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?



--

Stewart Hargrave

A lot faster than public transport


For email, replace 'SpamOnlyToHere' with my name

On Sun, 19 Oct 2003 15:40:10 +0100, Stewart
wibbled about:


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

Take it to Barry at BPV Servicing in Slough. He seems to know what he
is doing...

He's done my car and fixed others I have reccomended to him...

01753 692386

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
Reply to address is valid

From a point at sea, to the circles of your mind, this is hugh:

In message , Stewart
Hargrave writes

Tried both reducing and increasing spark plug gaps.

What gap on your plugs?

I've tried 28 thou (standard for petrol)
25 thou (what I usually set them to for LPG)
22 thou
I think I also tried 20 thou, but this is getting to be a very small
gap.

Currently set at 25 thou

None of the changes made any noticable difference.

I have been thinking that a hotter plug may do something.

It seems to me that the problem is centred around number 4 cylinder,
and that somehow the spark is not igniting the fuel properly. As the
fuel mixture must be the same in all cylinders, and there seems to be
a big voltage across the plug, I guess it has to do with poor spark
formation somehow, but I haven't come up with a plausible reason yet.



--

Stewart Hargrave

A lot faster than public transport


For email, replace 'SpamOnlyToHere' with my name

From a point at sea, to the circles of your mind, this is Sean Nugent:

On Sun, 19 Oct 2003 15:40:10 +0100, Stewart
wibbled about:


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

Take it to Barry at BPV Servicing in Slough. He seems to know what he
is doing...

He's done my car and fixed others I have reccomended to him...

01753 692386


Thanks Sean. As I'm running out of ideas, it seems like the next step.


--

Stewart Hargrave

A lot faster than public transport


For email, replace 'SpamOnlyToHere' with my name

Hugh,

Have you tried looking at the engine running at night in a dark garage it is
if there is any flash overs etc.

John



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In message , Stewart
Hargrave writes
From a point at sea, to the circles of your mind, this is hugh:

In message , Stewart
Hargrave writes

Tried both reducing and increasing spark plug gaps.

What gap on your plugs?

I've tried 28 thou (standard for petrol)
25 thou (what I usually set them to for LPG)
22 thou
I think I also tried 20 thou, but this is getting to be a very small
gap.

Currently set at 25 thou

None of the changes made any noticable difference.

I have been thinking that a hotter plug may do something.

It seems to me that the problem is centred around number 4 cylinder,
and that somehow the spark is not igniting the fuel properly. As the
fuel mixture must be the same in all cylinders, and there seems to be
a big voltage across the plug, I guess it has to do with poor spark
formation somehow, but I haven't come up with a plausible reason yet.



25 thou is the norm for gas. Presumably they are standard single point
plugs.
How are you turning off the petrol on your system?
Could it be the distributor itself?
--
hugh
Reply to address is valid

From a point at sea, to the circles of your mind, this is hugh:


Could it be ...



Oh joy, oh glory, oh bliss...

Finally nailed it. Finally and permanently (notice my air of
confidence).

It was, in fact, due to a split inlet manifold gasket.

This was a surprise because I replaced that gasket when I took the
head off earlier this year.

I had checked for induction leaks right from the start of the problem
appearing, and then again more thoroughly when Tim suggested an inlet
leak. Today I checked a third time.

So I'm feeling a little foolish that I missed it before.

On the plus side, the car now idles better than it ever has before.

So thanks to everyone who has tolerated my incompetence and helped
with suggestions.


--

Stewart Hargrave

A lot faster than public transport


For email, replace 'SpamOnlyToHere' with my name

"Stewart Hargrave" wrote in message
...
From a point at sea, to the circles of your mind, this is hugh:


Could it be ...



Oh joy, oh glory, oh bliss...

Finally nailed it. Finally and permanently (notice my air of
confidence).

It was, in fact, due to a split inlet manifold gasket.

This was a surprise because I replaced that gasket when I took the
head off earlier this year.

I had checked for induction leaks right from the start of the problem
appearing, and then again more thoroughly when Tim suggested an inlet
leak. Today I checked a third time.

So I'm feeling a little foolish that I missed it before.

On the plus side, the car now idles better than it ever has before.

So thanks to everyone who has tolerated my incompetence and helped
with suggestions.

Glad you got it sorted. I've seen many times a sucked in, or blown out, or
broken manifold gasket on one cylinder cause a "loping" effect at idle,
which disappears as soon as you raise rpm just above idle.

The last call on a car which lopes at idle and you are 349% sure there isnt
a vacuum leak or any other problems is a bent conrod from water injestion in
the past. Most often with diesels though, as their starters are powerful
enough to bend the rods when the engine is hyraulic-ed.

Tim..


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