Failing Ignition Module?

My '87 740 UK spec. has started to misbehave.

It can run on both LPG and petrol, and on both fuels it behaves well
at low load, but under moderate acceleration there is a slight loss of
top end power. Under heavy load (eg. accelerating hard or pulling up a
steep hill) this is more noticeable, and often accompanied by knocking
when on petrol, and backfiring when on LPG (LPG has a much higher
octane rating than petrol, and provoking knock is very difficult).

I have checked the ignition timing; the idle setting is right, it
advances when revved, and the vacuum advance works when I suck on it.
The valve timing is right. I have changed coil, leads, distributor
cap, rotor arm and plugs. I have checked thoroughly for induction
leaks.

The engine idles perfectly and revs well in neutral without any
hesitation or roughness.

I've never had a failing ignition module before, so don't know if
these are typical symptoms, but I can't think of anything else to
check. Anybody any clues? Is there a way to test the module? How much
is a module in the UK (I'm sitting down with a large glass of whisky)?
--

Stewart Hargrave


For email, replace 'SpamOnlyToHere' with my name

 

This sounds more like an air mass meter problem. Does the LPG system
get a load signal from the air mass meter?

--
Mike F.
Thornhill (near Toronto), Ont.

NOTE: new address!!
Replace tt with t (twice!) and remove parentheses to email me directly.
(But I check the newsgroup more often than this email address.)

 

My similar problem was addressed by a new O2 sensor and hollowing out
the catalytic converter (which was partially blocked). If the exhaust
is only partially obstructed then power loss won't occur until higher
revs (which require higher flow).

Lots of power at the top end now but still some pinging likely due to
the timing being set by eye/ear. I don't know if unblocking the
exhaust would call for a re-evaluation of the existing timing
settings.

Maybe it's the old 'banana in the tailpipe', got any enemies?

Just kidding.
blurp

 

Sometimes I think that 1987 in the UK must have been the dark ages. No
O2 sensor, not catalytic converter.

The whole exhaust system is not much more that a year old, so I would
be surprised if it was blocked. Also see my other post, that today the
problem seems to be absent.

But you're right, a blocked exhaust can give an engine a tough time. I
once reversed the tailpipe into a raised grassey bank and killed the
engine stone dead until I managed to push it away from the bank and
clear the pipe out.

It might do, but more likely require a change in the fuel metering.
I'm a little surprised your knock sensor isn't taking care of the
detonation, though. But as you can tell, these newfangled (1987)
control units are a bit of a mystery to me.

Well now you mention it...
--

Stewart Hargrave


For email, replace 'SpamOnlyToHere' with my name

 

No. Petrol fueling is by basic K-Jet, and the LPG system is open loop,
with a venturi mixer (like a very simple carb.) The only electronic
wizardry is the EZ-K ignition system. The sensory inputs are - idle
position switch, engine temperature, knock and manifold pressure. The
only ones I have doubts about are the knock sensor or the temperature
sensor. I'm pretty confident that a bad temperature sensor wouldn't
create these symptoms, and my tests on the knock sensor (tapping the
block with a steel bar whilst watching the timing marks) were
inconclusive.

What I figure may be happening is that the ignition keeps on
advancing, under load, beyond normal limits, until I back off the
throttle due to knocking/backfiring. Even if the knock sensor was bad,
this shouldn't happen (or am I wrong about this? Consider that
provoking knock on LPG is much more difficult, so if the only normal
limit to ignition advance was the knock sensor, then using LPG would
result in ignition too advanced for good running).

However, today I decided to run it with the knock sensor disconnected.
Went perfectly - couldn't provoke any knock or backfires under any
conditions on either fuel. To confirm the faulty part, I reconnected
it - still runs perfectly.

I supose it could be down to dirty contacts, but I'm still not really
convinced that the knock sensor is the culprit; I have wondered if the
hall effect unit, or power stage amp could produce these symptoms.
--

Stewart Hargrave


For email, replace 'SpamOnlyToHere' with my name

 

Well you may not have a catalytic converter but I'm pretty sure the O2
sensor (aka Lambda Sond) was not optional as it's one of Volvo's
bragging points. It should be screwed into the exhaust manifold and
mine wouldn't come out without a fight. When it did come out it was
clogged shut with a dense chalky residue (I'm told that's from burning
coolant).

As for knock sensor, my car's a 1983 240 Turbo so it predates that
radical technology. Additionally I believe the grage that replaced the
timing belt got the timing wrong and I didn't recognize the "funny
sound" as detonation and drove that way for months. Ugh.

Now the pinging is likely permanent but I won't know until my mechanic
of choice gets a timing gun.

blurp

 

Well they didn't brag about it in the UK. Not until a year or two
later. I was going to add one for the sake of making the LPG system
closed loop. I had a boss welded to the pipe, but the welder
positioned it so that the sensor would have pointed downwards
underneath the car leaving about 2" ground clearance. Twit. Anyway the
whole exhause system has been replaced since then, and I haven't got
round to progressing that idea yet.

