Revs drop when quikcly pressing the accelerator pedal

[Rhyse30]

Hi guys.

I think this has been asked before in some way or form.

Driving an 89 320i with an 89 m20b25 in there.

Still having a few engine problems. Throttle response is not very good, mpg not so good, struggles to get up to correct idle speed when starting sometimes struggles around 400rpm . The thing that's mainly bugging me is, after the idle sorts it self out, when I rev the engine quickly (like a quick pulse on the pedal), the RPM goes down to about 200 like it's going to die then settles back around the 700 mark.

I was just wondering if anyone could tell me what do I need to look for that could be causing this that's not part of the actual engine. It used to do this on the m20b20 engine that used to be in the car. I was thinking fuel filter or brake vacuum lines maybe?

I realise it could coincidentally be the same fault with both engines but I've checked the b25 engine as well as I can and all seems ok.

Also, I took the spark plugs out just to check them about a week ago. They're only about 4-5 months old. Gaps checked before putting them in. The plug in cyl.3 had a gap half the size of the others, anything that would cause this?

The car has got worse since it had an MOT about a month ago. I swear the mechanic carrying out the MOT fiddled with a screw on the throttle body as the head of the screw has now snapped off, which I'm 99% sure it was fine before hand. Would incorrectly adjusting the idle screw on the throttle body cause this? (car did have a hole in the exhaust so would he of adjusted this screw to alter the emissions?)

Cheers!

[kam-325i]

messing with that screw would screw up the idle, There is a switch under the throttle body, this tells the ECU when the throttle is closed and to use the Idle control Module to regulate the idle speed. If this switch is not working right, it will give the symptom's you've described.

[Rhyse30]

Thanks for the reply Kam

I took the throttle body off last week and check the TPS. It seems to be working as it should. I did think i solved it by cleaning the TPS. But it was actually the fact that I disconnected the battery for long enough so the ECU reset itself. The problem seems to go away if i reset the ECU but the reappears after a day or so of driving.

[Rhyse30]

Just a thought... If I have a hole in my exhaust (around where the down pipe attaches to the rest of the system), would the guy MOTing the car have possibly adjusted the idle screw so it would pass the emissions test and cause the problems above?

[Grrrmachine]

The TPS might work on its own, but if the screw on the throttle body is in the wrong position then the TPS will never work. Try winding it all the way out, then gently back in until you hear the TPS click. Once you've found the click point, wind it back out half a turn, and operate the throttle arm to confirm it clicks as soon as the cable is pulled.

Besides that, check the part number on your AFM to ensure it's the correct M20B25 version, not the old one from your 320i. I've seen these symptoms with a duff/non-connected AFM, so if you've carried this unit over from your original engine then that may also be the cause.

Other than that, I'd suspect an air leak on the intake side, particularly from the rubber bung underneath the throttle body. Any air leak will cause the air/fuel mixture to be way out, leading to hesitation/stalling/power loss/lumpy idle.

[Rhyse30]

Thanks for the reply Grrrmachine.

I bought the AFM from someone on here and I'm sure he would have given me the correct one but I'll check this anyway.

When I went to check the throttle body (which I also bought from someone on here and he usually very good) the screw had snapped. This was straight after an MOT. Hence my thought on the MOT guy tampering with it. I have adjusted the TPS so that it clicks as soon as the throttle cable it pulled but if the idle screw has been messed with, would it still cause this even if the TPS seemed like it was working properly (tested with a multimeter)

I checked for and air leak. I do suspect a possible air leak on the intake manifold. I checked for that rubber bung underneath the TB but I couldn't see it. And I couldn't see any kind of hole underneath where the TB would need to have a plug in it...

I'll check the part number on the AFM just to make sure. Is the rubber bung or the whole where it plugs really obvious on the bottom of the TB?

Thanks again!

[Grrrmachine]

On the bottom of the TB should be a stub pipe. This connects either to the carbon cannister pipework, or is blanked off with a rubber bung. If you have neither, then you have a short metal pipe that's letting unmetered air directly into the intake, and you need to block it off.

As for the idle screw; moving it in any way will bugger up the TPS, which is why you need to wind it back out as I wrote before.

