Air Flow Meter

[pentoman]

Hello,

What sort of running differences can (mis)adjustment of the Air Flow Meter make? Or having the AFM disconnected completely? I am of course talking about old style moving-plate air flow meters which sit in the intake and move down as air flows in.

I'm trying to solve a poor throttle response ("surging?" issue on my 1990 Mercedes 2.5-16. It feels just like turbo lag - there's good actual initial throttle response but then it pauses and takes a second for full acceleration to come in. At the weekend I tested my AFMs resistance throughout the sweep and it was progressive with no areas of zero or infinite resistance, so I think the contacts are fine. So in time-honoured tradition I disconnected it and drove around like that for a bit to see what happened :
When cold, my car's problem was amplified - throttle response was rubbish with a 2 second delay between pushing the throttle and getting full acceleration. No misfire, it just felt exactly like huge turbo lag.
When hot, it felt the same as normal, possibly a little more 'turbo lag'.


So, please can you tell me if this tells you anything? Should I look at adjustment of the AFM plate? I have the factory manual with instructions on adjusting it and can do it myself so it's a "free" option. I've checked it's not jamming and has the 1-2mm of play at rest.

Russell

[GreenV8S]

I don't know anything about the engine management system on that vehicle, but if the throttle pot is playing up it can often cause poor throttle response, because the throttle pot is usually the thing that triggers transient enrichment as you open the throttle.

[Graham]

When the afm on my td5 discovery goes tits up ( 3 of them now) it hesitates and judders under acceleration, as if it has a misfire..

G

[pentoman]

Excellent OK, so it does sound like it's potentially the cause of the problem. I just have to figure out how it is the problem... is it misadjusted? faulty? etc..

I believe Mercedes will not supply the pot on its own, only the whole throttle body which is jolly expensive, though I think you can source the pot on its own via other sources. Nevertheless it requires precise adjustment when you replace it and as far as I can tell mine has no problem anyway and might just be out of adjustment itself. The factory manual has steps for adjusting the position of the plate of the air flow meter (the throttle body has little pins you tap in to adjust) so I will look into this. Those Mercedes people thought of everything back then, although I'm not sure if they thought about whether anyone outside of the factory was smart enough to understand it all!

[100SRV]

I agree with GreenV8S - check the throttle potentiometer as well. The engine ECU relies on the throttle pot and airflow meter to indicate load / demand on the engine. Also try checking the fuel pressure regulator (if fitted) to ensure that it functions correctly (I am not familiar with any EFi system other than Lucas 14CUX) as this typically uses manifold depression to regulate the volume of fuel returned to the tank.. If this is faulty or there is a vacuum leak this may cause a momentary drop in fuel rail pressure and the resulting symptoms.

100SRV

[steve_d]

My fuel regulator failed on the RR classic 3.5.
When I went to replace it I was advised to fit a rising rate regulator which obviously is aimed at improved power rather than economy. Works well.
Just another thought.

My vote is still with the TPS.

Steve

[pentoman]

Unfortunately the car doesn't have a TPS. It's just got the air flow meter which connects to a pot. That's what I tested and seems OK. I have checked the adjustment (rest position) and that seems good.

[steve_d]

pentoman said:

Unfortunately the car doesn't have a TPS. It's just got the air flow meter which connects to a pot. That's what I tested and seems OK. I have checked the adjustment (rest position) and that seems good.

Sorry a little confusion here.
TPS = Throttle Position Sendor i.e. the pot you were talking about earlier when saying Merc only sell complete throttle body.

Steve

[pentoman]

steve_d said:

pentoman said:

Unfortunately the car doesn't have a TPS. It's just got the air flow meter which connects to a pot. That's what I tested and seems OK. I have checked the adjustment (rest position) and that seems good.

Sorry a little confusion here.
TPS = Throttle Position Sendor i.e. the pot you were talking about earlier when saying Merc only sell complete throttle body.

Steve

Ah sorry yes the 'pot' I meant was the potentiometer on the air flow meter, which generally comes as part of the whole throttle body.

Been on the diagnostic machine yesterday..

[Xenocide]

Compression's a bit low isn't it?

[pentoman]

Xenocide said:

Compression's a bit low isn't it?

Yes! Cylinders 1 and 3.
Could do with a top end rebuild as I knew already. Preparing myself mentally and financially for it as I'd also want to do chain, tensioner, etc..........