Air in Fuel Injection

[Big T]

So after installing the clear line with a roller coaster loop to eliminate any kinks, I continue to experience air in the system. On a cold start after sitting overnight, the engine starts immediately, but it will experience sputters, or missing while under acceleration for the first 3 to 4 miles, then it clears and engine runs perfectly smooth.

I’m wondering what the source is. I went out to check the clear line after siting overnight and it is 90% clear. What do the experts think or suggest?

 

[dbrannon79]

Looks like the return is emptying it's self. I wonder if now that the weather is getting colder hoses, clamps, and o-rings are shrinking just enough to cause a drain back to tank or create a suction of air. the drain back into the tank would be something in the LP allowing backflow of fuel or a vacuum being created within the tank by a vent or the fuel cap.

I would start off by double checking all the connections that were messed with during the engine install, then move on to the rest of the truck.

if you have access to a rubber hose near the LP, a dirty test to see if the LP is draining back to the tank is to take a pair of vice grips and pinch off that hose and leave it overnight. unclamp in the morning and see if that solves anything.

 

[MrMarty51]

Roll it in flour and look for the wet spot.

 

[Big T]

The air in the clear line cleared within a couple seconds of starting and the push of fuel came from the IP.

The line from the FFM to the IP is braided stainless.

I suspect as you say, I need to go through every connection.

Just got back from getting the AC charged on a day that it snowed in the am.

 

[Big T]

Just caught the part on rubber hose from lift pump. I can do that test tomorrow night.

 

[Will L.]

So GM says a quick check on the accessible connections then remove and reinstall the fuel filter. Retest. Still air- then replace the line from ffm to ip with clear. Both lines with bubbles means testing ffm and rearward.

If no bubbles between ffm & ip, but you have bubbles at the ip- the ip is the problem. Check fasteners & fittings on the ip. Still bubbling- pack plahdoe (we had a GM version of it but playdoh works fine) around areas of the ip until the bubbling stops. Replace the appropriate seal. If you can not seal the leak with playdoh- it is likely the main shaft seal and ip needs rebuilding to repair.

If the bubbles are coming out of the ip, disconnect the feed line to the ffm. Install clear line before the ffm and watch for bubbles. — I bet you are seeing a pattern here now….
Yup, just keep adding the clear until you isolate where the bubbles start.

Many times I have wished for a clear rated fuel line that can be used on the entire system. The pvc clear line at hardware stores will fail in a couple years, is subject to early failures from weather. Easily rubs through. Not a good idea to be everywhere.
The clear line from fuel-line.com is rated for the fuel, uv resistant (not uv proof), but is not rated for abrasion like the fat sae30r9 is. So that some lawyer doesn’t sue me I will say permanently replaced black rubber lines with the rated clear stuff isn’t my advice for anyone else. But when MY fuel tank gtes dropped, guess what is going in place of the original. And everywhere else as well. But I will go to great lengths to ensure nothing can rub against it. I will inspect them all frequently.

 

[Big T]

So GM says a quick check on the accessible connections then remove and reinstall the fuel filter. Retest. Still air- then replace the line from ffm to ip with clear. Both lines with bubbles means testing ffm and rearward.

If no bubbles between ffm & ip, but you have bubbles at the ip- the ip is the problem. Check fasteners & fittings on the ip. Still bubbling- pack plahdoe (we had a GM version of it but playdoh works fine) around areas of the ip until the bubbling stops. Replace the appropriate seal. If you can not seal the leak with playdoh- it is likely the main shaft seal and ip needs rebuilding to repair.

If the bubbles are coming out of the ip, disconnect the feed line to the ffm. Install clear line before the ffm and watch for bubbles. — I bet you are seeing a pattern here now….
Yup, just keep adding the clear until you isolate where the bubbles start.

Many times I have wished for a clear rated fuel line that can be used on the entire system. The pvc clear line at hardware stores will fail in a couple years, is subject to early failures from weather. Easily rubs through. Not a good idea to be everywhere.
The clear line from fuel-line.com is rated for the fuel, uv resistant (not uv proof), but is not rated for abrasion like the fat sae30r9 is. So that some lawyer doesn’t sue me I will say permanently replaced black rubber lines with the rated clear stuff isn’t my advice for anyone else. But when MY fuel tank gtes dropped, guess what is going in place of the original. And everywhere else as well. But I will go to great lengths to ensure nothing can rub against it. I will inspect them all frequently.

It happens as it sits overnight. Once engine starts the air in the clear IP return hose gets quickly cleared. Once it is cleared, there are no bubbles in the clear line.

 

[Will L.]

Possibly the return line is leaking.
The return line is supposed to be sealed all the way into the tank and submerged at all times in the fuel. This traps fuel in the return line all the time. If the line has a leak, the fuel can drain out (into the tank sometimes not necessarily on the ground). This will create a fit with starting and running until the air is purged.

If you use hose pinching pliers and after shutting off, go pinch the pliers half way in the clear line trapping fuel on both sides.
In the morning look at it and see if it has drained on either side or both.

 

[Big T]

Possibly the return line is leaking.
The return line is supposed to be sealed all the way into the tank and submerged at all times in the fuel. This traps fuel in the return line all the time. If the line has a leak, the fuel can drain out (into the tank sometimes not necessarily on the ground). This will create a fit with starting and running until the air is purged.

If you use hose pinching pliers and after shutting off, go pinch the pliers half way in the clear line trapping fuel on both sides.
In the morning look at it and see if it has drained on either side or both.

