9'97 850, skip and pulsing

[Doug and Lora]

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

 

[Rob Guenther]

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)

 

[Stephen M. Henning]

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.

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.

 

[Barry Leslie]

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

 

[Mike F]

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

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.