My prior thread about a miss at idle resulted in no faults wre found.

There's a recent unrelated thread as well concerning mileage and premium to which I responded that in my '95 Passat (also recommended to run on premium), I saw virtually no difference in mileage. But that's a VW, and a different engine management system, maybe it doesn't matter on it, but does on the 850.

A post in the mileage thread stated that they had seen a 2 MPG difference in milage in their Volvo dependant on fuel type. A quick calculation shows that if regular gas resulted in 27 MPG and premium at 29 MPG and an 18.5 gallon tank, and assuming regular at $3.25/gallon and premium at $3.45/gallon (I know that price is 2 weeks old) that regular costs $0.120 per mile and premium results in $0.119 per mile...an almost negligable difference, if I get 2 more MPG.

But switching back to the rough idle thread, quite frankly I forgot that Volvo recommends premium and maybe the regular is causing some of my miss at idle, since nothing could be found wrong. So for the next 2 tanks, I'm going to run premium, check my mileage and also see if that rectifies my rough idle problem. I'll keep everyone posted on the results!

Ron

It's interesting how the difference in cost between the different octanes has stayed about ten cents all the while the price per gallon has double. You're still paying an extra dollar for 10 gallons of 89 vs 87 octane but at $2/gal it was added to $20 now it's added to $40. I try to justify this by thinking how long it would take to walk 24 miles and how much I would pay someone to carry me that distance. At $4 it still doesn't seem that bad.

You might also want to try 2 more things, because of your differences in compression from cyl to cyl.
Get a bottle of Auto Rx and very slowly feed it in to the intake manifold to clean the carbon deposits in your cylinders. This will help with any possible detonation problems. Follow the directions on the bottle.
Then, drive the car for about 1/2 hour in 2nd gear, trying to keep the rpm over 3,000rpm. This speed will allow the valves to rotate and help clean up the valve seats.
Verify the compression by testing with the engine cold and hot.

Klaus
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The 164 has a new home, all I am left with are a 95 854T and a 98 V70R :)

Oh, and BTW, I generally do not lug the engine, often redline it, and once every week or two run at higher RPMS for a while just for this reason. Still the AutoRx stuff sounds interesting.

I did wonder about that one high cylinder pressure, I checked it twice and wondered how it, in relation to the other cyls, might cause this roughness.

I assume that this stuff can be found at my local FLAPS? HOw do I slowly add it through teh intake? Maybe through a hose connected to the vacuum tree?

Thanks Klaus.

Ron

The NA engine is supposed to be 185-213psi according to the Haynes manual, so it isn't carbon buildup.
Correct about the Rx stuff, a hose connected to the vacuum tree. Just let it sip, there is enough vacuum to drain the bottle and cause a cylinder to sieze!

Klaus
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The 164 has a new home, all I am left with are a 95 854T and a 98 V70R :)

The autoRX stuff's website says to use it in the oil.

My 1st 1/2 tank of premium made no difference in idle, it's too soon to know about milage, though it was never bad at about 24 in mixed city/highway, and nearly 30 on the road.

I'm hoping for stuck rings at this point ( based on the compression), because at 95,000 miles they should not be worn. We got the car with 65,000 on it, and *think* it had decent care and use, it's been well maintained since we've had it, but this is a new-ish problem. Since I had itr on hand and it's worked well before I added a quart of Marvel Mystery oil...old school stuff, but it works.

Could a poor injector spray problem also cause this? How can you check teh pattern? A bottle of Techron did not clear the injectors if that's the problem. And again, there are no codes.