At hwy. speed engine stumbles when letting off accelerator

[dosbricks]

'98 S70 NA, 230k. When driving in top gear a single intermittent jerky stumble will often manifest with changes in throttle position. The car has lived in flat country all it's life, but we have moved several hundred miles north where it's hilly, and this has become a bothersome issue.

The TB is clean and the ignition is all fresh except the coil. It has a new Bosch fuel pump. Trans fluid is clean and at the correct level. I'm using mid-grade 87 octane at about 300 ft. elevation.

I would be grateful for any insight on this problem.

dos

[Clemens]

How old is your oxygen sensor? If you can afford it, get a new one if it is over 100.000 miles because this might be the problem. Hope someone smarter than me chimes in on this.

Also, have you tried 91 octane?
300 ft of elevation should not be noticeable at all.

[abscate]

How is the fuel pressure regulator looking , clean of fuel?

Our old friend fuel pump relay is a possible Culprit , too

[dosbricks]

Thank you for your replies. I swapped in a new fuel pressure relay before changing the fuel pump.
Unlike my wagon which has the pressure regulator on the fuel rail, this one is in the rear, so I will have to read up one how to check that.

The O2 sensor may be near 100k. The car set the CE light this evening for the first time, but before I could get it home to read it, it went off by itself.

[abscate]

You might still find the code pending, try reading it out

[mrbrian200]

Momentary spike in MAP vacuum when you release pedal: The 98 uses a map sensor, it isn't reacting/reading properly during this moment of what would be the highest vacuum reading seen during normal driving conditions. Or a bad check valve/loose connection/gasket somewhere on the intake side. Though I doubt it, a very slow responding O2s might be a contributing factor depending on how the ecu software handles things, but without some other underlying problem a bad O2s wouldn't do this.

There's nothing else I can think of that would cause this under this specific condition unless there's slop in the drivetrain and you're misinterpreting it as an engine/trans problem.

[dosbricks]

Momentary spike in MAP vacuum when you release pedal: The 98 uses a map sensor, it isn't reacting/reading properly during this moment of what would be the highest vacuum reading seen during normal driving conditions. Or a bad check valve/loose connection/gasket somewhere on the intake side. Though I doubt it, a very slow responding O2s might be a contributing factor depending on how the ecu software handles things, but without some other underlying problem a bad O2s wouldn't do this.

There's nothing else I can think of that would cause this under this specific condition unless there's slop in the drivetrain and you're misinterpreting it as an engine/trans problem.

mrbrian, I think you have identified a very likely source (vac leak or MAP), because this stumble never happens when accelerating or decelerating--only in the equilibrium point in between. The drive train has 230k on it, but never neglected, and this is now more pronounced than possible slop there--plus if that were the cause I believe it would show up in low speed driving also.

The manifold gasket should still be good from the last PCV repair. I've replaced some vacuum lines but not all the difficult to access ones like the connections to the charcoal canister, and this car is certainly of the age to have vacuum leaks. At the moment we are still in the moving (changing homes) process, so the easiest thing to try at this moment is a new MAP sensor. As I've said, the problem has worsened since we changed from sea level to 300 ft elevation.

Thanks!
dos

[dosbricks]

Hmm....no MAP sensor where it is pictured to be located, and fcp does not list this part on '98 S70 NA. So vacuum leak or fuel pressure regulator remain suspects. We are heading back to Houston in the morning (2-1/2 hrs) where I will have facilities to check it out. Hopefully this not a car killer issue

[mrbrian200]

-edit- My bad. It does use a MAF/Air Flow Sensor. I looked at this momentarily before commenting on it having a MAP. I must have poked in the wrong year or model by accident. In which case these are symptoms I would definitely associate with a contaminated or failing MAF. I would try cleaning it first. Then rule out vacuum leaks before spending $ on a new one. The oxygen sensor could still be in play, but as I stated that wouldn't by itself cause such severe symptoms.
When you get home, if you can read DTCs from the car it'll likely be for fuel trim/O2s or the MAF itself, but mind, these DTCs can also pop up with a vacuum leak.

Also, do a sweep test on your throttle position sensor. I found a video if you're not familiar with the process. Since most of us don't own a scope I prefer an mechanical voltmeter (with a needle) and move the throttle plate very slowly while watching the needle for glitches/dead spots. Don't use a digital meter or try to watch live data on a diagnostic tool for this particular test.


As don't have electronic throttle, this simplifies things considerably. Except I have a question for anyone else watching - what is this 'throttle body control rod' business about? What does it do?
https://www.fcpeuro.com/products/volvo- ... 70-3507883

[dosbricks]

Thanks, mrbrian. The MAF is genuine Bosch two years old so pretty confident it isn't involved. So will continue on the same repair track. I'll post back if resolved.