High idle - Sorted - THANK YOU for everyones help

[bss325i]

Coincidentally i am doing some work to a 263k E24 635CSi Highline which has the E34 engine and it has the exact same symptoms as yours with the revs holding momentarily when dropping on the overrun. It also intermittently revved up on its own to approx 2500 rpm.

This is all when hot.

Disconnect the idle control valve when its revving up by its self and the revs drop back to idle speed approx 800rpm.

I've tried a donor AFM and ICV and its the same, checked resistance of the temp sensor hot and cold and is within spec. but i have found that the TPS is doing bugger all so i'm waiting on a new one. Its an auto with EGS so has the six pin TPS, i guess yours is the 3 pin manual type?

I'll let you know when ive sorted it.

[bss325i]

No cable rubbing either.

[jimbom30cab]

3 pin manual type

yer, i've got a 6 pin TPS i could have sent you also

To mine, i fitted a new OEM TPS and set it up perfect and still have the same problem

i've put a crude vacuum guage on the hose going to the FPR and it was only reading 15 which is a bit low, should be more like 18-20. Mine is also running super rich also

i'm going to check the less obvious vacuum leak areas tonight, oil filter housing, oil cap, dipstick etc etc.

if you listen right down under the inlet menifold it does sound a bit "hissy", i initially thought this was just worn alternator bearings.

My inlet menifold only has 3 connections on the botton, one has a new OEM cap the other 2 have new OEM vacuum lines fitted and secured tight with a jubilee clip for good luck

Thanks for letting us know Barry if you can ID the problem, i'll definately buy your dinner and drinks at the next meet in appreciation

[jimbom30cab]

WOW

I think i've got a lead on this

checked everywhere for vacuum leaks - zip, nothing, nil, used a whole can of carb cleaner almost and nothing.

BUT

I've found something and i'll be buggered if i know why this is happening

Everything is plugged in and normal, engine is up to temp

Driving along at any speed from walking pace to 70mph and the revs hang up when I lift off the gas and coast in neutral. The revs just hang at 1200 and NEVER drop below 1200 no matter how fast or slow i am going as long as the car is moving along in neutral.

The i stop the car on the brakes, count to 3 and the revs drop to 800. Its exactly the same time, 3 seconds every time. I repeated the test about 30 times, every result was identical

3 seconds after the car stops dead the revs drop to normal. (i coasted along at 20mph with foot off the gas in neutral for about a minute, revs steady at 1200. 3 seconds after i stop the revs drop)

WTF is going on there then!!!

[jimbom30cab]

Forgot to mention

Not sure if it's relevent or not but if i dissconnect the ICV - everything is perfect and nothing above is seen

[Speedtouch]

Sounds like you may have a leaking brake servo, or leak in the vacuum hose leading to it - does it consume brake fluid?

[jimbom30cab]

Thanks Mr Speedtouch, it doesn't consume any fluid mate and i had a good check of the hose, looks perfect and sprayed carb cleaner all round it. However i'm not so sure i've fitted it properly, i'm not sure how it should be fitted but this is what i have on mine (i think i've reused the M40 hose if i remember right) I would check on the other turds but they are both not at home at the moment. 2 secs i'll get a pic up.

[jimbom30cab]

Here we go

It's got an arrow on it, didn't come out tin the photo so i have drawn it on

Have i got this fitted the wrong way round do you think?

Ta

[Speedtouch]

It's possible - it's probably a one-way valve, so has to be fitted a certain way round.

Looking at these pics, you may well have it the wrong way round:

[jimbom30cab]

Here is a photo of the engine when i bought it and although it looks like i have used a different hose than this one originally fitted as it looks different

However the white bit (pic below) is nearest the servo and the white bit on the car is nearest the inlet men, so maybe i have got it on back to front ?

I'll swap it round and see if it makes a difference in the morning

[Speedtouch]

Yes, looks like the T-piece take-off needs to go to the servo, and the straight section to the manifold.

[jimbom30cab]

Thanks mate, i hope this does the trick, i'll be soooo happy !!

[DanThe]

If it was back to front the brakes would be shite, no servo assist. The arrow should point with the air flow, towards the engine, which it is, so is correct.
To rule out the servo take the vac hose off and plug the manifold, then test.

Strange how its only happening while the car is moving, maybe its to do with the speed signal from the clocks, unplug the diff sensor and drive to rule it out

[jimbom30cab]

Thanks Dan, i'll do both those.

I have just fitted a replacement diff, it is the one from Mick's 323 silver breaker, a 3.46 ratio

I wonder if i could have wired in the speed sensor the wrong way round, it only had 2 wires but was a very different connector to the diff than what was on the old diff.

Does that mean then the ECU gets a speed signal then?

[Brianmoooore]

I started to reply to this over half an hour ago, but my internet connection decided to throw me out, and I've only just got back on!!!
I see Dan has already commented on the servo valve, and is correct that the only effect of it being the wrong way around would be no assistance. You haven't mentioned poor brakes, so if it is the wrong way around it must be leaky.
The engine ECU gets information about whether the car is moving or not on the black/white wire through pin 14 of the C101. Cut this wire somewhere where it can be repaired, and see what effect this has.
Shouldn't make any difference which way around the diff. sensor wires are connected - the sensor is just a simple switch.

