hanging on to revs

[szklarek]

I'm back home and, as suggested, disconnected the battery for 10/15 mins to reset the ECU. Unfortunately this has made no difference.

So to date I've cleaned the ISV and 'reset' the ECU.

Over the past couple of days the problem seems to have got worse - the engine, on idle, now sits between 1100 & 2200 rpm - hunting between these two points; occasionally it will steady at around 1300 rpm. When idling it seems a bit rough although at much higher revs it seems to run OK.

Am I looking at an ECU failure or could the other items suggested such as the MAF, throttle position sensor (is this the same as the throttle relay?), vacuum leaks, O2 sensor be at fault? Before I get my credit card out and head for Northways is there anything else I can easily clean/check (where are these other items that I mentioned above?).

Kind regards
Anton

 

[Guards Red]

Hi Anton,

sorry to hear this. First thing to check is the throttle position sensor. There is no relay by the way. This sensor tells the ECU what position the throttle plate is, in effect how much you've pressed the throttle pedal. It uses this information to decide what demands you're placing on the engine and what fuel to supply and what ignition map to use. If this is faulty the ECU will not get a signal saying the engine should be idling and accordingly it will not idle right. You can't test the sensor without equipment but you can check the connector is on properly. Is your car a varioram or none varioram? I ask this cause access to the throttle position sensor is different.

Another thing to check for is vacuum leaks. As said in a previous post, you need to check the rubber couplings between the inlet manifold and the injector pipes. These are about two inches deep and sit right at the bottom of the inlet manifold where it mounts onto the heads. They are held in place by a jublilee clip and a metal band at either end respectivily. These often perish and let air in which will make the engine run all over the place. The pershing will be apparent in the form of little cracks along the circumferance of the rubber tube. Squeeze with your fingers and see if the cracks go right throgh the rubber. Any pics you can take would be helpful.

Keep the faith, ECU failure is rare, it's likely to be something else.
GR

 

[szklarek]

Thanks for the input it's really helpful. The car is a C4 varioram ('96). I check out the points you mention tomorrow.

Kind regards
Anton

 

[szklarek]

Well guys I finally have a solution to my idle problem!

Having cleaned the ISV and checked for air leaks I was no nearer a solution and realised that I now needed to get the other sensors checked. So off to Northway's who checked the data on the ECU - running rich at idle and lean at higher revs. The suspicion was the MAF meter or possibly the ISV (it might now be a clean non functioning ISV rather than a dirty non functioning ISV!) and the car was booked in for these to be checked.

The solution was cheaper than replacing the MAF meter or the ISV or the ECU - replace the air filter! The car is due to get its clutch changed as its slipping - the muck from the degenerating clutch was getting sucked up the breather and into the air filter, blocking it and strangling the engine of air - I guess the ECU was trying compensate. I have never seen an air filter that dirty - it was completely black with muck. The air filter was changed and the difference was unbelievable - the idle is back to normal and the engine sounds much much better.

So my thanks and well done to Northway's (especially Paul)!

Anton

 

[Guards Red]

Glad it worked out and that it was something as easy to replace as that. Must admit it would never have occurred to me, my hope is that I would have discovered it taking the filter out to get to the MAF.

Full points to Northway, I guess it's why they have such a good reputation around these parts.

GR

 

[cobalt911]

Mine still doing it :-(

Very intermittent.....you could eat your dinner off the ISV now and filter clean. I reckon it is something sticking something....throttle switch or something....

In for a 48k service at Northway on Wednesday so we shall see but they didn't find anything on the OBD before....

Grrrr.....

 

[Guest]

ORIGINAL: MoC2S

Well, check the throttle mechanics for binding, check for vacuum leaks ... [8|]

cheers, Maurice []

What IS the best way to check for vacuum leaks Maurice? I think I have one. Carb cleaner didn't affect the revs when sprayed around possible leak points. As for the smoke pellets that I got from eBay, well, waste of time - all burned out in seconds.

