This one has me stumped. I have a '98 S70 non turbo with 284,000. For about the last 9 months the check engine light comes at random times with codes P0300 (random cylinder misfire) and any cylinder from 1-5 (usually 2 and 5 codes P0302, P0305). Sometimes the check engine light will come on and then go off by itself while other times it will stay on until it's reset and then come on after another 100-300 miles. Very inconsistent. I've read many other threads on this issue and I've done nearly everything such as new oem Volvo spark plugs, wires, distributor cap and rotor, fuel pump (that died 12/12 so was replaced), and fuel filter all within the last 2 years. I've also replaced a few vacuum hoses and elbow connectors that were pretty worn out.

The one thing that's puzzling to me is that the car doesn't seem to actually be misfiring or running poorly. Once in a great while there will be a tiny buck but it's very inconsistent and doesn't correspond to the check engine light. I've owned the car now for 8 years and driven it over 150,000 miles and it runs and drives like it always has.

Any thoughts?

 

Check compression and cylinder leakage next, you've got it well covered otherwise

 

Hi Petmbro, welcome to the forum.

Compression Test - Volvo S70 GLT, 1998

 

Thanks for the reply's I never thought to check the compression. Back in 2008 the timing belt snapped after the tensioned failed so it had a full valve job, not sure if that did anything. I guess the next question is if it is a leaking cylinder is that something that can/has to be fixed?

 

I ran a compression test this morning and here are the results:

Cyl 1: 202psi
Cyl 2: 205psi
Cyl 3: 189psi
Cyl 4: 185psi
Cyl 5: 211psi

Now as I've never done this before I'm hoping someone can tell me if these numbers mean anything as they seem to be all over the place, especially 3,4, and 5.

In addition I noticed that there is this yellow/orange residue on the plugs and part of the wires that go in 3,4, and 5 (see pic below). I've never seen this on any of my cars before and have no idea what it is At the very least I'll be replacing the plugs.

 

That's above the block/head so it's from water getting on top of the motor. Don't worry about that. Just replace the plugs with the old school plugs. The 850's and S70's (pre-2000) do not like those plugs. I may be wrong about the plugs but mine are Volvo but look like the ones below. Some people use a certain Champion plug, I only use Volvo and gap them to 0.0028.



Those compression numbers are good. They should be within 30 psi of each other. If you used my instructions in my link it tells you about all that.

 

If you still have problems after the plugs are replaced (and like rspi said, pay attention to the gap; no more than .028") then could be a weak ignition coil and beyond that, maybe injectors. Assuming those are the original injectors, that's a lot of miles!