1998 S90 170k miles, origin owner.
CAN SOMEONE PLEASE TELL ME WHAT’S CAUSING CYL #3 & #4 TO MISFIRE?

My 1998 S90 had been acting up. The Cam Position Sensor and Secondary Air Pump were not working. Both parts were replaced and both OBD scanner codes went away.
Same day, the OBD scanner detects 3 codes, P0304 P0305 & P0300 .. Cylinders# 3 and 4 not firing.
I check all the coils and also move the suspicious coils to other cylinders to see if the problem goes to other cylinders. It did not. Can I assume the coils are Ok?. I get 20 Ohms across the two pins on all coils.
While the car is running, I disconnect the coils on the failing Cylinders and the engine does not change.
I did the same thing with the injectors, while the engine is running and with all Coils and connections in place, I remove the connector that actuates injector but, the car does not respond; doing the same thing to other Injectors, I get an RPM drop and the engines sounds like it want to stall. So I replaced those two injectors, #4 and #5.
I have checked for blown fuses, bad relays and bad connections all check OK.
Can it be the main computer ?
Thank You Very Much in advance..!

CRT, Anaheim CA.

 

I had a misfire at #3. The problem I had was the wires inside the connector was not making connection with the coils. Unplug the coil and look into the connector. You may see that the wires inside have backed out a little. Causing no real connection to the coil. I pushed on the wires on the back side of the plug and it fixed my problem.

 

Thank you RSPI, I‘ve changed injectors and coil plugs on #4 & #5, I am still getting the same error codes.
I will revisit that. Thank you for your reply and I will post results.

 

Might also be a good idea to do a compression test.

 

Compression test ?
Injectors were replaced.
Ignition coils were replaced.
New Spark plugs were installed.
ECM main computer was replaced.
Harness / wiring to Coils packs checked ok.
The two Ignitions Modules were swapped, checked ok.
#4 and #5 still misfiring, error# P0304 P0305.
After running the engine for a while, the spark plugs on these two cylinders look like brand new with its original shine while the others show normal signs of smoke and fire. While the engine runs, I lifted and separate the coil pack from those spark plugs; I do see and hear physical sparking in those cylinders.
I have not done a compression test yet but, with or without a good normal compression in those cylinders, wouldn’t I still be able to have a spark or full combustion in the missing cylinders?
Do you have to have compression in order for the cylinder to have a complete ignition?
ANY IDEAS ..?

 

If the valves are burnt then you will not have a complete ignition cycle.
But if the compression checks out ok next place to look is the power stage on the bottom of the intake on each side. They are known to go bad as well.

 

I already swapped the two Ignition Modules; I installed the front on the back and the back on the front, I did not get any change or misfiring pattern. I also did a Compression Test and it checked better than expected. 1cyl-210 .. 2cyl-195 .. 3cyl-210 .. 4cyl-210 ..
5cyl-230 .. 6cyl-210.
I will revisit the Ignition Modules and will post results …!

 

I would still take a close look at the wire harness, specifically where the wires go into the plug ends.

 

What causes valves to get burnt?

 

I am not 100% sure. But I can tell you when the gas prices went through the roof everyone went to regular. Then we saw a ton of cars with burnt valves. We went from seeing like 1 every 2 months to like 1 every 2 weeks.

 

Dang. I still run 93. I also put in some lucas every 10,000 miles. Hopefully that will help. I don't buy cheep gas either, from like the grocery store or places like that. usually only Exxon or BP when I travel. I want BP to be able to pay for their spill. LOL