Running rough after engine warms up

[industryspark]

Hi, ive been experiencing problems with my Vw polo 1999 1 litre which ive mentioned in another thread (starting a new one as ive narrowed the problem down))

The car runs fine for 2-3 minutes of driving and then suddenly the engine seems to 'lose' power and its very sluggish to accelerate. I think this is a result of it running 'lean' so to speak. I had one engine tappet fail about a week after this problem started which could be a coincidence but im starting to think this was as a result of the lean running, i.e excessive heat in the engine.

I have 2 thoughts of what it could be thats causing the lean running.

1, i understand the engine runs at open loop when started and switches to closed loop (i.e taking info from sensors) when warmed up- my line of thinking is that my problem starts when switching to closed loop hence maybe i have a sensor that is faulty.

2, The other possibility is that i have a vacuum leak causing the lean running. What i want to know is would the vacuum leak not cause me problems all the time? and not just when the engine warms up after a few minutes? If it would show up all the time then i can eliminate this as a potential cause as like i said-the car runs fine for 3 ish minutes and then bogs down on acceleration quite noticeably.

changed the fuel filter yesterday so ive now eliminated this.

Any comments appreciated

 

[industryspark]

Right, further update.......

I ran the car today and problem still persisted, so i turned the engine off and disconnected the coolant temperature sensor (from the base of the thermostat housing i think it is).........and guess what, the car runs fine.

If i run the car with it connected the fault occurs and persists even with it disconnected unless you turn the engine off and restart it. I.e, u need to 'reset' the car.

Is this indicating the CTS is at fault or could it be another sensor monitored by the ECU? To me it appears entering closed loop is when the car suffers the problems.

The question i really could do with answering though is if i can now discount a vacuum leak, i.e at the throttle body or associatted hoses? Surelly this would still effect the car in open and closed loop?

 

[jonnyfishing]

The ECU responds to the last signal it received, so change the ECT sensor
and you should find problem solved.

 

[industryspark]

Thanks for that johnny, am going to order the part tomorrow.

The only other comment i can make is that the engine appears to idle roughly. It looks a bit shaky and sounds like it jumps and splutters occasionally. Would this be symptoms of a vacuum leak?

When diagnostics were done it flagged up

1, Idle air control
2, knock sensor
3, lambda sensor

The sluggishness definetly only occurs when the ECU takes over running of the engine which suggests ECU or sensor problem to me. (Would think a bad sensor would be flagged in the diagnostic though). Diagnostic guy said he believed the lambda code was a red herring.

The rough idling seems to suggest a vacuum leak to me but would this not be evident when the car was in open loop aswell? Had a look for leaks today and couldnt find any obvious ones.

 

[Avocet]

I'm not familiar with the injection system concerned, but they tend to go "closed loop" within a few seconds of starting, rather than minutes. As I understand it, the term "closed loop" only really refers to the mixture control - which is done using feedback from the lambda sensor. If it senses the gas coming out of the manifold as being too "weak" it sends a signal back to the ECU to hold the injectors open a bit longer - and vice-versa. I think most systems do this about once a second or maybe a bit less.

If you're running fine for "minutes", I think your suggestion of CTS is the best. If you google around on the 'net, you might find some resistances for your CTS at different temperatures. You could always put it in a pan of water with a thermometer to check those. It seems like it has failed at the right resistance for a cold start, so if anything, you might be running way too rich as the engine warms up. That might be what the lambda reading is - it's gone "out of range" because it's so rich.

Most modern cars have a "get-you-home" mode when a sensor fails. The ECU will substitute an "average" value and log a fault code. The problem is that when you start up cold, it doesn't think there's a fault because the CTS resistance is about what it would expect to see on a cold start. It MIGHT not be "wrong" enough to go into get-you home mode when the engine warms up, but if you disconnect it, the next time you start the engine, it will see it as "open circuit" and would definitely subsitute a get-you-home value.