[Rhyse30]

I get what your saying about the idle screw and TPS. But I think the mechanic moved the screw while the car was in for it's MOT, then I tried to fix the problem by adjusting the TPS by loosening the two screws at the bottom and tightening them so that the TPS clicked when it was just off the stop.

I don't quite understand the idle screw. Doesn't the screw just control how much the throttle is open when there's no tension on the throttle cable? I'm not even sure if I'm looking at the right screw to be honest!

If I have already adjusted the TPS to click when the throttle is just opening, how can I reset both the idle screw and TPS back to their original position without a correct reference point to start with. To properly adjust the idle screw back to factory, wouldn't I need my engine to be running in perfect condition and have a exhaust gas analyzer?

Sorry if this seems frustrating to you guys, but I'm just trying to understand what's going on and how to fix it.

Thanks again!

[minesapint]

I'm getting confused here, which screw are you talking about? Is it the operating lever limit set screw, shown with yellow sealing paint on pic below, or idle screw which I thought was on AFM under red cap (never to be touched according to Mr Moore)



I thought at idle the ICV was the control & it lets the air into the manifold (by-passing the throttle body) via the AFM & the screw under the red cap (never to be touched) regulated this. The throttle body being, as near as damn-it closed.
Have I got this wrong.
So which screw are you talking about?

[Rhyse30]

I think I'm a bit confused between the two screws. I'm talkin about the yellow screw in your picture. Mine has been snapped off which is why I think someone's adjusted it. What difference does this screw make if it's tampered with? I thought the idle control valve controls the idle when the tps is closed, but if the yellow screw has been adjusted , could it be opening the throttle a little so some air is getting in? Or would this not matter?

Again sorry for any confusion.

Any advice in this welcome!

Cheers!

[Brianmoooore]

There is no such thing as an 'idle screw'. The one with the yellow paint is the one that sets minimum throttle opening, and in the absence of electronic idle speed control via the idle control valve, would be the idle screw. It should never be adjusted.
The one under the red cap is either a mechanical adjustable bypass for the AFM, or a potentiometer doing the same thing electrically. It trims the fuel/air ratio, with its most effect at idle, and again, should never be adjusted. If the air/fuel ratio is wrong, then there's a reason for it, which should be tracked down and sorted, not by making adjustments which will counteract the fault at idle, and mess up the mixture at other engine speeds and loads.

[Brianmoooore]

Back to the OP: If the engine takes several seconds to 'sort itself out' after starting, as well as being slow to start, it suggests that an injector (or more than one) (or just possibly the fuel pressure regulator) has leaked fuel into the manifold while stopped.
The lack of initial pressure will make the engine slow to start, and the engine will run rich until the surplus fuel in the manifold has been drawn into the engine and burnt.
Hesitation when accelerating from idle can be caused by an incorrectly adjusted throttle position switch. The switch must switch off of the idle map as soon as possible after the throttle leaves it's stop, consistent with switching back when the throttle is closed again.
Hesitation is also caused by a weak fuel mixture, which is likely to be the result of one or more air leaks into the inlet tract after the air meter, or by fuel injectors that are way down on performance, and badly in need of a rebuild.
The nipple that should have a rubber cap on it, in the absence of emissions equipment including a carbon canister, is on the underside of the throttle body, and almost touching the throttle position switch.

[Rhyse30]

Thanks for the explanation Brian! Much appreciated.

I'll check for all the things you've mentioned after work.

I just want to ask, if the min throttle opening screw has been adjusted one way or another, how would this have affected idle?

I was having trouble starting and a low, pulsating idle. So I took the TB off to test the TPS and adjust it so it was open as soon as possible after the throttle leaves it's stop. If the stop screw was adjusted incorrectly in the first place, and I adjusted the TPS to work with the stop screw in it's current position, would this cause anything to happen to the idle or hesitation when accelerating from idle?

The screw has sheered off since it's last MOT so I'm sure the mechanic played around with it. Either way, I need to get the screw out and replace it. Are they available to buy? And how do I set it correctly?

Cheers!

