hanging on to revs

[cobalt911]

Hi all,

My car has a tendency to get annoying little faults! This one is the most annoying to date. It is 'holding on to the revs', which the only way I can describe it! It takes a while to settle to an 800rpm idle too but once there it is fine. When I change gear it is almost like I am pushing the clutch with my foot still on the throttle as it sits in the rev band a while before lowering. This is irritating as it a) sounds like you can't work a clutch and b) has the unfortunate effect of lurching the car forward on the remaining revs when in the higher gear.

Any thoughts? In the old days it sounds like a slightly sticky throttle cable or something but I am now guessing it is more complex than that ha ha. IT had a new ECU about 2 months ago as well.....

Thanks,

Mark

 

[cobalt911]

Thanks Maurice,

I cleaned that critter out about 4 months ago until it was pristine so I will be a little surprised but you never know!! I was hoping not as it is the biggest pain getting it in and out (or rather doing up the clips!)

Thanks for the input though

 

[cobalt911]

the vacuum leaks bit may have exceeded my knowledge Maurice ;-)

What about the MAF? I hear it all the time and I have no idea to clean it - its the other side of the airfilter box right??

Cheers,

Mark

 

[38F3E1]

I would be interested to see if you get to the bottom of this. I think I have the same problem. The way I describe it is that as the revs fall (with foot off accelerator) they exhibit a dead cat bounce type of syndrome where at about 1000RPM they bounce up again a little to about 1100RPM and then drop and settle at 800RPM. Drives me nuts. I have only had the problem since the battery was disconnected for quite a while having the front bumper and bonnet sprayed for stone chips. I have also cleaned out the ISV with no effect so I don't think it is ISV related. I thought it might go away with time but it hasn't in 18 months.

Ian.

 

[Guards Red]

Not sure if the engine will run without it, but I'd check the throttle position sensor plug too ensuring that it's fully plugged in. This is easy on both the varioram and none varioram cars, but adjusting or replacing it is a lot harder. Durametric software can check that it's functioning and closing properly which is some thing it may not be doing.

I'd check for vacuum leaks too as Maurice suggests. This would be my first thought.

 

[cobalt911]

Will have a dig around over the next couple of days and report back. My credit card is already quaking at the thought of another Northway visit this year [&o] already had a new ECU which was, apparently, very unusual....

I suppose this is what can happen on a 12 year old car! My big concern is if the previous ECU failure (which meant more fuel going in and flooding a couple of cylinders) has done some lasting damage...we shall see!

Thanks for your responses...

Mark

 

[sjd]

I bought my 1997 targa a few months back and has had the same irratic idle as described on this thread. I am pleased to hear that this is not right and will see what else I can find out on this fault.

 

[zrcg1]

I would start with checking/cleaning the throttle position sensor (as already suggested) and looking at the oxygen sensor.

I had this problem, albeit much more pronounced, a few years back. OPC thought it may be the ISV which they cleaned but didnt fix the problem, then they thought it could be the ECU...... at which point alarm bells started ringing so I took it to an indy.

Turned out to be the O2 sensor, it hadnt completely failed but was on the way out.

 

[Mark Elder]

I find it quite tough to see how an ECU can fail in a way that causes poor running. It's just a simple piece of computer hardware with no changeable software (without going to a remapper). It has data that can change (adapt to fuel, driving etc) - but even if the data gets corrupt you can reset with a battery disconnect.

If an ECU fails, surely it fails big time. The car is not going to start. I think any indy or OPC suggesting an ECU swap for rough running is grasping at straws (and charging a load of money for it too).

I can see how rough running might come from problems with the loom that feeds data to/from the ECU - but not the ECU itself.

Any views?

 

[cobalt911]

My ECU went over a period of time. It started with a rough idle and tardy pull away in low gear. It then got worse and started flooding a couple of cylinders until I took it to Northway who said it was very rare but swapped it and the change was immediate...

anyhow....

OK - cleaned the ISV today - it was filthy after 4 months (surprising) and even more surprising is that it has made no difference whatsoever so I am gonna have to go back to Northway and get a diagnostic on it....

Will report back once more but I thought the ECU info would interest you - appears it can degrade before failure!

Mark

 

[pse_SC]

Turned out to be the O2 sensor, it hadn't completely failed but was on the way out.

I reckon the O2 sensor could be a good starting point - although this should show up on the diagnostic[8|].

