Hi all, I'm in desperate need for some troubleshooting help here.

The car is a 1997/98 GT Turbo/WRX, with about 140k miles on the counter.
The last couple of months, it has been running mostly fine till it's hot, then it starts stumbling at random times and idle gets unstable (300-1200 rpm, sometimes cuts out). Sometimes, it'll have problems when starting on a cold morning (after about 10 secs of running).

When the stumbling happens it can go away on it's own, or carry on until I either shut down the engine (it's fine after a couple of minutes) or till I breach the ~4k rpm barrier (it runs fine after that). When the stumbling persists (if i try and keep it below the 4k rpm), there is a distinct smell of raw fuel, black smoke coming out of the tail pipe and it sounds like it's running on half the cylinders (the rest must be choking). If I let it idle after a stumble, it'll idle rough or cut out (fuel smell again).
It happens primarily when hot, lesser when really cold - about once per 30 miles; but I think I can provoke it by running it hard for a few minutes. I've felt it stumble once or twice while running it hard too.

The rough idling can start at random times too - it'll idle fine, then fluctuate slightly, then more and more. It's usually fine if I keep it steady with the throttle; but sometimes it'll start choking no matter what (raw fuel smell, runs on fewer cylinders, cuts out after a long struggle). I'm not sure about this, but with the fuel smell, I may also have smelled exhaust (as if there was a small leak somewhere) - but it was hard to tell with the fuel aroma.

I recently did the following:
- changed spark plugs (NGK BKR6E - standard gap @ .075 - .8mm)
- changed fuel filter
- changed pcv valve
- cleaned IAC valve
- cleaned throttle body
- changed air filter
- changed speed sensor (had gone bad)
- unplugged the MAF (died immediately)
- reassembled the intake and made sure there are no visible places for a leak (not fool proof, but I really took my time)
- under a rough idling session, I checked for flying sparks from the spark plug wires; nothing to see. Normal idle doesn't go bad when shaking/moving the wires
- oil was changed < 1000k miles ago

Any help is GREATLY appreciated. Thank you.

 

well i had a similair problem like you with mine problem it where the ECU that had a bad ground on the inlet manifold. when i measured the lambda sensor in the downpipe i would get very high voltage's i don't know any more what these voltage's should be but you could try that!

also at some points it was totaly normal it would also drive fine but when it started you couldn't get over like 4k rpm or drive around the city without holding the RPM's at one point.

 

Sadly, the bit about the MAF is an urban myth, the only sure way to know is to fit a known working one.

What you have does sound like the MAF is dead.

 

+1

 

Well I just ordered some new spark plug wires that I'll install, just to make sure. I might as well get myself a new MAF too, while I'm at it.

Thanks a lot for clarifying the urban myth! I had completely ruled out the MAF with my "test"...

About that bad ground - I don't know how to check for that, but I've taken my time looking through wires, and everything seems correct nothing is loose, corroded or looks abnormal); that said, it could be invisible to the naked eye.

 

Run a decent thick cable from the bolt holding the ecu in place back to battery -ve. Use soldered connections and ring terminals.

The cable needs to be thick and flexible, the stuff sold for ICE installations is good.

 

Maf +1

 

Right - I'll do the ground check tonight and I just ordered a new maf which will arrive within the next couple of days. I'm really looking fwd to get the car running well again.

 

Well, I changed the Maf today and spark plug wires yesterday - the problems persist. Any other ideas?

There is definitely a leak where the uppipe joins the turbo - not bad enough for me to hear it, but I can smell it a little when the hood is open. Could a small leak there provoke those symptoms?

 

Fairly sure it's your O2 sensor.

 

I replaced the o2 sensor this summer, so it shouldn't have gone bad yet - unless I got a bad one, but I guess the odds of that are low...

 

CTS as its temp related

 

Were the lambda and MAF genuine Subaru? So many times people chase around in circles due to using crap ebay electronics (we've all done it and learnt the hard way)...
If you got them second-hand were they proven to be working?

 

As above, if you fitted a cheap O2 sensor then you might be requiring a new one.

 

The Maf should be ok: I got the right part number, second hand but tested.

I have no idea about what o2 sensor is in there - I didn't buy it myself. I thought even the worst would work ok for at least a few months... I'll try and get a new one.

 

I'd only use a Denso one personally.

 

New o2 sensor in, didn't fix it. Could it be the FPR going bad?

 

Mate, take it to a specialist, you could be replacing parts all day otherwise!

 

Have you had the fuel pressure checked? Have you checked for any fault codes?
If the fuel pressure is too low the o2 sensor will read a lean mixture and and the ecu will try to dump more fuel in hence the fuel smell and also it would cause your poor running.
As above you need to get it looked at by a specialist as you are just slinging money at it when a few simple tests may find the fault.

 

After a couple of days of driving, I've noticed an improvement after the o2 sensor change. There's still some bad idling left - I'll just take it to a specialist as you guys suggest.

I think the main thing here was the o2 sensor, apparently the one I got this summer was bad - it has gotten gradually better since, even after ecu reset.

Thanks for all your suggestions people!