850 NA engine and spark plugs enormous confusion Topic is solved

SomeGuy537 · Post by **SomeGuy537** » 14 Dec 2020, 03:53

Hello, i am seriously lost when it comes to my 850's engine. According to my VIN it's a B5204FS and this makes sense since it's a 94' model year italian import GLT. It also has Bosch LH-Jetronic 3.2 (separate control units for fuel and spark) aswell as the funky VVIS inlet manifold that i think only 20 valve "FS"-series engines have. However the timing belt cover says B5204S. The VIN engine code "45" that i have means B5204FS according to some sources and B5204S according to others. I suspect it's used for both. Anyways, a couple of days ago i replaced the spark plugs. The plugs that were in it had three prongs. The engine ran well with them but they were very old and replacement was due. I replaced them with a set of Volvo OEM single prong plugs made specifically for the B5204FS and they had the part number 271603-3. The engine runs like shit now. Vibrates slightly at idle and RPM fluctuates. Vibrates heavily under hard acceleration. I visited my local Volvo dealer today and asked for OEM plugs for my engine. They told me those were part number 271603-3 (same as i already have). Asked if they were sure they are for my engine and they said yes and insited they were for the B5204FS. On the packaging of the spark plugs it says 92- and newer B5204. Now, how do i know what plugs to get? I asked if the three prong pluugs for the other NA-engines were all the same but they didn't know and i don't dare take a chance because they are expensive. I couldn't find a part number on my old plugs sadly. I have heard of other people with the same problem but can't find any helpful info. I'm very curious as to why this occurs!

Kind regards and sorry for long text!

Post by **manovlov** » 14 Dec 2020, 04:03

If I understand well you should use these ones :
https://www.ebay.com/itm/Volvo-850-S60- ... xyzfNRxFz6

: Capture d’écran 2020-12-14 à 12.02.00.png (523.09 KiB) Viewed 1694 times

Post by **abscate** » 14 Dec 2020, 04:30

Those plugs will work fine. Make sure you got the wires back in the right place , that sounds more likely the problem

hipcheck · Post by **hipcheck** » 14 Dec 2020, 05:17

It sounds like you are describing a misfire.

There are only 3 things that come after a spark plug change to cause a misfire:

Wrong wire order
Damaged wire from removal
Massively mis-gapped spark plug.

Check each wire for proper order, this is most likely the case.

Confirm wires are in proper order and let us know.

SomeGuy537 · Post by **SomeGuy537** » 14 Dec 2020, 10:45

The wires are tied together on these engines and besides that i don't think it's possible to mix them up because they don't appear to have the same length. Today i visited my Volvo dealer again and bought the plugs manovlov suggested which if i interpret the info on the packaging correctly are made for all 850 NA engines after 92' except the B5204FS which i strongly believe i have. These spark plugs (serial number 8642660) are now installed and the car runs like a swiss watch, kinda what i expected. just like with the old 3-prong plugs which were probably the same. Nothing else was done to the engine so the driveability issues i experienced earlier must have come from the single prong plugs (271603-3) that supposedly are made for my engine. The question remains, how can this happen? Is my engine not a B5204FS? Are all B5204FS not the same? I read that other people's engines of the same model run fine with the single prong plugs. Any ideas?

hipcheck · Post by **hipcheck** » 14 Dec 2020, 13:18

Check the engine SN# of the back of the block.

It could be a swapped engine

SomeGuy537 · Post by **SomeGuy537** » 14 Dec 2020, 13:39

hipcheck wrote: ↑14 Dec 2020, 13:18 Check the engine SN# of the back of the block.

It could be a swapped engine

I will definitely check that out! Where more precisely is this number? And when i get it how do i decode it? How do i know which engine the number corresponds to? It would be strange if the engine has been swapped since it only had done 113k miles / 184k km when i bought it but i guess heavy abuse or something could have killed it prematurely.

hipcheck · Post by **hipcheck** » 14 Dec 2020, 15:18

Volvo puts the engine serial number in two places: A sticker on the timing cover and a stamping on a engine block surface. It is very common for the sticker on the timing cover to be missing or damaged but the stamping on the engine block is always there.

Post by **manovlov** » 15 Dec 2020, 04:16

Anyway, the issue seems to be solved, which is great. Congrats !

Manov

SomeGuy537 · Post by **SomeGuy537** » 15 Dec 2020, 10:23

manovlov wrote: ↑15 Dec 2020, 04:16 Anyway, the issue seems to be solved, which is great. Congrats !

Manov

Yeah, it's pretty much solved now really, thanks a lot man! I'm just really curious as to why it happened in the first place

I'm guessing the 8642660 spark plugs are right after all and that i have a B5204S like my timing belt cover says and that my Volvo dealer just was wrong. I guess i'll dry to dig deeper and try to find out the differences between "S"- and "FS"-engines in my other thread about that topic.