 

[zrcg1]

ORIGINAL: pse_SC

Turned out to be the O2 sensor, it hadn't completely failed but was on the way out.

I reckon the O2 sensor could be a good starting point - although this should show up on the diagnostic[8|].

Thats very true - I think the OBD gives you the voltage outputs, rather than an explicit functioning/non functioning indication....

 

[cobalt911]

OK - so here is the latest as I see a few of you are interested!!

Car was misbehaving all day Thursday and Friday morning. Call Ray at Northway and drive over there. About 15 mins blast down the M4 and when I get off....problem gone.

Ray plugs in diagnostic kit and checks it all through. He tells me, 99/100, it is the ISV or the throttle on/off relay not actuating. Either way diags all look 100% and the engine even idles nicely at 880rpm....Hmmmm.....

Much annoyance from me as I am sure you know whats coming! Drove home, no problems, this morning no problems until I go up a hill then, its back!!

Looks like my personal favourite - an 'intermittent' problem' ARGH!

So, may well re-clean the ISV and electrical contacts in the connecter and also give any throttle linkages I can get to a blast with something like WD40 I guess! Then we shall see if it clears off forever...

Anyway, Ray - as usual - had great advice about what it could be and despite all the other possibilities, we are looking at ISV and throttle relay as the ones he will always attack first. BTW, if you listen carefully with the engine off you can hear a 'click' when you get someone to prod the throttle (get your ear next to the ISV roughly) which gives an indication that connection is working fine....

Onwards and hopefully upwards!

Mark []

 

[szklarek]

Hi

I have what appears to be a similar or related problem and I would appreciate any comments on the possible cause.

The car's a 993 C4 (Varioram).

When I start the car from cold the idle sits at around 1000 rpm and, if the vehicle is left to warm up ,without touching the throttle, the idle will settle down to around 900 rpm (once the engine's warmed up). Having reached this point, if the engine is then rev'd, the idle rises to around 1200 rpm - the idle speed then never goes below that point (1200). The engine appears OK in all other aspects although there is sometimes a slight hint of 'rough' running when idling. As suggested, I have checked that the ISV 'clicks' when the throttle pedal is blipped (whilst the engine is not running but the ignition is on) and it does, quite audibly.

Anybody got any ideas or suggestions?

 

[jboult]

Mark,

A while ago I had exactly the same problems as you report and it turned out to be the throttle position switch. Changing that fixed it immediately. Is your indie friendly enough to 'try before you buy'?!

For what it's worth, in my experience vacuum leaks tend to cause the revs to die too quickly (and the car to stall with the clutch down) as do problems with the MAF, whilst ISV problems usually result in the car 'hunting'. Go on, ask me how I know... Having said that I'm sure everyone you ask will have had a different symptoms from the same faults!

Cheers/John

 

[cobalt911]

Hi,

I think the Varioram is unlikely especially as it is now very intermittent. The throttle position sensor was working fine at Northway on diagnostics but it has to be a suspect! It is also incredibly sensitive!

I actually think I am going to give the ISV another pummeling with carb cleaner and see if it finally goes away. Drove 7 miles today with no incident so it is seriously intermittent!!

Cheers for all comments so far....

Mark

szklarek - the erratic idle mentioned in your post is almost certainly the ISV.....buy a flexible screwdriver and have a crack at it []

 

[szklarek]

Hi Mark

Thanks for the suggestion.

Earlier today I took the ISV out (7mm spanner did the trick) and gave it a thorough cleaning (I hope!) with brake/clutch cleaner. At first the idle appeared OK - back to the correct level, however, after a short run it was oscillating between 1000 & 1500 rpm, after another run it was back to the correct level! Now it's back to the original problem that I had! So either I didn't clean the ISV well enough or it's something else. I'll monitor it over the next day or so to see if it becomes consistent.

Kind regards
Anton

 

[Guards Red]

Just one thing Anton, and it's a long shot but worth a try, disconnect the battery and reconnect it. Make sure that the ignition is off before you do so and then drive it as normal and see if that helps. Long shot but the ECU will do without a none critical sensor if it needs to. Disconnecting the battery resets it. It's free at least!

 

[cobalt911]

yep - infuriating!!

Mine did it again tonight after behaving all day. MUST be something 'sticking' somewhere...

 

[szklarek]

Thanks 'Guards Red' for the suggestions, I'm away from home 'til Friday and will struggle on with the car 'til I'm back. I'll post the results of the 'battery disconnect' then.

Kind regards
Anton