406 GLX - 2.1l TD R-reg - engine hunting/pulsing

Hope someone can help with this or at least point me in the right
direction.

Got a problem with my car where the revs suddenly start to rise and
then the engine "pulses" or "hunts" between 1500 and 1900 revs. It can
happen if I'm at a set of lights without touching the accelerator. At
tick-over the revs are pretty much correct and it's never stalled (not
without my help anyway! LOL). I can find the bite with the clutch and
that pulls the revs back down to normal but once I press the clutch in
the revs go back to the pulsing bit.

Sometimes I can be driving along and if the revs drop down to around
the pulsing area (around 1700-1800) the car starts to kangaroo a
little because the engine tries to maintain the 1700 revs. Sometimes
that can be quite a strong pull and others the engine will happily
drop below that level.

I just don't know where to turn first. I don't know if it's a valve
problem somewhere or maybe a flow sensor problem? It can't be a
sticking accelerator cable because it can happen even when I'm not
touching the accelerator. The air filter has been changed and so has
the fuel filter.

Any ideas would be very much appreciated. I'm a designer and just
haven't a clue where to look. If it's fixable without going to a
garage then I'll definately try and do it myself (not THAT useless
LOL).

Daz

 

sounds like a flow sensor or fuel pump not keeping up with the feed for
the engine,

 

*snip*

Someone at the parts place mentioned a flow sensor when I was buying
the fuel filter. Are they expensive and can they be fitted at home or
is it a dealer job?

Just finished trawling through 3 years worth of posts in here
(giganews retention is HUGE! lol) and read a post about a dirty idle
valve causing pulsing revs too. Is that a possiblity? Where is it for
me to clean? God I feel dumb! LOL. I think I need to get a Haynes
Manual! lol

Thanks

Daz

 

It might be worth checking for small amounts of air getting into the
fuel lines between the injection pump and the tank, perhaps through a
fuel filter seal or a worn o-ring on a banjo union if that model has
any. If I remember correctly this can be unnoticeable at higher rpm but
cause instability at idle.


J

 

Hi J
That may well be worth looking into. It only seems to start playing up
once the engine has been running for maybe 5 mins so things will hav
started to warm up by then so if there are any gaps for the air to get
in they would be much more likely to open once warm.

Now I have to go and get a Haynes from somewhere cos I have no idea
what a banjo union is! LOL I'll also wait until it's starting to
pulse then I'll go round the fuel path in the engine compartment and
see if covering things drops the revs back to normal. That may help
track down any air leaks. If there's a point where air is getting in,
would I see a drop or two of fuel getting out??

Thanks for that

Daz

 

Darren wrote:
.. That may help

Unlikely. If a section of clear tubing could be inserted in the inlet
side of the injection pump you could look for air bubbles in it but if
you're not sure take it to a specialist because diesel fuel under
pressure can penetrate the skin and kill.


J

 

Wow OK. I think the problem IS air getting in somewhere. There's a
clearish section of tubing that goes from around the priming bulb-pump
thing to the fuel filter tank and I could see little bubbles of air
flowing through it. Just don't know where they are getting in. I think
I'm going to have to take it to someone who actually know what they
are doing rather than trying to do it myself. At least there's a good
chance of being able to narrow it down and fix it pretty quickly now
(hopefully).

Daz

 

The dangerous high pressure is between the pump and injectors, and the pipes
are metal,
all other pipes are low pressure, supply and return,
quite right to point this out though J,
The most common places that cause air ingress with this engine,
1, The primer bulb its self, (take it out and by-pass it)
about £10 - £12 for new part
2, large "O" ring situated under the plastic fuel filter bowl,
(remove filter, 13mm bolt in bottom of bowl, lift bowl and seal up with
silicone or replace "O" ring
can be about £5 - £6 from dealer ,, a bit expensive for what it is ??
these are two of the easier , quickies to attack first ,
These are tips I got from this NG a few years ago, and they worked for me ??
HTH
Fitzy

 

Thanks Fitzy
It's got a chance of being the primer bulb then. The air bubbles seem
to be going towards the fuel filter tank. Hopefully it's that cos it's
nice a cheap to fix and I can do it myself! LOL

I'll give it a whirl at the weekend.

Daz