S70 2.5t misfire

radek93 · Post by **radek93** » 31 Jan 2020, 15:41

I have S70 2.5t 98r B5254t and I have no idea. I replaced:
Spark plug
Ingition cable
Rotor
Cap
Ingition coil
Crankshaft sensor
Camshaft sensor
TPS
O2 sensor
Coolant sensor
Compresion is ok. No air leak i thing. No fault code.
When I drive, car have 1s misfire on 20 kilometers. Sometimes more often, sometimes less often. Sometime when engine working on idle, rotation degrese to 500 a grow up back to 900 in one sec.
I have idea that a ecu is broken.

Post by **misha** » 31 Jan 2020, 15:48

Parts that had been replaced....are they OE,OEM or aftermarket?
Which spark plugs are used?

radek93 · Post by **radek93** » 31 Jan 2020, 15:59

I used oem cable, tps. Other parts aftermarket. I used three brand of spark plug. First on oem. I forgot to add, car have LPG. On LPG or petrol is that same.

Post by **misha** » 31 Jan 2020, 17:17

Rotor & cap,o2 sensor,crankshaft,camshaft sensors...coil MUST be OEM Bosch.
ECT especially MUST be OE Volvo or at least Vernet.

Since almost all parts are aftermarket it's hard to diagnose the problem.

LPG could also make problems.

Spark plugs should be OE Volvo for turbo engines or cheap copper ones (NGK or Bosch)gapped properly because gap is critical for turbo engines.

Post by **RickHaleParker** » 31 Jan 2020, 21:31

radek93 wrote: 31 Jan 2020, 15:41 When I drive, car have 1s misfire on 20 kilometers. Sometimes more often, sometimes less often. Sometime when engine working on idle, rotation degrese to 500 a grow up back to 900 in one sec.

Sounds more like something dragging the engine down but it is intermittent, like an Alternator bearing starting to go or a A/C compressor clutch going out and binding. Check everything on the Timing belt and the Auxiliary belt.

Post by **wizechatmgr** » 31 Jan 2020, 21:34

Put the old plugs back in. I know someone recently that got a plug that went bad after a week or two. Drove the guy nuts...

radek93 · Post by **radek93** » 31 Jan 2020, 23:10

wizechatmgr wrote: 31 Jan 2020, 21:34 Put the old plugs back in. I know someone recently that got a plug that went bad after a week or two. Drove the guy nuts...

I have this problem since four years. Oem volvo plugs. Ngk plugs. Brisk plugs. All that same problem.

Post by **misha** » 01 Feb 2020, 02:13

Then...LPG system might be the cause of your problems.

radek93 · Post by **radek93** » 01 Feb 2020, 07:42

misha wrote: 01 Feb 2020, 02:13 Then...LPG system might be the cause of your problems.

It was like this before lpg assembly

michael8888 · Post by **michael8888** » 01 Feb 2020, 09:18

I found after going over a year with the same issue and replaced everything that the culprit was the thermostat sensor that's at the thermostat housing. A $13 part plus a new thermostat while I was there. ECU reads the temp wrong from the sensor and adds more fuel to the mix to cool the temp.
Also, check that all electrical connections are good and clean. use some dielectric grease. Be sure the wires from the cam sensor are not near the ignition wires. that may cause some em feedback.