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Up to Temperature - Problems

bobster

bobster

PCGB Member
Member
I've exhausted the search and can't find an answer to this problem.

I took the car out today for the first time after my new paint job and it's decided to play up. It ran fine until it got up to temperature, then kangaroo'd at constant low throttle openings and just cutout when coming up to roundabouts with no throttle. Basically it wouldn't run at less than around 2000rpm.

I've checked and there are no vacuum leaks that I can see, I've cleaned the ISV and Air Flow sensor thinking they may have gummed up. I didn't manage to get the black cap off the air flow sensor as I wasn't sure how to. It seems pretty well stuck on.
I did happen to notice that the tamper proof plug on top of the sensor was missing! I'm hoping someone who knew what they were doing made some adjustments in the cars previous life. But that's been like that all the time I've had the car.

The only link that I could find that had anything to do with temperature from my search was a mention of cylinder head temp sensor, Not sure what that really does or where it is. Could it be that?

Any thoughts? Anything I should be looking for?

 
Unfortunately that was the first thing I tried Colin, that didn't make any difference.

Thinking back to the paint work that was done, the passenger seat was removed, I know the engine management module is under there. Is there anything else that could have been disturbed in that area?
 
If it wasn't having this problem before it went in to the shop I would ask them if they needed to disconnect anything in the engine bay. I don't see why they should when they were only painting it though [&:]

Check that when the car is warm that the throttle cable in the engine bay is making contact with the idle microswitch. Push the throttle cable away from its stop and as you let it back you should hear the microswitch click.

The black cap inside the air flow box that covers the air flow meter just pulls off with a bit of force and then snaps back into place.

Other simple checks on things that might have been disturbed:

Is the thin plastic vacuum pipe connected to the cylinder at the rear of the air box?
Is the oil filler cap on correctly and tight?
Is the oil dipstick pushed in correctly?
 
Hi Steve,

I did have the lower 6 plugs changed and the rear engine tin changed as it was rusted away. I took the Cat away and rewelded the broken heatshield attachment points, so all of the exhaust was off and has been refitted. Can't think of anything around there that could have been disturbed.
I wouldn't have thought I could have damaged the oxygen sensor by Tig welding the cat. But would that give the same symptoms?

Throttle cable is adjusted ok and the microswitch clicks on it's return.
The thin plastic vacuum pipe is connected to the cylinder at the rear of the air box and as far as I can see all the rest are connected.
The oil filler cap and the oil dipstick are secure.

I found mention somewhere of the Fuel Carbon Cannister Evaporator system working from 40degrees heat onwards and wonder if that might throw up bad running if it's blocked or disconnected, but I can't find anything showing the routing or how it works on a non turbo engine. I believe some of it's in the LH Rear wing area?

Starting to get hacked off now!
 
ORIGINAL: bobster

Hi Steve,

I did have the lower 6 plugs changed and the rear engine tin changed as it was rusted away. I took the Cat away and rewelded the broken heatshield attachment points, so all of the exhaust was off and has been refitted. Can't think of anything around there that could have been disturbed.
I wouldn't have thought I could have damaged the oxygen sensor by Tig welding the cat. But would that give the same symptoms?
Click to expand...

Hi Bob this is exactly what I was getting at. In my experience a problem can often be traced back to what was done last. The above is giving me a few clues.

1. The O2 sensor would have been disconnected so check that they have actually plugged it back in when refitting the cat. It can be easy to forget to do it. I believe the O2 sensor plays a much more important role at low revs than it does at WOT.

2. If you mean the rear tin near the front of the cat, the cylinder head temperature sensor needs to be unplugged when changing it (although the CHTS is more associated with cold starting problems). Check that it has been plugged in. You'll find some useful pics in this thread on Rennlist:

http://forums.rennlist.com/rennforums/964-forum/235360-head-temp-sensor-replacement-urgent.html?highlight=cylider+head+temperature+sensor

 
Bob, just spotted a couple of things in Adrian Streather's book related to stalling that might be linked to removing the exhaust. He says that:

1. Check for a blocked catalytic converter - did anything untoward happen when it got removed?
2. Modified exhaust system leading to lack of back pressure - is it leak tight after reassembly...however I guess this would be obvious as you would hear it blowing or it would be very noisy.
 
Hi Steve,

As far as I know the cat was reinstalled OK and the cup pipe has been on the car for some while now. There aren't any leaks, a couple of clamps were renewed on reassembly and all hardware is new stainless steel and tight.

I checked the evap cannister and pipework in the LH rear wing for damage or blockage and all seemed ok. Couldn't see any connectors unmade either whilst under there.

I'll take the air flow sensor off again tonight and get the black cap off the top. I guess I'd be looking for contamination or signs of damage on the contacts of the board would I? Do I clean it with electrical contact spray cleaner?
But then again why would the problem only occur when it's getting to temperature? ...I think I'll do it anyway.

 
I'm not sure what you mean regarding the evap canister on the left side? It's mostly heating stuff on the left side of the engine near the wing i.e. the rear blower and heat ducting. The O2 sensor plug is hiding between the rear blower and inside wing. You should be able to see if it is plugged in if you remove the rear fuse box cover and peer down to the left of the blower with a torch. It's a cylindrical shape plug. There's a plastic clip on the wall of the engine bay that it should really be clipped to.

