91 740 Wagon problems redux

Ok, I posted the other day about this. Someone said it could be the air
mass sensor, or the coolant temp monitor.

It just got harder and harder to start.

Here's the history again:

This car has been leaking about 1qt of oil per month for a couple of
years , and I ran low on oil several times, and there's likely lots of
gunk in the engine. Not sure what this would do to the fuel system though.

Then, one day, it just started hemmoraging oil. I took it to my regular
shop, and they said the flame guard was clogged, and it blew out my
seals, and something on the side of the engine. Luckily I was near the
shop at the time.

They fixed that, but it was still leaking oil as usual, I took it back,
and they finally got the damned seals in right, because the leaking
pretty much stopped.

However, right after that, it began having problems starting.

I finally took it into a shop near where I live now, they replaced the
distributor cap and rotor, said they were shot. That gave them some
spark and they were able to start it, but by the time I got there and
tried to start it, it would almost catch, then die, then wouldn't start.
They started it by spraying carburator cleaner into a line on the top
of the engine.

I brought it back in, and they did some more work on it, cleaned the
coolant sensor, cleaned a bunch of crud out of the ?mixing chamber?, and
it was working better for a while. Now its getting worse. I've been
relying on the carburator cleaner to get me started, but its just
getting worse, running rougher, etc.

They tested the pressure on the fuel pump, and said it was only getting
about 30psi, where normal was above 40, but they didn't feel this was
the ultimate problem and didn't want me to throw more money at it in
case it didn't work. They checked the fuel pump relay and that was ok.

I'm wondering if it might be something as simple as a bunch of crud in
my gas tank.

I bought some fuel system cleaner, but am leery of using it in case I'm
going to make something still worse.

The shop who just worked on it said they don't know what else to do and
the next stop should be a dealer. My experience with dealers is that I
will be screwed to the wall, but I can't find any reviews for any of the
independent Volvo mechanics in my area (Fremont/Newark, Ca.)

Any further advice on this would be appreciated.

 

I'm more concerned about the low fuel pressure than the shop is. The fuel
injection system is basically a reckoning system (fine tuned by the O2
sensor when that warms up) that opens the injectors for an amount of time
based on the air consumption reported by the air mass sensor and assuming
the proper fuel pressure. Too low a fuel pressure means too little fuel
injected.

Might as well go ahead with the cleaner. If there is grunge in the system
you'll be better off with it dissolved off the sensitive surfaces. I don't
think the pump is bad - low pressure isn't its style. I'd focus more on the
fuel filter and possibly on the fuel pressure regulator. Any competent
mechanic can change the fuel filter; unless things changed between my '85
and your '91 it is under the car below the driver's seat. It isn't hard to
change with the car on a lift but it is a pain to do with just a creeper.
However, I dunno about the pressure being low. At least in the older 740s
normal pressure is 36 psi, 43 for a turbo.

You can also try the procedure for starting that is used when the check
valve is leaking (allowing fuel pressure in the rail to bleed off when the
engine isn't running.) To do that, hit the starter for half a second or so,
then let the key spring back to the "run" position. Leave it like that about
two seconds while the fuel pump brings the fuel rail up to pressure and try
starting.

Mike

 

Is there a way for me to test the air mass sensor, to determine if it
needs to be changed? I just read online that its an easy switch-out.

 

If you have access to another car with the same fuel system then yes,
otherwise no, not without creating a test setup that will power the sensor
and let you read the output as you vary the amount of air blown through it.

 

ere is a good place to startgathering info:

http://www.brickboard.com/FAQ/700-900/

Good luck,
joe

 

Great. Thanks. Looks like the last mechanic has done me a disservice
by telling me to use carburator cleaner to get it started.