1.4 16v tickover/idle rpm

[isambard]

Hello! 

Question for all you 1.4 16v owners out there; what RPM does your car idle at?

Mine sits at about 1,100 when it's started, but then once it's warm it sits at around 700, which seems a little low to me

Is this ordinary, and maybe for reducing emissions, or is it lower than it should be?

Cheers

[sepulchrave]

700 is indeed low, I think mine sits at about 850, a little higher when started from stone cold but only about 950, 1100 sounds high, I suspect you have a vacuum leak allowing additional air into the manifold bypassing the throttle body.

 

My reasoning is simple, when started from cold additional fuel is injected to richen the mixture to compensate for fuel droplet condensation on the cold port walls, the leak is allowing extra air to be burned with that fuel raising the fast idle speed above normal, once the engine is warm enough the ECU cuts the fuel right back to allow a lean burn, the additional air from the leak doesn't have any extra fuel to burn so the tickover is slower due to the even leaner mixture.

 

I expect your engine 'pinks' noticeably off idle as you accelerate under load.

[Breezy_Pete]

What year/engine code/transmission type?

 

Here's the spec for the various combinations.  Note 1 = A/C switched on.

 

 

Edited 24 March, 2017 by Wino

[isambard]

Wow... thanks guys, very helpful posts just with opposite implications

Sepulchrave-i too suspected a vacuum leak would be the issue if indeed there was one, but my engine is indeed a BBZ so this table looks like it could be right. But just for general info, any suggestions where you would look for vacuum leaks? Somewhere between the throttle body and the intake manifold?

 

Wino-thanks for the table! Is it from a reliable source? If so it seems like i maybe don't have an issue after all

 

 

[Breezy_Pete]

That's from an erWin document about Mk1 Fabias with those engine types.

I don't know of a more reliable source.

 

No problem with your idle speed.

 

Checking the brake servo vacuum pipe for cracking at all the pipe junctions is a very good idea anyway, if the car is new to you.

Edited 24 March, 2017 by Wino

[isambard]

Thanks, and thanks for the advice

 

I'll be sure to have a look to be certain

[Breezy_Pete]

No problem.

If you can connect the car to VCDS, the Actual and Specified idling speed are found in Measuring block 50 of the engine module, fields 1 and 2 respectively.

Fields 3 and 4 tell you if the A/C system is on or off, and if A/C compressor is on or off.

[JWvrs25]

I had a problem with my engine running rough on idle, turned out to be a brake vac pipe problem as they seem to split by the one way valve

Check for any splits in it, I used some electrical tape over it to temporarily sort the problem

Might be something to check

 

[sepulchrave]

6 hours ago, isambard said:

Wow... thanks guys, very helpful posts just with opposite implications

Sepulchrave-i too suspected a vacuum leak would be the issue if indeed there was one, but my engine is indeed a BBZ so this table looks like it could be right. But just for general info, any suggestions where you would look for vacuum leaks? Somewhere between the throttle body and the intake manifold?

 

Wino-thanks for the table! Is it from a reliable source? If so it seems like i maybe don't have an issue after all

 

 

 

Yes, mines an AUB, hence 850 RPM, but 1100 still sounds too high when cold so I'd definitely check for an air leak between the brake servo and the manifold.

[Breezy_Pete]

Cold start revs may well be higher on the EU4/BB engines to speed up precat light-off. Mine did 1200 this morning.

Edited 24 March, 2017 by Wino

[sepulchrave]

AUB is also Euro 4 compliant and is almost identical to the BBZ apart from the ignition system.

[Breezy_Pete]

Aye, so it is. No reason to assume that the different ECUs would have exactly the same cold-start strategy though.

 

I can't find any specs for either for cold start revs. Variable with coolant temperature, within limits, presumably.

[sepulchrave]

So BBZ doesn't have Marelli 4-M ECU?

[Breezy_Pete]

AUA and AUB have 4LV, BBY and BBZ have 4MV. (BKY has 4TV).

[sepulchrave]

The only difference will be the ignition module and the static map. The lower tickover is entirely down to the higher energy ignition system on the BBZ.

[Breezy_Pete]

Maybe, got any evidence?

 

 

 

[sepulchrave]

It's a deduction, ECU's are modular, they both share the same model number 4, revision letter has changed from L to M, the next letter in the alphabet, the engines are mechanically identical and therefore the basic pumping efficiency is unchanged, the only change worth noting is that the ignition module has been changed from 2 channels to 4 for the individual coil packs rather than the dual wasted spark pack on the AUB.

[Breezy_Pete]

Seems reasonable.

AUB higher tickover than AUA because of hotter cam/shifted-higher torque band of AUB??

Why is BBY/BKY versus BBZ the other way round? My BBY in the Polo idles at more like 680 than 760, so possibly the erWin data is wrong there?

Don't have the equivalent document with Polo at the top of it, but I guess I can see what VCDS says. Edit: found a log where at anything above 82C coolant temp, 678 is target idle rpm. Would it be different in Polo to Fabia? Can't think why right now?

 

Edited 24 March, 2017 by Wino

[Breezy_Pete]

Come to think of it, when I look at that log, which is measuring block 53 amongst others, excel column D seems to be target rpm, which starts at 790 for a warm start at coolant temp 62C (column I).

Suggests that you can see target rpm for any given coolant temp in that second field of block 53.

If I remember, I'll plug in the laptop in the morning and see what it says at lower temps.

[sepulchrave]

I think you're right, the lumpier cams in the BBZ would necessitate a higher tickover than the BBY.

[Breezy_Pete]

Log of measuring blocks 50 and 1, starting at 6C coolant temperature suggests 1200 rpm is as specified for idle speed for about the first 30 seconds on my Polo 9N, 2003, BBY engine.

It's even a little higher for the first 5 seconds. 

Will upload file later.

 

Here you go, various irrelevant data columns deleted from the spreadsheet, and a scaled version of coolant temperature so the plot (below the end of the data) can show it on the same scale as rpm.  Only ran the log for just under 2 minutes, will do a longer one tomorrow if I get a chance, to give more of a flavour of the coolant temp/cold start idle speed relationship.

Quite impressed by the closeness of fit between specified and actual rpm after the first 5 seconds.

 

Edited 25 March, 2017 by Wino
Link to data/plot added

[isambard]

Thanks for all the replies guys-although it got a bit technical there at the end!

I have checked the brake servo vacuum line as you all suggested and it seems to be ok

I drove the car A LOT over the weekend and the only other thing i noticed was that in crawling traffic the revs would often dip down to just above 500 before bouncing up to ~680

Does this sound ok or do i still need to be looking for a vacuum leak?

[JWvrs25]

Could be an EVAP system problem

Purge valve  would be a place to check for any hose leaks to the inlet manifold

[Breezy_Pete]

2 hours ago, isambard said:

Thanks for all the replies guys-although it got a bit technical there at the end!

I have checked the brake servo vacuum line as you all suggested and it seems to be ok

I drove the car A LOT over the weekend and the only other thing i noticed was that in crawling traffic the revs would often dip down to just above 500 before bouncing up to ~680

Does this sound ok or do i still need to be looking for a vacuum leak?

 

Do you mean when you're just sitting still waiting, foot off the go-pedal, or as you start to pull forward?

 

I find mine will drive itself without accelerator input, in traffic on a flat road; just dipping and raising the clutch as required to trickle forward with the queue. If you lift the clutch too abruptly it will make the revs dip as you describe.

 

 

[sepulchrave]

Starting to sound like EGR circuit with that dip in revs in traffic, clean the EGR system and see if it's better.