1997 V70 loss of power; no acceleration possible.

[giovannibianco]

1997 Volvo V70 GLT B5254S motor 410,000 km.

The story:

In July 2019 5000+ km round trip Geneva-Finland-Sweden-Geneva with no problems. September 2019 Geneva-Carcassonne-Geneva also no problems. Highway driving 130-160 Km/h with approx 9l/100km (30+ mpg).

My wife complained that the car seemed to be losing power on motorway to/from work. Eventually when I had a test drive of the car it was so weak I had to turn around on the first slight uphill! Now the car will barely achieve 30 km/h on the flat. When driving with gas pedal in roughly the idle position it's "OK" but any attempt at acceleration the power drops off. Seems to lose power in any off-idle situation.

At rest (i.e. in the driveway... runs rough at idle and will NOT rev past 4,000. Wavers at 4,000 rpm and then drops to idle. Pushing the gas pedal to the floor results in drop in revs to approx 1,000 and rough running.

So we have the triumvirate of Air/Spark/Fuel to consider:

Air:

* Changed the throttle position sensor (TPS).
* Changed the camshaft sensor with known good one... no change.
* Unplugged the MAF to see if the engine behaviour changed. Indeed, the idle got a bit more unsteady but the car drives as before. Stalls when re-connecting MAF. As expected?
* Checked the air-cleaner to throttle plate path for leaks or loose connections. None found.
* Verified that all vacuum lines are attached. Tested vacuum by unplugging blanking cap on vacuum tree at idle. Engine changed speed etc as expected. Vacuum at idle approx 15 in-Hg.
* Checked the Vapour Canister Purge Valve. This is a bit of a mystery. Measuring vacuum on the charcoal canister side I see a constant 15 in-Hg. 9.x volts to the connector. Pulling the vacuum line on the intake side has the predicted effect... idle stumble and unsteady due to vacuum leak.

Spark:

* Changed: cap, rotor and plugs for new.
* Old plugs looked OK, if gapped a bit optimistically (not by me).

Fuel:

* Changed the fuel filter for new.
* Fuel pressure tested at rail. The fuel pressure is steady at 43 lbs at idle and also through rev range (up to 4,000 max as above).
* Checked the fuel pressure regulator (FPR)... vacuum line looks terrible. No vacuum observed at the input to the FPR at the rear of the car. Need to check the other end of the line at the charcoal canister. See above.

So I am at a bit if a loss at to what the problem could be... MAF still?
I attach a plot of the MAF sensor output as a function of the throttle position sensor output.

Screenshot_2019-12-15_22-37-32.png (4.91 KiB) Viewed 1844 times

[Empigt]

Partially plugged catalytic converter?
Had this issue with a watercooled vw, it would idle but bog down giving it gas. Just enough air flow to idle, but any more than that was bogging it down.

[giovannibianco]

Thanks for the reply. I made a check on the cat/exhaust, first by giving it a good few whacks to see if anything loose inside.
Seems solid. Then a test with the engine at 2500 rpm... vacuum drops from idle but stayed steady. Dropping back to idle, vacuum snaps back to before. Seems like a blocked exhaust is unlikely.

[RickHaleParker]

Seems like a blocked exhaust is unlikely.

A conclusive test is to remove the front O2 sensor and see if the power comes back, doing so relieves any back pressure. If the power comes back, you do have a clogged exhaust.

Could be Muffler, Resonator or Catalytic converter. It could be a potato stuffed up the tail pipe if you pissed off somebody lately.

How old are the Spark plugs?
Does it pass the PCV glove test?
Compression test results?

[giovannibianco]

Seems like a blocked exhaust is unlikely.

A conclusive test is to remove the front O2 sensor and see if the power comes back, doing so relieves any back pressure. If the power comes back, you do have a clogged exhaust.

Could be Muffler, Resonator or Catalytic converter. It could be a potato stuffed up the tail pipe if you pissed off somebody lately.

How old are the Spark plugs?
Does it pass the PCV glove test?
Compression test results?

