Chugging

[Jona1971]

Might not be the best description but my car doesn't like partial throttle. Idles fine and drives sweetly through the rev range when I've got my foot down but is awful when on light throttle - hesitates, like its on/off or misfiring.

I've got new ht leads, distributor cap, rotor, plugs, even sent the injectors off to injector tune and it's still doing it. Timing and tappets fine too.

The car is a 1986 318i M10 auto.

Any ideas?
Thanks in advance

[Cloggy Saint]

I have the exact same problem with my 2.5. It was suggested I check the TPS.

[Jona1971]

Hi,
Yes I did wonder about that. I checked it when I bought the car but thought it was ok, perhaps I did it wrongly or overlooked something.

Any suggestions how to check, I'll give it another go...

[Brianmoooore]

TPS doesn't just have to click, it has to click when it should, and that's immediately the throttle leaves its stop.
The hesitation is more than likely the result of a misfire caused by a lean fuel mixture.
The mixture should be lean at part throttle, to increase efficiency, becoming closer to maximum power ratio when the throttle is fully opened and the WOT switch turns on. The WOT switch in the TPS could be faulty, so needs checking, although it's usually reliable.
Another possibility is an air leak into the inlet system. At light throttle openings there is a considerable vacuum in the inlet system, which will draw large quantities of air through a very small leak. At wider throttle openings, the vacuum in the inlet is much less, so fat less air will be drawn through the same leak, and may not affect the running at all.
A leak of this type would also allow a lot of air to leak in at idle, and would also upset tickover, but the engine is running on a different map at that point, and may just run OK.

[Jona1971]

Thanks Brian,
The car idles beautifully. There is a definite audible click immediately after the throttle moves off it's stop.
I'll check the TPS and for an inlet leak. Any advice on the best ways to do this would be greatly appreciated.
(I must have overlooked something last time)

[jmec]

This may help... it may not! My 325i 1990 suffered from 'chugging'. TPS was doing what it should. Boot was renewed. New leads cured it, for a while! The injectors were refurb'd and all seemed fine until a few days ago when it returned about 200 + miles from home. A specialist tracked the fault to the AFM sticking intermittently. The cover showed signs of being resealed and the mechanism 'tweeked'. The only replacement that could be sourced to allow me to continue on my travels was from an ETA. So that was fitted. All, so far, has been fine with a 100 + miles since. Question: couldl there be problems with the ETA AFM fitted? PS: thanks to Hurst Automotive Leicester.

[Brianmoooore]

The ETA AFM is incorrect for your engine, and will prevent it working at its best, but how much effect it will have is impossible to say.
I would guess the the 'E' meter is set to "max out" at considerably less airflow than that for the "i", so may be going weak at peak power. Of course, it may be that the 'E' meter is already rich at part opening, and might just head back towards correct mixture at peak power.

[Jona1971]

Thanks guys.
I been watching YouTube videos of smoke tests and will try that this weekend, see if there is an air leak.

I've also ordered another TPS to swap on to see if it makes any difference.

[jmec]

Interesting Brian, thanks. I'm just happy for the car to work right now and hopefully let me continue with my 'Grand Tour'. As you rightly say it's not the correct part and there must be some downside. I will see about replacing it when I get home.