Idle Surge

[Rod 0]

My M30 engine has caught the idle surge disease. It runs fine off idle with no apparent loss of power, however intermittently it will not idle at the correct RPM. This idle surge occurs idependent of whether the engine is hot or cold.

This is a common problem with these engines, however so far I have not been able to ascertain exactly what is the cause. I have the Bently manual and I have done plenty of reading on the internet but I am none the wiser. Below are some comments and observations:

Possible very minor vacuum leaks in engine oil dipstick, and in connection of hose between idle stabiliser valve and intake manifold. These possible leaks however are not significant enough to cause this idle problem. No other apparent leaks after checking of all soft components, the three small vacuum hoses have been replaced.

AFM (Air Flowmeter) - have not yet checked the electrical performance, the vane however moves freely without binding. The AFM was replaced about 25000km ago with a brand new one, I have therefore assumed that this is probably working fine.

Idle Air Stabiliser Valve - This checks out perfectly okay with the electrical resistance values and the rotary plug moves without binding or friction. As the tests are static I am suspicious of this item.

Cold Start Injector - have not yet checked this, could be leaking?

Thermo-Time Switch - When hot, the first terminal of this switch is supposed to have a resistance value with respect to earth within a range of resistances, which it does. The second terminal is supposed to have an infinite resistance with respect to earth, however has a resistance of only 138 ohms. When cold, the resistance value for the first terminal is within range, and likewise the second terminal has 0 resistance with repect to earth, as it should. Possibly this switch could be faulty, however given this I would expect that when cold there would be no idle problem.

Fuel Pressure Regulator - This has no diaphragm leaks however I have not checked the actual pressure within the fuel rail. If this was faulty I would expect a continuous problem rather than an intermittent problem only upon idle.

Ignition System - Distributor cap and rotor cleaned, spark plugs are new, complete condition of spark plug leads not checked.

The wiring loom also appears to check out okay, the Motronic unit however could be the problem?

If anyone else here has experience with this idle surge problem I would like to hear you comments and conclusions given the above. So far I have not been able to identify what I believe is the problem component.

Rod

[nz320i 0]

pleeeease pm me if you know how to fix this, my 320i was doing it so badly tonight and i feel real nervous in it keep thinking the engine is going to die completely or something!

I will check all of the above if i have time tomorrow and see if i notice any difference!

It sometimes idles as if it has a huge lumpy cam in it?!

This is different but i get a whining turning over sort of sound from the engine, gets faster as i increase idle speed, sounds like a belt squealing or something!?!?

Do i need a cam belt change or something i dont know!!

cheers

[Gus 5]

i checked all the normal ones but it drove me nuts for ages....was just the idle mixture on the AFM...the allen key. tighten for richer, loosen for leaner

[Rod 0]

I have thought about the idle mixture as well, but the problem is intermittent, and why would this change all of a sudden? I will inspect the AFM to confirm its physical condition, but my suspicion is that there may also be another problem. This issue is also driving me nuts.

Rod

[Mwizi 1]

Check coolant temperature sensor as well - located on thermostat housing. Resistance should be 2100 - 2900 ohms when cold, dropping to 270 - 400 ohms when warm.

Fixed a similar problem on my 320.

Replacement unit $30 from Repco.

[bravo 35]

Also could be incorrectly adjusted valves.

Common of M20's too - you have identified all of the common causes, now I guess it is a case of elimnating them one by one which by the sounds of things is what you are doing. Infuriating I know!

[DCEIVN 0]

what do you know. Mine just happened to play up today. I blame you lot for starting the topic.

Cut out on me twice now, even when the engine was warm. Usually it just plays up on a cold start. Just get a quick start button so when it stalls you dont be bothered with the keys hahaha

[Carl 3]

what do you know. Mine just happened to play up today. I blame you lot for starting the topic.

Cut out on me twice now, even when the engine was warm. Usually it just plays up on a cold start. Just get a quick start button so when it stalls you dont be bothered with the keys hahaha

Have you checked that you haven't blown the intake apart again???

[DCEIVN 0]

HAHAHA

YES it s the first thing I checked!

ICV is still in the right place and everything else there looks fine...

[Rod 0]

Problem identified: Worn Thottle Body & Butterfly Disc

When I had previously inspected the throttle body I could detect any wear, however I took the car to East City Auto Electrical in East Tamaki who are Bosch agents this morning. They identified the problem very quickly. They have stated that they have managed to correct the problem for the time being through adjustments, and that the bill will be $67+GST. They also stated that they have scanned the Motronic system and have found no faults in fuel or electrical systems. Complete diagnosis in about one hour, spot on and economical.

This makes complete sense as butterfly valves are inherently unstable at less than 25 degrees of opening due to high gain. Any leakage flow would therefore be unstable causing the fluctuations in the idle speed. To avoid this unstable characteristic some high-end throttle bodies and carburettors use a piston or gate(flat slide) concept, however this design poses another problem in trying to eliminate clearance flow from around the piston/gate. Enough control valve perfomance installed flow characterisitics theory for today.

Since my last post here I had also checked operation of the following myself and found no faults:

Possible vacuum leaks - eliminated all known possible leaks, no impact on performance.

Idle Air Stabiliser Valve - replaced with new one to eliminate this as a cause.

Cold Start Injector - Confirmed that it is not leaking or stuck open.

Thermo-Time Swich - Confirmed that it is operating correctly using a noid light

Fuel Pressure Regulator - tested fuel pressure with accurate gauge, pressure was stable and within specification

Ignition Leads - all cylinders firing and all spark plugs correct colour therefore no issues with ignition components.

Always good to have a problem idenified and solved, just need to keep an eye out for a mint throttle body. Though I may be better off to buy a new one which is a little expensive at $656+GST but not prohibitive and may be a better idea long term.

Rod

[Coffee&Doughnut 0]

Me too, engine does not cut out but idle speed increses and often settles at 1500rpm

then will go back to normal 750rpm seems to spend half the time between the two speeds.

I recently changed the TB used one of course. But the idle problem only occurred when I switched the stock ecu chip back in?

Looks like I will be going down the same route, If I find out I will let yooz know.