Hello Volvo-experts and others!

I have been working on this 1991 Volvo 960 B6304 3.0L 24v for a while now and nothing seems to be "wrong" with it!

The symptoms are intermittent, the car can run fine for a couple of minutes and then start to run really bad, severe hesitation and bogging.
Not really temperature related because it can happen when the car is hot OR cold. It can run rellay nice for 10 minutes, then after a restart it runs bad.
Some other time it may run bad from the start. The car is almost driveable above 3000rpm if you hold it in lower gears and shift manually.
The car belongs to a friend of mine and I have been helping him sort out these problems for a couple of weeks now (1 owner car 340000km).
He owns 2 identical 960 3.0L so most of the parts are just changed between the cars hoping to find the problem.
I have been working on cars for all my life (Im over 40 years of age) but this one is the hardest to fix by far!

Following components have been either replaced or tested repeatedly:
- All usual service items (sparkplugs, oil, filter etc.)
- No specific fault codes (codes checked several times with the built in lightbulb).
- Engine compression is checked.
- All six ignition coils.
- Wiring harness to coils have been repaired (coil-connectors).
- All six fuel injectors with pressure regulator.
- Air mass meter (tried several different ones).
- In-tank fuel pump and wiring.
- Fuel filter near the main fuel pump.
- ECU Motronic-box.
- Crank trigger sensor on bellhousing.
- TPS-sensor and cleaned throttle body.
- Intake hose with Idle-valve.
- Fuel pump relays and fuses.
- Oxygen sensor and harness.
- Catalytic converter is checked and NOT clogged.
- Exhaust system is checked and NOT clogged.
- Engine grounds is checked and additional ground wires mounted.
- Automatic gearbox PNP-switch replaced.
- Battery, cables and alternator are checked.
- Crank-case breather hoses are checked (and flametrap).
- All vacuum hoses are checked.
- Spark is checked via extension-cable and strobe light (no misses).
- Charcoal canister is checked.
- Various wiring connections checked.
- No EGR-valve or pulsair-valves on these engines here in Sweden.

Here is a video on how the car runs when these problems occur:
I would be very grateful for any help or input that solves this problem! Thanks!

 

I would call this a fuel delivery issue. It does not seem that you lose spark but fuel.

 

Yeah it may be fuel related, going to make me some adapter with a gauge to check fuel pressure that should be 3 bar in the rail.
If I pull the fuse on either the in-tank (new) pump or the main pump under the car engine dies after 3 seconds.
But maybe the main fuel pump is bad although I can hear it buzzing, would be the first time for me a fuel pump behaves like that..
My experience with pumps is that they are either completly dead (and will sometimes come back to life with a blow from a hammer) or working fine.

Other parts I haven´t checked is knock sensors, cam position sensor but it just doesn´t make sense.
I have had many old Audis with very bad knock sensors through the years (have owned and restored many Audi 200/100 turbo quattros).
None of those have behaved like this Volvo 960 does...

 

I'm trying to remember. Are these premium fuel engines?

 

The car has always run fine before on either 95 or 98 octane fuel and 95 is whats probably in the gas tank right now, fresh fuel also.
It cannot be a fuel quality issue because the car runs this way intermittently...the fuel quality doesent improve from one day to another.
And I have also had cars sitting for years that ran just fine with old gas in the tank...

 

The problem is solved! Finally!!!
I found the cause of this poor running condition and it was the main fuel pump underneath the car.
Had the car idling when it was running bad and gave the fuel pump under the car a light tap with a hammer!
Immediately the idle changed and became normal, you could clearly hear the change and the car runs good.
The pump must be worn out but still operating to some degree and the shock from the hammer made it run better.
Repeated this procedure 5 times and the car behaved the same so that confirms it has a bad fuel pump.

Im going to order a new pump tomorrow. Im so happy that I finally can fix this 960.