ok- so im not a noob, been working on subarus professionally for 16 years- but sometimes a problem needs other ears and eyes

 

 

a friend has a 2002 outback with a sohc 2.5 and 5 speed

it has about 20k on a complete rebuild, bearings the works, 200 on the car

when i rebuilt it i installed a mild set of delta cams, a header, thats it

 

thing runs like a scalded dog

 

but- he has a 700 mile commute, sets the cruise at 70-75 and it will randomly try and accelerate, and slow down, without driver input.

it does it with and without the cruise on

 

when he reduces speed to 60-65 it will not do it as often, maybe once in 200 miles

 

randomly it will stutter and hesitate, occasionally it throws a random misfire, nothing consistent, and randomly it will throw a po172

 

i have checked for vacuum leaks- none. i replaced the oem fuel pump last week and he noted an improvement everywhere, but problems occurred three times in 700 miles

 

he noticed it happens more often at half tank and below, as opposed to full tank

 

help please and thank you

Very possibly the cam grind.  If it was done at Delta Camshaft, Scott can probably tell you which grind it has.  Some are better for low end torque, some for constant high rpm...but all will usually produce a loping idle and as a side effect, a "hiccup" at constant speeds. 

 

Afterthought: have you checked the O2 sensors?

o2 sensors were replaced at 30k ago

 

it ran flawlessly with the cams until just recently

Have you put a scanner on it and checked sensor outputs? Computer thinks its too rich so I would assume front O2 sensor is reading rich and fuel trims are probably going negative.

Pull the plugs and see how they look.

Every time we pull plugs they look fine. So far I have only used an el cheapo scanner

bump

You almost have to put a scanner on it and see what the computer is seeing and doing.

If plugs look OK and fuel pressure is good, the answer is probably going to be a sensor dealing with fuel that's out of range. MAF/MAP or O2 sensor. TPS maybe?

We have a scan gauge 2 on the car. It's not showing anything weird. We plan on trying a map sensor soon

I was leaning toward a speed sensor, but I also looked here http://www.cargurus.com/Cars/Discussion-t9991_ds508275 .  Lots of possibilities.  Worth looking over anyway.

Knock sensor getting flaky perhaps?

So what happens if you try to hold the gas pedal at 70-75 and not use cruise?

Is it predictable enough that he can loosen the gas cap after it happens to see what the car does. This may throw a code (loose gas cap) but ignore it for now.

It occurs under cruise and foot control

hrmmm

 

Is it just engine rpm or vehicle speed that changes

Both

Could it be the lockup torque converter kicking in and out? Maybe not an engine issue and more of a transmission issue?

5 speed

I don't like you. 

 

I'm going back to a flaky knock sensor. You can watch the engine timing with a scanner, or  if you can get it cheap enough just swap it out. I rarely suggest just swapping out a part but its not a lot of money. Flaky TPS is also possible. 

Edited March 7, 201511 yr by nipper

I'll look into it

Wondering about the cruise and throttle cables/throttle body - something compromised might cause drift up or down via the throttle body/cabling?

 

I had a non-starting 2001 EJ25 with no engine codes and no known issues.  I swapped the entire intake manifold and it ran beautifully.  How's that for terrible diagnosis and not at all helpful!?!?!?

 

when it does it (with and without the cruise control on) is there a corresponding movement in the gas pedal?

how significant of an increase/decrease are we talking.  a tiny bit, noticeable, or a lot?

 

Last resort guess work:

$10 ebay knock sensor, swap crank and cam sensors, MAF, TPS to test.

swap the entire intake manifold

swap in an EA81