The M20 Crap Throttle Response When Cold Debate.

[scott180gtb]

There seems to be loads of post about poor idle/rough running/heavy petrol smell relating to the M20.

I'm trying to get to the bottom of an annoying cold throttle response problem I'm having.

The only way to describe it is when the engine is cold the idle is smooth and steady at 850 rpm. approx

The problem comes with light throttle inputs. The engine seems to slightly miss/cough until it reaches about 2000 rpm and then pulls clean. Its really prevalent between 1500/2000 rpm

This just makes the car a bit kangaroo like when cold.

Is this just one of those M20 characteristics that is normal and I'm expecting too much from a 20 year old engine.

I checked quite a lot last year but I'm going to start a fresh today.

What's first?

I'm thinking of checking the valve clearances as I never did this last year?

[Grrrmachine]

Have you had the injectors rebuilt? They'd be first on my list.

[daimlerman]

Some-one scanned in a road test report from back in the day a year or three back.
The writer made the same comment,so it seems to be something that we have to learn to live with.

However,a live re-map by some-one who understands the engine/ECU does help.

[scott180gtb]

Have you had the injectors rebuilt? They'd be first on my list.

Yes Its had the injectors cleaned.

I'm going to adjust the valves first and see what happens after that.

[Rav335uk]

Blue temp switch overfuelling the Car scott?

[Brianmoooore]

There seems to be loads of post about poor idle/rough running/heavy petrol smell relating to the M20.

And nearly all of them will be cured by eliminating inlet side air leaks, having the injectors rebuilt, checking that the TPS is working (I.e. not full of oil), that all the adjustments that shouldn't ever be touched are at their factory settings, and the few that do need adjusting are correctly set.

[Morat]

Scott, did I see you driving a red 2 door on the Ring Road near Monks Cross on Saturday morning?

[scott180gtb]

Scott, did I see you driving a red 2 door on the Ring Road near Monks Cross on Saturday morning?

Not me dude.

I'm sure I checked the blue sensor last year, but will check again.

Is it meant to be 4500 ohms when cold?

I've got a bently manual and worked through a few of the checks last year.

What is the best way to check for an air leak? I don't have lumpy idle issues its only on light throttle when cold that there is hesitation/miss.

[Rtaylor2208]

For air leaks you want to spray carb cleaner on all the vacum pipes or basically anywhere that air can leak. The usual suspects are:

1. failed rubber bung on the underside of the Throttle body
2. Failed orings on the oil breather pipe between the inlet manifold and the block
3. Breather pipe from the throttle body to the rocker cover
4. All vacum pipes particularly the large boot between the airbox and the Throttle body
5. Throttle body gasket
6. inlet manifold gaskets
7. Dipstick orings
8. Oil filler cap

And potentially the D Shaped bungs that fit in the ends of the cylinder head. Usually you would also have leaking oil as well if these are on there way out.

[scott180gtb]

What is the optimum gap for the valves?

The wiki say's 0.25 mm I thought that it was best to set the inlet a little slacker although I can't remember were I read it.

[daimlerman]

They idle better with a slightly wider gap,but this affects hot idle rather than cold,as when hot,the clearances are reduced.
Downside to a wider setting is the clattering when cold!

[Ant]

set the throttle gap to spec, clean everything inc TPS, set the TPS so it goes open circuit @ just off the idle stop.

and Listen to Brian and fix ALL the air leaks in the system, Injector O rings and inlet gaskets plus the oil breather system are always the main culprits, as is a badly fitting oil cap and dipstick.....

its a bad transient mixture that causes the lurchy flat spot, not the OEM mapping.

all IMHO of course

[scott180gtb]

Right

I'm all over this and will report back.

[BenHar]

set the throttle gap to spec

How, and what is the spec?

Thanks
Ben

[Morat]

When looking for leaks, how much carb cleaner can you spray round the bay before you create a serious fire risk? I bottled it after about 10seconds....

[clipper]

When looking for leaks, how much carb cleaner can you spray round the bay before you create a serious fire risk? I bottled it after about 10seconds....

Dunno, but I used a whole can and found nothing that I could relate to as a symptom of a leak.
Brianmooore, amongst others, has convinced enough people that air leaks are the root of most of these types of woes though, so despite finding no leaks I stripped the intake manifold and admission side off the engine and VERY carefully put it all back with new gaskets, checking torque settings etc.
Solved all my problems and turned a hunting, poorly responsive, wobbly idle b25 into a different motor.

I still don't know where the leak was (although I suspect manifold to head being poorly fitted by me in the past was to blame) but I'm convinced that there was one.

[scott180gtb]

I think for the cost of a few paper gaskets and o rings it beats throwing carb cleaner all over the engine.

I'm going into the stealer's tomorrow to order my bits.

I'm thinking that the head has been off my donk for the head gasket so quite a lot of the gaskets will have been off. The O rings between the inlet manni and block I think are common sense culprits.