[minesapint]

http://www.e30zone.net/e30zonewiki/inde ... ottle_Body
This is good simple reading from the Wiki.
I guess your throttle stop screw could be holding open the throttle valve slightly & allowing air to pass into the manifold along with air via the ICV, even though you've adjusted the TPS at idle. The stop screw has a transparent anti-tamper cap on the screw head, (see photo above) as well as sealant on the threads for a reason!
Throttle stop screw shown as not available, but is M5x20.
Don't think this is your major problem, although it is an irritant.
Cheers

[Rhyse30]

Thanks for the link. I've read that a few times now. This might be a very stupid question but what's an 'xx feeler gauge'? Is this just a normal feeler gauge of an 'xx' thickness? Any idea what thickness gauge to use?

I don't have an anti-tamper cap or any sealant on the threads.

Cheers for the screw size, I'll see if I can find one.

Ye I don't think it plays a big part of my problem but now I've come across it, I might as well get it fixed properly. Next thing it to test the FPR, I've checked the hose from the FPR to the manifold and there isn't a hint of petrol in there. I'll also check the fuel pressure.

Then manifold off, send the injectors off to injectortune as I wanted to do this anyway. While I'm there I'll replace TB/manifold gasket and I've heard it's a good idea to replace the o rings on the oil return pipe?

I'll see how the car runs after all this is done.

On a side note that could be relevant. When driving to work and on my lunch break today, I noticed two high pitch whine coming from somewhere. It sounds electrical. The first lower pitch whine occurs when the revs are around 2000-2500 rpm, and then when its about 2500 - 3500 rpm the whine pitch changes to a higher pitch. The whine pitch doesn't increase with engine speed or car speed. Just two different notes depending on where in the rev range I am.

I couldn't tell exactly where the whine was coming from. It didn't sound like it was the fuel pump but then again, I don't know what a struggling/failing pump sounds like. I did think it sounded like it was coming from the ECU but again, I don't know why the ECU would be making any sound at all. It definitely not the alternator/PAS or belts.

Also, every time I disconnect the battery to take the TB off or something similar, when I reconnect everything the car starts fine and idles where it should be hot and cold. It's only after a few start-ups or a day of driving the problems come back again.

Sorry to go on but I remembered something else that could be related. Since this problem has started, if I lift off the throttle so it's now closed while the car is moving, the car judders (or kangarooing as I've heard some call it). I don't think it's anything to to do with the way I drive as it's only started recently haha.

Thanks again!!

[Grrrmachine]

what's an 'xx feeler gauge'? Is this just a normal feeler gauge of an 'xx' thickness?

Yes. Whoever wrote that article ( ) used xx as a placeholder while researching what the thickness was, but never updated it when the actual thickness was found. The answer is 0.0015in.

[Brianmoooore]

The correct way to set the throttle stop, AFAIK, is to replace the ICV with a special BMW ICV that has a fixed opening, and then to adjust the stop for a specified RPM.
I'm not a fan of the feeler gauge method - the way I set them is to remove the ICV, block off both hoses, and then set the stop for a very unstable idle speed of around 400 - 500 RPM. The sort of revs. where it looks like the engine is going to rip itself off of its mounts. I arrived at this figure by blocking off the ICV on an unmolested engine in good condition, and noting what happened.
Seems unlikely, but this engine doesn't have a lambda sensor, does it?

[Rhyse30]

Haha thanks Grrrmachine!

Would you recommend I try your way of setting the screw Brian?

.... I found out last night that.... I may have made a BIG mistake when doing the engine swap...!

If there's a zone award for the stupidest mistake done I think I should be in the running for it!

I found my old ECU from the previous b20 engine last night. Or so I thought.... it ended in 173. Took out the ECU that's currently in my car ... It ended in 172! So it looks like I installed the b20 ECU back in the car while doing the swap and have been running it for the last 6 months or so. Could this be causing some of my problems??

[shedrool83]

The correct way to set the throttle stop, AFAIK, is to replace the ICV with a special BMW ICV that has a fixed opening, and then to adjust the stop for a specified RPM.
I'm not a fan of the feeler gauge method - the way I set them is to remove the ICV, block off both hoses, and then set the stop for a very unstable idle speed of around 400 - 500 RPM. The sort of revs. where it looks like the engine is going to rip itself off of its mounts. I arrived at this figure by blocking off the ICV on an unmolested engine in good condition, and noting what happened.
Seems unlikely, but this engine doesn't have a lambda sensor, does it?

Brian do you set the throttle stop with the engine hot or cold?

[Brianmoooore]

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