97 legacy GT. No code hesitation when trying to maintain speed.

[buddyalan]

I have a hesitation problem that gets worse with fast acceleration. And can completely go away at times with normal driving. Car will cruise at 60 to 75 for hours and then act up as you slow to 40 to 50. Gas pedal acts like an on off switch. Not really noticeable under 40. Also idles high in gear and sometimes shifts erratically when cold. If you take it out of gear the idle drops to normal. (Auto transmision) Changed TPS, Maf, cleaned EGR (it works) cleaned IOC.(Closed at idle) Checked and replaced vacuum hoses. Replaced fuel pump and filter. Checked fuel vent line from tank to engine. No vacuum at idle from canister to EGR. New plugs and wires. Never found vacuum at idle at purge control solonoid, press. reg. or press. sensor. Auto zone checked, - no codes - no light. Wiring to eng. computer looks fine. Going to change timing belt when I get the chance. Car has 188,000 miles on it. I know the previous owner, he got rid of it for this problem. I'm at the point where its costing to much to fix. I still believe its a manifold press. problem. But I am now lost. Any suggestions would be appreciated.

[unibrook]

When is the last time your front 02 sensor was replaced?

Does symptom only appear when engine is warm?  If so, that is another pointer towards front 02 sensor.

[Fairtax4me]

Agreed. Possibly front O2 sensor is bad.

Engine coolant temp sensor is another possibility. There are two sensors, one for the gauge, one for the ECU.

 

Did you calibrate the TPS when it was installed? ECU can get confused if it thinks the throttle is open all the time because the TPS is slightly out of range.

[idosubaru]

is the idle controller working properly?   i'd swap another if it's so linked to gas/throttle opening and closing.

 

spray starting fluid around the engine to check for minor vaccuum leak?  i found a cracked vaccuum hose on one of my daily drivers the other day...though oddly it drove fine.

 

knock sensors crack at the base...though typically throw a check engine light if causing issues.

 

it shouldn't cost too much to diagnose if you can do it yourself - MAF, TPS, and those sensors almost never fail so they are commonly replaced with used for pennies, particularly as a troubleshooting routine like this.  never buy new costly sensors unless they are common failure points and you know they are bad.

 

i had strange shifting issues in my 1997 automatic impreza EJ22 (in which case the engine and transmission are plug and play compatible with your EJ25).  there's a hose that goes from the engine to the passengers side strut tower with a quarter sized piece in the middle - that's a filter.  i replaced that years ago and the shifting went back to normal - a friend has driven that car for years since, no issues.  i can't think why that would cause issues, no one else ever seems to bring it up on Subaru forums, and maybe it was coincidence, but easy enough to check.

[Fairtax4me]

Ah, the MAP filter. I had one that plugged up solid as well. I don't remember it causing any wierd running issues but I'm sure it's possible.

The filter is there because of the solenoid by the MAP sensor. It has a port thanks open to the atmosphere because the ECU uses it to switch the pressure source to the MAP sensor. It does this so it can use the same MAP sensor to read both manifold and atmospheric pressure.

Just for a split second the engine is able to pull vacuum through the open air port on the switching solenoid. There is a dust cover there, but no filter. So Subaru put a filter inline to the engine vacuum to catch any dust that gets sucked in when the solenoid switches. It's only for a very brief period of time, but its long enough that if there is any dirt or dust in the dust cap of the solenoid it can get sucked in.

Eventually, this causes the filter to clog. The clogged filter lowers the amount of vacuum on the sensor side of the filter. Since the ECU uses the MAP sensor to determine air density, it thinks engine vacuum is low and will alter the fuel /air ratio some to try to compensate.

With the right combination of conditions this can lead to lower power output from the engine. You use more throttle to do the same work as before, which you may not notice, but the TCU is getting a higher TPS opening percentage than is normal, which causes it to alter shift points.

Domino effect.

[buddyalan]

Hello everyone, I'll do these one at a time.

The O2 sensor was probably never changed. I may be able to check it. Prob. does get worse as car warms up. (most times)

I've been told Idle control just needs to be adjusted for idle speed. Oddly small adjustments make no changes.

The map filter was changed first. It was filthy, Subaru mech at the parts store said he'd never changed one.

Sprayed propane at maifold gaskets and vacuum hoses and changed hoses that didn't fit tight in the winter.

I should have time over the Holiday to do some more troubleshooting. But I have two shocks in the trunk that need to get put on first.

Thanks for the help. Have a great weekend!

[Fairtax4me]

I've been told Idle control just needs to be adjusted for idle speed. Oddly small adjustments make no changes.

 

There is no adjustment for idle speed. The ECU handles that 100%. If you've been messing with the throttle stop screw on the side of the throttle body you have probably moved the TPS out of adjustment and will need to reset the throttle stop and recalibrate the TPS.