96 Volvo 850 Multiple Missfires

Post by **larrybum** » 05 Dec 2013, 19:44

Solved a serious problem today by accident. Some work had been done on the car several weeks ago and here were the symptoms:'

Car runs fine until it warmed up. Then it bucked on acceleration and sometimes stalled taking off from a stoplight. Check Engine light came on with multiple codes for engine misfires.

Replaced Throtte Position Sensor since problems were bad at acceleration. No effect. Suspected a plugging fuel filter...replaced..no effect.

Began to suspect air leak at hose from Air Mass Sensor to the intake. It looked good but I took it off anyway to examine for holes. When I did, I realized the adapter the hose was attached to was loose. This adapter has a major hose on it just prior to the intake and I could easily twist it. The clamp at the actual intake was VERY loose. I tightened multiple turns, put the hose back on (there were no holes) and the problem vanished.

Theory: Movement of the engine when accelerating caused the leak to worsen or change so the air mess measurement was affected by the leak. Also, when the engine was cold, more fuel is fed to the injectors, making the engine less sensitive to the Air Mass computer input and the rubber hose itself would soften as the engine warmed up and make it easier for a leak to occur.

The misfire codes being thrown by the bad fuel mixture threw me off a bit and so did the good running before it fully warmed. Simple fix, but a difficult to diagnose.

Post by **rspi** » 06 Dec 2013, 09:09

Is the car turbo or NA?

Post by **erikv11** » 06 Dec 2013, 10:22

Sounds like NA (non-turbo), this hose:

: intake.jpg (10.37 KiB) Viewed 406 times

Good info Larry.

Post by **JimBee** » 06 Dec 2013, 17:30

My 5 speed manual '96 850 had this issue a month ago. The exhaust note was snapping and popping on deceleration while in gear. Also, acceleration was weak and rpm's dropped very low at stoplights.

The accordion hose had developed a small hole half the size of a postage stamp, but there was also a larger hole actually burned through the top of the hose about a third of the distance from the throttle body to the MAF. I replaced that hose last year so where the initial hole came from is a mystery.

My theory about the burn-through is the small hole caused misfiring, including backfiring, so flames shot out through the intake manifold and into the hose which just has a light plastic sheath. If not that, I have no idea how the burn could've happened. But with two holes in the hose it was running really bad.