P.S. Dan's idea of just pulling the diff. plug is probably a better idea than cutting the black/white wire, at least as a first test!

[jimbom30cab]

Thanks Brian, will do

[jimbom30cab]

RESULT !

I checked the vacuum line to the servo, it's the right way round, i tested it with some blowing and sucking !!

Then dissconnected the speed cables from the diff - just pulled the plug out

And.......

Perfect idle back again

It still has a little hessitation at 1500 rpm but maybe only for half a second but the revs drop straight back down to 800rpm where it idles perfectly. I can live with a little pause at 1500 and probably wont even notice it after a while.

So now i've got no speed displayed - i wonder if that diff speed sensor/switch is duff?

Brian said the cable goes through pin14 on the c101 so i might run some continuity checks on that, is the second cable to the diff an earth wire then?

Thanks guys, you've been really great help, Am feeling so much happier now.

[jimbom30cab]

I might remove the sensor and see if it needs a clean, looks like a simple magnetic switch.

Looks like the brown (earth) goes through the C2 plug on the instrument cluster to earth

and the brown/red (signal) goes to the same plug pin 12.

I'm still totally puzzled how this would effect the idle - unless there is some c101 incorrect wiring (i did fit my own c101 plug on the loom as it originally had a x25 plug), the WIKI states:

PIN 14 Black/White Speed signal out to ECU pin 10 white plug inst cluster

[jimbom30cab]

Brian, I have been looking at the above and to me it looks like the speed signal comes in to the instrument cluster to the solid state bit (presume that's a PCB) then there is a speed out to the C2 pin 10 to the "injection electronics" - is that the feed to the C101 plug you mentioned where it then goes into the engine loom and ECU?

Thanks

[DanThe]

The sensor itself does not give the signal to the ECU, the clocks do
ECU takes the signal to help the engine idle when slowing down, and yours seems to be working backwards
I would try a different ECU first if you havnt already

[jimbom30cab]

Thanks Dan, i've tried 3 ECUs - all known good ones, same issue

I guess it could be a fault in the clocks PCB or maybe one of the those signal wires into or out of the clocks could be damaged and earthing against something ?

I might test for continutity from the speed out (of the clocks C2 pin 10) wire to the back of the ECU harness if i can find out which pin on the ECU which looks like "29 BK/WT Vehicle Speed input from instrument cluster"

If i get continuity there - would it proove the connection is sound ?

[Grrrmachine]

Reconnect the diff sensor and confirm the Speedo reads correct when driving. If it does, then your fault is either in the dash-to-ecu loom, or the cluster itself.

If the previous owner removed a stereo he might have yanked one the cables out of the cluster, which is an easy fix. Another thing is to try resetting the service lights. If you cant, the si batteries may have leaked, which has damaged the cluster.

[jimbom30cab]

Thanks Mr Grrrmachine

The speedo reads correct and the SI board is really nice, no leakage and everything like the service internal and reset all works perfect

I'm sure it's a wiring fault tbh and as it idles perfectly with the diff speed sensor not connected what i might do is re-connect it so i get correct speedo read out

Then snip the speed output wire from the cluster - after the C2 plug there is plenty of spare wire there (the one that goes to the c101 plug) - where i have put the 2 yellow lines below

I'm guessing that should give me correct speedo readings but remove the speed signal to the ECU (fixing the dodgy idle)



If that works i could then jump a cable direct from the snip to the ECU pin which would bypass the c101 and any broken cables?

Off to work now so i'll give this a go later tonight

[Brianmoooore]

The problem isn't that the signal isn't getting there - it's what the ECU is doing with it when it does, and why. If the speedo. works OK, then it's difficult to see that anything could be wrong with the diff. sensor and its wiring.
Having said that, isn't there something slightly 'different' with the M30 loom concerning this pin (and another one) when fitting a M30 into an E30?
Are you sure the throttle stop hasn't been 'adjusted' on the throttle body?

[jimbom30cab]

it's what the ECU is doing with it when it does, and why

yer, that's the bit i'm unsure about also mate - all very odd !!

isn't there something slightly 'different' with the M30 loom concerning this pin (and another one) when fitting a M30 into an E30?

could be Brian, i'll do some research, much more interesting than work !!

Are you sure the throttle stop hasn't been 'adjusted' on the throttle body?

yer, its fine, i've tried a couple of TBs also.

Thanks v much

[jimbom30cab]

Right oh gents, am hoping this will be the last post on this issue!!

I snipped the blue with yellow strip wire in the location in the pic above with the 2 yellow lines

The car is now running and idles perfect - i also have a speedo reading.

I guess the ECU is not getting the road speed signal but it seems to make zero difference to thw way the car drives

Job done as they say !!

Funny how about 8 hours of diagnosis ended up in a split second snip of one little wire to rectify !!

A sinceer thank you to Brian, Mr Speedtouch (buy his chips they are ace), Grrrrrmachine (aka mr Wiki) Barry and Dan, thanks guys, i really appreciate your help.

[Speedtouch]

Good to hear you licked that annoying problem in the end.