94 850 has spark and gas, doesn't run.

timo · Post by **timo** » 10 Nov 2012, 09:14

I can't figure out what's wrong with my sister's 850 non turbo.
The code showing is 312 in mode 2. I can't find that code on the internet nor can my mechanic friend find it. Not in mode 2.
Before this volvo quit totally, it was running rough but still going. A month later it quit completely. It will crank, fire and promptly die.
I have spark, the plugs are wet with gas, and I have good fuel pressure to the fuel pressure regulator. I'm not sure beyond that. How would I check that?
I'm told that, cam sensor, crank sensor, rpm sensor would all cut spark it they went bad. Is this true?
I tried unplugging MAF, not change.
Would a vacuum leak do this?
I'm lost and need some guidance.
Any ideas for the most informative volvo website on the internet?

Post by **northernlights** » 10 Nov 2012, 11:09

I believe that the RPM(crank) sensor prevent starting by not enabling the fuel pump relay if it can't tell that the engine is turning. I think you would have dry plugs with a dead RPM sensor. The cam sensor tells the ECM when to fire the plugs, so that seems like a plausible explanation with wet plugs, except for the fact that the problem gradually got worse.

It might be worth checking the coolant temp sensor. If it is running worse and worse as the weather got colder, the car can act almost like the 'choke' isn't working. If the coolant sensor is delivering the wrong value, but a believable value, then the system might not set a code. In the old days of K-Jet injection, the system would default to some default lean value if the enrichment actuator failed. This would allow you to get home but probably not restart when cold. That sounds similar to this in that an increasingly lean mixture would make it worse and worse until some cold restart limit is reached. In this case the input is the wrong temperature to the ECM.

These also have some desgree of self protection for the catalytic converter. If there is too much raw fuel in the exhaust, say from cranking it for a long time without starting, the ECM can prevent the car from running. I don't know what the limits are. I assume this data would be from the O2 sensors.

-Eric

Post by **Pauloil** » 10 Nov 2012, 12:01

312 on this site: RPM(crank) sensor

timo · Post by **timo** » 10 Nov 2012, 16:53

Thanks. According to other websites, 312 should only come up in mode 1, which I was not it, however, I'm gonna try it. Do you know if (like the MAF) I can unplug it and temporarily bypassing the problem?

timo · Post by **timo** » 10 Nov 2012, 17:12

Also, according to my research, a faulty RPM sensor should kill the spark. I have spark, so now I'm thoroughly confused.

Post by **rspi** » 10 Nov 2012, 19:24

How old is the car? If it's older than 15 years it may have a burnt valve. Check compression.
http://www.atthetipwebs.com/technologyi ... n_test.htm

timo · Post by **timo** » 15 Jan 2013, 06:03

I finally got around to checking compression and the one cylinder has zero compression. Guess I'll just part it out or something.