Hello all, haven't posted in years (was an active member since 1999) and yes, I still own both 1998 S70s.

Maybe someone can help or give some suggestions, my 1998 S70 GLT started to give a rough idle during a warm start (engine turned off for less than 30 minutes and started again). This happens 60% of the them.

My mechanic suggested I try putting fuel injection cleaner before doing more investigative work, which I did.

Now the rough idle would also happen when the car is idling. The CEL then sometimes comes on and then the rough idling disappears. The CEL later disappears that same day or the next day. Twice my mechanic was able to get the error codes when the CEL was lit and it read multiple misfires (I saw his scan tool readout but didn't write down the codes) but then the CEL later disappears.


The spark plugs are new, the cap and rotor were replaced less than 20,000 miles/4-yrs ago, and the MAF was just replaced with a new one (I replaced it myself with a BOSCH OEM part) hoping to eliminate one of the probable causes.

I have an appointment to bring it in next week and my mechanic is thinking of swapping the ECU to see of it is the ECU.

Any thoughts or suggestions?

Thanks in advance!
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Brian M. - 1998 S70GLT, 1998 S70 GTA

Hmmm, having trouble posting.

Anyway, I would go to Auto Zone or download the Torque Pro app (5 bucks) if you have an Android phone and a Bluetooth dongle and see if there is a trouble code. I'm guessing there is, it may be something simple.

OK, lets try this again with some helpful words!

I would go to Auto Zone and see if they can pull the code, although the light goes off, the code is in there unless you disconnect the battery. It may be something simple.

Some of your cylinders are misfiring, but not all at the same time. A real multiple misfire will show a blinking CEL, not a steady light.

So you go about 5K miles/year. I also suspect that your are not a speed demon. So the plugs could be fouling. You could warm up the engine and then ease the rpms up to 5000 in second gear, followed by an immediate lift off the throttle. That will clean the plugs, but could also get you a speeding ticket!!

Check the crankcase breather system for plugs, leaks.

I doubt the ECU swap will show anything nor improve the situation. A descaling of carbon from the piston chambers might work. Seafoam comes to mind for that.

What kind of spark plugs are in there now? Some of those cheap plugs will stop working after 10K miles. The multi prong iridium have been known to loose a prong.
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Keeping it running is better than buying new

Thanks Klaus. I decided to postpone the repair since the CEL was off again. The mechanic said that they would have to check different things (plugs, fuel injectors, cap & rotor, etc). He suggested to just drive it and the problem could be more obvious when it arises. So far the car runs fine.

BTW, the plugs were replaced in March 2017 and the invoice says the plug part no. 8692071. I might try your suggestion of easing the rpms to 5k and then release.

We'll see in the next few days or weeks. Will keep you posted.

Thanks again.

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Brian M. - 1998 S70GLT, 1998 S70 GTA

Try running Chevron premium gas. Eight years ago I had a similar problem with a different make vehicle. It ran fine but would throw "multiple misfire" codes every few months. I was running Costco gas when the codes lit the check engine light. Since running the Chevron premium, which has techron, no check engine lights in 8 years. That was my shops recommendation and it worked. Also, you need to take freeway runs and don't dog it around town. Enjoy the sound of 5 cylinders above 3k. Merry Christmas!

Thank you for the suggestion. I started going to Chevron. So far no CEL (so far!)
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Brian M. - 1998 S70GLT, 1998 S70 GTA