3000RPM fluttering and backfire before carrying on just fine.

[Stevieabz]

Hey guys,

A little back story first.

325i touring 1989 sat for 6 years.

I bought the car sight unseen and not running. It arrived on a truck with the promise of a new fuel pump arriving soon after. A couple of days a later the in-tank fuel pump the previous owner had sent me arrived. As I was so busy I didn't have time to fit it myself so I gave it to my local garage.

Before fitting the new pump he wanted me to have new fuel AND brake lines front to back. So I did. He then got the car running but it was only running on three cylinders. I then took the car back off him.

Once home, I found time for it and did the following: minus the top three.

New fuel pump
New fuel lines front to back
New Brake lines front to back
New Brake pads all around
New Brake discs all round
New Fuel Filter
New Oil FIlter
New Air FIlter
New Spark Plugs
Injectors professionally cleaned
New Throttle body Boot
New Oil
New Coolant
New TPS
New Thermostat
New HT leads
'New' second-hand ECU as this was why it was running on only three cylinders.

After that, the car was running great and has been for months now.

About a week ago or so I got in the car to go to the garage for new tires, as the old ones were from 2001 lol. Everything was fine. It was only when I got onto the bigger roads that I noticed that the car was absolutely fine until it reached 3000rpm then it would stutter through to 4000rpm backfiring like crazy and then rev absolutely fine to 6/7000rpm.

You wouldn't know it was happening if you didn't push the car a little. It only happens under load. I can rev the car all I like in my driveway and not replicate it. only when I'm driving. If I drive extremely eco it's hard to tell. But, the more I put my foot down the worse it gets. The bigger the stutter can be.

Any help would be great. I put some Radox in the fuel tank today before filling up to see if that helps and it did a tiny bit, I'm worried its rust in the fuel tank as it was sat for so long and the fuel pump itself was rusty as ever.

But I'd really love for it not to be that, so any suggestions would be great.

Cheers.

P.S I need to drive this to Spain very soon.

[paultv]

It sounds like you are missing sparks during that rev range, and dumping un-burnt fuel into the exhaust system, hence the backfire.

There's no mention in your extensive list of replacement parts of the distributor cap and rotor, are they in good shape?

Is your crank position sensor correctly mounted, and distanced? or covered in crap? as this affects the ability of the management system to fire at the right time or at all.

I had a random loss of power in the same range - just missing, no backfire - turned out to be a knackered HT cable with a burnt out resistor causing internal arcing, but you say yours are new.

Good luck with it

Paul

[Stevieabz]

It sounds like you are missing sparks during that rev range, and dumping un-burnt fuel into the exhaust system, hence the backfire.

There's no mention in your extensive list of replacement parts of the distributor cap and rotor, are they in good shape?

Is your crank position sensor correctly mounted, and distanced? or covered in crap? as this affects the ability of the management system to fire at the right time or at all.

I had a random loss of power in the same range - just missing, no backfire - turned out to be a knackered HT cable with a burnt out resistor causing internal arcing, but you say yours are new.

Good luck with it

Paul

Cheers Paul,

The cap and rotor looked new, I was surprised. The HT leads aren't brand new but came from my other E30 that had no issues. I will take a look at the CPS.

Much appreciated.

Steve.

[Brianmoooore]

The other thing I'd do is to check out the AFM for a 'drop out' at a particular position of the door. This is best done with a battery and a volt meter.

P.S. Not entirely convinced that a new ECU was the cure to running on three cylinders only, although it's possible. The usual cause is corrosion of a wire inside the lower rubber boot of the injector loom plug and socket, fitted to the black metalwork under the inlet manifold.