Former VWDieselParts.com Forum

So, I've always wondered, what actually causes the clacking sound on a diesel? The noise I'm hearing on VO sounds an awful lot like when I start my car when its cold; that clacky, rattly sound that eventually goes away once warmed up. I'm just trying to understand how my car works a little better.

It's the fuel exploding suddenly. Reason it happens when it's cold is that the fuel does not ignite right away after being injected into a cold cylinder -- it sits for a split-second, then explodes with a bang. That's why you get all that extra rattle, shaking, and vibration when it's cold. Once it's warm, the fuel ignites smoothly and instantly as it's injected, so the power strokes are of longer duration and more controlled. You experience this as a smoother, quieter engine.

My thought was the reason it clacked on VO was that the fuel was not hot enough, so it was thick and came out of the injector as a stream rather than a spray. That would cause it to ignite later, like if the engine was cold. Like I said, when mine is just starting to clack (about 5min after switching over to VO), I can make it stop by pulling the advance handle out partway. You should see if the same thing happens on yours.

I guess I really just need to get those heaters installed to know if my theory is right. Waiting on a few electrical parts, then that will happen in a weekend or two. I don't have a vac or temp gauge on my fuel line, and I know I need at least the vacuum. Once I have the heaters installed I won't care so much about fuel temp, because it only has to be hot enough to be able to flow through the pump, and I know it must be at least around 120 degrees based on measurements other people have taken off Greasecar systems.

Fatmobile, assuming your reasoning is correct about why it takes time for the problems to start occurring...does that shed any more light, do you think, on what the issue might be? It should not be necessary to have the timing set significantly differently for VO use, right? I mean, it should not be true that the timing is right-on for diesel and biodiesel, but way off for VO, which is how my car acts now. No?

I'm not sure about different timing being required for vegy and diesel.
I've always believed it might need to be advanced a little when on vegy.... probably not to the point that it doesn't run well on diesel.
I'll know more this summer when I get a dash mounted gauge for internal pump pressure and get more time to mess with my timing light.
V8, I still think you have a problem with fuel temp, due to a restriction in coolant flow in the HWH.
So I think a temp gauge is important for figureing out your problem... at least to rule it out and know to look elsewhere.

OK, I finally got my electric line heaters on. And they made no difference whatsoever. The clacking noises that I and wewantutopia have described were still fully present and occurred just as quickly after switchover to VO as before. I know the lines were getting hot because I could see them smoking as they burned off my fingerprints and the lines were much too hot to touch as they entered the injectors. So surely the fuel was sufficiently hot too.

However, I made an important discovery as I traveled across the country again (just returned from driving from Boston to Los Angeles to Seattle, a 5600 mile drive). At one point I was running on diesel and about to run out of fuel in the middle of nowhere in Utah on a very hot day. I decided I would try to mix some of the heated VO in my oil tank with my diesel fuel, so I disconnected the ground wire for the 3-way solenoid valve in the fuel return side, then switched it to run on VO, so that it would pull fuel from the heated VO tank but return to the diesel tank, creating a diesel-vegoil blend in the stock tank.

I ran it for long enough that I would guess by the time I switched back to pulling from the main tank the blend must have been at least 50-50. The engine ran perfectly, in fact better than it did on straight diesel (higher idle speed on the blend -- maybe because of better lubrication? the diesel was 15ppm ULSD). I could tell by the smell that it was running on at least half veggie oil, but it made no bad noises at all and ran smooth and powerful. I was not using the electric line heaters and I would guess the fuel mix was not very hot, certainly not over 100 degrees.

So at this point I am thinking that either there is something funky in the fuel lines or fuel filter on the VO supply side, or there is something about my injection pump that reacts to having hot fuel pumped through it -- like something expanding and binding up the dynamic advance mechanism, jamming a part, or a seal getting weaker and the pressure getting lower, changing effective timing. My next places to look are internal pump pressure and a complete re-do of the fuel line routing for the VO system.

Just thought I'd give my latest input on this problem. Anyone else have any more discoveries or words of wisdom? I'm determined to solve this weird issue this summer and will keep you all posted on what I figure out.

A little late posting but try cleaning the coking around your heat shields, then see how long before the problem comes back using your new injector line heaters.

I have absolutely no experience or expertise in this questions.... but:

If the lubrication of the injectors was changing between diesel and VO, (particularly in the unworn parts of the new injectors), then a shift in spray pattern/breaking pressure could take a while for onset due to the time it takes for the last of the diesel to be flushed from the injectors.

Is onset time similar to the time it takes for the problem to go away when you shift to diesel? VO and diesel may have different "wash away" times. Did heating the VO to higher temps make onset/go away faster or slower? It could be that >any< diesel in the mix lubricates the injector system parts, and so the return to good behavior could be quicker than onset.

One can do breaking pressure measurements - but doing them at temperature would be a bit more work and sounds like what may necessary.

Allen

I found some very interesting applicable reading on the biodiesel and SVO forum, by crossbones. Hope it's okay to post this. Hope the link works. His post is on page 2. Phil

http://biodiesel.infopop.cc/eve/forums/ ... 03902/p/2