What number do you need for the intake manni gaskets. Number 2 or 3?

http://www.realoem.com/bmw/showparts.do ... g=11&fg=40

[Rtaylor2208]

You need 4 of number 3 and 1 of number 2.

[Royalratch]

Vacuum laks seem to cause hell, even tiny ones.

Having said that, M20s always have poor throttle response.

Helps if you have a a fast flowing river nearby to power the watermill though.

[Brianmoooore]

Having said that, M20s always have poor throttle response.

Throttle response with a flap type air meter is always going to be worse than that with a MAF, but with everything set as it should be, it's good enough.

[scott180gtb]

This morning I had a play with the valve clearances and set them all at 0.25 mm

After building everything back up the idle was still the same.

So given the pap weather I decided to strip down the inlet side of things to try and get to the bottom of things.

With the inlet manni off I found the rubber pipe that sits under the manni had a crack along were the clamp sits.

Also the bottom O ring on the pipe between the sump and manni looked squished and miss shaped.

All the other gaskets looked good but I'm going to replace them seen as I've got it stripped down.

Had a major cock up though. There was an old nut sat behind the connecting pipe that dropped into the sump when I lifted the pipe off.

WAPITA I've tried fishing it out with a magnet but can't find it.

Will I get away with leaving it or does the sump need to come off again. Not happy!

[scott180gtb]

Is a black spark plug proof that the car has been over fuelling?

The plugs were all black and stunk of petrol.

[Brianmoooore]

Red 325 in my sig. has a 6mm nut lying in the sump as well, and many of the M40 engines out there have a bolt or two rolling around loose in their lower sump.
Black plugs are a sign of rich running on a half decent engine. On one from a lesser maker it could be a sign of worn piston rings as well.
Did you measure blue temp sensor resistance at the sensor or at the ECU? The injector rail plug and socket to the engine loom causes more trouble on M20s than the sensor itself.

[BenHar]

set the throttle gap to spec

How, and what is the spec?

Anybody know?

Thanks
Ben

[daimlerman]

set the throttle gap to spec

How, and what is the spec?

Anybody know?

Thanks
Ben

I am given to understand that a cigarette paper should be a sliding fit between the butterfly and the body.The gap is adjusted by the throttle stop screw on top of the T/B.As soon as the butterfly moves,the TPS should be heard to click.

[Brianmoooore]

set the throttle gap to spec

How, and what is the spec?

Anybody know?

Thanks
Ben

Special tool, which AFAIK, is a tube with a fixed orifice that replaces the idle control valve. With this part in place, the stop is adjusted for a specified engine speed.

[scott180gtb]

Red 325 in my sig. has a 6mm nut lying in the sump as well, and many of the M40 engines out there have a bolt or two rolling around loose in their lower sump.
Black plugs are a sign of rich running on a half decent engine. On one from a lesser maker it could be a sign of worn piston rings as well.
Did you measure blue temp sensor resistance at the sensor or at the ECU? The injector rail plug and socket to the engine loom causes more trouble on M20s than the sensor itself.

Brian I think I checked this last year but I'm going to go through it all again.

I'm certain everything belled out o.k.

Could an intake air leak cause rich running?

I took the car to a local VW/Bosch specialist and he measured the Co2 when the car was hot.

It was all within parameters but the plugs tell a different story.

Is this going to be a case of slowly working through all the areas until I find the problem.

As for the nut I'm going back out with a big magnet. If I cant get it out then it will stopping in!

[scott180gtb]

Brian does this dizzy looked past its best?




I'm thinking the state of the cap could be causing the slight miss when its cold?

Would some new plugs and dizzy help or should I just clean this up and refit?

The blue sensor belled out o.k as well onto the harness pins 1 and 2 I think. I just need to check from the female end to the ECU.

[Brianmoooore]

An air leak can cause high HC measured at the exhaust, indicating rich running at first thought, but this is just fuel mixture that is too weak to ignite being pumped straight through the engine.
Black plugs, with a dry matt finish (after letting any fuel evaporate) is caused by a rich mixture, whereas black and oily is just that - burnt oil.
If all the plugs are evenly black, then it's likely to be a fault lengthening the injector signal pulse, opening the injectors for too long a time.
If one or two plugs are noticeably blacker than others, then it's likely to be weeping injectors.
Your post says you checked CO2. Do you mean CO2, or CO? Two very different gasses.
From what I can see in the pics., the distributor cap and rotor look OK.
For the blue temp sensor check, leave the sensor end all connected and everything under the bonnet undisturbed. Unscrew the engine ECU and let it hang down. Remove the plug, find the appropriate two pins in the plug, and measure the resistance across them. Be careful not to permanently open up the contacts of the plug by pushing meter probes into them - use two short pieces of bare thin wire.
Reconnect the plug, let the ECU hang, bring the engine up to full temperature, then switch off, unplug the ECU again, and re measure the resistance, which should now be around 330 ohms or so. With the meter still connected (the resistance will slowly increase) wriggle the wiring to the injector loom plug and socket under the inlet manifold, and see if the meter reading jumps.

