. You can try a new O-ring, or you can switch to a fuel filter that eliminates the T.
 
V

vgmnstr

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2001 New Bettle TDI
I need to replace the fuel lines between the fuel filter and the hard poly lines (quick disconnect). They are cracking and introducing some air. Anyone tried to just pry the clamp off of the quick disconnect and change the rubber portion of the hose? Or do I have to just get a new hose with fitting?
 
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greengeeker

greengeeker

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Cambridge, MN
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2002 Jetta GLS
Pull the line completely off, plug on end and pull a vacuum on the other end. Ifthe cracks are an issue sthe vacuum will bleed down.
 
deerleg

deerleg

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N E Ohio
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Jetta GLS 2002 Black
TornadoRed said:
When your injection pump is pulling fuel with a lot of bubbles in it, that air is probably entering the fuel filter through the thermostatic-T. You can try a new O-ring, or you can switch to a fuel filter that eliminates the T.
Click to expand...
I also have a broken clip on the thermostatic-T(currently just using the bigger half) I should replace...anyone know if it has a part number?
Did not know there is a filter without the tee...anyone know a part number on that by chance?
 
FUCHT

FUCHT

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Something else to check/replace, the rubber lines going to the injectors and te one from the pump, if they are leaking, you will get lots of air bubbles, I had this with mine and as soon as I replaced them, the bubbles were gone, these lines should be hard to put on and pull off, if not you'll have bubbles... I used the mercedes fuel hose as it's a super tight fit. Mercedes line goes on super tight!(dark-greyish black in color) part number# 6154760326, you can also use the original vw line (contitech with yellow stripe) which is part# N0203535, you'll need about 2ft total in length.

Anyway, it's worth a shot, I know mine started leaking after I changed the glow plug harness and disturbed it, the rubber cracked right at the injector, and this was the rubber line coming from the injector pump.
 
Last edited:
FUCHT

FUCHT

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deerleg said:
I also have a broken clip on the thermostatic-T(currently just using the bigger half) I should replace...anyone know if it has a part number?
Did not know there is a filter without the tee...anyone know a part number on that by chance?
Click to expand...

Part Number 1J0127247A
 
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