6.5L diesel surge on low fuel

Had an odd thing happen today. Crusing along at 70mph and fuel tank is close to empty but not quite, and suddenly I get a surge of power that felt like a turbo kicking in (not a turbo vehicle), and it went up to
80mph without moving the pedal. Then as suddenly as it happened it dropped back to 70. It did this about 5 times over a 10 minute period. I swapped tanks and filled up a few miles down the road and the tank proved not to be empty. Could this be bubbles coming through the fuel line and changing the air/fuel mix ? I would think this would drop power off not boost it. I will be interested to hear some ideas.
woodee
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Greetings,
Truthfully, I've never heard of this happening to a diesel from a low-fuel condition. Diesels don't mix air and fuel like a gas motor does - instead it squirts the fuel into the cylinder at or near top dead enter when it is full of compressed air and that's what causes it to ignite and burn, giving you the power stroke for the cylinder (hence the reason for the ultra-high fuel pressures). It doesn't mix the fuel and air prior to compression like a gasser.
Usually whenever you get a certain amount of air in the fuel lines it will shut the motor down. It's never a good idea to run out of fuel in a diesel (especially an older one) because some require that you prime the motor before you get it started again.
There could be a few non-engine related issues that may cause an increase in power (like a sudden decrease in static load on the motor such as the AC compressor cyling off) but 10mph takes quite a bit of power and there isn't anything I can think of off hand that would cause that kind of uncompensated boost. It could also be something like your transmission's torque converter unlocking and locking again, but I can't say with any authority.
What year is your truck, and did your Service Engine Soon light come on when this happened? Non-turbo 6.5L diesels weren't made for many years, and dual tanks tells me this truck is possibly from the late 80's. Some of the early non-turbo 6.5L's had the all-mechanical DB2 fuel injection pump.
I hope you finally track down this issue. Good luck - Jonathan

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Thanks for the reply. You are almost spot on the mark with the truck. Its a 1986 F350 4x4, but has been fitted with a 6.5L diesel conversion. see - www.brunswickdiesels.com.au It is very common here in west oz. I understand the mechanics of diesel injection on a basic level as it differs from a gas engine. The truck has done about 20K miles over 2 years since new install of this engine with regular oil and filter changes. It has a manual transmission and was in 5th gear when this occured. The surge wasn't a minor change, it really leapt forward for about 5 seconds each time. I must also add that I tried flooring the pedal and it wasn't anywhere near as much jump. Otherwise the motor runs sweet. Starts easy, runs smooth and pulls like crazy. I had it hooked up to a set of harrows a few days ago after sowing grain. I am just curious what would cause such a boost, and how I can maintain it :-)
woodee
Jonathan wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Greetings,
An interesting mix of motor and chassis. Good luck with it.
Boosting a diesel is just as easy as boosting a gas motor. On the expensive side you have turbochargers, intercoolers and similarly costly items.
On the cheaper side you can use propane injection, or even nitrous like in a gasser. This is the only thing I can think of that will give you the boost you described on purpose.
Cheers - Jonathan

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.