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Problem: 1992 940 b230fd with automatic started to rise fuel consumption.
Ive owned car 5months. First two months mpg at mixed driving was 25mpg. Now even 100% highway it goes about 24mpg and mixed is near 18mpg.

This is not my first volvo with this technic and this is too bad mpg.


Car runs well, idles well, starts well hot and cold, wont bog or hesitate or nothing. No fault codes, many times checked.

What ive check:

-Brakes wont drag or stuck

- No exhaust leaks, new manifold and flange gaskets. Cat is old but passed well emission tests after i bought car

- New bosch o2 sensor and ive tested it by multimeter and it works

- New ect sensor

- Spark plug, wires and rotor new, cap good condition. Plugs are white not black

- FPR tested with vacuum pump, no leak

- Egr works, pulsair blocked because valves leaked

- Automatic works well, red fluid, wont slip, tcc works

- No fuel hose/line leaks and new fuel filter

- New intake manifold and vacuum hoses

- Vacuum gauge test gived steady 22hg and ive ultrasonic cleaned injectors, no vacuum leaks




Maf shoudl give 1.4v at ignition on and 2.3v at idle. Mine gives 1.49v and 2.48v, resistance test passed.




Help me? And thank you for all hints and tips as far!

maybe their is a loose connection at the header to cat connection??

There could be an air leak at the heaDER TO CONVERTER CONNCECTION. tHE BRAKE PADS COULD BE BOUND UP & NEED TO BE LUBED W synthetic brake lube,,the brass fuel parts that connect to the fuel filter could have hairline holes..There is a machinist in NH that sells them on ebay

I got tomorrow or monday used but low mileage car bosch maf, will see what happens and what volts it give

With old maf and new ect no effect, fuelup yesterday was 18mpg / 13l.


Now got full tank and "new" maf.

Here again.



Problem still there. Ive noticed that my o2 sensor is reading 400-800mv at most time, wont jump fast 100-900mv. I have tried two oe bosch sensors now so they cant be bad.


What can make o2 sensor to lean mixture? If i do vacuum leak o2 sensor drops 100-200mv so it will richen mixture like it should.



And again all checked

If the exhaust is lean, engine control responds by enriching the fuel to air ratio. QA vacuum leak no at the intake manifold can cause this, like a hole in the large accordion style hose at the AMM downstream to the throttle body. Or the PVC piping has failed and leaks vacuum, or the AMM has gone wonky and engine control applies the ultra fuel rich 'limp home' mode.

Also, is the silver accordion preheater hose between the exhaust manifold heat shield connected the air filter box? If you have not replaced the air filter box air temp thermostat, only heated air passes through the air filter box and AMM / MAF and can ruin the AMM. However the this failure appear to cause a lean running condition until engine control can't tolerate the fault and goes ultra rich limp home mode and sets a check engine light.

https://www.fcpeuro.com/products/volvo-engine-air-box-thermostat-1266826

Air intake manifold vacuum fuel pressure directly effects the intake manifold vacuum controlled fuel pressure regulator. With your foot off the gas pedal, fuel passes more easily from the injectors, with 'high' vacuum air intake manifold. Flooring it reduces vacuum to closer to ambient pressure and causes the fuel pressure regulator to restrict fuel passing to the return back to the tank.

The fuel pressure regulator fails in a few ways, usually unresponsive to the air intake manifold pressure, whether the FPR leaks or not, whether you floor it (high fuel injector pressure) or coast or are stopped at a light (high pressure, leaner fuel ratio)
- Restricts fuel return so fuel pressure is higher than should
- Allows too much fuel to pass
- May leak through the vacuum line with or without these other two problems

You checked fuel pressure?

No brake drag?

The oxygen sensor you originally testes was old.

And you are certain the odometer is accurate? Or do you use GPS to verify mileage? Old odometers may be unreliable narrators over the miles.

Hope that helps.

Quiet Veggie Black Bean Soup Boyeeeeeeeeeeeeeeeeeeee
--
Beh.

My exhaust is rich because o2 sensor gives mainly high volts. So it tries tell ecu that my exhaust is rich, lean it?



I had two bosch oe o2 sensors. I drived 1k miles with first one. Lazy readings but me thinking about worn sensor. Now a week ago i ordered another new bosch oe sensor and it reads still same.

You removed the oxygen sensor and replaced with twice new Bosch oxygen sensors.

Excellent engine compression numbers!

With your electric multimeter the replacement oxygen sensors appear lazy. Yet show a limited range delivering a voltage output at 0.5 volts DC and above with occasional dips to 0.4 VDC volts?

