98 forester, 5 speed, intermittent stall and shake, need help

[hhoowwiiee]

I have been doing research for over a month now, and I could use some advice on my 98 Forester manual transmission.


In April, the speedometer stopped working altogether, but the car drove
fine at first. Subaru dealer here in Indy said that they diagnosed the
specific problem as a stripped plastic part inside of the transmission.
The cost to repair was four figures, I can't recall but maybe $1900.

They suggested I drive with a GPS as my speedometer and save the money.

A few days later, I had my first engine shake problem, after slowing
down to match traffic on the highway. I took it back to Subaru, they
said they could not recreate the problem. So they have not been any
help since then. So for 3 months, I tried to fix the intermittent stall
and shake problem.

The shaking and stalling is intermittent. I can go for two weeks with
no problems at all, then they come back. The stalls are more frequent
that the shakes. The stalling usually happens when I am going downhill,
or when I am putting in the clutch to downshift. Obviously the CEL is
on, and I get the codes PO500, P1507, and P1540. I am told these are
speed sensor, idle air control (failsafe), and speed sensor 2,
respectively. (Autozone)


I have had the following work done this summer, at several different auto shops.

early June - Auto shop 1 cleaned Idle air control valve, and changing
out arcing spark plug wire (Ran good for two days, then it slowly went
back to Intermittent stall and shake.)

late June - Auto shop 2 replaced knock sensor because this was the first
code that came up after resetting all codes. This did not help at all

late June - replaced dirty air filter on recommendation of Autozone tech
(because of P1507). Helped overall operation, but did not fix the stall
and shakes.

Early July - Auto shop 3 cleaned fuel injectors. Helped overall operation, but did not fix the stall and shake.

Late July - Auto shop 3 diagnosed IACV needing replaced, but they did
not replace it because to me, it wasnt the high idle problem that
members have descrbed, instead it was a low idle problem if anything.
Also, that part was cleaned by Auto shop 1 in June. Auto shop 3 replaced
the Neutral Safety Switch at my request. I thought this was the problem
based on some forum posts from ferret, here on the boards. This did not solve the stall and
shake.

At this point, I did even more research on the message boards, and there
are some posts that say the engine computer needs the speedometer code,
and if it doesnt get it, then the IACV failsafe code gets triggered,
along with the appropriate speed sensor codes.

Does anyone have any insight on the cheapest way to proceed? The discussions here have been helpful for me, but now I need to
zero in on my specific symptoms and get some input.

At this point, I see two paths.



1. Fix the speedometer. Either bite the bullet and open the
transmission, or install an aftermarket speed sensor and try to get it
working with my speedometer. (Is this second idea even reasonable? It
seems a lot cheaper, but is that going to work long term?)



2. Change the IACV. I can get one from ebay, used, but tested.



Its a good car, 181,000 mi before speedometer quit. But its old, and I
don't want the expense of the transmisssion work if I can avoid it.

I would like to try the aftermarket speedometer fix, but I have never done this, and I will likely need to find somebody to install it for me.

Thanks any advice you have. Please let me know if I need to be clearer with my descriptions.



Howie from Indiana

[Fairtax4me]

It will not help to change the IACV. That code is being set because of another problem, likely the speed sensor issue. The ECU is trying to set idle speed when you lift your foot off the pedal at speed because it doesn't know the car is moving. It can't set idle because the wheels are turning the engine, so it determines there must be an issue with the IACV and sets a code.

 

Speed sensor 2 is in the instrument cluster and has to get signal from the main speed sensor 1 on the transmission. Speed sensor 2 converts the 8 pulse speed signal to a 4 pulse speed signal that it sends to the ECU. ECU uses that signal to determine vehicle speed and uses it along with throttle position and engine RPM readings to alter idle valve opening.

 

The gears for the speed sensor output in the transmission are a fairly common failure. Most people just live with it but I've never heard of anyone having stalling problems because of it.

 

The problems you're having sound like the symptoms of a bad or stuck nuetral position switch. This is a switch on the side of the trans that the ECU uses to tell when the transmission is in gear. It doesn't care which gear, just needs to know if its in gear or not in gear. It uses this switch to hangs idle settings and fuel/spark mapping. A bad nuetral position switch can cause erratic and/or low idle and stalling.

It's a simple switch, either on or off, so its easy to test. It's the furthest back on the drivers side of the transmission.