Flat spot!

[Mr Tank]

Hi again!

My winter project is coming up with a few interesting problems.

Tonight's is its running!

Starts ok, the tick over is around 900 rpm but hunts a little!

Increase the revs to around 1200 and it hits a flat spot which if you leave it at that the engine nearly dies! If you push above 1200 it goes like a train with a gloroius exhust noise!

We think it's also over fueling or just a bit rich.

We have fittied new plugs, tried my spare refurbished air flow meter and it's still the same, then we tried the air flow meter off no 1 , and it's still the same.

Any ideas?

Andy

[chedder]

Possibly the timing is out, is the vacuum advance working?

Chedder

[honestjohntoo]

Mr Tank said:

Starts ok, the tick over is around 900 rpm but hunts a little!

We think it's also over fueling or just a bit rich. Any ideas?

Having been a failed clairvoyant and not knowing what fuelling system or engine you are working on, your two observations cause a dilemma. So Guessing:-

First. What colour are the plugs. Overfuelling or just a bit rich is easy to analyse with anything from black to grey.

But that would NOT normally cause a flat spot that nearly results in dying because a slightly rich mixture generally gives more torque for the engine to work with.

However you may have a small rogue air leak causing an uncontrollable weak mixture at 1200 rpm but as the throttle is opened more the leak becomes less significant and the engine roars.

Thus the flat spot is explainable as the torque is just too low to sustain the rpm and your plug colour would defo not be black or dark - more likely tending towards very light biscuit or even whitish.

There are easy ways of checking for air leaks - click here to find out how.

Now to the second horn of the Dilemma: The normal reason for 'Hunting at Idle' IS a slightly over-rich mixture as you can read all about by clicking here

Obviously weak and rich can't normally co-exist, but at idle its completely different to when the throttle opens, so my best guess is that your system is way off tune in several areas and all you can do is study the available reading material, mug up on the overall Efi System and carry out tests related to air leaks, wiring/connection problems and component integrity, as detailed in many other pages associated with the above links.

Now the winter project gets more interesting. An Ops Manual is good reading too.

Edited by honestjohntoo on Monday 7th December 22:51

[Mr Tank]

honestjohntoo

Thanks for your comments. My error!

It's a 1988 350i running the normal fuel injection system fitted to the 350i V8.

The old plug's were black and very sooty.
Thought it was the cold start injector causing the problem, so we disconected that. No! still the same!
Any more ideas!

Andy

[honestjohntoo]

Mr Tank said:

The old plug's were black and very sooty. Thought it was the cold start injector causing the problem, so we disconected that. No! still the same!

Any more ideas!

• study and learn the system.

• Test all components that can be responsible for over-fuelling - viz.

• Temp sensor AND its connections to the ECU

• Throttle pot to see if its noisy

• FPR to check it is controlling the fuel pressure down to 26 psi when needed

• ECU - temporary swap with a known good unit, as ECU over-fuelling faults are common.

• Cold start injector system - disconnected - but possibly not removed, see next item.

• Weeping injectors - any one from nine.

• Badly soiled air filter

• Because they are responsible for 80% of all Flapper faults, dont overlook air leaks and wiring/connection faults

• NOT likely to be AFM, Breathers, fuel pump, fuel filter, EAV, ignition.

[convert]

Mr Tank said:

honestjohntoo

Thanks for your comments. My error!

It's a 1988 350i running the normal fuel injection system fitted to the 350i V8.

The old plug's were black and very sooty.
Thought it was the cold start injector causing the problem, so we disconected that. No! still the same!
Any more ideas!

Andy

Could be the Coolant Temperature Sensor. I had a similar problem on mine and this fixed it (albeit a couple of years later it has returned, but the CTS I used was an intermotor one!)

[adam quantrill]

Andy - if you're talking about no-load revving, after the CTS I'd check the throttle pot is smoothly varying voltage. They commonly get worn out low down in this area. If it's a big one I can refurb it for you (just drop it off, you know where I work ;^)

Edited by adam quantrill on Wednesday 9th December 12:13

[mac123yre]

The symptoms are the same as I had, when stripping the engine I found some of the inlet bolts loose and letting in air

[Mr Tank]

Hi Adam

The flat spot is there on and off load!

I am not happy with the air inlet pipe to the plenum from the airflow meter its in a bit of a sorry state.

I need to find a new one?

I will be going over all the items mentioned at the weekend work permitting! 7 days and 24hr a day are not enougth this time of the year, in fact my next day of is Christmas day!

Andy

[rev-erend]

Lots of flat spots .. sounds like the advance retard is not working .. would be very easy to see with a strobe.

[AC Motors]

Maybe worth trying the little servo motor up near the plenum, has a cone shaped air seal on which can get stuck or be leaking, has a large black 4 way connector on the back and the whole thing unscrews with a 22mm (I think) spanner. The symptoms aren't usually as bad as you describe but worth a check.

[adam quantrill]

Only on hotwire - but it's a flapper isn't it Andy?

At least you have lots of donor parts to try off the shiney one ;^)

[GRIFFONMARK]

As you have a tatty inlet pipe you might want to look at pics of my old car which is back on Pistonheads for £3,300. Should never have sold it. I bought a polished aluminium tube plus rubber reducing pipes and clips for about £30 from my local max power type car accessory shop. The beauty of this set up is the pipe doesn't go oval at the bend or suck in if worn like my old one did. And it has a nice smooth interior. My engine revved its nuts off (luckily not conrod nuts!). The air intake was a cheapy one from JTF (£10?) but looked good! Good luck with your flat spot, Mark