Regarding the black cap on the airflow meter - I thought you meant the funnel shaped black cap inside the air box which routes the air into the air flow meter. I now see you mean the cover for the electrical bits. Personally I wouldn't bother to try and clean the board because I've heard they either work or they don't. And they're very robust compared to modern MAFs.

Is there any chance that they didn't torque the new plugs down properly? Did they change the oil and if so what's the level like?
 
The Evab Cannister is located between the inner and outer LH rear wing above the plastic wheelarch shield. The thread I read mentioned that if it was restricted or blocked the engine would display strange behaviour. It only circulates vapour back to the engine above 40 degrees and after partial throttle opening, so I thought it may be connected to the problem. However the vent pipe is open to atmosphere behind the LH sill cover and there are no signs of damage in the plastic pipes anywhere.

I saw the O2 sensor connection and it has been made and secured in it's retaining clip. Could a faulty sensor affect the running to this degree?

Re the plugs, without trying them I don't know but I trust the mechanic on that.

The oil level is on the minimum mark right now, I will be topping it back up as soon as it's running OK. I presume you were wondering if it was overfilled?

 
OK, with you on the cannister now. I know it as one of the flapper valves and it's only related to the heating system. The air that goes through it only comes from the main engine cooling fan and the heater blower motor so it won't have any effect on engine performance.

Glad to hear that you've verified the O2 sensor was reconnected and eliminated that as a potential cause. Whether the O2 sensor itself is the problem, I can't say. There's some evidence for it in this thread:

http://forums.rennlist.com/rennforums/964-1989-1994-911/208133-diy-oxygen-sensor-replacement.html

However he also says he changed the distributor caps and fuel filter so he doesn't know for sure that the sensor was causing a bad idle. So you could change the sensor (approx 100 quid) and be no better off.

At the moment I'm scratching my head wondering why your car was fine before the paint job and what could have been done that caused the problem?

Regarding the oil, yes I was wondering if there had been an overfill but obviously not.

As we've not been able to find something simple unplugged, it's sounding more and more like you need a specialist to look at it who can interrogate the DME.
 
Just one more thought before I have to go and do some work. Since the car was standing a long time, probably with the battery disconnected, did you do a DME reset?:

http://forums.rennlist.com/rennforums/964-1989-1994-911/198001-how-to-do-a-dme-reset.html

And here's a thread about coils causing problems with idle:

http://forums.rennlist.com/rennforums/964-1989-1994-911/200088-1991-c2-cutting-out-stalling-after-trip-to-body-shop.html
 
Mentioned this before but..... on ours(1990 C4) after a battery disconnection (e.g after vist to bodyshop) our car needs to be plugged in for a "system adaption" with some high current sapping accessories running at the same time- like rear demister and lights on,if you don't carry this procedure out the car has a tendancy to stall with a weak idle.Got this "Fix" from Specialist cars of Malton after we bought the car from them a few years ago.Whenever we recconect the battery now our OPC carries this procedure out,sorts it every time,car runs perfectly after and idles like a dream.
 
Mark, I've found references on Rennlist of others that have had to go through the same system adaption routine. Any idea if it's C4 specific as I've never had to do it on my C2 (DME reset procedure has been sufficient after disconnecting the battery)?
 
Maybe it is C4 specific then?,our car is also one of the wonderful earlier cars aswell maybe that has something to do with it?Definetly works everytime though.
 
The "System Adaptation" is not specific to the C4 - it's basically the mechanism whereby the DME adjusts it's settings for the ISV. The idle speed is controlled by the air volume passed by the ISV. A "DME Reset" - i.e. disconnecting the battery for a while - has no effect on the DME settings for the ISV control so it will just revert to using the same settings as before the "reset". The "System Adaptation", however, done with a Bosch "hammer" or, even the Scantool interface, causes the DME to re-adjust the settings to get the idle speed correct. It is essential that the engine is up to running temperature when a "System Adaptation" is done otherwise the idle speed will get reset to the wrong value when it IS at temp.

Hope that helps.

Regards

Dave
 
Have you checked if the distrubutor drivebelt has snapped,also disconnect oxygen sensor plug and drive car,if car runs ok,replace sensor.

Chris
 
Hi Steve,

Not yet, the weathers been crap the last couple of days and I've been working late. But finishing early Friday so will do the DME reset as described above. The disconnect and 'Spirited drive' method.

If that doesn't work I'll try the O2 sensor.

Thanks for everyones help so far, here's hoping!!!

I'll let you know tomorrow.
 
Eueka!!!!!! DME reset carried out by an early morning thrash today and it's now OK.

I disconnected the battery and charged it overnight. Connected it up this morning and left the engine running for a few minutes before giving it a right good thrashing (after letting it warm up of course) It now pulls cleanly and ticks over a treat.

Steve thanks, you came up trumps. Thanks to Mark ,Dave and all for taking the time to respond.

My God how I've missed this car since taking it off the road for the painting in September. I'd forgotten just how much I love it!

So MOT in the week and we're back in business.

Thanks again Steve and all, see you around soon.

Bob.
 
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