Setting Fuelling - Why does the DMC PRV Hunt? [long]

["mgutkowski@xxxx" <webmaster@xxxx>]

Hi All

Bit of a lengthy one this. Let me set the scene.

I've bought a new project car, VIN#4426 from Germany. It needs a new
engine. As you are aware, the PRV was used extensively over here by all
three manufacturers and a couple more besides.

By an extraordinary stroke of luck, I was put in touch with a guy by the
name of Darren Bowker who hails from Manchester. Not only is he a mad keen
Renault fanatic (and owns a concours Renault 30), but he has built several
PRV's from scratch before, and was more than happy to take on the work of
building me my new one. I even had the option of a performance version in
the 200-210hp range...

(some might ask "why not buy a new crate engine from Houston?", and the
simple answer is that it'll be far cheaper to have Darren build me one)

Anyway, at the NEC last weekend, I finally got to meet him, after numerous
e-mails and phone calls, and gave him the lump that had come out of #4426
(which came from a carbourettored Peugeot 504 circa late 70s, with a 2664cc
PRV). This engine had never run in the DeLorean and was merely a block and
heads by the time I got to it.

I had decided that I wanted a completely stock DMC engine, and this gave
Darren the task of establishing exactly what parts cross-reffed to what
version of the engine, and more importantly, how to set up the lambda and
idlespeed systems which are not used on the engines he has built before.
For interests's sake, you might like to know that the DMC PRV uses a
Renault sandwich plate and sump (although the Renault sump was not
aluminium and Darren was quite jealous about this) but uses Volvo pistons &
liners, crank, heads, cams, intake manifold and injection. The block is 99%
identical across all PRVs of this age.

One of my choices sometime ago was whether to stick with the DeLorean's
"over complicated" idlespeed system. My decision was based in part on the
archetypal "hunting" of a DeLorean engine when cold, and I told Darren
this. He thought I was being daft and that "there must be something wrong
with a lot of DMC engines then" until he heard Chris Parnham's 111-mile,
museum stored car doing it. We also worked though how to set up the
idlespeed system, and how the DeLorean manual tells you not to adjust the
setting screws - they're meant for the older Renault system (the later
R25's used the idlespeed system found in the DeLorean, but don't hunt
either according to Darren). This is the e-mail I got from him this
morning. It should make interesting reading:
-----
Went to see the font of all PRV injection knowledge yesterday, Steve
Whitehead and learnt all about setting up lambda K-jet. He also leant me
the manual that shows you how to do it. It appears that DMC didn't want
unskilled people messing with the settings but Renault had no proplems with
giving the dealers the info to set them up. To set the idle you clamp off
the pipe to the idle valve and set it with the brass screws as per engines
without idle system,ie remove and clean the screws,refit them and screw
them all the way in.Back off the two front ones 2 turns and the set the
idle with the back screw to 900 rpm. It is absolutely critical that the
control pressure is correct (3.4 bar) and that the pressure from the pump
is correct(4.8-5.2 bar). The only thing you can't do on the DMC is set the
co which should be 0.7-1.0 before the cat. The idle should not vary when
cold and the manual gives all the testing proceedures and values of
electrical sensors etc.

What was interesting was the crude manner in which the lambda system
operates! All it does is vary the fuel pressure in the lower chamber of the
metering head with an oscillating valve to weaken the mixture if the lambda
sensor senses the mixture is too rich. Simple. If yours runs better with
the lambda system on, then it is running too rich when the lambda is off
[#1458 stops hunting when the lambda system cuts in - Martin], so either
the control pressure is too low, the pump output pressure is too low or the
mixture is out of adjustment. A way to set the mixture screw with out a co
meter is to bridge the fuel pump relay with the engine off,pull out an
injector and put it into a container and put a washer (about 0.5mm-1.0 mm
thick)under the air plate stop to hold it in the same position as at idle
and turn the mixture screw untill the injector "cracks" which is the point
at which the injector starts to spray.Then set the idle and robert is your
fathers brother!
-----

I've already learnt to respect Darren's knowledge, so wanted to clarify the
"hunting" of the DeLorean engine - why does it do this when it apparently
shouldn't?

Martin
#1458
#4426



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