Original description of problem:

My 1993 Volvo 940 has been running smoothly however, most recently and suddenly, while going at 30 mph (@ approx, 2000 RPM), I stepped on the gas and the power and RPM dropped... No power for about 2-3 seconds. Just as suddenly, the power came back and the engine ran smoothly. This happened a few times on the way home from work. When I was stopped in traffic, it stalled once but I was able to immediately start the engine which ran smoothly...

Action taken:

(1) Replaced the fuel pump relay with new one

(2) Installed new Bosch Main fuel pump and fuel filter

(3) Checked all visible wiring and fuses. All O.K.

The problem has persisted. Today the car stalled twice while stopped in traffic but immediately started and ran smoothly.

I welcome any suggestions as this intermittent issue is a difficult one to diagnose.

Thanks in advance to all!

OnlyVolvo



Thanks,

OnlyVolvo

I suspect the CPS.

Try this. Go over any speed bump and see if the engine dies then. The engine will rock a bit when going over such bump and if there is any internal wiring problem at the CPS it might show. I got similar issue when using AIP Electronics's CPS last time. Threw it out after about 3 months of use. Yes engine was able to start immediately after it died. No such thing happened when using OE Volvo even with splayed wiring insulator.

Check your OBD for any codes too. Mine gave out no code as the interruption was intermittent.

Regards,
Amarin.

I have to lean toward RSTARKIEs coment. A Crank positioning sensor (impulse sensor) will give you that sharp . with no 'out of gas' coughing.

If this happens when teh gas tank is about 1/2 full then you may have a crack in the small hose in the gas tank that joins the intake pump to the sending unit.

Check #6 fuse for good conductivity.
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How many miles?

How much fuel in the tank?

You reside in the SF Bay Area or somewheres in CA-state?

No on-dash CHECK ENGINE light? Though the CHECK ENGINE light illuminates with the key turned to accessory II, where in all the dash lights illuminate, before you start the engine?

Check for fault codes. Merely a guide. As the ignition and fuel engine control can be rather forgiving.

You have the Haynes or another service manual (no Bentley for 900, afaik)? Maybe Clymer or Chilton?

To check the codes, please see:
https://www.brickboard.com/FAQ/700-900/EngineOBDCodes.htm

Specifically, for your Bosch engine control:

Fuel control and emissions:
https://www.brickboard.com/FAQ/700-900/EngineOBDCodes.htm#LH24FuelInjectionFault

Ignition Control and emissions:
https://www.brickboard.com/FAQ/700-900/EngineOBDCodes.htm#EZK116IgnitionFaultCodes

Off the cuff, what you describe is the engine, using a commonwealth English term, is "bogging." Engine hesitates when pressing accelerator.
- Dirty or failing throttle position switch (TPS)
- Weak ignition spark. fuel / air compression overwhelms the weak spark, so it 'bogs' on more fuel. Wiring check include disconnect / reconnect of wire harness connectors, such as the Power Stage. Take care not to lose the seal inside the wire harness connectors!
- Indeed, early faulty crank position sensor (CPS) failure
- Mass air flow (MAF) or air mass meter (AMM) - same thing, two of four different names for it, fault. (Air filter box preheat flap valve stuck to hot air inlet only?). Sort of doubtful.
- Fuel starved, yet you replaced the main fuel pump. Fuel pressure regulator?
- Dirty idle air control valve (OBD codes may show fault)
- If in CA-state, the EGR valve or the system itself maybe faulty. You may have the Pulsair system that introduces air into the exhaust manifold, yet on the air induction side.

Or other fault I cannot imagine now. Please check the fault codes.

You replaced the head gasket on this 1993 Volvo 940 and the neutral safety switch?

You also complain of surging idle in this post when (I presume) the engine is fully warmed up. Idle surges between 650 to 900 RPM when stopped with the transmission in Drive.

https://www.brickboard.com/RWD/index.htm?id=1611701&show_all=1

And here, earlier date:

https://www.brickboard.com/RWD/index.htm?id=1572233&show_all=1

Did you solve the issue?

Also, you replaced the ignition power stage:

https://www.brickboard.com/RWD/index.htm?id=1528078&show_all=1

If no, the issue you are encountering now may be a worsening of the idle surge. So, maybe air intake vacuum leak between the air filter and the air intake port manifold to cylinder head gasket. That includes all connected vacuum lines. Oh, and dirty throttle body, possibly.

I would dare to suggest the fuse and relay platform board may also be considered at issues. Corrosion forms at the bonded contact where you insert a relay with the wire harness terminals secured under this board. I believe it is Dave Barton and other that would suggest the 700-900-90 board a serviceable item for inspection and repair. Some may encounter melted and even burnt plastic around the fuel pump relay on this board.

You have to get under the board, inspect the connections. Treat with a corrosion dissolving and inhibiting dielectric grease or spray such as DeoxIT. May not be your problem, yet good prevention. Also, Use DeoxIT at all wire harness connection; certainly engine control. (And don't lose the seal inside the connector housing!)

About the FPR:
https://www.brickboard.com/FAQ/700-900/FuelSystem.htm#Fuel_Pressure_Regulator

More knowledgeable 900-redblock series folks like Spook can better help you.

Hope that helps you.

Sorry to go on so long.

Questions?

Earl Grey Tea MacDuff.
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Have you ever replaced the crank position sensor? It it loses power while you are driving (as it did at 30mph) take a quick look at the tachometer- if it is registering zero it would be a good indicator that the CPS is failing.

Randy