850 Miss at idle

A dad recently passed away. He had a '97 Volvo non-turbo 850 my mom would like to sell. It was running fine but has now developed a miss at idle. The miss only occurs at idle in Park. It seems to improve while idling in D.

Initially, there was no CEL and no codes.

Since it seemed to be worse when cold I replaced the CLT sensor. No change.

I tried a spare MAF. No change.

I replaced the aftermarket distributor and rotor with JY Bosch stuff I had on hand. I replaced the copper plugs with new NGK platinum plugs. The miss didn't go away, but fault codes appeared for a misfire on cylinder 3 and sometimes cylinder 5. The only consistent misfire codes appear for cylinder 3.

I pulled the vacuum hose from the FPR, and there was fuel in it to my surprise. I replaced that with one from an engine I parted out a while ago. Again, no change.

The miss is only detectable at idle. If I drive the car hard it's not present and the motor appears to make good power. The motor never dies.

A Seafoam really smoothed out the idle but that didn't last.

What else is there to check besides maybe compression for cylinder 3?

One other possibility I've considered is that the gas is simply bad. My dad retired in 2019. This car was the car he used to drive to work and back. After he retired he only drove it 7 miles into town and back for cigarettes I imagine. This car wasn't used for long trips or even grocery shopping. With the pandemic, I imagine it was driven even less than normal. Could bad gas cause a miss that only occurred at idle?

An air leak could be the problem.

ZVOLV said:

An air leak could be the problem.

Click to expand...

I've wondered about this as well. The car has had PCV service in the last few years, but all the other little vacuum lines are probably original. I did order a cheap smoke machine to investigate the possibility.

I would try to look at fuel trims with a scan tool, but on a 97 850 you would probably need VIDA. Misfire counter would be great too.


You can try to spray around with brake cleaner and listen for idle changes.

Maybe disconnect an injector one at a time to see which cylinder is misfiring. (Cylinder drop test).

Misfires at idle that go away with higher rpms sound like an air leak.

You could try your luck with an IR gun on the exhaust runners and see if one is cooler

The smoke test didn't reveal leaking hoses. Most of the smoke came out from under the TPS and throttle return spring of the TB. I guess I'm not sure if this is normal or not?

Check compression

yeah, start with basics. Compression, spark plug, spark plug wire. Work your way up the ladder from there

I know these cars burn valves up.

What's the result of the cylinder drop test?

It passed a cylinder drop test.

Finally got the tank down to 6 gallons of gas. After topping it off with fresh gas the miss is gone so far. Could it be that simple?

ovlov760 said:

Finally got the tank down to 6 gallons of gas. After topping it off with fresh gas the miss is gone so far. Could it be that simple?

Click to expand...

No idea...

I spoke too soon. The miss only occurs when the car is warmed up now vs. all the time previously. Until then it idles fine.

Well, now I think the miss is solved. I broke down and spent $75 dollars on a new Bosch cap and rotor.

Idles smooth as glass now. No CEL. It even sounds quieter overall.

My *known good* collection of caps and rotors failed me.