[BenHar]

How, and what is the spec?

Anybody know?

Thanks
Ben

Special tool, which AFAIK, is a tube with a fixed orifice that replaces the idle control valve. With this part in place, the stop is adjusted for a specified engine speed.

Interesting, thanks.

Wonder if Barry has the gear.

Ben

[Brianmoooore]

My technique is to blank off the ICV and adjust for a really low idle that makes the engine threaten to rip itself off of the engine mounts.

[BenHar]

My technique is to blank off the ICV and adjust for a really low idle that makes the engine threaten to rip itself off of the engine mounts.

Thanks again, Brian.

I might give this a go.

Would be nice to get it to the correct factory setting though.

Ben

[scott180gtb]

An air leak can cause high HC measured at the exhaust, indicating rich running at first thought, but this is just fuel mixture that is too weak to ignite being pumped straight through the engine.
Black plugs, with a dry matt finish (after letting any fuel evaporate) is caused by a rich mixture, whereas black and oily is just that - burnt oil.
If all the plugs are evenly black, then it's likely to be a fault lengthening the injector signal pulse, opening the injectors for too long a time.
If one or two plugs are noticeably blacker than others, then it's likely to be weeping injectors.
Your post says you checked CO2. Do you mean CO2, or CO? Two very different gasses.
From what I can see in the pics., the distributor cap and rotor look OK.
For the blue temp sensor check, leave the sensor end all connected and everything under the bonnet undisturbed. Unscrew the engine ECU and let it hang down. Remove the plug, find the appropriate two pins in the plug, and measure the resistance across them. Be careful not to permanently open up the contacts of the plug by pushing meter probes into them - use two short pieces of bare thin wire.
Reconnect the plug, let the ECU hang, bring the engine up to full temperature, then switch off, unplug the ECU again, and re measure the resistance, which should now be around 330 ohms or so. With the meter still connected (the resistance will slowly increase) wriggle the wiring to the injector loom plug and socket under the inlet manifold, and see if the meter reading jumps.

Brian I belled this all through this morning and everything checks out A1.

I spoke to Matt and he said he had no issues with the car in his ownership but did say the previous owner to me had the head gasket done.

I'm hoping the route cause is an air leak. My new gaskets arrive in the morning and I should get it built back up over the weekend.

I'm not giving up until I'm happy. Please keep checking up and passing on your wisdom.

Thanks

Scott

[Rtaylor2208]

Did you rule out these other parts for air leaks as well as it should be easy with the inlet manifold already removed.

For air leaks you want to spray carb cleaner on all the vacum pipes or basically anywhere that air can leak. The usual suspects are:

1. failed rubber bung on the underside of the Throttle body
2. Failed orings on the oil breather pipe between the inlet manifold and the block
3. Breather pipe from the throttle body to the rocker cover
4. All vacum pipes particularly the large boot between the airbox and the Throttle body
5. Throttle body gasket
6. inlet manifold gaskets
7. Dipstick orings
8. Oil filler cap

And potentially the D Shaped bungs that fit in the ends of the cylinder head. Usually you would also have leaking oil as well if these are on there way out.

[scott180gtb]

Did you rule out these other parts for air leaks as well as it should be easy with the inlet manifold already removed.

For air leaks you want to spray carb cleaner on all the vacum pipes or basically anywhere that air can leak. The usual suspects are:

1. failed rubber bung on the underside of the Throttle body
2. Failed orings on the oil breather pipe between the inlet manifold and the block
3. Breather pipe from the throttle body to the rocker cover
4. All vacum pipes particularly the large boot between the airbox and the Throttle body
5. Throttle body gasket
6. inlet manifold gaskets
7. Dipstick orings
8. Oil filler cap

And potentially the D Shaped bungs that fit in the ends of the cylinder head. Usually you would also have leaking oil as well if these are on there way out.

I will go through and list what I have done as future reference for anyone tackling this problem.

1. Checked the bung. All good and air tight.

2. Both O rings replaced. (The bottom O ring was past its best)

3. Replaced with new from BMW (Old one had a crack at the
clamp end of the rocker cover)

4. Vacuum pipe checked O.K.

5. All new gaskets on the throttle body.

6. All new gaskets on the inlet manifold.

7. New O rings on the dip stick.

8. Oil filler cap is nice and tight.

In addition to the above I have gapped the rockers at 0.25 mm.

I have also removed and cleaned up the Dizzy and arm.

And last but not least I have tested that the ECU is getting the correct values from the blue sensor at pin 24 and 48.

I should have it all built back up today if the parts arrive soon.

Has anyone got the torque settings for the bolts on the manifold?