Power loss (misfire) under load, help!

[havenhse1]

I have a 96 subaru legacy wagon problem that has me and

 

some mechanics stumped. There is a loss of power under

 

load - when accelerating up a hill - and I get the code for

 

misfire in cyl 2 & 3. The short term fuel trim goes way up

 

when this happens. I have checked fuel pressure. It has

 

new wires and plugs, a new knock sensor ( in my hunting

 

for the cause, it was found to be kaput.) Also tested

 

manifold vacuum, indicated no blockage in exhaust, and

 

did a compression test which turned out to be fine. If

 

anyone can help me, direct me, I would be very grateful.:-\

[Adnan]

Hi,

 

Power loss is related, amongst other things, to incorrect sensing of the load. You've checked several of the suspects but the mass flow sensor could be one problem area. If it does not report accurately, the engine management system will cause improper fuel metering.

 

I don't think that engine uses a manifold pressure sensor since it has a mass flow sensor, but that's another possibility.

 

Regards,

Adnan

[blitz]

I'd be tempted to say that the coil pack was breaking-down under load, but the cylinder pairing doesn't fit the pattern. Normally it would be a problem in pair 1 & 2 together, or pair 3 & 4 together.

 

Nonetheless, you could temporarily gap the plugs tighter (something like .025-.030) to reduce the secondary voltage, then take it for a test drive to see if that a positive effect on the problem.

 

If doing that helps the problem somewhat, it would point in the direction of an ignition secondary breakdown.

[mtsmiths]

So, does a faulty MAF sensor throw a CEL and code?

 

Can it create totally random intermittant missing (feels just like a plugged fuel filter)?

 

How much do they cost?

[havenhse1]

Hi,

 

Power loss is related, amongst other things, to incorrect sensing of the load. You've checked several of the suspects but the mass flow sensor could be one problem area. If it does not report accurately, the engine management system will cause improper fuel metering.

 

I don't think that engine uses a manifold pressure sensor since it has a mass flow sensor, but that's another possibility.

 

Regards,

Adnan

 

Thanks for your input Adrian, I did a visual inspection of the mass air flow sensor - and it seemed in good shape. The reason I discounted it being the source of the problem is two-fold, 1: If it were faulty it would set a code, and 2: I would think it would affect all 4 cyls ?

Robert.

[havenhse1]

I'd be tempted to say that the coil pack was breaking-down under load, but the cylinder pairing doesn't fit the pattern. Normally it would be a problem in pair 1 & 2 together, or pair 3 & 4 together.

 

Nonetheless, you could temporarily gap the plugs tighter (something like .025-.030) to reduce the secondary voltage, then take it for a test drive to see if that a positive effect on the problem.

 

If doing that helps the problem somewhat, it would point in the direction of an ignition secondary breakdown.

 

Good thinking. I will try the gap change.

 

I had wondered if the coil pack could affect these two cyl's? I checked it's resistance and it seemed ok, however I noticed something I have never seen before, when I put a timing light on the wires going to the affected cyl's the timing light would flutter eratically, whereas on cyl 1 it was steady and rhythmic.

 

Something else that I have seen on other postings - and have not done yet - is a valve adjustment. but it seems doubtful that both are tight.

 

Thanks for your input, Robert