9'97 850, skip and pulsing

Our '97 850 wagon, with 78K miles has developed an intermittant problem of
"skipping" or pulsing at speeds of 45-65mph. The "skip" occurs in a no load,
constant speed condition and is most notiable when slightly de-accelerating.
There is no abrupt change in engine RPM ( no big dip on the tach)....leading
me to conclude it's not ignition related.

Switching out of overdrive doesn't seem to have an impact... I once thought
it might be a shifting problem with the trans. searching between D and OD at
45 mph.

We've replaced plugs, wires, timing chain and always done all Volvo
recommended periodic maintenance. A check of fuel pressure has also come-up
normal.

No codes are showing up and because the problem is intermittance, our
mechanic and the dealer haven't been able to identify the cause.

Any one have any experience with this problem? I'm thinking checking the
fuel pump pressure at highway speeds might be a could help to futher
identify if it's a fuel pressure problem. Also, it's been suggested that
Throttle Position Sensor could be the source of the problem.

Thanks,

Doug

 

If it was the throttle sensor then it would make sense for these problems to
present themselves. It could be that there is a connection going to it that
very intermittently has a bad connection, and for that split second
registers no throttle or whatever. (I don't know all that much about
throttles, but I am an electrical person, and it sounds like a bad
connection at this sensor... see if you can clean all the contacts up
nicely... You should't really see these type of electrical problems for
another 5+ years tho, unless you drive your car in a lot of rainy
conditions... connections take years and years to deteriorate usually, but
if it is a sensitive one going right into the computer, it might pick it up)

 

Does it occur with cruise control on?

One problem with 850s is sticking throttles. They can stick off and
then suddenly accelerate. I haven't experienced them sticking on and
then suddenly decelerating.

Water in the gas is a real possibility. A can or 2 of dry gas will rule
out that possibility.

Does this occur when the gas tank is at a certain level. If so it is
related to the fuel pickup or intank fuel pump.

Another possibility is slime in the gas tank. This is usually caused by
mixing brands of fuel. Some budget brands use additives that can
combine with other brands to create this slime layer. I have seen the
slime layer, but can't confirm the chemistry that caused it.

 

Hi,

When you say you have replaced the "wires", does that include the plastic
extenders that reach the spark plugs. I had a subtle problem like yours
that took at least 6 months before it got bad enough for me to diagnose
properly as a "miss". I was suspecting all sorts of things. The plastic
insulators break down eventually and from what I've been told this is not
peculiar to only Volvos. My car had done about 90,000km. Normally these
are supplied if you buy a spark plug lead kit.

Regards
Barry

 

I had this problem on my 98 S70 during light acceleration at 120 km/hr.
I had the benefit of having access to a Volvo System Tester, and noticed
injection duration varying from 8 to 10 ms, sagging occurring at 8 ms,
running fine at 10. No lambda (check engine) light, so I guessed
(correctly as it turned out) a new front oxygen sensor might cure the
problem, since nothing else could really affect the mixture that much.