99 S70 hesitates at low RPM - why?

[krenekj]

I'm now able to easily reproduce the problem. Giving it full throttle (almost) always makes it sputter until 2500 RPMs are reached, then it takes off just fine. The gear, time of day, hot/cold engine, or inclination do not make a difference. There are still not codes thrown that I can read.

And it's not coughing or varying the RPMs. It just slowly sputters up through 2500 RPMs. It's never stalled. Idle is fine and so is anything above 2500.

I've checked all the vacuum lines and intake. I'm starting to suspect something in the ignition system, perhaps a plug or coil pack.

Any thoughts are much appreciated.

Thanks.

[gswede]

Had my tcv replaced (850R 97`) some time ago. Noticed that something wasnt right after a few miles of driving. Hesitation, then full power. Very similar to what you describe here. Idled fine although the exchaust note had changed, richer smell on exchaust too. Like ignition timing had been changed.

Opened hood and found that the hose from TCV to intake pipe had come off. Put it back on. Still had the hesitation problem.
Checked fault codes with my OBDII scanner and smartphone app.
Checked for codes, no codes found. As i was sitting there, idling, I opted to press delete codes although none had appeared.

5 seconds passed and all of a sudden a black plume of smoke came out the rear. Lasted 30 seconds or so, then cno smoke, exchaust note back to normal. Hesitation gone. Odd but true.

Reckon dealer would have found a code and deleted with same BRILLIANT result.

Worth a shot.

[Rondo]

This may sound too simple to be true, but after I replaced a vacuum line, I had nearly the same issue, only just at idle and lower rpms. No codes were showing and I deleted anyway, but the issue persisted. The only thing I hadn't done was disconnect the battery. Once I did that, the car ran perfectly. Now I get better acceleration and gas mileage. Simple fix. May work for you. Disclaimer: I'm no mechanic and everything I know about these cars is what I've learned from fixing my own. Feel free to not take my advice.

[krenekj]

Thank you for all the suggestions. Here's what I did last night:

- disconnected the battery for 1 hour
- pulled all the plugs / coils. Volvo plugs have 12k miles. Plugs, wires, loom, coils look good. No oil.
- checked the top fuel pump - wires, hoses - looks fine, smells like fuel though
- reconnected the battery
- hooked up the ScanGauge II and took it for a drive

Monitored the following digital gauges:
- Throttle position sensor - varied correctly with throttle between 10 and 90, yet somewhat delayed readings
- Manifold pressure - 3 at idle to 15 or so at peak and smooth in-between
- O2 sensor - flipped between open and closed
- Fuel pressure - blank - would have loved to have that one.

Still no change in behavior. Next thought is to replace the O2 sensor. I've read that they can begin to fail w/o throwing codes and could cause these symptoms. The current one has about 60k miles on it. But they are spendy, and don't want to spend the $$ if I don't have to.

Any other thoughts?

Thanks again.

[krenekj]

Problem solved. All 5 of my original ignition coil packs had cracks and was causing the engine to misfire. I replaced them and the symptoms have vanished. You can see pictures below.

The dealership wanted $95 each but IPD had OEM Bosch coils for bit over $50 each so the choice was easy.

Just to summarize the symptoms:
- sputtering and slow acceleration through 2500 RPMs, and then no issues
- did not occur until car was warmed up
- no codes or CEL
- no idle issues

Then I tried:
- replacing some suspect vacuum elbows
- cleaning MAF and throttle body (TB) did not alter the situation
- checking spark plugs and ignition coils for abnormalities
- consulting with tech at XeModex confirmed it was probably not a yellow sticker TB issue
- calling local dealership who estimated $600 just to start diagnosing, even after I told him everything I'd done
- calling local indy shop I trust, took a test drive with tech, suggested ignition or fuel pressure problem

Indy shop suggested pulling the rubber back on the ignition coils and checking for cracks. This never occurred to me. They see a lot of cracked coils with over 100k on them. And you know the rest of the story.

Thanks again for all the suggestions. Hopefully this will help someone in the future.

[regent]

Yes,it definitely will.
Thanks for posting back with good report and detailed findings.

[alexk243]

Having the same issue with a 93 850, but since I only have a single coil would that be a similar issue or no?