I can't come up with anything better than your current diagnosis but if a new CTS doesn't work, post again. If you have an air leak, the common ones are the big convoluted rubber hoses between the air flow meter and the engine - especially at the bottom of the convolutions where the oil collects. Also any other hoses round the idle speed control valve. Ihe idle speed control valve itself can cause rough idle but disconnecting the CTS shouldn't fix it! Similarly, I can't see a knock sensor fault only giving symptoms when warm. As far as I know, most cars only run "open loop" when you work them fairly hard - maybe above 4000RPM and 3/4 throttle. that's outside the load / speed range that they would do their type approval emissions test on so they don't care what happens outside that!

 

[industryspark]

Avocet, thanks for the write up, you seem pretty clued up so ill pick your brains again if i may.....

Ive been running the car the last couple of days with the CTS disconnected and have noticed the problem does occur (unfortunetly) but takes a lot longer to become noticeable and the loss of power isnt as extreme as when the CTS is connected........so my theories maybe are wrong.

You seem to have a better understanding of closed loop and open loop than myself and about cars in general. What if anything do you think of the fact the car still runs rough with the CTS disconnected, yet not as extreme? does this tell me anything? Ive ordered the CTS but will take a few days to get here so will no soon hopefully. I would think the CTS would throw a fault code up though which it didnt??????

Also with a vacuum leak would i feel the problem through the full range of the throttle pedal or would it just be at certain points, i.e gentle acceleration. i was thinking that surelly at full throttle there is that much air entering the intake that a little bit of unknown air from a leak wouldnt make much difference?

One last question, with me disconnecting the CTS does this stop the ECU working or will it still be taking information from other sensors and using these to run the car?

Regards

 

[Avocet]

OK, this is a wild guess, and I could EASILY be wrong, but my guess is that the "get-you-home" setting that the ECU substitutes is not quite rich enough for a really cold start (but adequate for most) and TOO rich for correct running at normal temperature. There's probably an engine temperature where it's just right, but as your car warms up, you go "through" it and out the other side. (if that makes any sense)!

You say the CT didn't throw up a fault code. Have you had it on a fault code reader? If so, that's worrying! Depending on the managment system, not all failures will throw up a fault code, but I would have thought that a failed CTS would! The options now are as follows:

1. I'm wrong! (always a distinct possibility)!
2. It's not far enough out of range to throw up a fault code. Does it throw up a fault code when you disconnect it completely? If so, at least you know it's one of the parameters the that ECU logs. If it doesn't, we could be barking up the wrong tree!
3. The fault could lie elsewhere. In general, the "get-you-home" mode is a fairly rich bunch of settings that give the engine the best chance of running when something is wrong. If, for instance, you have a bit of a misfire, richneing the mixture slightly might make it easier for a weak spark to fire it. Disconnecting the CTS might put you into get-you-home mode and mask some other fault that you haven't yet found!

I agree with the airleak - a smal lleak at full throttle is unlikely to make much difference. HOWEVER, you can sometimes get the situation where (if for example it's a split hose) that as the engine moves on its mountings with the strian, it opens up the hole more than at tickover.

No, disconnecting one sensor shouldn't stop it listening to all the others (at least, I don't THINK it should)! Part of the problem is that cars of tat age tended not to use a standardised on board diagnostic protocol, so each manufacturer would have their own and marketed their own fault code readers to go with them. Cars designed after about 2000 tend to use the OBDII protocol so generic fault code readers are much more likely to give good results.

 

[industryspark]

I see your point about the code reader probably not being as usefull on my car as other later models.

The diagnostic i had done only showed the first 3 faults that i mentioned in the first post. I have got a lambda sensor from a scrappy to try out in the morning as this was one of the things that was thrown up so worth a go. I would have though that the CTS would throw a fault but as we've discussed, its not a given!

I havent had the code reader on since the CTS has been disconnected so that is to be investigated. Am taking it to be read again at the weekend so i will find out.

Im convinced its a sensor fault somewhere as i seem to have eliminated most things apart from a vacuum leak-i would think any other fault, i.e fuel filter blockage would show up all the time and not just under different circumstances and after 3 minutes of running.

A mechanic who replaced a tappit in the engine commented on a 'flat spot' on the throttle body. Would you have any idea as to what he meant by this? Cant get intouch with him to ask but i know there's the throttle potentiometer sensor actually on the throttle body which i suppose could give the problem, am unsure of what he meant by 'flat spot' though.

Again, many thanks for the input.

