My MY97 Sti has started hesitating or missing below 2000rpm ie. when pulling away or on an early gear change that would cause the rpm to drop below 2k. The hesitation is quite aggressive, causing the whole car to lurch a couple of times and then all is well. It's been doing it intermittently for a little while, but is now getting more common. Hot or cold engine makes no difference. I've got another crank sensor to try and also another set of plugs on the way - but I don't really expect these to help. Other than this it's driving fine at higher rpm....

Any suggestions??

 

Check for leaks in the air hoses then maybe take a look at the Lambda/O2 sensor, this will be controlling the fuelling at mid throttle.

 

Thanks for the reply. I've been all through the pipework to check for leaks and not found anything. I wondered about the O2 sensor, but wouldn't I expect to have problems throughout the rev range at part throttle rather than just below 2k??

 

The ECU should only be using the output from the O2 sensor under normal throttle conditions, at full throttle the ECU ignores the output.

I doubt its the crank sensor, duff crank sensors normally result in no starting at all.

 

Agreed - that's my point and why I am assuming that the O2 sensor isn't the culprit because I only get the problem below 2k. Also agree that the crank sensor is unlikely.

Any ideas of where else to look?

 

I assume at under 2k you are not flooring it though, so the output of the O2 sensor is being used which means it could be the culprit.. What happens if you floor it does it feel any different?

 

No this is on gentle acceleration. It comes on just after the clutch is released ie. if I know I have to pull out or across a junction, I can slip the clutch through it.

Okay, I've just been out and changed the O2 sensor and crank sensor and the problem still remains...

Anyone got any suggestions

 

Anyone else got any ideas please......

Have had coil suggested, thoughts?

 

Just a thought,
Do the symtoms feel as if the car lurches as if you are about to run out of petrol, so you push the gas pedal down a little harder to sort of get through it ?????????????????????????????????????????......... ...............and then you find yourself taking off ????????????????
And all this lurching is normally 40mph and under ?
i.e. higher speeds and under boost it seems normal?

If the above describes your car let me know I know what it is likely to be

Kev

 

Kev

On the whole yes, although I don't have to stamp on the pedel to ride it through.

So come on mate, share your thoughts....

 

After changing plugs/coils/crank sensor and even trying to get a mapper to look into my car problem I changed the air idle valve ( I cleaned it out about six months previously) for a brand new one £200 plus from Subaru

I believe the reason for the lurching is under 2000rpm you are getting close to the idling rpm at standstill, and the slider inside the air idle valve is staying slightly open when driving along under 40mph rather than being fully shut.
This results in extra air being dragged into your manifold putting your mixtures out causing the lurching.
I changed mine and my car was perfect again.
Hope this helps

Kev

 

Well that certainly sounds like it could be the culprit, but £200+vat to try it - yeowch!! Can these be cleaned, or is it a case of buying a new one. Can you point me to where I should be looking in the engine bay to find the beasty?

Thanks Kev

 

I'm sure there are plenty of threads on these if you do a search
If your car is similar to mine being a Jap import, then as you look into the engine bay where the intercooler meets the manifold, to the right handside you will see a beige top with three wires coming out of it.
This will unplug, then if you look around the back of it there will be four screws holding the sensor onto the manifold............undo these......and lastly there should be two pipes halfway down on either side........these supply water to this sensor...........so when you take these off don't worry if some water comes out.
Hopefully the whole thing should be in you hand.........be careful as there is a gasket between the sensor and the manifold so try not to damage it.
Then looking inside the air idle valve,you should see a sliding door type of thing..............mine was green, you should also notice how much black carbon crap is around it.
Obviously this is what you need to clean out.
Check the door slides by moving the top of the sensor........it should be quite obvious.
Spray lots of brake cleaner or petrol through it............let it dry out and put it all back together again.
Hopefully you will notice the difference straight away as your car should not be lurching.
It may well be no matter how much you clean it, if the signal to the ECU is faulty because of the sensor then it won't make any difference.
I would also take a close look at your plugs (how old are they and have they been gapped properly?)
And coil packs (how old are they?) 60,000 miles plus is a normal time for the coil packs to start to play up and crack.

I think thats it pheeeeeeeew
Kev.

 

Kev

Many thanks for all the help mate - the old fingers must be sore! I'll pull the sensor off tonight, give it a good clean and see how we go. I've got a new set of plugs on the way, so they'll go in as soon as they arrive. I wondered about the coil pack - I've got the single pack on top of mine, but past experience with dodgy coils leads me to expect problems throughout the rev range.... I'll see how it goes.

Cheers

Dan

 

The reason I didn't think it was my coils is because if they were faulty then you would REALLY notice it under boost aswell as normal driving.
My boost was fine no hesitating at all.
So I knew my mixtures were out at normal rpm.
One other quick thought..........if you have an after market dump valve check that aswell, sometimes after time the rubber insert breaks down and leaks inside.

KEV.

 

Touch wood, the problem seems to have been fixed. I took the idle speed valve out and sprayed a good dose of brake cleaner through it as Kev suggested and all now seems well.

Cheers Kev.

Dan