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MonkeyChap

Hunting and stalling: 3.2 Carrera

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Posted (edited)

Morning,

I brought this up a while ago, and got some really good suggestions about what might be going wrong. Basically, the car hunts at idle when cold. So much so that it will regularly stall when braking with the clutch down, and when sat at traffic lights. I've replaced the air bypass valve. I've warmed the engine up, bridged the idle contacts and set the idle screw. The problem still persists, and might even be a bit worse since I replaced the valve (with a brand new one). 

A while back I found a thread on one of the forums about early 3.2s having a different ECU map to the later ones, and the later maps causing this hunting problem (I now can't find the thread on google otherwise I'd link it). I currently have a later ECU in mine (courtesy of Sladey) and I wondered if that might be the problem. I also have a CANEMS ECU, but that has a single map in it, and I don't think there's an alternative. It is programmable though, so if I knew what to tweak, I could have a go. Anyone else had this problem ?

Ta

Ian

Edited by MonkeyChap

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I had similar on my old standard 3.2, although it wasn't as bad as didn't stall, just dipped to 500 rpm and then back up. Are you running rich as I thini this was mu issue? I did all the idle speed correction stuff like you to no avail. I took it to a specialist who swapped the air flow metre for a known good one on another customers car that they had in and it resolved the issue. Had to give that back though so i was back to square one but at least it pointed at the AFM as the cause. I opened it up and moved the little needle that moves across the conductive surface to a cleaner in worn area (there are some instructions somehwere on how to do that) which helped a little. Sold the car soon after.

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I haven't looked at the AFM. I'm tempted to go back to the CANEMS box and fit their upgrade AFM. Not sure they're still in business though.

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Further to last mail - CANEMS are still in business and going strong. Their website just takes a little while to load. I spoke to David at CANEMS and he confirmed that early 3.2s do require a slightly different fuel map to the later ones. 

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When I first got my car, using the cruise control would make the idle go ragged afterwards.  "The Internet" suggested there was some stiffness in the throttle control system not allowing the idle position switch to be fully engaged, and indeed, a little light oil on all the moving components involved fixed the problem instantly.

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I'd not thought of that - got to be worth a try ! 

I can't help thinking stickiness in the throttle cable would more likely produce a high idle though ? My car will actually stall on closed throttle and there's a a definite lag in it picking up the revs - like the idle control valve is too slow. 

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I would also think worth looking air fuel mixture, it's pretty easy to test and fix on an SC but I don't know for a 3.2.

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Pretty sure there is an idle mixture screw somewhere and (at least on the stock ECU) there is the fuel quality switch which also has some impact on mixture. I remember having to turn that a notch when I fitted a Steve Wong chip.

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This sounds quite similar to the issue that my car used to have which was caused by the AFM.  This was diagnosed by someone who has worked with these cars for 3 decades plus and as:

- I wasn't doing many miles for a couple of years, I left it (and learned to drive around it) and
- Knew how much the part cost

I left it for a while.  Last summer I finally accepted my fate and had it changed before I went to drive a few thousand miles in it around a 

If this is the fault, changing the part will transform the car…but I would make sure this is the fault as a new (even refurb) AFM was £695.58 last year.  Some say they can be cleaned / re-tracked but this is, I believe, really just kicking the can down the road.
 

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Thanks for all the tips. It sounds like CANEMS is making their AFM kit again (the previous sensor was out of production but now there's a new one) so I might go ahead and just fit that. It's a much more sophisticated sensor than the stock 3.2 one, so probably worth doing even if it isn't the problem. I wasn't aware there was a mixture screw on the ECU - got to be worth a look too. If I go back to the CANEMS unit, I can set the mixture with my laptop. Tempted to go the whole hog and fit a wide-band lambda sensor. I must be getting bored with all the lockdownitis. 

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There is a hole on the back of the ECU that you can access from the rear footwell. Its usually plugged with a plastic cap. Behind this is a triangular shaped female adjuster and it turns with 'clicks' you can feel. More info here:

https://www.911chips.com/fuelsys.html

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Thanks - I don't have a gas analyser so is it just a case of suck it and see ?

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I would just baseline where you are set now so you can always go back to how it was and then just see if the hunting and stalling stops or improves with a different setting. If you find its fixed, you could always ask a friendly MOT man stick his tester up your pipe. You can feel when the adjuster moves to each of the positions as you turn so just count them.

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I think I’d try to identify the root cause before changing to a different setup with the Canems EMS - this may just make it harder to work out what’s going on.

I may be able to help - I’ve got a spare early 24pin ECU here and a spare AFM that I know’s good too. If you can get over to mine (once lockdown permits..) you’d be welcome to see if either of these solve it. I’ve also got a portable lambda kit that we can use to check your idle mixture with.

Have you check your cylinder head temp sensor resistance versus temp - is it one of the later two wire ones?

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That's a good point - I should work through the possible issues one by one. I've already replaced the air bypass valve (which actually made it worse). I spoke to Wayne at Chip Wizards (the ECU is one of his) and he reckons the usual culprit is two transistors on the board that open and close the air valve and which tend to deterioate over the years. Easy job to fix, apparently. In order of likelihood (biased by how easy to fix) I reckon:

1) swap in the CANEMS box with a stock map on it - that'll tell me if it's an ECU problem or something in the air / fuel system

2) if the ECU is the problem, replace the transistors

3) check the cylinder head temp sensor is working (maybe do that while the CANEMS box is in, as I can see the result on the laptop)

4) adjust the idle mixture screw

5) swap out the AFM (may be go with a CANEMS one)

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to fix the problem could as simple as lean the fuel mix..if the engine dies  it is too rich..when was the last time you have seen or did fuel mix adjsutment??also how old is your fuel filter??