Let’s collectively think this through.

Starts immiediately and does not run rough. So probably not leaking injectors (they are brand new).

Some sputtering misses on acceleration for up to 3 miles. So air getting in the system.

In my roller coaster loop for the clear line. Fuel had drained off both sides.

Would hose over needle nose vice grips do the trick on the clear line?

 

[dbrannon79]

Hose over needle nose vice grips are the best at pinching a line without damaging it. pinch it vertical with the loop or bend so it will expand out after removal. sometime on that PVC hose if you pinch it in the bend (horizontal) the pinch can eventually create a crease in the hose that won't come out.

 

[dbrannon79]

I'm sure you used new injector return hoses but maybe there is something there? I have seen hoses and seals leak under a slight vacuum but not leak under pressure, also with large temp swings leaks can form where it was perfectly fine before. This can be one of those that can drive a fella nuts trying to find it LOL.

 

[Will L.]

Yes just like described pinch the hose.
I’m shooting from the hip on pinching it off, never tried this before.
It shouldn’t drain. I’m thinking pinching it and which ever side drains should have the leak.

My normally process is what I described before- throw in clear and chase that way.

 

[Big T]

I'm sure you used new injector return hoses but maybe there is something there? I have seen hoses and seals leak under a slight vacuum but not leak under pressure, also with large temp swings leaks can form where it was perfectly fine before. This can be one of those that can drive a fella nuts trying to find it LOL.

We had an isssue with incorrect return line hose leaking profusely. All driver’s side has been replaced stopping leaks there. Passenger side (PS) rear replaced, stopping that leak. I did a squeeze test on the remaining PS hoses and they felt the same as the good stuff. That said, they are on the list for replacement.

 

[Will L.]

On those little hoses: the good realmones with good clamps that are annoyingly hard to get on/off work well. The cheap hoses fail often. The weak clamps are worse than useless. Unless you are auditioning for the tole of hulk and need something to make you angry.

My favorite hose is bought at a Mercedes dealership and used on the
1970’s OM 617 5 cylinder engine. It cuts well with sharp razor or side cutters. It pushes on and needs no clamps. It lasts longer than the engine does. Make the loops long enough to cut them off once and reuse for when you need to remove them replacing your injectors in 100,000 miles. You can’t remove it easily so vertically cut the end to remove, then cut off that section with side cutters.

It is sold by the foot and is priced low, especially considering how well it works.

 

[MrMarty51]

I have a fair abundance of 1/2”, 3/8ths” and 5/16ths” SAE30R9 hose setting here.
I also just ordered in 10’ of 1/4” SAE30R9 to plumb from the FFM to the water drain valve.
If You need hose let me know how many feets of each and I will box and mail it to You.
The 1/2 and 3/8ths is what came with the Air Dog LP system so it should be some mighty good stuff.
The 5/16ths is Continental brand and made in the USA.
The 1/4” I just ordered I am not sure of the brand on it, yet.

 

[Big T]

I have a fair abundance of 1/2”, 3/8ths” and 5/16ths” SAE30R9 hose setting here.
I also just ordered in 10’ of 1/4” SAE30R9 to plumb from the FFM to the water drain valve.
If You need hose let me know how many feets of each and I will box and mail it to You.
The 1/2 and 3/8ths is what came with the Air Dog LP system so it should be some mighty good stuff.
The 5/16ths is Continental brand and made in the USA.
The 1/4” I just ordered I am not sure of the brand on it, yet.

My first order of business is getting this truck with tractor and other stuff on the trailer, down to SoCal. I can work on the truck down there in the warm California sun…

Timing for vehicle work is flipped when I am in Montana. There you do the vehicle work during the summer warmth, but it takes away from things like fishing. In SoCal the heavy vehicle work is done during the winter, then you have fun with the vehicles the rest of the year.

 

[Big T]

Have not done leak down test by pinching the clear line. Started it to show how long it takes to purge the air and it was much longer than previous start:

Also looks pretty obvious to me that it’s leaking back towards the IP.

 

[jrsavoie]

I've never heard of air leaking back into an IP through the return.

Something else I learned

 

[dbrannon79]

Something tells me the fuel is being pulled back into the tank by possibly vacuum in the tank and or the built in check valve in the LP along with the return not being air tight. check to see how secure the clear return hose is attached. it might be there. but also if the tank isn't well vented, as the fuel cools in the tank and lines it contracts as well as the air space in the tank creates a vacuum drawing fuel from the lines back into the tank. suction is then created in the return sucking in air at the weakest point. something I learned was that clear hose is a thin wall where clamps don't really do a good job clamping it, also if when that hose was removed in the past where your might have had to cut or split it and scored the barb on the steel line, it can be sucking in air there.

I would test this theory out in two parts. first with the return hose. on my 95 I had to take a slightly larger size piece of clear tubing and slide it over the 1/4" clear hose at the ends so my clamps had a thicker hose to clamp on to. verify that, then leave your fuel cap loose overnight.

I think Will had mentioned on some other posts about the built in check valves in factory LP's tend to fail but the pump would work just fine. by placing a one way check valve in front of the LP and behind the FFM would fix that.

some fuel caps are vented like a check valve as well where it's supposed to vent pressure out but not allow the tank to go into a vacuum state. since it most likely came from China, they might have put that in backwards where it won't allow air in but will expel pressure. Me, I would just take a small drill bit and make a vent in the cap.

 

[ak diesel driver]

Stock fuel caps are designed to have a slight vacuum.