740 keeps running very rich

I am really starting to pull my hair out with this car. It is an '89 740
(non-turbo), 140,000 miles. Should be a good solid runner, but it just
won't stay running. It just belches black smoke from the exhaust and runs a
few hundred miles until plugs are so fouled it won't start. It doesn't
sound bad when it is running and has pretty good power. I have replaced the
air mass meter with a different unit from a junked car. Also replaced the
fuel pressure regulator. Should I break down and buy a new AMM just to rule
it out as a problem? What other components may be causing this and how easy
are they to replace for the do-it yourselfer.

TIA,

Kirk

 

Check the coolant temperature sensor, not the one for the guage but the one
for the injection. You can test it with an ohm meter.

 

Thanks for the very fast response. Do you have a site with instructions or
know what reading should be?

Kirk

 

No, I don't remember what it should read, however if you warm up the engine
a bit, clip an ohm meter between the sensor and grounded metal under the
hood you should get a resistance reading which changes smoothly as the
engine cools. Also how's the wiring harness look? I forget what year this
car is but pre-'89 cars had crummy insulation on the engine wiring harness
which deteriorates.

 

I have the same problem with my 86 740 were is this ting plased. I want
to fix my Volvo too.

 

It's on the cylinder head under the intake manifold, small round sensor
threaded into the head. There's two of them, I don't remember which does the
guage and which goes to the injection but if you unplug the one to the guage
the guage will be pegged. Another option is to simply test both.

 

On my 85, the sensor is somewhere around cylinder #3, under the intake
manifold and hidden by the throttle body and idle air control valve. On my
turbo, the new engine harness doesn't make good contact with that sensor and
it causes the engine to occasionally run very rich under boost until I
reseat the connector.

Mike

 

Under #3 intake runner.

Bob

 

Thanks I'll take a look at it tomorrow!!

 

Thanks wonderful to have so many helpers!!!