I hope not - you should be able to adjust the timing to prevent knock,
unless there is something else amiss; badly coked up for example. I've
seen the harm detonation can do to a piston. The local forces and
temperatures involved can be very damaging and spell the end of an
engine. Try and avoid it at all costs.
--

Stewart Hargrave


For email, replace 'SpamOnlyToHere' with my name

 

To me it sounds like it's leaning out (mixture too lean when the problem
is happening). At higher engine loads the restriction provided by your
LPG mixer and the air flap in the air flow sensor would make the effects
of any vacuum leak worse. Did you perchance have the boot from air flow
sensor to throttle body off while checking the knock sensor?

The ignition system is programmed with an ideal advance curve, based on
engine speed and load. If knock is detected, timing will be retarded
from this ideal to eliminate knock.

--
Mike F.
Thornhill (near Toronto), Ont.

NOTE: new address!!
Replace tt with t (twice!) and remove parentheses to email me directly.
(But I check the newsgroup more often than this email address.)

 

Weak mixture was my first thought. Running on LPG, it felt like I was
running out of fuel. I checked the LPG system in close detail to
establish that there was no blockage or somesuch, and also checked for
induction leaks. I then discovered the symptoms were evident when
running on petrol, too. I haven't taken the boot off yet (the LPG
mixer sits in the top of it - have a look here if you're interested in
the details http://www.hargrave.me.uk/lpg/volvo740.htm); I'll do that
tomorrow - I may even unbolt the manifold for a look-see just to
eliminate it from the equation. A couple of weeks ago I renewed the
injector seals to resolve an uneven idle, so I've been pretty diligent
about looking for induction leaks.

I figured this must be the case. Presumably, also, the control unit
will detect the absence (or malfunction) of the sensor and retard the
ignition a little to a 'safe' advance curve. Thinking on, then, maybe
the knock sensor has now failed totally, and the control unit is
responding with the safe curve, which is why there was no apparent
difference between the disconnected and re-connected sensor earlier
today. Is there a definitive way of testing a knock sensor?

--

Stewart Hargrave


For email, replace 'SpamOnlyToHere' with my name

 

ALL MY MANUALS ARE FOR THE 200 SERIES SO I DOUBT THAT WILL HELP YOU BUT
HAVE YOU TESTED THE FUEL PRESSURE REGULATOR? ARE THERE ANY SENSORS THAT
WILL MAKE THE INCREASED LOAD NOT CHANGE FUEL/AIR DELIVERY WHEN IT'S
OBVIOUS THAT THE ENGINE BEING PUT ON AN INCREASED LOAD ie UPHILL SHOULD
REQUIRE ADDITIONAL FUEL? IS THE ENGINE RUNING ON A DECENT COMPRESSION?
THIS TEST SHOULD BE DONE TOO ESPECIALLY IF IT HAS A LOT OF MILEAGE!

LET US KNOW HOW YOU MADE OUT WITH MY OR ANYONE ELSES ADVICE.

 

I feel I may have wasted my reply to original post cause I didn't notice
on my browser that there were several replies there already, if what I
have written makes no sense please disregard it and accept my apology in
advance
Dan
1987 244 DL 196,xxx klms needs blasted new seats augh
1988 245 DL 237,000 Klms and damn it, it also needs at least one front
driver's seat double augh it's leather too and costs a lot i'm sure
last was big mistake but wife was scared of underbody rust on the 1987
so she urgently needed a car and settled on 91 Park Avenue piece of crap
car lemon with green cap top too.
but she likes the power in V6 3800 blarney to that I say

 

No probs, Dan. I posted it to get different perspectives on the
problem.

As it is, the car is now running fine (for the moment, at least). I
still have no real idea of what the problem was, and can only think
there may have been a dirty connector that I've made good by pulling
it off and pushing it on again (I'm not entirely convinced by that
explanation, but I'm at a loss for any other).
--

Stewart Hargrave


For email, replace 'SpamOnlyToHere' with my name

 

Very interesting system.

The only way test the sensor is to tap the side of the block with a
small hammer and watch what happens to the timing. You have to keep
tapping, once the "knocking" stops, the timing is readvanced almost
immediately.

--
Mike F.
Thornhill (near Toronto), Ont.

NOTE: new address!!
Replace tt with t (twice!) and remove parentheses to email me directly.
(But I check the newsgroup more often than this email address.)

 

Well I've replaced the knock sensor, because there was some doubt in
my mind about it. I found the tapping test inconclusive on both the
old and new sensors.

But the car has been running perfectly for some days now, with no
recurrence of the symptoms it displayed last week. I've no real idea
what caused this - I can't figure out a way to make the knock sensor
entirely responsible.

Thanks for your help, anyway.
--

Stewart Hargrave


For email, replace 'SpamOnlyToHere' with my name