Ivan

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Good questions. No idea if Nick put a new filter in when he restored the car - I assume he did as the tank was out. I probably changed it when I replaced the pump in 2013. It's only done a few thousand miles since then.

I wanted to fit a wideband lambda anyway, so this is a good excuse to do it, and then I can set up the idle mixture.

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idle mix can be set by listening to it by good Porsche 911 mechanic...at least i can.....;-)

 

Ivan

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Had time to tinker a bit today:

* Fuel filter is only a couple of thousand miles old

* Attaching the CANEMS ECU showed that the cylinder head temp sensor is working fine

* ...and there was also hunting at idle with the CANEMS unit. It never dropped low enough to stall the car, but it did rev higher than the Motronic (up to about 1500 rpm at peak)

* Throttle position sensor is working

So it looks like there's nothing wrong with either ECU. I've swapped out the air bypass valve (the old one passed the shake and resistance tests). It's also unlikely to be a rich mixture problem as both ECUs are causing hunting. AFM seems an unlikely culprit as it shouldn't be playing any role at idle when the throttle position switch is reading closed. I guess it's possible i bought a duff bypass valve (new) but that seems unlikely. A few people mentioned air leaks at the inlet manifold gasket, but apparently they tend to happen when the engine is warm. 

I bought an old handbrake when the car was restored which has a manual throttle lever. I'm tempted to remove the whole bloody air bypass system and just connect in the throttle lever !

Ian

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Posted (edited)
12 hours ago, MonkeyChap said:

Had time to tinker a bit today:

* Fuel filter is only a couple of thousand miles old

* Attaching the CANEMS ECU showed that the cylinder head temp sensor is working fine

* ...and there was also hunting at idle with the CANEMS unit. It never dropped low enough to stall the car, but it did rev higher than the Motronic (up to about 1500 rpm at peak)

* Throttle position sensor is working

So it looks like there's nothing wrong with either ECU. I've swapped out the air bypass valve (the old one passed the shake and resistance tests). It's also unlikely to be a rich mixture problem as both ECUs are causing hunting. AFM seems an unlikely culprit as it shouldn't be playing any role at idle when the throttle position switch is reading closed. I guess it's possible i bought a duff bypass valve (new) but that seems unlikely. A few people mentioned air leaks at the inlet manifold gasket, but apparently they tend to happen when the engine is warm. 

I bought an old handbrake when the car was restored which has a manual throttle lever. I'm tempted to remove the whole bloody air bypass system and just connect in the throttle lever !

Ian

My standard 3.2 hunted at start up and then runs unevenly when warm.  I now have all of the inlet side off, originally to do the triangle of death but suffered mission creep, as I discovered air leaks.  Simple to check by spraying carb cleaner around the various joints.  I had 2-3 leaky joints between manifold and cylinder heads. 

Edited by Fuchs915

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very simple test can be done..if you have A/C put it on and step on the throttle then let go if it dies it is too rich.....these cars are not complicated;-)

 

Ivan

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44 minutes ago, proporsche said:

very simple test can be done..if you have A/C put it on and step on the throttle then let go if it dies it is too rich.....these cars are not complicated;-)

 

Ivan

Well...for an 80s car I think they've got a pretty convoluted air and vacuum system. Not to mention the two-bolt inlet manifold design on a head that's designed to run a bit hotter than a water cooled one. No AC, but I take your point about running too rich. It seems unlikely as the problem exists for two different ECUs with different maps in them unless the injectors are a getting a bit worn and dumping too much fuel for the car to cope with at low revs. Oh well, I'll put the Motronic back on tomorrow and twiddle the idle enrichment doo-dah just in case.

Remaining candidates are..

1) air leak at manifold gasket

2) vacuum leak to fuel regulators

3) running too rich at idle (might have to wait for the lambda to arrive for that one if the twiddling doesn't work)

4) AFM is buggered (after 129k miles this isn't that unlikely)

5) ...and I only just read about this one...as well as the throttle position sensor (open /closed) there is also a microswtich that activates on fully closed throttle. I hadn't even spotted that one, so maybe I'll check that out too

 

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you are running the lambda? for a reason- emission? These engines run much better without the system in circuit..i do have mine but i do also have a bypass when i have to go for test every 2 years...Back in LA i have passed the test even without the cat and lambda..mind you California emission standards are the toughest in the world..

Now i do it in France and always pass with flying colors...

 

Ivan

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FWIW, my money is on an air leak somewhere - classic symptoms are hunting tick-over - as mentioned earlier, I'd start by spraying brake cleaner around the running engine and watching for a rise in revs.

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Quick update - I sprayed carb cleaner around the base of each manifold leg. No change in behaviour.

What is weird though is that the car was absolutely fine starting from cold yesterday. On the Motronic ECU there was no hunting, and the idle settled quickly down to 900rpm and held there. There was a bit of hunting with the CANEMS box plugged in, and the idle was very high, but nothing compared to the day before. The only real change was the weather - from overcast and rainy to sunny and dry. If the AFM had been a hot-wire job, or the whole thing was MAP I could see barometric pressure being a factor, but not with a flap sensor. I suppose the other change was that I unbolted the Motronic from the car floor as it was a bit of a faff constantly plugging and unplugging it. Can't see that affecting anything though. 

With the car running cold(ish) at idle, I tried adjusting the fuel quality switch on the Motronic. It made no difference whatsoever on any of the settings. 

@proporsche- the reason for buying a (wide band) lambda sensor is that it's cheaper than buying a gas analyser, and all the garages are shut at the moment. Also, with the CANEMS ECU, I can wire the sensor into the ECU and use it to control the fuel map at partial throttle and (maybe, not sure about this) at idle. 

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