A working oxygen sensor:
- Encounters a rich, oxygen poor (relatively) fuel to air ratio and so the fuel ECU will lean the fuel to air ratio in what is called fuel trim control. Like an exhaust leak at the header pipe output to catalytic converter input leak that is in front of the oxygen sensor (more oxygen in the exhaust)
- Encounters a lean, oxygen rich environment, and the fuel ECU enrichens the fuel to air ratio. Like a high up exhaust leak at the cylinder head to exhaust manifold interface.

With exhaust works cold, a spray bottle with some dish detergent and soak the exhaust unions upstream of the catalytic converter. Use a remote start or use an assitant as you hurridly soak the unions for bubble.

The critical exhaust union at the header pipe output to catalytic converter input. Just upstream is a bracket secured to the bellhousing. There is an intermediate carrier angle plate between this bellhousing bracket and the header pipe. This exhaust support failed on my 1991 240 sedan and that exhaust union leaked both ways (sucking ambient air between exhaust pulses) and lowing MPG considerably. I had to replace the forward pointing studs at the catalytic converter input with beefier thermal proof hard (nuts, washer, bolts)

Yet the Bosch ECU uses multiple sensor and telemetry sensor inputs. There is a hierarchy of show the ECU uses and responds to sensor input.

Have you verified you mileage using a GPS device? Old Volvo odometers lose accuracy?

And you've replaced the FPR. A new part can be faulty. You've tested the fuel pressure and applied vacuum to the FPR vacuum line. High vacuum is throttle closed and less fuel injected. Low vacuum near ambient is gas pedal floored and throttle wide open and the FPR partially closes to raise fuel injector pressure.

Your 1992 Volvo 940 does not have a cold start injector, yet it may be useful to check for faulty fuel injector spray pattern? Maybe an injector is delivering too much fuel that shows a relatively rich exhaust.

I've not pulled an injector since the early 2100s. There appears description for testing injectors on the car so long as you have a receptacle large enough to contains the spray and fuel that passes into a container.

You get good voltages at the battery terminals with engine off, engine on not started, and engine start. A weak battery, poor charging less so, and corroded ground connections can cause poor fuel economy. I'll guess you've tested the electrics for these items.

And no brake drag. Lift your 940 and you can easily turn the wheels. Or after a sort of long drive, no one wheels is hotter then the others as brake drag can cause. You brake fluid is clear, not black? Black brake fluid causes calipers to become slow to engage, release, ultimately binding open or closed.

You've also verified proper air intake manifold vacuum be in inches of mercury or another fluid pressure scale at the air intake manifold?

And I'll guess you've read the Volvo 700 / 900 Series FAQ here or at the large UK site?

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

https://www.volvoclub.org.uk/faq/FAQSummary1.html

If you have searched this forum for posts and threads that address poor fuel economy? Search at the UK site. Doubtful yet turbobricks.com in the Maintenance and Non-Performance forums may offer you insight. Helps to have a Turbobricks log in account.
--
Woo.

I got fuel pressure meter and i try use it soon.


My workmate drived yesterday behind me. I was driving normal and accelerating easy, not hard or high revs.


He asked today is my volvo diesel because there was black smoke coming in exhaust!


That proves at least there is real rich problem and its not just " its a volvo it is thirsty"

Dear volvomies,

Hope you're well and stay so. Pull the engine oil dipstick. Put the dipstick's tip close to your nose. Inhale. Do you smell petrol (benzin, gasoline, l'esence)? If so, that suggests an injector is stuck open, spraying excess fuel into a cylinder. When that extra fuel burns, it creates sooty (dark black, carbon-rich) smoke.

Rather than trying to figure out which injector has failed, replace 'em all with factory-reconditioned items. The replacements ideally should have four ports, versus the single port on factory-installed injectors.

Hope this helps.

Yours faithfully,

Spook

Dear Spook,

Hope you're well during these trying times.

I've experimented with 4 holes injectors years back.

First using modified 4 holes Bosch EV1 injectors brought at eBay. Better fuel economy, yes!

Then upgraded to Bosch EV6 injectors (4 holes, plastic body type, same electrical plug, more accurate pintle control) from Cadillac Northstar engine. Much better economy!

But with every upgrade I experienced gradual loss of the car's gliding motion ie wafting (borrowed from Rolls Royce's terminology)

So I installed back one hole OE injectors. And the whole world smiles again!


Amarin.

Oil didnt smell fuel


Now took fuel pressure: steady 35psi at idle, with fuel pressure regulator vacuum off 45psi


Stays pretty good 35psi when i rev engine

Now at least i got some drivebility problems.


