180e AFM pot replacement

[MLC]

After searching this site I've been able to diagnose a problem I've been having with my 1991 180e. There's been a slight hesitation on acceleration from a standstill, which may not sound like much but these things are pretty sluggish even at their best (82kw for a 1200kg car is not a lot of power) and this problem was making it pretty much undriveable. Anyway, I was been able to isolate the problem to a worn air flow meter potentiometer. Disconnecting the EHA provided an instant fix which has transformed the car - no hesitation and better performance all round.

I was tempted to just drive it around forever with the EHA disconnected but wasn't able to find a clear answer anywhere as to whether this would do any harm in the long term. I'd read that cold starts would be harder but this hasn't been a problem - if anything it's starting better - and that fuel consumption may worsen, which hasn't happened either. It does run a bit richer which I can fix easily enough, but having the EHA disconnected does make it ping too, which has me confused because I thought that was caused by a lean mixture, not a rich mixture.

Anyway, the pinging is not a good thing to let happen for any length of time, and I'm not sure if having the EHA disconnected is doing any other bad things, so I decided to change the pot. For the information of Australian members I ordered the part (Bosch part no. 3437224035) through Repco. It was $120AUD and arrived in three days.

I found this post Aiflow meter pot source - Bosch CIS which gives some really clear instructions on how to replace and tune the new pot, but there are still a few questions I have that I hope someone can answer.

1. Based on advice from this site I believe I will have to disconnect the fuel regulator to replace the pot (capacity aside, the 180e engine is as far as I can tell exactly the same as the 190e 8v). That's OK, but my Haynes manual has a procedure for de-pressurising the fuel system which runs to about half a page, and I'm wondering if this is really necessary. From the posts in the thread given above it doesn't sound like it is, as long as you have some rags to catch the escaping fuel. I don't know much about fuel injection but am assuming that the fuel system won't need to be bled afterwards.

2. Summarising what I've found on this and other forums about tuning the new pot, the engine should be idling and when the pot is correctly positioned the output voltage from pin #2 to ground should be 0.7v, +-0.25v. That much is pretty clear, but should this be with the pot's wiring plug connected, disconnected, or doesn't it matter?

3. Are there any fault codes that need to be cleared after this has been done? I'll be getting 70,000km service done pretty soon and if anything's going to show up as a fault I'd like to let the service guys know beforehand so they don't try fixing what ain't broke anymore.

Thanks

[sbourg]

I removed the pressure regulator from only the fuel distributor end, and not much fuel leaked out, and it still gave me the access I needed. Mark the position of the old pot when removed - the new one will be close if set in the same position.

Do not count on replacing the pot to solve your problem. The EHA is controlled by analysing many different inputs, so disabling it simply points to a problem with one sensor. When the pot was causing problems with ours, the symptom was an irregular idle with hunting. This is to be expected, because the pot wiper wears this position (idle) more than off-idle spots. Even though the pot determines acceleration enrichment, this is off-idle operation and is not as likely where it would be worn.

Steve

[MLC]

Originally Posted by sbourg

Do not count on replacing the pot to solve your problem. The EHA is controlled by analysing many different inputs, so disabling it simply points to a problem with one sensor. When the pot was causing problems with ours, the symptom was an irregular idle with hunting. This is to be expected, because the pot wiper wears this position (idle) more than off-idle spots. Even though the pot determines acceleration enrichment, this is off-idle operation and is not as likely where it would be worn.

Steve

Thanks for the reply Steve
Do you have any thoughts on what else might be causing the hesitation, if not the pot? With only 65000km on the clock I admit that I was a bit surprised that the pot would be worn enought to cause this problem but from searching this and the 190revolution forums there seems to be a fairly consistent link between a worn pot and acceleration hesitation. I'm open to any other possibilities though. I had considered the O2 sensor and the OVP but couldn't find any posts that indicated a correlation between a dud O2 sensor or OVP and this kind of hesitation.

Thanks

[sbourg]

Instead of unplugging the EHA, unplug the pot. If you notice a difference in the hesitation, the pot is likely good. Confirm by marking its position, then removing and inspecting for wear.

Steve