Flat spot on accel.

[jmec]

Just given my 1990 325 for a run after 6 months hibernation. I've done plenty of work during those months. So the engine has been run to temp many times.
All was well until pulling away from a junction onto a slight uphill gradient the engine momentarily lost all power. Then a split second later all was well and has been since. Any thoughts people?

[paultv]

Does it feel like someone switched of the sparks - quite aggressive sudden drop in power? very unpleasant when pulling away hard.

Mine did this randomely for ages - sometimes never, sometimes twice - then settled down - having checked everything, I decided to swap the AFM and it's been fine
ever since ( about 3000 miles or so )

I did "re track" the AFM some three years ago and it was fine for a while but in the end it started to get pretty bad - so I bought a second hand one.

Although I have some serious test gear, oscilloscope's DVM's and so on, I could never get the AFM to show a fault - unitl in the end I could see drops in the
track resistance in a couple of spots.

I paid 70 euro's for a replacement off Ebay - still had the mixture screw cover in place so reckoned it hadn't been abused too much.

By the way I had ruled out the cables/plug under the inlet manifold for the injectors, the distributor cap, cables to the plugs, the CPS and so on.

Good luck, maybe this will help,

Paul

[Brianmoooore]

While the above is possible, a more common cause is a split in the convoluted hose between the air meter and throttle body, usually in the bottom of one of the convolutions and out of sight at the bottom of the hose. The split is normally closed, but engine movement under hard acceleration opens it up, letting in umetered air, and then, when cruising, the split closes again.

[aimlessrock]

when i overhauled my B20 i carefully examined most of the air horses (off the car) and found a number of them had small splits - most look fine when on the car.

age and heat makes the hoses brittle and ultimately crack.

I would remove the hoses one by one and examine them for peace of mind.

[jmec]

Gentlemen, thank you. I've checked through my service records (sad but essential) and found that I changed the Boot, new item, in 2015 and the AFM, second hand item, in 2016. Also a similar lack of power happened last year when I pulled the car out of hibernation and that was it. It never happened again. I do any work needed over winter rather than cosset the car in the garage to stop it getting wet. Hence the post hibernation runs. I'm aware that replacing the above items doesn't mean either is sound at the moment. However could it be that the AFM arm is fine on the carbon track while it's being moved 'gently' and then when some rapid accel' is demanded of it it momentarily sticks, after said period of inactivity? Once that sticking is overpowered it will work without repeat, as in last year's experience. I'm doing around 3K a year.

[Brianmoooore]

and then when some rapid accel' is demanded of it it momentarily sticks, after said period of inactivity? Once that sticking is overpowered it will work without repeat, as in last year's experience.

What sort of clearance does the door in the AFM have? Does it come close to touching the side at any point? Could be a bit of 'white rust' forms on the alloy over winter, and is just enough to touch the door, making it stick until it rubs off.

[jmec]

Mmmm! That's an interesting thought. It passed its MOT this morning so I've been trying to replicate the fault by driving over the same route. Same time on the road etc. The car seems fine now! But if, as you suggest, it was corrosion there maybe some trace of it in the AFM. I'll try and take a look over the weekend. Thanks Brian.

[jmec]

Checked over the boot and AFM on the bench. There was some corrosion in the AFM duct but nothing catching or obviously imparing movement of the door. However in view of not being able to replicate the problem I think, as suggested, the corrosion was the problem just catching the door at that moment of acceleration. Gave the duct a clean with carb cleaner and toothbrush. All seems to be well now. Thanks again for the help.

[paultv]

You should be able to tell the difference between a mechanical fault such as the door sticking and an "electrical" error such as a connector or AFM track damage - the elctrical error will create a very sudded and harsh interruption , akin to switching off the ignition for a second whilst accellerating, very nasty.

The mechanical error will be a soft "weak spot" as you feel the car become unresponsive to you foot, but it won't feel "terminal"

That was my first question about the type of flat spot you were experiencing.

Paul

[jmec]

Hi Paultv, In answer to your question; it was as you describe, 'as if the ignition was suddenly switched off.' But, and I neglected to mention this in previous comms', looking through my records again I had re positioned the arm on the carbon track sometime ago. If the AFM door is moving through an arc associated with gentle acceleration and then it enters a different section further on that arc brought about by harsher acceleration and that section has, as described above, corrosion on it, it could have momentarily stuck. What are your thoughts on that theory? I have had no problems since and have put the car through some foot to the floor stuff to try and simulate the original snag. As mentioned I've had the intake system off and cleaned off the corrosion in the AFM duct.

[paultv]

I would guess that as the car has been standing the carbon track has some degredation but the wiper may have cleaned it up after use. This is exactly what mine did, it was fine for ages but then some random drop outs when the car was first run after a rest period started to occur - despite my attempts to clean and alter the track, it just became worse until my huge drive back from Ibiza when it really started to cause more regular problems, especially when fighting through city traffic - as I say I replaced it and have had nothing since.

Maybe keep an eye out for a cheap but healthy replacement to stash away in the event.

Paul

[jmec]

The joys of running a 30 year old car. I'll keep an eye out for a halfway decent AFM!

[TheWolfman]

"until my huge drive back from Ibiza" I see what they mean about your covering a lot of miles now Paul TV!!!

[TheWolfman]

Good info in this thread.I was just checking it out because I'm also suspecting my high-idle may be now be something to do with AFM and whilst tinkering yesterday noticed that the 'Idle mixture screw' has been turned almost all the way to 'rich' and has no cap so clearly been previously mucked about with....

[jmec]

FYI... just back from a trip of 624 miles in 7 days mostly on A roads. Car didn't miss a beat (bold statements like that usually come back to haunt me!). Looks like the flat spot drama was down to lack of use over the winter.