E32 M30 engine runs rough on moderate to full throttle

[Remerus]

Hi guys,

I'm battling with my 730 and would appreciate some input to sort her out.

A bit of history first: the car stood for a few years due to a blown head gasket, top was redone, the company doing it went belly up shortly after I handed the head in and it took some time to get it back.

Last year we started to rebuild her and all was right again for a while after replacing fuel tank, fuel pump and obviously the filter too.

After a 500km trip last weekend, she has developed a miss/rough running under load.

As long as I keep a light right foot she pulls fine and runs smoothly but if I try to accelerate harder, it's like she runs into a wall and I can feel a roughness.

I've noticed that as long as I keep the economy meter (which responds to vacuum as far as I know? ) under about 20l/100 she's smooth but if I go beyond that she misses.

I've replaced the 2 vacuum lines from the intake manifold to the fuel regulator and side of the MAF/throttle body coupling because they were very old and dodgy. I thought it might be a vacuum leak somewhere but it made no difference at all.

What's interesting to me is that under steady throttle conditions the needle in the economy meter seems to move all the time - on our E39 it tends to be very stable under the same conditions.

Where do I look next - fuel regulator, injectors, timing? Somewhere else?

 

[lordicon232]

I had a similar problem with my E28 and it ended up being a burnt spark plug misfiring.

 

[Remerus]

That's certainly an option to investigate - thanks!

 

[sclass]

How sure are you that the headjob was done right? At least so that can be eliminated as a possible source of problems.

Didn't you replace the plugs anyway seeing as you'd done the fuel pump + filter and a few other maintenance jobs?

 

[Auditor]

My money would be on the volume airflow meter (AFM). That particular airflow meter has a mechanical contact track that runs on a potentiometer that just wears out over time. You can effect a workable repair by moving the contact arm to a point in the track that isn't worn out. Remove the AF, take the plastic cap off and visually inspect the track. If it's worn through let me know and I'll post up a link for the repair.

 

[scunick]

This does sound like the AFM, but if not i would check the ignition system from scratch and also check the earthing in the car. A bad earth can also cause some of the symptoms you are describing...

Later
Nick

 

[GPGrobler]

My E30 did this, the first time it was a partially blocked fuel filter before the electric fuel pump, the second time the fairly new FEMO pump was the fault.

 

[JayDrft]

Im with Auditor on this one....if the e32 has the afm, then its quite possibly the issue.

The only thing it sounds like is that when fuel is demanded, its not being fed....fuel pump, maybe, but I dont think so, it wouldnt run well at lower throttle demands....

Does this happen when its idling? You could put an inline pressure guage on the fuel line inlet, if it happens when idling, and revving up - monitor the fuel pressure, if it revs up smooth, cool....once you "rev" it and it starts to hit that wall you spoke about,,check for any change in fuel pressure.

This sounds like a fueling issue.

Hop it helps, and keep us posted....Subbed!:thumbs:

 

[Remerus]

Some feedback on this for everyone: I finally got round to replacing the spark plugs over the weekend and it looks like lordicon232 got it spot on in one. raise:

No more "running into a rough wall" feeling when accelerating :joy:

Another bonus: average fuel consumption has dropped by more than 1.5l/100 according to the OBC :rollsmile:

To answer some other points:

Re: sclass - the shop that helped me put her back together didn't, which I didn't realise at the time.:argh:

Re: Auditor, scunick and JayDrft - something added on the To-Check list, to be scheduled. Please post the link to the repair if you get a chance, Auditor?

She still has a rather unstable idle (cycles between 600rpm and 800rpm in neutral, slightly lower in drive) but methinks a hard reset after all the fiddling might be required (never been done to my knowledge - I certainly haven't)

Thanks for all the input folks - I really appreciate it!