1995 850 Intermittent misfire at idle / accel.

jnitschke123 · Post by **jnitschke123** » 08 Dec 2015, 15:25

Hi members, a recent misfire issue has risen in my 1995 850 Wagon. The car is an automatic and is not turbocharged.

The problem persists when the car it either hot or cold. When it misfires, it almost always does it when it is idling or when the car is given light acceleration from a stop.

So far the spark plugs, cap, rotor and wires have been replaced with Volvo OEM parts. When the car is running without this intermittent misfire, it runs strong and smooth - the car has 185,000 KM on it.

What parts should I be diagnosing as a start?

Post by **scot850** » 08 Dec 2015, 16:03

Check the routing of the cam position sensor wire. It runs near the distributor and if it gets too close it can stall the engine by killing the signal to the sensor as the distributor sparks. You may have disturbed this while doing your repairs.

As the guys here always recommend, check all the rubber pipes for small holes or cracking as this will affect the engine.

If you have not done so, have a look at the MAF. They do fail with age and this could be the start of it. You can get dedicated spray cleaners for the MAF. Just don't touch it with your fingers. A clean may help, but once you have sprayed it, give it time to dry before re-fitting.

Neil.

jnitschke123 · Post by **jnitschke123** » 09 Dec 2015, 09:42

Thank you Neil. I will look into these culprits today!

Post by **erikv11** » 09 Dec 2015, 10:09

When the misfire is always at a certain throttle position like idle, one thing to consider is a bad throttle position sensor (TPS). there is a mechanical, sliding mechanism inside that can get a flat spot. Unfortunately the only way I know how to test this, is by swapping in a known good one. Junkyard unit should be totally fine.

jnitschke123 · Post by **jnitschke123** » 11 Dec 2015, 17:57

-All vacuum hoses are okay and aren't cracked

-Cam shaft position sensor doesn't seem to affect the running condition when I jiggle the wire or tap the sensor. But the misfiring is an intermittent problem, so it could very well be an internal part of the sensor. Is there a way to test it out more thoroughly?

-I don't think it is the throttle position sensor as the misfiring occurs at any RPM and at any speed, but it is a possibility.

Post by **rspi** » 11 Dec 2015, 18:03

I would check compression. It is a certain cylinder?

Also, pull the cap and rotor to see if there is an oil leak in there.

jnitschke123 · Post by **jnitschke123** » 11 Dec 2015, 22:06

Hi rspi, I can rule out that it isn't a compression issue as the car runs very strong and has lots of power when the intermittent electrical issue isn't occurring. The issue only happens maybe 10% of the time I'm driving. It happens in any random circumstance (idling at a light, accelerating, cruising). When the tune up was done, I check the distributor cap for oil leaks and there weren't any.

Post by **erikv11** » 11 Dec 2015, 22:21

Full circle now, especially given the clarification that "It happens in any random circumstance" So +1 to Scot850's suggestions: MAF or CPS wire routing.

Try unplugging the MAF, does the problem go away? Unplugging it will set a code but the ECU should go to default air mixture values and run without hiccups. Cleaning the MAF almost never does any good unless it is visibly dirty/oily. On some 850s, unplugging the MAF just makes it run like crap, if that is the case then you simply can't test it this way.

On the CPS you want to make sure the placement of the wiring is away from the cap and the ignition wires. It is about where it rests, not what happens if you jiggle it.

jnitschke123 · Post by **jnitschke123** » 11 Dec 2015, 22:54

NEW UPDATE:

After driving the vehicle for this way for two weeks, I have come to the following conclusion; the car seems two driving modes.

Normal- The car has good acceleration, idle speed, good fuel economy, and runs essentially normally.

Yuck mode- The car has sometimes good / sometimes bad (misfires, kicks in and out) acceleration, high idle speed (1150 rpms), bad fuel economy, runs rich (smells like gas when running).

I have visibly inspected what I assume to be the MAF sensor (refer to photos) and cleaned the sensor and plug with electrical contact cleaner. This made 0 difference. I also unplugged and plugged it back in while the car was running. No difference in idle speed and CEL did not come on.

I also inspected the CPS wiring and it runs away from the distributor and is clear of any wiring obstructions (its not tangled or mangled or anything).

I'd also like to point out that the car underwent a tune up before I purchased it. It was performed by a used car dealership that specializes in older Volvos and Subarus. The car still has a 3 month warranty on it so I might just have to take it back for them to look at. That being said, it may be hard for them to diagnose as the issue only happens sometimes and the car doesn't have the CEL on :/

Post by **abscate** » 12 Dec 2015, 03:38

What's the history on the ignition coil? That's a possible culprit here and is rarely included in a tuneup as it has a long, but finite, life,