Volvo 850 engine stuttering at low RPM, not sure why?

Krisemann · Post by **Krisemann** » 26 May 2020, 11:15

Instead of necroing my old post from 2019 (viewtopic.php?t=92418) I decided to make a new one, since I've done quite a few things to my Volvo since then.

My problem is that my 850 stutters during acceleration, sometimes idle is bad but not always (see video at about 30 secs): It's much better with the O2 sensor plugged out, but can sometimes stutter a little bit.

What I've tried so far:
- Replace O2 sensor (twice, put a lot of money into a Bosch one)
- Replace ECT and thermostat
- Replace fuel filter
- Clean out gunk from some white tubes with black tips (not entirely sure what these are, but they were connected to the vacuum tree, so I believe they might have something with the vacuum to do).
- Clean off throttle body
- Checked intake manifold and its gasket for leaks (Fairly good shape)
- Changed oil filter, oil and air filter
- Checked compression (Excellent)
- Checked timing (Excellent)

In my ownership I've also replaced the PCV and pretty much the entire ignition system (coil, rotor, distributor, spark wires, spark plugs) before this started happening.

My car is an 854 4-speed automatic, 1995 (new late 94) model with a 10v 144hp 2.5L non-turbo engine. It does not have any SAS, EGR, A/C or MAF, uses a titania O2 sensor (it only has one O2 sensor in front), has an IAT and is completely original. I should mention that it's an uncommon variant of the 850, whereas the PCV housing only has one hole on top, as well as some other minor things.

Is it possible that my problems are related to a vacuum leak? I can't seem to make any sense out of it and I've spent so many hours trying to figure this out. Norwegian MOT is coming up soon, and if I can't figure this out by then I'll probably end up without a car the next few months...

I'm not a mechanic, and pretty much self-educated on this, so I apologize for stupid answers in advance.

Post by **misha** » 26 May 2020, 11:26

10v cars doesn't exist is U.S.
Did you checked for codes?
You have diagnostic ports under the hood.

I just re-read your old post about this....
Cur runs rich....
10v car can run rich if MAP,IAT,O2,ECT sensors are bad...furthermore....FPR could also leak fuel into it's vacuum line...check it.

Krisemann · Post by **Krisemann** » 26 May 2020, 12:17

misha wrote: ↑26 May 2020, 11:26 10v cars doesn't exist is U.S.

It's a European car, sold new in Norway.

misha wrote: ↑26 May 2020, 11:26 Did you checked for codes?

Knew I forgot to add something to my post :p I get a code A2-521 when I disconnect the O2 sensor, which is natural. I've also gotten A2-512 sometimes, which seems to be related to the MAF, according to another post? Not sure about that one.

misha wrote: ↑26 May 2020, 11:26 10v car can run rich if MAP,IAT,O2,ECT sensors are bad...furthermore....FPR could also leak fuel into it's vacuum line...check it.

I'll look into this. O2 and ECT sensors have been checked and replaced, I could try buying a new IAT sensor. I don't know if my car has an MAP sensor, I'll check that out. I'll also have to figure out how to test the fuel pump.

Post by **misha** » 26 May 2020, 12:30

IAT sensor have exactly the same spec as ECT.
Multimeter is all you need.
A2 5-1-2 :
https://www.scandcar.com/volvo-850-error-codes/

You have a vacuum leak somewhere or o2 sensor connector is bad.
FPR-fuel pressure regulator should be checked if it leaks fuel into vacuum line.

Krisemann · Post by **Krisemann** » 26 May 2020, 12:55

misha wrote: ↑26 May 2020, 12:30 You have a vacuum leak somewhere or o2 sensor connector is bad.
FPR-fuel pressure regulator should be checked if it leaks fuel into vacuum line.

Doubt it's the O2 sensor connector, but will check it out anyway (if the connector was bad I'd assume it'd throw me a 521 while plugged in). Will check out the rest, I'll probably end up replacing the IAT as well. Still haven't gotten myself a multimeter since I moved so I'll do that tomorrow

Post by **misha** » 26 May 2020, 13:11

Do you have exhaust leak between exhaust manifold and cat?
If so...o2 sensor reading will be corrupted.
I can't remember if 10v cars have flexi pipe between exhaust manifold and cat...but if it have and if it leaks...o2 sensor will not work properly.It will send false reading to ecu.

Intake leaks will also affect o2 sensor reading.

This is MAP for 10v car:
https://www.binckhorst.com/nl/onderdeel ... 282/589255

It's vacuum line or elbow might be bad...if it is ok.
10 valve cars rely on data from the the air temperature sensor, the MAP sensor (behind the radiator shroud), and the front oxygen sensor.

Krisemann · Post by **Krisemann** » 26 May 2020, 15:51

Alright, so I've had a look at the vacuum lines again, and there seems to be a tiny bit of residue on the vacuum line from the FPR.

- Vacuum lines seem fine, no apparent leaks.

- Residue on the vacuum tree.

Is this enough to make the car stutter this much? Should I replace the FPR? (And should I replace MAP and IAT while I'm at it?)

I dried off some residue and tried smelling it - it does indeed smell like petrol.

Post by **misha** » 26 May 2020, 16:18

That oily residue is from oil vapors.It's normal.

What you can try about FPR....remove it's vacuum line from vacuum tree,cap the vacuum tree to prevent vacuum leak and start the car...see if idle improves.
If it is improved....than you found the culprit.

Sometimes FPR can start to leak in vacuum line after a few minutes of engine idling or driving.

1st check IAT values depending of ambient temperature(hairdryer with cold air option and standard heat would be helpful).

2nd check MAP sensor's vacuum line....there should be a line just like for FPR or a little elbow.
Map can be easily be checked with a multimeter while applying vacuum to it's vacuum line.

You shouldn't smell fuel in intake,unless evap purge valve isn't stuck open or fpr is leaking.

Post by **misha** » 26 May 2020, 16:24

Is that black silicone...holding a vacuum tree?
MAP is highly sensitive to vacuum leaks and it measures the Manifold Absolute Pressure.

That silicone is the source of your problems....what about the bolt hole?
That's where the main vacuum leak could be...messing with MAP and O2.

Where that white vacuum line goes to?
Fpr is at fuel rail....that line should go at the opposite side!

Krisemann · Post by **Krisemann** » 26 May 2020, 17:29

misha wrote: ↑26 May 2020, 16:18 Sometimes FPR can start to leak in vacuum line after a few minutes of engine idling or driving.

This is very common. From a cold start the car runs for a few minutes, then once the engine heats a little bit the stuttering begins. Usually happens at around 2000-3000rpm (so when I start from a complete stop or when I drive at a steady speed on the motorway).

misha wrote: ↑26 May 2020, 16:18 What you can try about FPR....remove it's vacuum line from vacuum tree,cap the vacuum tree to prevent vacuum leak and start the car...see if idle improves.
If it is improved....than you found the culprit.

I'll try this first thing tomorrow, thank you

misha wrote: ↑26 May 2020, 16:18 1st check IAT values depending of ambient temperature(hairdryer with cold air option and standard heat would be helpful).

Values as in with a multimeter, or just how the engine responds? I don't have a hairdryer but maybe I can use a lighter if I'm careful. I can't really check any OBD2 readings either as my car is pre-96.