I have a continuing problem with stumbling while boosting, a common reply I see for stumbling or misses is clogged cat. How exactly do you check for a clogged cat or other exhaust restriction?

 

Other posted threads on the problem

http://www.ultimatesubaru.org/forum/showthread.php?t=105402

http://www.ultimatesubaru.org/forum/showthread.php?t=104058

Have you changed the plugs or at least gapped them recently? The higher cylinder pressure's while under boost can cause a poorly functioning plug to not fire. I had the same problem with my Turbo Legacy and new plugs/wires solved every ounce of it.

 

GD

Have you changed the plugs or at least gapped them recently? GD

 

Yes, new NKG plugs (0.040 gap) & wires, also new cap, rotor & stock coil. Took plugs out again last weekend just to be sure gap was correct & wire boots were seated, I did notice 1&3 plugs were tan but 2&4 were black and I think I caught a whiff of raw gas after the "stumble" yesterday.

What year is the car?

If its 87+, it will have a hotwire MAF and that can get dirty.

Most Auto Stores carry MAF cleaner for this.

Mine had A clogged cat, and the only way I figured this out was I took my o2 out and drove it, and all of a sudden I had boost once again. Now if you have a boost gauge or not would be easiest, but you should notice a difference in hesitation.

 

However the stumbling sounds like you may have a vacuum leak, or a weak coil. Does it stumble like it wants to buck you out of the car, or just hesitate slightly, kinda like a soft revlimiter.

What year is the car?

If its 87+, it will have a hotwire MAF...

 

86 GL-10, it has the flapper style MAF.

if you have a boost gauge

 

I got the combo boost/vacuum gauge.

 

Does it stumble like it wants to buck you out of the car, or just hesitate slightly, kinda like a soft revlimiter.

 

No bucking more like a bad miss.

Stupid question, but you are running Premium fuel right?

 

You could unbolt your downpipe, and this will bypass all the catalytic converters. The turbo itself creates enough backpressure for the engine. this would be one way to test to see if your cats are clogged.

 

Things will be alot louder, i suggest a short test drive

 

Other culprits could be bad timing, bad knock sensors or filthy MAF. Also fuel filters if those havent been changed in a while.

I am in agreement with GD but also check the fuel filter and pressure as more pressure is needed under boost,10psi boost needs 10 psi more pressure to maintain mormal mixture strength,that is why you upgrade the fuel supply system when increasing boost over stock levels.

Also this is why fuel supply has to be maintained at factory set levels for a turbo engine as they have factored in the required extra fuel pressure needed to compensate for an above atmospheric engine design.

Stupid question, but you are running Premium fuel right?

 

Not stupid, no just regular gas.

,that is why you upgrade the fuel supply system when increasing boost over stock levels.

 

The turbo is all stock and boost is currently only around 5psi (working on that) and fuel pump is new & the pressure is ~ 40 psi.

I would stop running regular, unless that was a joke. Definatly run the highest octane you can for your area.

 

If you do not understand why, I suggest you research what octane ratings mean & how it relates to turbo cars etc...

 

I am sure it will help solve your problem.

 

Also an intercooler will help greatly.

and before cranking up the boost research this engine. they are not boost friendly in their stock trim, like say.... a chrysler conquest. those cas can double the stock boost and not hurt anything, but on these, my research suggests that building the motor is nessicary to properly run high boost, and reliability becomes a problem, the higher the boost goes.. my 2 cents