Low idle causing stall when warm.

[superdamon]

Hi All,

I have an 87 325i cab which seems to have an engine from an 88. They kept motronic 1.0 but it started to have issues so I've converted to 1.3. Everything seems to be working except for the idle, it's not hunting much but the RPM is too low. When warming up it's around 750 and drops to around 450-500 when warm (based on tacho). When you blip the throttle, it drops below that before bouncing back up to where it should be. When warm, it often stalls if the revs drop too sharply.

The throttle looks to be set correctly (barely open) and the TPS is producing the correct readings at closed and open throttle. There is a bleed screw on the housing but it doesn't make much difference as I'm guessing the ECU is accounting for the change in mixture.

The coolant sensor is producing resistance in the expected range and the signal is reaching the ECU.

I've got an intake boot on order and a new FPR to intake manifold hose on the way. I've changed the FPR as I had a spare one. If I unplug the hose to the intake manifold, the revs rise to a more acceptable level but that's probably due to the unmetered air being sucked into the manifold.

I've switched out the air filter but haven't made any changes to the AFM yet. If I unplug the ICV, the engine stalls but isn't it meant to keep running with a hunting idle?

I've noticed a section on setting the base idle within the motronic 1.0 wiki though does this still apply for 1.3 (seeing as I have the early throttle housing)? The bit about shorting the TPS is interesting, does that still work for 1.3 as perhaps I need to tune the idle before the ECU kicks in.

Cheers

Damon

[Speedtouch]

Could be a number of things, ranging from the timing belt fitted 1 tooth out, to inlet manifold leaks (try spraying WD40 around it with the engine idling; if it temporarily raises the idle, then you have a leak.

I assume you've checked the basics such as spark plugs of the correct type and gap, and that all ignition parts are in good order?

Try cleaning the ICV out with carb/brake cleaner.

Yes, there is an idle map programmed within M1.3; try disconnecting the battery, leaving it for an hour or so to reset the ECU, then let it idle for at least 10 minutes to enable base idle settings to be learnt. Switch off, then take it for a drive throughout the rev range; this ensures full data transfer from the EPROM chip to the processor.

Best not to touch the AFM, unless you happen to have a CO meter, as they are factory set and changes to it shouldn't be necessary.

[Brianmoooore]

Is the TPS adjusted properly? Not only must it switch off the idle map when you start to open the throttle, but it must switch immediately it leaves its stop, otherwise you'll get a flat spot as you describe.
What condition are the injectors in? There are very few E30s out there that aren't in need of an injector rebuild, if they haven't been done before.

[Speedtouch]

May also be worth changing the fuel filter, if it hasn't recently been replaced...

[superdamon]

Thanks all, some good suggestions to check through. Fuel filter wise, I ordered one earlier today as it's a nice easy one to check off and it probably hasn't been changed in a while. I forgot to mention that the head has just been skimmed and the valve ports have been done. It's got new rockers, shafts and related gaskets. I did a basic service of the injectors with fresh filters, caps and o-rings but I didn't get them tested (sorta wish i did now!). The engine seems to pull pretty well though I haven't put many miles on since the rebuild. I'll check the TPS is switching off as soon as the throttle starts to open as that's a good point.

I've just done a more thorough clean of the ICV tonight and I've disconnected the battery so I can try a fresh map tomorrow. Once I've got the new intake boot I'll give the vacuum leak tests a go. I'll take a look at the timing belt too as I haven't checked that. It's got new spark plugs but I'll give them the once over nonetheless as you never know.