Any ideas? [8|][][][]

 

[Guards Red]

I would (carefully!) grab the intake runners at the base of each one, just above the rubber couplings, and give it a tug. Rises and falls in RPM would then be revealed. failing that you can buy a mechanics stethescope for around £10 from tool suppliers which should help. Porsche remove the MAF and replace it with an adapter - basically a metal plate witha schroder tyre valve in it and apply pressure. They block off one pipe going to the oil tank too.

Mine had a check valve completely go to the varioram tank and it did not effect the idle at all. I believe the leak would have to be of a reasonable size and the most likley candidate it the rubber couplings.

I'd check the fuel pressure regulator and the vacuum pipe to it too.

 

[cobalt911]

Just a note...

Mine still doing it - very intermittent and had a 'big' service at Northway last week - still unable to find it!!

Must be something sticking somewhere ARGGGH! []

Still - intermittent is better than all the time, as long as it does no harm to the engine! []

 

[caherass]

I have a 1994 C2 and it has being doing exactly the same thing for YEARS. Drives me around the twist. I had it at a few OPCs and of course it never happened when I was there, so nothing turned up as a fault with the system. I left the car with them for a couple of weeks and something did turn up as a fault; the throttle switch. They changed it, I left, and still have the problem.

It is definitely an electrical fault, as the problem will come and go at the drop of a switch. Even if you turn the ignition on and off a few times, that can cure it sometimes. I've done all the ISV cleaning, changing throttle switch, new lamda senor in exhaust, etc. Absolute pain in the ass, but it does seem to be becoming a common fault on all our cars, so we'd better try to find the exact fault. Otherwise, we might all end up paying for garages to trial and error their way around the problem.

Has anyone actually changed the ISV. I haven't done this yet, but at £300 I don't fancy a mistake. Have the cars that the problem has arisen been Vario ram or non-varioram.

James.

 

[cobalt911]

Good point James, there seems to be a few of us with the same issue!

Looking back in the history of my car it seems there was a 'lumpy rev' issue once and the ISV may have been changed. Certainly the one I am cleaning looks in very good shape and the 'valve' moves very easily and is smooth. Now I can see a perfect reflection in it I am sure it isn't that.

My one seems to clear itself once very hot and been on a long 'motorway' run. I am making the assumption that it is something 'sticking' but the throttle linkages move ok and the throttle sensor was tested at Northway, although I have to say it is unbelievably sensitive!

So, no real conclusions and no further. When it comes to checking anything more detailed I do struggle (vacuum etc) so I am looking for us to maybe 'pool' together and see if there is trend....

Mark

 

[mbrands]

So far, it seems that the only one that has had any notable success in remedying this problem has been zrcg1, who replaced the O2 sensor.

Has anyone else tried this ?

 

[cobalt911]

ok - more interesting info...

My air-con appears to have packed up (or is at least not sending cold air!). Funnily enough, whenever I 'engage' the aircon the revs issue appears - every time! whereas aircon off seems to reduce or almost eradicate the problem. Paul at Northways fitted a new carbon filter last time which may have helped but the aircon problem seems directly related. Could the compressor cause the engine to go a bit loopy revs-wise?

Also - now I have no cold air is there a quick way to check if it is the compressor itself or a simple refrigerant issue going on?

Cheers as usual...

Mark

 

[zrcg1]

mbrands/Mark,
I can confirm mine has been fine since, absolutely no trouble after changing the o2 sensor. It was changed at around 100k and im up to 169k now, and have never had a problem since the day it was changed......

I had a quick look the other day and the undertray with the protection guard seems pushed out of shape slightly from what it was, so I guess they can get damaged very easily - In my case road humps are taking their toll, I guess.

When these fail, they usually dont show up on the diagnostic at idle that there is a problem. You need to take your engineer on a test drive with the OBD connected and they will need to monitor the voltage changes for it while driving. I had one hell of a job convincing the OPC that this was the problem and my hunch proved right.

Other suggested causes (from rennlist members) were ECU, throttle position sensor, ISV (which I cleaned many, many times prior), cyl head temp sensor.
Chris