[XC70 2.5T 2006] high fuel consumption, fluctuating idle + ECM-400C

[elmozgo]

Hi All!

My XC70 has 185k miles and a problem of bad fuel economy - its 20-21 mpg when slowly crusing 55-60 mph (it is has LPG conversion, on LPG its even worse). Also, idle revs fluctuate - they get low, so that all lights (running and interior) dim and pulse. My subjective impression is that it should also have more performance, but can't measure that or compare to other XC70 with the same engine.

Its been like this since I bought the car, for about 30k miles - its a second car so never bothered too much about it.

Recently I started to get sporradic engine misfires - after turning off/on the engine everything was back to normal, no check engine light. I'm planning to take Volvo on a longer trip and started to look into the issue.

I scanned codes with VIDA... and:

- There was a code about detected misfire and exhaust VVT solenoid signal
- I once managed to "catch" the misfire with my laptop plugged in: it was misfiring on cylinder 1
- idle rpm fluctuated 620-710 rpm
- long term fuel trim lower part: 1.21
- mass air flow on idle: 11-12 kg/h

I launched the parts cannon and replaced all inexpensive and easily accesible things:

- new coil on cylidner 1
- new exhaust VVT solenoid
And also after Google's LLM hint, that there could be a vacuum leak:
- new boost control solenoid (the 3-hose T junction + electrical connector)
- hose that connects the plastic over engine pipe to the intercooler: it was covered in oily mist and rubber looked poor
- cleaned MAF with a dedicated spray cleaner

No noticable difference

I drove 20 miles, checked VIDA and a new code appeared:
ECM-400C Three-way catalytic converter efficiency, bank 1 - Signal too high

I cleared codes and engine adaptations and drove additional 5 miles, the same code came back.

I checked oxygen sensor values and on reving the engine while parked, front oxygen sensor shows values as high as "15" - it then switches to the open loop mode.

it looks like that:
https://drive.google.com/file/d/1sRw7t8 ... sp=sharing

I'm suspecting:
- vacuum leak
- exhaust leak
- bad front oxygen sensor
- bad MAF

I could try to replace the oxygen sensor and MAF myself, but would struggle to futher diagnose the leaks myself. I don't have a smoke generator, I tried to spray break cleaner/engine starter at various hoses and connections in the engine bay to see if that gets sucked in, but no effect.

Do you thing that the VIDA values for MAF and the oxygen sensor are so off nominal, that its worth buying parts and replacing them myself, before bringing the car to a shop for proper leak detection?

Thanks!

[scot850]

Have you tried cleaning the MAF and the MAF connector with MAF cleaner?

If you have Vida, if you can access live data, check the Duty cycle of the fuel pump. It should be 35 +/- 5%

Neil.

[jonesg]

I'd throw money at vida, not parts.

[scot850]

Owner says he checked Vida, so I assume he has Vida.

Neil.

[elmozgo]

I got a hint to record front and rear oxygen sensors readouts, on idle, with warmed up engine:
https://drive.google.com/file/d/1MsRDhv ... sp=sharing

Apparently this readout means that the catalytic converter is on its last legs and works only when its well warmed up - rear oxygen sensor starts to oscilate after some time on idle, when the cat cooles down. But would it impact performance and fuel consumption? Could it be clogged up?

I recorded fuel pump duty cycle: it stays ~42%, then when I lift off the trottle it jumps to 85%, drops to 25% and continue on 42%:
https://drive.google.com/file/d/1Ceq3Q6 ... sp=sharing

I also cleaned the MAF connctor using contact cleaner - i cleaned the actual MAF with MAF cleaner before.

I have a chance to drive 1000 km and check real fuel consumption with full to full refuelling on gas stations - will see if there is any difference after replacing parts.

[elmozgo]

After a 1300km (800mi) highway trip, 100-110 km/h (60-70 mph) speeds, no overtaking, on LPG:
- average fuel consumption was 14l/100km (17mpg)
- had to stop and restart the engine once, because it felt like one cylinder lost fire (vibrations) - had a CEL on, but it disappeard after the restart. Also no fault code has been saved for it.

The car doesn't accelerate well over 80km/h (50mph). Also at this speed and low rpms on the 5th gear, the onboard computer's fuel economy calculation show reasonable values in ranges of 7 to 8 l/100km... but when I kept the same speed but reved it up on 4th or 3rd gear, reported values tripple or quadruple.

Also two additional things that I've noticed:

- Whistling sound on high engine load (when I floor it)
- On P, when I push and imediately release the throttle I can hear a low frequency "pop" sound, coming back from the rear of the car.

Time to schedule a service appointment.

[vtl]

When my then stock XC70 was doing a lot of highways at 65+ MPH, the average MPG was around 21. Now it's a motorized grocery cart, it gets only 13ish MPG in summer and 11 in winter.

2.4 FWD V70 was getting 30 MPG on highway, easy.

High ground clearance 2.5T AWD is thirsty.

[elmozgo]

On the way to the workshop, I tried to keep the car in 3000-3500 rpm, in hope that this would make it register a new error code. It turned out, that it revealed a serious problem - when I stopped in front of the garage, the coolant reservour changed into a water fountain.

the mystery is solved: the engine has no compression (7 bars on the worst cylinder, 9 on the best), there are traces of oil in coolant and these are exhaust gases that push it out of the reservour.

the engine definitely has:
- a blown head gasket or cracked cylinder wall (camera doesn't show it, the head has to come off for proper inspection)
- burned out valve sockets in the head

Well.. now that I think of it, it was an obvious thing from the begining and I could definitely do the head gasket test myself, but we naturally try everything to avoid a messenger with bad news

I invested in this car during my ownership term (suspension, breaks, transmission) and I don't think I could find another car in the engine replacement budget. Its time to find another used B5254T2 - there are still some shops around me, willing to do the job. Unfortunately volvo specialists quote the FU price - they are fully booked with newer nicer cars, with better margins

[scot850]

Hopefully you can find a good used engine out there. I found over here the cost of a warranted good engine was close to the cost of rebuilding and repairing the damaged threads of a spark plug in the cylinder head. I had found an engine that the Specialist Volvo breaker had that only had 77kkm on it which would have been great. In the end the cost of repair was not worth the effort as the owner decided to sell the car as is.

Neil.