740T bucking when cold

After years of solid running, in the past few weeks my '87 740 Turbo has
had a strange intermittent problem when cold. Either right after it's
started or shortly after the engine will suddenly start idling very
poorly, RPM dips down to 300-500 RPM and stumbles for several seconds,
then it's fine. First time it happened I was pulling out into traffic
and the car lost all power, started bucking heavily and sputtering, then
suddenly roared to life with a spectacular burnout. The next time I was
coasting down my hill when it started stumbling so I pushed in the
clutch and coasted for a few moments until it recovered. Once it goes
back to normal it runs great for the rest of the day, haven't had any
other problems at all.

What do you guys think? I'm leaning towards the beginnings of problems
with the AMM, I'm afraid I might be chasing my tail trying to track this
one down since it happens relatively rarely. It really feels like a fuel
problem though.

 

Distributor leads? That's what caused similar probs with mine, specially
when it rained.

 

I can check them, they're Magnecore wires though and only a couple years
old. Hasn't rained lately either. I haven't checked the distributor cap
lately though.

 

That is odd. How about the operation of the throttle position switch? Maybe
it's getting balky when cold and isn't coming out of idle mode. Listening
for the click with the engine off is a good start.

But it always could be the AMM/connector.

Mike

 

I hadn't thought of that switch, I think I've eliminated it though as
the last time I had this problem I was coasting with the clutch in and
the engine was idling very rough and stumbling still, just as it does
with more throttle applied, it doesn't seem to make a difference.

 

Ugh - I'm no expert on that but it does make me think of the AMM. The
connector on mine acted up, causing similar symptoms, and with it barely
idling I pressed on the connector a bit and the idle jumped up (like
starting) and then settled down. Cleaning the connector with alcohol
(because that was the best contact cleaner I had around) fixed it right up.

Mike

 

That had been my original suspicion, I'd just wanted to check if there
was anything else likely. I'll try cleaning the connector and see if
that helps, if it starts having the problem more regularly it'd be
easier to track it down. I do need to figure it out though, it's
dangerous, I'm afraid I'll forget and charge out into traffic and end up
broadsided when I lose power.

 

This is undoubtedly happening just as the engine computer systems are
going closed-loop. You undoutedly have one or two timed vacuum switches
screwed into the block and/or cylinder head. When the coolant is
warming up, these thermostatically timed vacuum switches open to allow
vacuum to various other components, one of them being the charcoal
canister, turbo overboost switch and in the case of my old '84 Turbo,
the distributor advance. It might be time to check ALL of your vacuum
lines and replace them. I just replaced all of my lines...took about
two weeks and made a HUGE difference. You should have seen the
condition of some of them, they were all sucking air. My old Volvo, from
the day I drove it from the dealer has had that idle decrease and a bit
of rough running during warm-up, particularly on very cold mornings just
before the engine goes closed-loop.

 

Have we determined whether or not the OP has a turbo or not? I know
that not all Volvo's with a "T" in the designation have been turbos in
the past. I don't think that any Volvo turbos had an MAF sensor.

 

"........................................................"

Mine does / :-(

Mike

 

It can't be going closed loop *that* quickly, it does warm up fast, but
the first time it did it the engine had been running literally no more
than 10 seconds and it was below freezing outside. I'd hopped in, put on
my belt, started it up, put it in gear and started to back out of the
driveway when it started lurching like crazy. The second time it'd been
running for maybe 30-40 seconds.

Also I'm pretty sure it doesn't have any of that vacuum stuff, my 240T
has that, but the 740 is LH-Jet EFI.

 

Yes it's a 740 Turbo with a B230FT, hence the T I put in the post, the
only Volvos with a T in the designation and no turbo were the GLTs. Only
the 240 Turbos had K-jet, the 700 and 900 series all had either LH with
the AMM or possibly Regina, not sure whether the turbos got that or not.

 

May be time for a new cap and rotor?

 

Sounds like a problem with the fuel enrichment, possibly the
thermo-time switch, injector, or wiring.

 

There is no thermo time switch that I'm aware of, wiring is a
possibility, the original harness is a bit tattered, I suppose I should
replace that too.

 

Have you tried cleaning out the slops in the air fuel mixture gizmo under
the intake manifold .Try cleaning its electrical connection as well .It gets
messy when you use normal oil and needs a good flush out so it can slide in
and out to alter the mixture .Its easy to remove and clean .Now your heading
for winter ,it might be more sluggish with thicker glug .

 

Maybe not. I'm only going by the generic L-Jetronic description, which
describes a cold-start valve and a thermo-time switch. These add fuel
under the conditions you describe. Or your auxiliary air device, which
adds extra air during cold-start conditions, is intermittent.

Why replace the harness? You can check the wiring using a VOM and
judicious bending and tugging.

 

I'm not aware of any of that stuff, there's a temperature sensor, I'm
not sure if that could cause this problem though? It has a CIS valve to
regulate the idle by bypassing the throttle plate, that part seems to be
working.

Because it's clearly rotting out, there's insulation falling off exposed
wires all over the place. I've taped most of them up and haven't had any
trouble with them yet in the 5 years or so I've been driving it, but it
definitly needs to be replaced. Classic pre-89 Volvo problem.

 

Air fuel mixture? You mean the idle speed valve? That has nothing to do
with mixture, it allows a variable amount of air to bypass the throttle
plate to regulate the idle speed. I cleaned it out about a year ago,
might be time for it again but I really don't think that's my problem
here since I've experienced it both at idle and all the way up through
WOT. Once it happens though it seems to not happen again until the
engine is very cold.

 

"It has a CIS valve to regulate the idle by bypassing the throttle
plate, that part seems to be
working."

That's the aux air device. It would open fully during cold cranking and
gradually close as the engine warmed up, finally remaining open just
enough to hold a steady idle. Some people clean them out with carb
cleaner when they start to stick.