My parents have a 1991 Plymouth Grand Voyager SE 3.3 V6 4-Speed
Automatic.

First week of this month, the van had problems:
1)One day, at 35 MPH, 3rd gear didn't engage upon 2-3 shift causing van
to fail to continue picking speed. After that, it lost speed,
hestitated/puttered, and engine quit. Restarted it but wouldn't hardly
go. Had to park it. They also reported it didn't kickdown properly to
1st/2nd at times other times.
2)Oil pressure gauge read 'nil' whole day showing pretty much no oil
pressure.
3)My dad told me of coolant leaks where level would go down with no
visible leaks over time.

Here was my observation when I drove the van over Christmas week. I was
borrowing it because my car was being worked on. Anyway, the van drove
fine. It never hesitated and ran pretty smoothly. Transmission shifted
like it was supposed to. Oil pressure was alright; 1/4 at hot idle and
a little less than 1/2 going down the road. It never ran hot as the
temperature gauge stayed at the 1/3 mark the whole time I drove it.
Only thing I noticed was that the heater didn't blow warm enough.
So, one night as I was getting gas, I decide to check the fluids. The
oil level was full, but the oil looked 'dark' brown if not blackish.
Now, the coolant was low. The coolant in the burp tank was well below
the MIN mark, and it looked like it had pure tap water in there as
well. The radiator also looked pretty empty according to my own eyes as
well. It took almost a gallon of 50/50 mix coolant to bring the system
up to par. Other fluids were fine.
Based on the oil bottles I see in the van, 10w-40 oil is used in it.

Anyway,
They towed it to the shop that day.
The shop did the following over the next week:
1)Pulled pan, cleaned pickup/pump area, then engine flush, then
multiple oil changes/treatments = this fixed the oil pressure problem.
2)Scanned vehicle while driving and found O2 sensor bad. Replaced
those. Fixed hesitation/puttering/cutting out
3)Checked out transmission module. Tweaked some parameters in it and
replaced RPM interface to trans module. Also fixed kickdown thingies on
there. Trans works properly again.
4) To tackle coolant leaks, they replaced radiator cap, fixed a heater
core hose because it had a small pin hole leak that they said was
squirting a little coolant when under pressure.

Anyway,
My parents got the van back on Friday the 13th. They told me it ran a
lot better.

Oil pressure problem fixed.

The mechanics got every issue on the transmission fixed. 3rd gear now
engages properly. Van kicks down to lower gears (passing, 1st)
properly. All they had to do was scan the module on top of the
transmission (TCM!?) and tweak the parameters within it and replace the
RPM interface thingy.

The shop said that the coolant leak was due to a faulty radiator cap
causing coolant to escape since the system wouldn't hold pressure.
also, the mechanic found a leaking heater core hose. On the hose, they
found a tiny pinhole which was 'just barely' squirting coolant when
system was under pressure. So, they fixed all of that.

Van just needs new brakes now, which my dad is going to put on himself.

But anyway,
Last Wednesday, my parents had to drive the van again to finish taking
some Christmas stuff off. He wanted to monitor the coolant level to
make sure it was no longer leaking and that it was staying steady.
Well, when they got to their destination, it went way down. Not wanting
to risk anything, they decided to drive it back to the shop for further
looks on it.

Here's the Bad News:
On Thursday, the shop called them and told us that the Head Gasket blew
(or had blown). They pressure tested system and it failed (didn't hold
pressure). They checked the oil and reported that there was some
coolant in it. They described the oil as a chocolatey-milkshakish
appearance. Also, they ran a compression test and reported that
compression was leaking on 2 cylinders.

They said that the reason for some coolant getting into oil and cyl's
was that when they replaced the radiator cap and fixed the heater core
hose leak, the system was able to hold pressure, and the coolant
started to get pressurized into some of the cylinders and hence, into
the oil and out the exhaust. So, the mechanics, therefore, told my
parents that the bad radiator cap was masking the blown head gasket
symptoms for a long time.

So, on Friday, my parents got the van towed from the shop over to a
friend of our's house to have the HG fixed. My dad's friend will be
able to fix it a lot cheaper than the shop will. So, over the weekend
his friend dissassembled the top end of the engine. And today my
parents picked up the parts. Hopefully, he will have it ready by
tomorrow.

What a headache these problems have been..And my dad still has to do
the brakes on it.

And, oh, btw, the total for the work the other week was approximately
$1,170. They spend another $150 late last week for diagnosis.