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.
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
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
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
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
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