Sometimes after hot start, idle is 1500rpm at one second then drops. Also car pulls sometimes 30mph without touching gas pedal. If i switch neutral on the fly idle is perfect.

And couple times it has jerked when i left at crossing roads.



I think maf is going to bad?

I already two months ago cleaned and tested injectors and they were fine. Mpg problem was then also.





Many cars with 1 hole injectors vs 4 hole injectors people speaks about better mpg and better throttle response, how is it in volvo if you but 4 hole injectors there?

I got fuel pressure meter and i try use it soon.


My workmate drived yesterday behind me. I was driving normal and accelerating easy, not hard or high revs.


He asked today is my volvo diesel because there was black smoke coming in exhaust!


That proves at least there is real rich problem and its not just " its a volvo it is thirsty"

And yes odometer works ive been checked by gps and google maps distances.

Thanks for big advices.


If i reas o2 sensor to multimeter it acts like this:

https://aijaa.com/9zAMmB
My video



So its swinging and changing yes, but lazy and upper volts insteas fast 0.1 to 0.9 swing

No leaks, header gaskers new, header and pipe gasket (two holes) new and that one at just near catalytic converter is tight




If i rev hot engine at idle 2000-3000rpm and stay there, o2 sensor voltage acts stil like that video

Have you done a compression test? Hot and cold?

Also, it might be worth pulling the engine coolant thermostat (with a coolant change, since the car is fairly new to you) to verify that it is operating and not stuck half-open.

Maybe i must do compression test but engine runs fine, wont burn oil or coolant etc.


I have new thermostat and it works i get very hot air inside if i want

A new thermostat is good. Any chance it was a lower temperature than the stock one? I think they were like 87-90C stock depending on turbo or not. Some people like to convince people to put super-cold thermostats in (like 70C instead of 87C).

I had an engine I thought was in good shape. No oil burning, minor leaking, no coolant loss, everything looked great. Cylinder 3 was leaking past the fire ring and into the oil return, and only had 60psi compression. Every used junkyard engine I checked had a bad #3 cylinder as well.

It's very worth checking.

Now i got compression test results. Hot engine and 8 rounds with start full throttle:


1cyl 200psi
2cyl 200psi
3cyl 202psi
4cyl 199psi


And 5 rounds with start was steady 170-172psi all four cylinders.


So engine is pretty healthy

You have to pull the pump relay before compression testing.

Yes i did that. Also took coil wire off and all plugs also.

Now i got compression test results. Hot engine and 8 rounds with start full throttle:


1cyl 200psi
2cyl 200psi
3cyl 202psi
4cyl 199psi


And 5 rounds with start was steady 170-172psi all four cylinders.


So engine is pretty healthy

I have stock winter stat ≈92c





That 3cyl example makes me worry, i have to take compression test soon.

I recommend you check the resistance of the ECT (engine coolant temperature sensor) from the terminals in the ECU connector. Remove the connector from the ECU, and use your ohm meter between terminal 13 and ground. There is a chart in the 700/900 FAQ which gives resistance to temperature.
--
john

ect resistance checked, old was faulty, new is working properly. It still seems that old fault wasnt effecting mpg.


(Old showed 290 and 136 ohms at 190-195F and new 189 and 190 ohms)

tHATS NOT TRUE,,THERE ARE LOADS OF PLACES TO LOOK FOR LEAKS.THERE ARE BANJO FITTINGS ON THE FUEL FILTER THAT CORRODE OVER TIME. YOU CAN FIND A MACHINIST THAT MAKES THEM ON EBAY,,PUT THE CAR ON LIFT & CHK FOR LLEAKS AKA SPRAYS OF CLEANED GREASE OFF THE UNDER SIDE,,ON A DARK NIGHT RUN THE ENGINE LEAVE THE LIGHTS OFF & HAVE A FRIEND SHINE A VERY BRIGHT LIGHT UNDER THE CAR TO SEE IF YOU CAN FIND A MIST SPRAYING OFF?THERE SHOULD BE SPRING CLIPS ON ALL THE VACUUM LINES FOR THE NEW HOSES TO KEEP IT TIGHT,,
THERE ARE 2 I BELIEVE TINY LITTLE ORINGS THAT ARE LIKE ABOUT 8MM THAT FIT ON THE THROTTLE SHAFT ON EACH SIDE ,TTO KEEP AIR MEASUREMENTS RIGHT

Fuel system pressure check with a gauge...Faulty check valve can be determined. Leaky injectors and sitting pressure.
ECU needs correct running pressure. Not much more you can check.