82 928 only runs when gas tank nearly full, Any ideas?
09-11-2016, 09:40 PM
#16
Team Owner
do you have a work shop manual?
09-11-2016, 09:44 PM
#17
Rennlist
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Might read "There you will find the Main fuel pump and fuel filter."
There's a fitting in the tank with a hose attached, hose running to the main pump. That fitting -may- include a couple electrical connections, and if it does it means you have a pump inside the tank. The in-tank pump lives on the backside of that fitting, with a short hose connecting the pump outlet to the fitting. There's a filter/strainer on the inlet of the pump.
Both the in-tank pump (if you have one) and the strainer should be visible when you have the level sender pulled out. Unfortunately, you can't easily inspect the condition of the connecting hose between the pump and the tank fitting through the sender hole.
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Curious if the problem appears only when the level is less than full, vs a short while after putting fuel in the car. The tank has a pressure-balancing hose between the main section and the top of the filler neck section. It also has a liquid separator and connecting hoses that allow air to back-fill the tank with air as liquid is withdrawn from the bottom. If you close that off anywhere between the tank and the engine bay, you risk creating a vacuum in the tank as fuel is withdrawn. Stupid-simple test is to remove the filler cap when the car is suffering from the fuel starvation. You might hear a littl whoosh as air goes into the tank. Will the car start and run OK then for a bit?
There's a fitting in the tank with a hose attached, hose running to the main pump. That fitting -may- include a couple electrical connections, and if it does it means you have a pump inside the tank. The in-tank pump lives on the backside of that fitting, with a short hose connecting the pump outlet to the fitting. There's a filter/strainer on the inlet of the pump.
Both the in-tank pump (if you have one) and the strainer should be visible when you have the level sender pulled out. Unfortunately, you can't easily inspect the condition of the connecting hose between the pump and the tank fitting through the sender hole.
-----
Curious if the problem appears only when the level is less than full, vs a short while after putting fuel in the car. The tank has a pressure-balancing hose between the main section and the top of the filler neck section. It also has a liquid separator and connecting hoses that allow air to back-fill the tank with air as liquid is withdrawn from the bottom. If you close that off anywhere between the tank and the engine bay, you risk creating a vacuum in the tank as fuel is withdrawn. Stupid-simple test is to remove the filler cap when the car is suffering from the fuel starvation. You might hear a littl whoosh as air goes into the tank. Will the car start and run OK then for a bit?
Leave the filler cap loose and try to start and run the car.
__________________
greg brown
714 879 9072
GregBBRD@aol.com
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
greg brown
714 879 9072
GregBBRD@aol.com
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
09-15-2016, 09:56 PM
#18
I removed the under tank pump and fuel filter. I ran 99% alcohol through the fuel pump forwards and backwards (I mean flushing by running the fuel pump -forward- and reversing polarity -backwards). Not a lot of gunk came out but after back in car pump was much quieter.
A large amount of crud came out of the fuel filter when back flushing (gravity) with alcohol. I didn't expect it the be that dirty but was clean after flushing with a lot of alcohol.
After reinstalling, car had same symptoms. I then added fuel and it ran perfectly again. So still unknown at this point and I'll keep it pretty full for the time being and dig into it again when I get time.
One thing I learned from this process is the fuel pump back flush idea from dr bob. It's so easy to do it seems like a person could do easily every time when changing fuel filter. I put both intake and outflow nozzles in a quart sized plastic cup of alcohol and powered with a "jump box" that has an on off switch. On/off switch and making sure good connections avoided causing a spark. The fuel pump was much quieter and smoother after the back flush (of course the clean fuel filter probably influenced that as well). I noticed that while not much crud can out of the pump, some of it seemed to be hard particles so it's good to get that out of there. once I get the problem sorted I'll probably replace pump and filter just for peace of mind's sake.
A large amount of crud came out of the fuel filter when back flushing (gravity) with alcohol. I didn't expect it the be that dirty but was clean after flushing with a lot of alcohol.
After reinstalling, car had same symptoms. I then added fuel and it ran perfectly again. So still unknown at this point and I'll keep it pretty full for the time being and dig into it again when I get time.
One thing I learned from this process is the fuel pump back flush idea from dr bob. It's so easy to do it seems like a person could do easily every time when changing fuel filter. I put both intake and outflow nozzles in a quart sized plastic cup of alcohol and powered with a "jump box" that has an on off switch. On/off switch and making sure good connections avoided causing a spark. The fuel pump was much quieter and smoother after the back flush (of course the clean fuel filter probably influenced that as well). I noticed that while not much crud can out of the pump, some of it seemed to be hard particles so it's good to get that out of there. once I get the problem sorted I'll probably replace pump and filter just for peace of mind's sake.
09-16-2016, 01:26 PM
#19
Chronic Tool Dropper
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"A large amount of crud" that made it all the way to the fuel filter came from somewhere. To get from somewhere to the filter, it had to make it through the suction sock/filter inside the tank. That filter is likely plugged with crud that didn't make it through, or torn and has a plugged fitting, along with the hose between the fitting and the pump.
You can eyeball much the filter looking down through the level sender hole on top. Replacement procedure has you pulling the fitting and sleeve assembly from the tank, and replacing. The pump draws fuel from the "well" you can see looking down from the top, so you'll want to make sure there's no junk sitting down in there. Easy enough to flush that area once the tank fitting is removed.
There are more than a few threads here discussing the joys of removing that tank fitting. It threads into a sleeve that's molded into the tank, right where any water in our ethanol-laced fuel will often cause a bit of corrosion and join the two pieces as one.
Start off the visual inspection from the top via the level sender hole, with fuel removed. You have an obstruction to flow somewhere between the top of the tank and the suction side of the pump. Or the pump itself has worn/failed but that's a bit of a longshot; your symptom points to loss of suction pressure at the pump. Just need to find the obstruction.
You can eyeball much the filter looking down through the level sender hole on top. Replacement procedure has you pulling the fitting and sleeve assembly from the tank, and replacing. The pump draws fuel from the "well" you can see looking down from the top, so you'll want to make sure there's no junk sitting down in there. Easy enough to flush that area once the tank fitting is removed.
There are more than a few threads here discussing the joys of removing that tank fitting. It threads into a sleeve that's molded into the tank, right where any water in our ethanol-laced fuel will often cause a bit of corrosion and join the two pieces as one.
Start off the visual inspection from the top via the level sender hole, with fuel removed. You have an obstruction to flow somewhere between the top of the tank and the suction side of the pump. Or the pump itself has worn/failed but that's a bit of a longshot; your symptom points to loss of suction pressure at the pump. Just need to find the obstruction.
09-16-2016, 02:40 PM
#20
Aha that makes sense. If the strainer was plugged up perhaps the agitation from adding more gas frees up enough of the crud to let it run until it's plugged up again. The idea sounds promising. I'll take a look at it!