740 Turbo issues

Got a couple of issues that I've noticed recently on my '87 740 Turbo.
Not sure if they're related but both are intermittent. Firstly, it has
started to idle occasionally at 1200-1400 RPM. I thought it was the idle
air valve sticking but I've tried tapping on that and it doesn't seem to
make any difference. Next step is to remove, clean, and bench test that.

The other issue is that once the engine is all nice and hot, it starts
to stumble when the boost is up near the top. A friend who was following
me commented that there was some smoke from the exhaust when this
occurred. My first thought was a split in some of the plumbing to the
intercooler, it behaves exactly like that except that it doesn't happen
when the engine is cold, in fact it may have been going on for some time
but I didn't notice it until I was going up a mountain pass on a hot day
with the A/C on.

Anyone run into either of these issues? Intermittent problems are tough
because they never seem to happen when I'm in any position to diagnose
them. Overall the car has been running really well and I got 25-28 mpg
on various legs of the recent trip.

 

Accumulation of oil in the intercooler is normal so a hot day up hills a
blow out is ok .I drain mine once a years and get a couple of teaspoons of
oil out of it .

 

Hi James. I have an '86 740 turbo intercooler. When I first got mine
on the
road a few years ago, it was during summer, and it ran great except at
higher rpms.
It would stumble and break up, kind of like a rev limiter when I wound it
out. I
tried everything I could think of. Finally, I can't remember which, but I
either ohmed
out the mass air sensor, or swapped in another that I had, and that cured
it. It was
only doing it when the car was fully hot, or during hot weather. Has been
fine since,
although at max rpm, it does break down like it has a rev limiter, but I
figure that will
keep me from blowing it up.

Your mileage! I can only get 22-23 max. I do have the mixture a bit
rich, because
I'm afraid to lean it too much. How do you set your mixture? Do you follow
the
procedure found in brickboard using the LED?

good luck, /glenn

 

MAS sounds right. & it does have a limiter\governor.

 

Use a ohm meter between pins 2 & 6 on the maf sensor adjust the screw
very carefully to 700 ohms

 

Thanks Glenn. I just did so. Before adjustment, I had 521 ohms.
That's
the setting that gave me the 22-23 mpg. I'll see how it does tomorrow
to/from
work. I'll be filling it up at the end of the day, so we'll see what effect
this has.

I do drive the car fairly hard, so that accounts for much of the low
mileage.
I have a 50 mile commute one way, and run between 75 and 80 mph on the
highway.

What else could cause low mileage? Mechanically that is?

One thing that may be affecting it. I believe that at one point in the
past, I lost
my O2 sensor. Due to lack of money, I put in the one from my '91 940 16
valve.
My research led me to believe that the same sensor would work on both cars.
Other than that, the car runs great.

thanks much, /glenn


Also, I can't get much more than 6-7 lbs of boost out of it.

 

A better solution is to install a wideband O2 sensor to read the actual
mixture, but those can be a bit spendy.

The automatic versions get 18-22 mpg but this has a manual gearbox and
consistently gets 22-26 on average, on one occasion I got 29.8 mpg with
a light foot on a long fairly flat highway trip.

I'll try a different air mass meter and see if that makes a difference.
I'm suspecting something electrical since the problem comes and goes,
experience tells me that most other problems come and then just keep
getting worse.

 

This was a repeatable puffs of smoke whenever the engine started to
stumble, which would occur any time I got up into a significant amount
of boost. It felt exactly like it did the time I had a loose hose clamp,
except the current symptom only happens when the engine is good and hot.
I've meticulously maintained this car so there is little oil
accumulation in the intercooler. I clean out all the intake plumbing
with Simple Green from time to time.

 

I haven't messed with the mixture at all, the car is stock and until
recently always ran great so I never touched it. Unless something is
broken, a fuel injection system should never need to be adjusted.

 

How do I know if it is a wideband O2 sensor? Or how do I find one?

Mine has an M46 w/electric OD.

My symptoms came and went. Mine ran perfect in cool weather (it was
during summer), but as soon as it got hot out, the problem came back. I do
remember coorelating it to a particularly hot day.

I didn't see any smoke in the mirror, but also did not have anyone
behind me
to check for smoke, so maybe it did puff, but I didn't see it.

thanks, /glenn

 

If you're not sure, then you don't have a wideband O2 sensor. They were
not OEM equipment until the mid 2000's or so, and even today many cars
still use narrowband sensors. Aftermarket wideband systems are available
and useful for diagnostics & tuning, but rarely really needed.

I couldn't see any smoke in the mirror either, it was only when I had
someone behind me that I found out. At any rate I swapped the air mass
meter tonight and it seems to be running noticeably more smoothly and
with a bit more power. I noticed when I got back that the main fuel pump
(which I replaced a couple years ago) was noisy and I can't hear the
in-tank pump running so I ordered a replacement for that. Fingers
crossed that this fixes any remaining issues.

 

The idle air control does wear out but often needs a good clean out only.

 

That's the next step, I've got a cleaned up spare that I'll pop in and
see if it fixes the fast idle. I swapped the air mass meter and it has
been running noticeably better, feels more powerful and smoother
although I haven't had the combination of a hot day and mountain pass to
really test it.

The pre-pump is shot as well, I can hear the main pump (which I replaced
last year) cavitating and no nose from the tank, replacement is on order
and will be installed shortly.

Changed some vacuum lines too that were getting ratty, they hadn't split
yet but can't hurt to have nice new ones in there.