Scott.T

iTrader: (2)

I have a problem that some of u may be aware that I have been trying to cure for some time.

I have recently found that I can provoke the 'hesitation/miss' at approx 4,700rpm in either 1st of 2nd.

Accelerating away fast from the line produces hesitation in 1st in between 4,000 and 5,000rpm (approx 4,700rpm). After 5,000rpm it goes like stink.

Holding the car at 4,000rpm in 2nd, then flooring it produces the same problem.

If it is held at 4,000rpm then the speed is 'Gradually' increased the problem does not occur.

Is the ECU changing modes at this point or is somthing cuting the boost. The gauge shows no sign of this ?

P.S Car is an MY94 UK....standard of tune makes no difference i.e bog standard or running 14psi with Prodrive BB.


[Edited by Scott.T - 2/19/2002 8:15:54 AM]

HarryBoy

Hi Scott,

I may be wrong, and if anybody knows for sure then please correct me.....I think the ECU switches from closed loop to open loop at around 4700 RPM IE: it no longer uses the Lambda sensor for AFR reference.

If everything else is OK, plugs, coil packs etc. then it maybe the Lambda......

Harry

Scott.T

iTrader: (2)

PE fitted a new one didn't fix it....but it was faulty non the less.

I have also tried all of the following (either new or borrowed)

Lambda (new)
Boost Control Solenoid (new/borrowed)
MAP Sensor (borrowed)
MAF (borrowed)
Ignition Module (borrowed)
ECU (borrowed)
Cam Sensor (borrowed)
Crank Sensor (new)
Coil Packs (2nd hand, but in a worse state)
New Plugs (tried at 0.6 - 1.0mm, made no difference)

I have just re-grounded the ECU to the Engine, but have been unable to confirm any improvement due to be coolant tank failure (see other posting)

I think that's it......

[Edited by Scott.T - 2/19/2002 5:26:09 PM]

john banks

From watching the AFR meter the ECU seems to go open loop beyond a rather lower figure than 4700rpm, or over a certain boost level whichever is lower. But with all the flashing lights it is difficult to tell. I heard there is a select monitor signal which says whether you are closed or open loop.

Scott.T

iTrader: (2)

Goes open loop at positive boost i.e above 0psi or 3000rpm whichever occurs first.

That can also be verified with my AFR.

Cosie Convert

I think this may vary by MY as my 95 WRX will stay closed loop up to 5000 rpm if the throttle opening is small enough to maintain a decent vacuum.(verified by Lamda output) It will go open loop at any RPM with application of throttle.

cc

Cosie Convert

Scott

Try the fuel pressure regulator, if it is a bit sticky it could delay the fuel pressure increase which should rise proportional to boost.
It may also be that the signal line from the throttle body to the FPR has become restricted (oil etc) causing a delay in pressure rise.
Either of the above will cause a transient weak mixture which could explain your symptoms ?

cc

Scott.T

iTrader: (2)

Cheers cc, I was thinking along the lines of the pressure regualtor as I think I have exhausted all the electrical type devices.

Where will I find the regulator and the mentioned pipework to the FPR?

Jay m A

iTrader: (2)

CC

I've got a MY95 WRX and on the rollers with select monitor attatched mine went open loop at 3800rpm or +ve boost, whichever came first. I'm pretty sure I didn't see it closed loop past that rpm even when pulling a vacuum. Wierd!

Justin

nom

Not anything to do with the 'keeping the noise level down' that the ECU seems to like doing is it? Doesn't sound quite the same, but, well, thought I'd mention it, just to show I'm paying attention

NENO

Very interesting thread, can't really add anything super constructive other than to say I had a similar problem with previous car - my96. I first noticed it after it through a code for the knock sensor, this was replaced and the problem started.

I'll keep reading to find out what cures it...

Scott.T

iTrader: (2)

As mentioned above were can I find the fuel pressure regulator. Couldn't find it at the weekend.

Only Electronics I havn't changed/borrowed are :

Knock Sensor
Throttle Position Switch/Sensor
Wiring Loom !!!!!!!!

Scott.T

iTrader: (2)

BTT.........

So where's the Fuel Pressure Regulator then ?

Should there be anything under the Knock Sensor as took it off at the weekend and could see (in the hole) what looked like either water or a ball bearing. It's difficult to see as it's so deep.

[Edited by Scott.T - 2/19/2002 5:23:59 PM]

Cosie Convert

The FPR is at the side of the turbo, on the rear end of the offside fuel rail beside No 3 injector. Not too easy to get at ! The signal pipe comes from a tapping on the inlet manifold.
Not sure what's under the Knock sensor, if anything !

cc

JIM THEO

Hope this helps you:
Subaru Service Bulletin Number 11-53-98 Date: 1/5/99
Applicability: 97~98 Legacy, Impreza, and Forester Manual Transmission vehicles with 2.5L & 2.2L engines.
Subject: Hesitation On acceleration
In the event you encounter a customer complaining of a slight engine hesitation between 1500~2500rpm’s when the engine is cold or hot, perform the following:
Eliminate all external influences, such s a incorrect or dirty air filter, loose or cracked intake duct, dirty fuel filter, low fuel pressure, PVC system, or low engine vacuum that would indicate an external leak or an internal engine component. Check for trouble codes and repair as necessary. Verify the fuel quality.
If all external components are confirmed to be operating within vehicle specifications, the hesitation may be caused by the ignition control logic in the ECM. Under certain low rpm driving patterns, the ignition control system can pick up engine vibrations through the knock sensor and may retard the ignition timing. This ignition timing is learned by the ECM and placed in memory. Note: This area of the memory can not be viewed by using the Select Monitor. When the vehicle is driven under these conditions, the timing may be retarded and could cause that engine to hesitate on acceleration. To confirm this condition, road test the vehicle while viewing the Knock Sensor Signal on the Select Monitor. If you duplicate the hesitation, and the reading on the monitor is around -10 Degrees, you will need to change the ECM to correct the concern.
Note: Some vehicles (shown on the following two charts) were produced with two different emission systems during the same model year. To identify which system you have, look at the Vehicle Identification Number. Refer to the chart to identify which system you have on the 98MY Forester and 97MY Impreza vehicles and choose the correct part number from the chart on the following page.
JIM

submannz

Just to add on from Jim, the knock sensors on legacies & some WRX's are too sensitive, I can't remember what models are affected but there was a mod for this. Which is what causes the hesitation in most cases.

Dan