Poor acceleration at high speeds

[Watosama]

Hello,

I've been searching forums for a clue to what may be wrong with my poor car.
Alas, that search failed, so here's a post to ask for help.

Basically, my 1998 318i has been running fine for most of it's life. Last week, I noticed a considerable drop in power while on the motorway. I couldn't get the car past 70mph/ about 3000 rpm in 5th, and 4th can't get above about 3500 rpm.

The engine sounds fine when idle, and is running steady. 1st through 3rd gear is okay. There is still a drop in usual power, but not that much, and the revs can go all the way up.

There's no obvious juddering while driving, it just simply runs out of power as speed increases. I've changed the plugs anyway, and replaced the MAF. The fuel filter was changed last year and looks fine. None of this has fixed the problem. Also, theres no engine check lights on, and diagnostics show no errors.... All in all, everything seems like it should be fine.

Other than the helpful suggestion of "it's a 318....", can anyone suggest things to try/replace because I'm out of ideas and things to try.

Thanks in advance for any help.

[iwannabeam3]

Its the fact its a ragged out 318?

[DJCarbine]

Not sure of your emissions laws on the other side of the pond, but could it be a failing/clogged catalytic converter?

[slocar]

^ Could be, but I would think it would be struggling in the first 3 gears at high revs as well. :dunno

Honestly if it was my car, I'd try to get my hands on some new HT leads and a new coil ... they can and do fail occasionally, and it very well could be bad spark (that makes itself noticable at high speeds in a high gear when engine loads are high).

[DJCarbine]

^ Could be, but I would think it would be struggling in the first 3 gears at high revs as well. :dunno

Honestly if it was my car, I'd try to get my hands on some new HT leads and a new coil ... they can and do fail occasionally, and it very well could be bad spark (that makes itself noticable at high speeds in a high gear when engine loads are high).

Sometimes a bad cat will only manifest itself when it gets fully warmed up or if the engine is called upon for high load

A heat-soaked or faulty coil could be at fault as well

[flyfishvt]

My first thought was your cat. But I just dont think you need to start there. Take the advice about the plug wires and new coil. Its probably due for those anyway. One thing to think about is the Throttle Position Sensor. Its easy to test since its just a variable resistor. My bet is that it is failing. If you have a Bentley Manual I believe they give the test values you should be looking for. One of the symptoms of a failed/failing TPS is when you press on the gas nothing happens. This could easily start to happen on the upper throttle range or lower throttle range

[Pierre3400]

Well, you say it runs fine with only a tiny bit of loss in power at in 1st till 3rd.

Sounds to me like you got the same amount of power missing all over, the only change is, at speed the wind resistance is slowing you more, and making you notice it more.

It seems like you have covered the basics. Fuel filter would have been my first choice to change, i have no idea how you have checked it, since its not see threw is beyond me.

Cat is the area, if its collapsed, just like a fuel filter could do.
Next up, air filter, restriction of air flow.
If your 100% sure non of this is at fault, start work on ignition, even though it doesnt sound like its the error you looking for.

Remember, you also got a filter in the fuel tank itself, on the fuel pump.