99 S70 hesitates at low RPM - why?

krenekj · Post by **krenekj** » 21 Jan 2013, 19:12

The car has developed an oscillating hesitation at low RPMs when starting off and accelerating from below 1500 RPMs. It's almost like it wants to die, but in a few a few seconds it always recovers and continues to accelerate smoothly after 1500 RPMs. The problem seems to be between 800-1500 RPM and happens about 25% of the time. Hot/cold engine does not seem to make a difference, nor does the weather. If I rev the engine above 1500 and start moving, then no problem as long as I keep it above 1500. It never happens when on downhill grade. I'm familiar with the ETM issues but I don't think that is my problem. I don't have any other symptoms described in the ETM forum. I'm looking for any pointers or insight that may help find the culprit. Here's some data:

99 NA Manual S70, 137K mi, completely stock intake, engine and exhaust.
ETM replaced by dealer at 73k mi (yellow sticker)
ETM & MAF taken off, cleaned in last 500 miles
New air filter, spark plugs
Vacuum lines and elbows checked, some replaced.
Same gas brand/octane since new

It's very annoying and since my kids will be driving this car a lot in the future (I'm aiming for a new Volvo soon) I want to get a handle on it before it develops into something more problematic. Thanks in advance for any guidance.

gswede · Post by **gswede** » 21 Jan 2013, 22:23

Check your distributor cap and rotor, while there, check your throttle plate for carbon/gunk build up.
Both are quick fixes that can do a world of good.

Post by **regent** » 22 Jan 2013, 07:04

I thought this one had individual ignition coils, no?
Have you looked for fault codes?

krenekj · Post by **krenekj** » 22 Jan 2013, 08:32

This car has ignition coils. Throttle plate is in the ETM, which I cleaned.

Should have added earlier that there are no fault codes. I've checked many times using my OBDII scan tool.

Thanks.

Post by **regent** » 23 Jan 2013, 06:25

Not sure which scan tool you have; when I mentioned codes I had in mind certain Volvo proprietary codes that remain obscure to generic OBDII scanners. Those codes may be stored with or without MIL and may sometimes help diagnose the problem much quicker.
Also, do you know when the PCV system was last serviced?
Thanks

krenekj · Post by **krenekj** » 23 Jan 2013, 08:25

I have a Scan Gauge II which I believe is pretty generic. PCV has never been serviced. I check regularly for signs of blow-back and nothing has ever surfaced. How would the PCV related to the symptom I have now.

Thanks.

Post by **regent** » 23 Jan 2013, 16:35

Increased crank case pressure (due to obstructed PCV passages) would cause hesitation at idle similar to what you described.

krenekj · Post by **krenekj** » 26 Jan 2013, 19:07

Today I placed a latex glove over the dipstick tube and oil fill opening. In both cases under idle air was being sucked out of the glove. I hand to grab it so it would not get pulled into the rotating cam. I also tried the same test at 3k rpm with the same result. Can I rule out the PCV or not? Any other ideas?

Thanks.

Post by **jblackburn** » 26 Jan 2013, 22:37

PCV system is fine, but I may have an idea.

The low-RPM hesitation makes me think that it's being bogged down with a too-rich fuel mixture.

Near your battery, and in the bottom left corner of this picture, there is a square plastic valve on the radiator. Remove the tube that goes towards the engine, and then put a piece of duct tape securely over both ends - the hose and the valve.

This is the EVAP purge valve that opens and lets vapors from the charcoal canister into the intake manifold to be burned off. Sometimes they stick open and will choke the car out until the RPMs get going.

krenekj · Post by **krenekj** » 27 Jan 2013, 11:38

I just pulled the purge value off and tested it. The unit holds a vacuum - no problem. And it releases it when 12V are applied. So I don't think that's the issue. Any other ideas that might be causing a too-rich mixture at acceleration from idle?

Another observation. The intake box has two sources: cold air from the front of the car, and hot air off the exhaust manifold. The amount of each is metered by a flapper that is controlled by vacuum lines to the airbox. I replaced and tested all those lines and elbows earlier and that did not change anything. However, I did notice that when the vacuum is applied to the flapper (which is when the car is running), the air source is the hot side, not the cold. In fact, I never saw it draw cold air when testing. It was always drawing hot air.

From a cold start, this would not matter much. But as the car warms up and the air density drops coming into the intake from the exhaust manifold, that would require less fuel. So it seems this could be a problem with a rich mixture at idle acceleration - just not sure what else could be causing it. Is the airbox operating correctly?

Thanks again for the pointers and suggestions.