90-91 Legacy AT vs 5 spd engine mgmt system

[Just enough to be dangerous]

I have inherited the task of sorting out some issues with a family member's

91 Legacy, following 2 big modifications

 

a) converted from AT to 5 speed earlier this year

 

recently has also had short block replaced with 2.5 L, retaining existing 2.2 heads, injection, sensors, ECU etc.

 

After driving this a couple hundred miles, it seems to run great at speed, but the idle is rather rough, with what I would call a "spitty" sound at the exhaust pipe--like rrrrrm-ppptt-rrrrrrrrrrmmm-pt-pt-rrrrrrrrrm.

 

Also, there is a bit of stumble or hesitation when letting the clutch out in 1st gear which sometimes accentuates into a fore & aft lurching sensation

 

The other thing I notice is that with the car warmed up when you shut it off for a while and restart, the idle speed goes up to maybe 1100, down to the lowest pt on the tach, back up again (but not as far), back down, through a couple of cycles before settling down to normal idle speed which is maybe 650 rpm.

 

I took it to a local independent Subaru specialist who confirmed that it was performing poorly at idle, and noted that the ECU, air flow sensor, injectors, harness, and idle speed control were different between AT and MT cars in 90 and 91. These components were not changed when the transmission swap was done. The shop said we need to get the MT-specific parts in order for the engine to run right

 

He also said the cat might be plugged

 

Interestingly, 92 and later use the same management systems for both AT and MT

 

I don't know if the 2.2 engine manifested these idle problems after the tranismission swap. I only drove it once, and that was on the highway, and would not have noticed any idling issues

 

Does anybody know enough about the differences between the AT and MT engine management systems to give an opinion on whether the mechanic is on the right track?

 

Or, given the difference in displacement and higher compression ration, is the bad idle something we just have to deal with?

 

Thanks in advance.

Mark

[Legacy777]

The engine management goes with the particular sensors that were used. If the AT sensors & ECU were retained, that's fine. Everything will work fine. If they mixed and matched sensors with ECU's, then that's not good.

 

There is a MT/AT identifier pin on the ECU. That will tell the ECU what transmission it has, and will adjust things accordingly.

 

Read my swap write-up. I go into detail the modifications that need to be done to get the wiring correct.

www.surrealmirage.com/subaru/swap

[Just enough to be dangerous]

Thanks Josh. Your web page is the guide he followed when doing the swap, and it is a very thorough and informative write-up, I must say.

 

I notice you went with '92 engine management which is the same between AT & MT.

 

For whatever reason it is not the same on the 90-91. What the differences are, and why Subaru would bother.....I would speculate as follows, (and remember I know just enough about this stuff to...make me dangerous...): 1) with AT the engine has somewhat of a load at idle 2) with MT the engine must be able to respond to much more rapid throttle changes especially backing off the throttle during gear changes as well as deal with emissions curing such conditions

 

Whether that's right or wrong, the question before us is whether this engine, with the increased displacement and compression would run any better with the MT-specific system

 

And I guess a question to ask first is are there other causes of poor idling, stumbe, and hesitation that we can check first

[Legacy777]

I went to 92 engine management for a different reason. I had swapped in 92 fuel injectors. They were different then the 90-91 models. Since I swapped in the injectors, the car seemed a little sluggish and my mileage was crap. I attributed it to the differences in EM programming and the different injectors.

 

I ran on my AT ECU for a while before I got my 92 ECU, and it worked fine.

 

As I said, the ECU is outfitted to be able to run in either AT or MT mode as long as the identifier pin and other associated wiring is corrected.