Hi Rick,

Thanks for the answer... Yes, pulling the front O2 sensor is next on my list... Well, it was yesterday but with the "Blue Beast" in such poor shape I cannot drive her to a lift. Jacking and supporting the car gives just enough room on the gravel to get my 22mm wrench on the sensor and do nothing but risk rounding off the nut and putting my back out. So... I wait for a O2 sensor socket to arrive and in the meanwhile get her higher into the air to protect my back. I'm debating whether to just cut the wires and use a 22mm impact socket. I do have a fully wired new sensor sitting on the shelf. Hmm.

* Spark plugs are good... as mentioned in the original post, I changed the plugs, cap, rotor for new. What I pulled were not bad though.

* PCV glove test is fine... glove slowly collapses into the oil filler hole. Could be better, but nothing like I've seen in the past with glove attempting escape velocity...

* Compression test... warm engine, dry cylinders (1 -> 5 (PSI)) >185 : 190 : >190 : >190 : >195
So all good.

Cheers, John.

[Klausc]

I would place the MAF in serious consideration. A disconnected MAF is supposed to idle, and not be able to get around the block.

If this were a turbo, I would have said that one of the intercooler hoses was loose or broken. For non-turbo, check the air filter box to throttle body tube very carefully, looking for any cracks in the tubing.

[giovannibianco]

Hello Everyone,

Thanks for the replies... I've now made some effort to catch up with this problem...

So I have received my O2 sensor socket... Allowing me to check out the O2 sensors and exhaust/cat.
I removed the front, before catalytic convertor, O2 sensor. The tip is VERY white. Then did the following tests:

1. Front O2 sensor removed... she now revs quite freely past 4,000 rpm and up to 5,000+ rpm.

2. Front O2 sensor replaced by bung. Goes back to the previous behaviour of not wanting to rev...

3. Front O2 sensor installed. Removed rear sensor. Still does not want to rev.

4. Front O2 sensor installed. Rear sensor replaced by bung. Still does not want to rev.

Given these tests, it looks like the catalytic convertor may be blocked. As removing the rear O2 sensor should
release back pressure that may be built up in the mid or rear exhaust sections. This therefore can tell me whether it's the catalytic convertor or the mid-section muffler that is causing the problem.


I will make a test tomorrow where I will remove the front O2 sensor and take her for a quick drive locally to see if the power is back. Then repeat with the rear O2 sensor removed. This should tell me what I need to know.

Any comments/observations?

[RickHaleParker]

1. Front O2 sensor removed... she now revs quite freely past 4,000 rpm and up to 5,000+ rpm.

2. Front O2 sensor replaced by bung. Goes back to the previous behaviour of not wanting to rev...

3. Front O2 sensor installed. Removed rear sensor. Still does not want to rev.

4. Front O2 sensor installed. Rear sensor replaced by bung. Still does not want to rev.

#1 Concludes clogged exhaust. Clog is after the front O2 sensor.
#3 Concludes clog is before the rear O2 sensor.
#1 + #3 = Clog is between the front and rear O2 sensor.

If you want extra assurance it is the Catalytic converter before you reach for your Portefeuille. Install both O2 sensors and open the exhaust between the Catalytic converter and the Resonator, give it a test drive.

[misha]

Disconnect Maf sensor connector and go for a test drive.
If it runs good...it's bad Maf.
Clogged fuel filter or weak fuel pump could also be the cause.
Did you checked fuel pressure at fuel rail?
Check engine light on?

[giovannibianco]

Hello,

A update on my 1997 V70 problems... I at last have taken her for a quick drive
locally to make a check. I removed the front O2 sensor to relieve the
back pressure, resulting in a rather "sporty" sound! Anyway, now she can
get to 60 km/h whereas before getting above 20 km/h was a struggle. The power
is certainly not fully restored by any measure. I am assuming that with the
from O2 sensor removed (disconected and sitting on my work bench) the engine
is in some kind of "limp" mode? Once everything is cooled down I will put a bung
into the O2 sensor hole and re-test.

I have in my possession a new catalytic convertor and exhaust middle section. May as well
replace if it's got to be taken down anyway. Also new front and rear O2 sensors.

Fuel pressure at the rail is a solid 43 psi over the rev range.
Disconnecting the MAF gave no appreciable change in behaviour.
Engine check light is on... if it was not I would be worried I'm in the wrong car ?

Cheers, John.