In message , Stewart
Hargrave writes
I thought I had resolved the uneven idle problem on my '87 Volvo 740
with a new distributor cap. It lasted about ten minutes until the
engine was fully warmed up. Over the past couple of weeks it seems to
have become a little worse.

Tickover on LPG has never been quite as smooth on LPG as it is on
petrol, but now it is significantly worse than it used to be. Sitting
at traffic lights is quite uncomfortable as the engine jiggles about,
shaking the whole car with it. Above 1500 rpm there is no evident
problem. There is no loss of power, no hesitation, no flat spots.
There is a stronger than usual smell of LPG at idle.

It's an open loop system and runs perfectly on petrol.

Here is the complete list of what I have done to try and resolve it:

Changed all the HT components - plugs, leads, dizzy cap, rotor arm,
coil.
Changed the air filter.
Checked the inlet track for obstructions.
Checked for induction leaks.
Compression tested all cylinders - all are v. good.
Checked for leaking petrol injectors.
Moved the location of the mixer.
Moved it back again.
Stripped and checked the vapouriser diaphrams.
Tried both reducing and increasing spark plug gaps.
Reset the valve clearences.
Checked the valve timing.
Checked the ignition timing on all 4 cylinders.
Varied the ignition timing.
Tried fuel from different filling stations.
Played about endlessly with the mixture control on the vap. as well as
the idle speed control and the LPG max. flow valve.

The whole exhaust system is nearly new. The cylinder head was off 6
months ago and was, to all intents and purposes, perfect. Valve stem
seals were renewed, and there was no carbon build up in the combustion
chambers.

A timing light connected to each plug in turn shows a strong and
regular flash. A plug lead held near to an earth point produces a
spark up to three inches long.

When shorting out spark plugs one at a time, number 4 seems to make
significantly less of a difference than the others; I'm figuring that
this, together with the stronger LPG smell, suggests incomplete
combustion in that cylinder. But I'm at a loss to account for this.
Switching around the leads and plugs does not move the problem away
from cylinder 4. Number 4 plug shows a slight amount of pale, dusty
deposit that is absent from the others.

My reasoning says that the problem can't be in the LT side of the
ignition - a good spark occurs at the right time on all four cylinders
- but I'd bow to superior knowledge here.

I can't think of anything else to check and nor can I figure out
what's going on; I'm utterly stumped.

Suggestions please.

Any LPG fitters/experts in the Bershire area who would be willing to
try and diagnose the problem?



What gap on your plugs?
--
hugh
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