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Matt
Matt, Mechanical Engineer
Category: BMW
Satisfied Customers: 21607
Experience:  BEng hons Mech engineering, in auto industry 22 years
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I'm looking diagnostic on light throttle surge from

Customer Question

Hi, I'm looking for a diagnostic on light throttle surge from idle. Car is e39/M54 engine.Symptom: light surge in throttle at around 1500rpm, but only when I throttle quick. It's almost like double-revving when it gets at 1500rpm. Video here: https://youtu.be/pC5C8eqR7s0I just had the intake manifold off and replaced every breather pipe, checked every rubber hose, caps, seals etc. just so I can rule out a vacuum leak. ICV valve and DISA were cleaned/checked also and they are operating fine. There are no error codes whatsoever and the engine is otherwise operating fine, both at idle and under load.Anybody can give me some pointers? I have a full diagnostic suite (DIS/SSS/INPA/NCS Expert) and can pull operating values if needed.
Submitted: 1 year ago.
Category: BMW
Expert:  Matt replied 1 year ago.

hello

if you're certain that there's no vacuum leak then I'd say to remove the air intake so you can see / reach the throttle and clean it out with some brake cleaner and a lint free rag

and its also possible that the fuel purge valve may be sticking and so flowing fuel tank vapour when it shouldn't so try disconnecting it to see if that helps

(see image)

and the best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.

Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.

Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

Customer: replied 1 year ago.
Thanks, ***** ***** off the throttle body and as far as I could see it's working perfectly. The seal is good too. I disconnected the air box/MAF sensor so I could have a clear view onto the throttle body. When I blip the throttle, the butterfly valve opens and then springs back closed as it should. The surge happens after the butterfly valve closes.I also disconnected the fuel tank purge valve pipe - no effect. Another thing I tried was to block the CCV to crankcase pipe, just in case it the CCV would allow oil to be sucked into the intake and effect the surge.Here is something interesting I saw in INPA. There's this thing called "OVERRUN" state. This overrun doesn't always come on, for instance it does't come on at all if I don't blip the throttle very fast, or if I don't let go of the throttle very fast, or if I don't rev it past 1200 rpm.The coincidence is, I only get the surge ONLY when I let off the pedal, and ONLY if overrun had briefly come on. Almost as if extra fuel is being injected after the overrun. If the overrun doesn't come on, no matter what the conditions I don't get the surge. Does that mean anything? I don't have an oscilloscope so I can't measure the signal to the injectors, but I've just re-flashed the DME unit, reset all adaptations and still the same.
Expert:  Matt replied 1 year ago.

hello

thanks for the extra information

I'm wondering if you have a faulty throttle position sensor or faulty pedal sensor if you watch the throttle butteryfly whilst your assistant slowly pushes the accelerator pedal down , then the butterfly should open smoothly and slowly with no sudden movement

Customer: replied 1 year ago.
Matt, the throttle actuates correctly but I've just discovered something about the ICV (which was already cleaned off). When I take the plug off, it still idles ok but the surge disappears. It looks to me it's causing extra air build-up by not actuating properly. The thing inside is very mobile and resting position is about what it should be, but if it doesn't actuates right - a.k.a. closing when getting the command from the ECU - it allows the extra air in. Am I close?
Expert:  Matt replied 1 year ago.

Hello

thats a good find, the ICV should be quite heavily sprung loaded so its held on its seat so it sounds like it might being moved about by air motion more than it should

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Customer: replied 1 year ago.
Ok Matt, that's us sorted then - thank you very much!
Customer: replied 1 year ago.
Hi Matt, can I pick your brain one last time about this?I just replaced the ICV and it didn't fix it. As I said, with the 3 pin plug disconnected it doesn't surge. Also, with the plug connected I physically jammed the valve inside so it won't move when the computer tells it to. Still surging, so we know the ECU-to-ICV signal is not responsible for it.Have you any idea what is actually happening here? Are we looking at an ECU glitch?
Expert:  Matt replied 1 year ago.

Hello

OK if the ICV is blocked and there's no vacuum leak I wonder if the issue is with the PCV / breather allowing air in when it shouldn't

I'd suggest tracing the breather pipe back from the cam cover to the inlet and consider replacing the PCV

Customer: replied 1 year ago.
Thank you Matt, i disconnected the 2 breather pipes at the top of the manifold and blocked the 2 ports to isolate it from the PCV. Still no fix. I even beefed up with some tape the rubber boot where the ICV is seated. No luck. At this point, there would have to be a crack into the manifold to account for a leak.I've noticed something else - when I turn on the engine from cold, it works fine for about 3-4min. and then it starts this surge on overrun. The idle drops to standard 700rpm way before the fault kicks in. Correct me if I'm wrong, but isn't the ECU ignoring sensor reading for the first few minutes? Might point out to a sensor sending out garbled data. Or the ECU.
Expert:  Matt replied 1 year ago.

Hello

well that's possible

but if there was cracked manifold then I'd hope that the vacuum leak test with the lighter gas would show this up?

however if the manifold crack opens up with heat then this could be possible

have you tried doing the vacuum test with a hot engine?

Customer: replied 1 year ago.
I did the propane test with a (relatively) hot engine. The thing is, fault comes in very sudden - revs fine up until one point and suddenly starts to surge on overrun.
Expert:  Matt replied 1 year ago.

OK

is this the plastic manifold version?

If it is then check along the seams for signs of leakage - what might be worth a go is running a couple of strips of heavy tape along the seams to seal it, just for a trial

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