Been having a problem on the scoob ever since i've owned her, the car pulls power back briefly in 1st and mildly in second when at full throttle and boost comes in, normally around 3500 rpm. I've been working along the lines of a fuel pressure problem as it also gives the sensation that it is leaning out at higher revs, above 6k. I've done some extensive testing today with the following results, hoping that someone a little more clued up than me could check them over.

Car is MY95 UK.

Fuel px at idle 2.1 BAR, vac line off 2.7 BAR. I put pressure into the regulator simulating boost and got a 1:1 rise (1 BAR air px gave fuel px of around 3.8 BAR) I also went higher and got a top fuel px of 4.3 BAR before my tester maxed out.

Does this mean the fuel pump is good as i can get a high pressure, and also that the regulator is good as i got the correct rise?

The only other thing noticed was that when the fuel pump stops running pressure falls straight to 2.3 BAR then holds, even with vac line disconnected.

With regards to the car everything else has been checked/cleaned/replaced, the only time the car doesn't pull back is in safe mode running 0.5 BAR of boost.

My only concern is that the pump maybe cannot give the flow required when engine is under load, just don't want to waste money on a new pump if not required.

Thanks.

 

personally I would reset the ecu first (assuming your on a standard subaru ecu) go for a test drive and see if you get a CEL (check engine light), then read for any error codes.
Another possibility is that the coil packs may be breaking down, they do tend to get brittle with age. Would do the sensor check first though as it is a lot easier than taking the coilpacks off!

 

Here is the list of what has been done so far.

New plugs, coil packs x4, O2 sensor, crank sensor + normal service items.

Following sensors checked for voltage and operation, and all good.
Cam, MAF, MAP, boost sol, throttle pos.

All pipework has been cleaned, checked and leak checked with no faults.

ECU has been reset and fault codes checked many times, known good ECU fitted with same results.

Car has also been datalogged, no knock detected, no ignition pull and all ecu parameters reporting good readings. The only thing we couldn't get done was wideband AFR readings.

This is why i've been checking through the fuel delivery system, JUST WANT TO FIND SOMETHING........ANYTHING...........HELP.

 

bttt for you, i know how much u want this sorted.

 

i think as well pressure should be 3 bar with the vac off at idle. I know its only .3 of a difference but I suppose there is a possibility that could cause it, especially at boost it may still be 1:1 but the gap will be more away from standard pressures if you see what I mean.

 

Cheers Rob, might stand a chance of catching you once i'm fixed

A friend of yours and mine will install his electrical gizzmo's and work his computer club magic hopefully next week, so we should get a bit more insight into this.

With regards to the pressure question, the only ref i have is from the scoob manual, 26 - 30 PSI at idle, i have 2.1 BAR thats just over the 30 PSI, so with a 1:1 FPR, and -0.6 BAR at idle that gives 2.7 BAR base fuel pressure, and that is also what i have. Think it could be all down to flow demand, just have to wait till next week to find out.

Mark.

 

you may ant to consider checking your furl pump it may be failing under load

just a thought

matt

 

Thats the conclusion we are already at, how would a fuel pump be checked??

 

well with out having a fuel presure gauge in the car wile driving i dont know but going on the age of your car 95 its all ready a good idea to upgrade it anyway

matt

 

mark - i hope thats after he's fitted his turbo, so he can crack on with mine!

rob

 

Hi Vessy

I’ve had exactly the same problem with my car for the past 8 months now
So Ill be watching this thread with baited breath done a lot of the things
You have done without any success.
I change the fuel pump but no change.
Currently waiting for my Apexi power fc & commander to arrive, should
Help with diagnosis.
I currently have a wideband AFR permanently fitted and can confirm it
Reads the car is leaning of when the hesitation and pulling back is happening.

Tony.

 

Two options..

Extend fuel pressure gauge pipe so you can read it whilst driving.
Sticking out the back of the bonnet is better than in the cabin for safety reasons.
Give it a full throttle run, a slight incline would load it nicely and watch to see if the pressure tapers off at the top (remember boost will likely taper at the top so expect a little drop..)

Get a wideband o2 sensor plugged in to the lambda boss on the downpipe and go for a full throttle run, should get richer at the top.

Andy

 

Tony, thank god, thought i was the only person to ever get this problem! I'll keep you informed if we find anything, it's interesting yours is leaning out, i really hope i find the same, least we can concentrate on a specific area. You say that the fuel pump was changed, 255 i guess? did you change FPR and check pressures under load as Andy suggested?

Andy, wideband will be in next week, like the guage at rear of bonet trick, didn't want to put in into the car! I'm more concerned with the pulling back as boost comes in rather than top end, although we will check that too.

All the datalogs taken have never suggested running lean, no knock etc, the only strange result we did have was high inj duty cycles, 98%, which we saw in 1st, as well as 100% load!!!