Regards, I-spark

 

[Avocet]

Don't know for certain. Of the three faults you mention:

1, Idle air control

This seems to be an odd sort of fault code. Generally, you'd expect it to say something about the idle speed control like "Idle speed control open circuit" or "Idle speed control shorted to earth " or "Idle speed control out of range". I know these units can be troublesome on many cars but you need to find out whether the unit itself is sick or whether it is working outside it's normal operating range whilst trying to overcome some other problem elsewhere in the engine. For example, if you had an air leak that made the engine run rough, the idle might slow down and the valve would try to speed it up again. It might be running right up against its end stop trying to get the idle back where it should be but there wouldn't be anything actually wrong with it.

2, knock sensor
Again, I'd expect to see more information. I don't know much about knock sensors or the sorts of problems that a faulty one can cause. I GUESS that if it failed, the ECU would substitute the readings from it with ones from its "get-you-home" map, so you'd see a more retarded spark than you otherwise would to avoid possible detonation. That might make the car feel sluggish but I doubt that would only happen after the engine had been running for a few minutes.

3, lambda sensor
Like the others, I'd expect to see more information. Also, like the idle speed control valve, I would have thought that it could throw up a fault code just by trying to "fix" another fault. Again, in the case of the air leak, it might have gone right to the "richen-it-up!" end of its scale in an attempt to cure the problem. Once you fix the air leak, it would go back to its normal operating range, so it's not at all uncommon to have fault codes for things that there's no fault with!

I agree it's odd that the CTS would not have thrown up a code though.

Something I've found out the hard way with generic fault code readers is that they often give out codes which might mean something on an OBDII engine but mean nothing (or worse, the WRONG thing!) on earlier engines. My dad tried one on my wife's old car (a 1999 Alfa 156) and we got a code (P1234 (or whatever it was)!). When he looked up that in his OBDII list of codes and found that number, it turned out to be a code for a something like an exhaust gas recirculation valve - something the car wasn't even fitted with!

Don't know for sure what "flat spot" means. It's often used to describe a problem with performance - a hesitation or misfire-like condition. If he was talking literally, however, I'm not sure. The throttle body is likely to have a spindle running through it with a butterfly valve (the throttle plate) on it. Maybe there was some wear on this and he could see or feel a flat bit? If so, you might get a small air leak. I doubt it would be significant though.

Getting back to the idle speed control valve, I know a lot of them get bunged up with deposits of sooty, oily carbon. That can cause all sorts of problems (though I admit it would sem odd to have one that only came as the engine warmed up). I've heard of people cleaning them out with carburettor cleaner to good effect. Might be worht a look at least?

 

[^neo^]

just change the bl**dy temp sender as i said in your other thread!

 

[industryspark]

Well, this weekends job is to completely strip down the throttle and take a look-might aswell clean it out whilst im at it and replace the gasket.

My first look at vacuum leaks didnt throw up anything though so im still more leaning towards a sensor problem. The problem did come on at first 'intermittently' over the space of 1-2 week so possibly a sensor going bad/failing and then finally giving up. The diagnostic should tell me more hopefully.

Thanks for the contributions anyway-uve furthered my knowledge a bit more and given me some more things to think about. Although the cars not worth a lot its only done 64,000 miles so im very reluctant to give up on it.

Regards.

 

[industryspark]

Lol neo, im waiting on one from VW, they asure me its coming in Friday so we shall see. Im not convinced thats the problem (but then again im not a mechanic!)

 

[^neo^]

if it makes a difference in the running by unplugging it, then i reckon it'll be the problem.
Be careful stripping the throttle body down as you may have to reset the ecu afterwards to re-learn the position of the throttle position sensor, which you may be able to do with a battery disconnect for half hour , or may be a VAG-Com (VW Diagnostics) job

 

[industryspark]

Right, changed the CTS and the lambda today and the problem still persists..........

Im hoping to get the other sensors tomorrow to try out. Can anyone else think why the car would be ok until the engine has warmed up? im hoping im on the right track with the sensors.

Regards.

 

[^neo^]

ok, i failed

It seems temperature related though.

if you wanted to send me an email (should be in my profile or if not my website is on there) i'll send you a .pdf of how to test the sensors and what readings you should be getting etc.
Might help you out