[billymade]

We have seen problems with bad coolant temp sensors; cause this type of issue. On cold start up, you will get a rough idle and when revving, the engine can stumble with a rich condition (smoky exhaust and unburnt fuel smell). After the engine gets to normal operating temps; the engine will smooth out, regular idle and hesitation will disappear. Most of the time, a bad coolant temp sensor; will throw a trouble code but not always.

It is a good idea, to keep an eye on the coolant temp light in the speedo and see if it acts "normally". On cold startup, you should see a blue light and when it gets warm, the light should go out. There should not be any other type of modes shown.

Here are the modes of the coolant temp light:

Blue coolant light = engine cold
Flashing blue coolant light = fault in the coolant monitoring system
Flashing red coolant light = low on coolant
Steady red coolant light = engine overheating, pull over and stop engine IMMEDIATELY and do not run until the problem has been identified.

The might be other issues, that are fuel delivery or ignition related; could be a possibility, dirty throttle body (clean and do a throttle body alignment procedure recommended), low fuel pressure, failing coil, old plugs, etc. etc.

It is best to have a scan tool; that can show live data and that way, you can monitor, various things, such as coolant temps, maf readings, fuel trims, pending codes etc.

As for your MAF replacement; why did you replace it? Was there any related trouble codes; that would indicate it was faulty? It is best to monitor the MAF readings and that will indicate, if it is reading in spec. A replacement MAF, should be a oem BOSCH branded part; not a cheaper aftermarket MAF that will not work right, causing the car to run badly. If you have the old maf; you might reinstall it and see if the run any differently.

On these modern cars; a scan tool is crucial to see live data and note how things are running, that way you can see if anything looks off (e.g. fuel trims, maf readings, coolant temps, trouble/pending codes, etc.).

Fuel Trim Info - Ross-Tech Wiki

Start looking at your readings (fuel trims, maf, etc), checking for pending/trouble codes, check for a dirty throttle body, condition of your plugs, swap in your old maf and we can go from there.

A UK based service manual that covers your 1.6L engine; would be a good thing to purchase and that way, you can go over testing, troubleshooting procedures for possible failing parts. We do not have the 1.6L in the North American spec New Beetles; so, we do not have service manual info to refer you to.

The Haynes UK New Beetle service manual; seems to be a good and affordable option.


VW Beetle Petrol & Diesel (Apr 99 - 07) Haynes Repair Manual (Haynes Service and Repair Manuals) Paperback – 13 May 2013

Note: covers your engine: Hatchback, Saloon and Coupe, Petrol: 1.6 litre (1596cc),


ISBN-13: 978-0857336545

https://www.google.com/search?ei=gS...8459.2.2.0.0.0.0.121.237.0j2.2.0....0...1.1j2.64.psy-ab..0.0.0....0.nXXJoJS2CnQ

 

[billymade]

I haven't worked on a 1.6L or have a manual; that I can look at but on the 1.8T here in the USA, the coolant temp sensors are known to fail. Your service manual; should have testing procedures for the sensor and you could confirm things are ok. The sensors, tend to not be very expensive and if you still are using a 10 year old sensor, it wouldn't' surprise me if it has failed after all those years.

Here is a video showing how the temp sensors fail:

https://www.youtube.com/watch?v=TwQ-2xp_TK4

testing:

https://www.youtube.com/watch?v=eXNBlH0c4_U

For your maf; I would confirm correct operation, using live data from a scan tool. As for using a Hitachi MAF; here we only see BOSCH as the stock oem for these but maybe VW uses Hitachi in the UK/Europe? Again, we don't have that engine; so, I cannot confirm this. If you were having a lean code; you could have other fuel delivery issues, that can cause that. Monitoring your fuel trims; can help you pinpoint if you have a possible weak fuel pump or vacuum leaks. Fuel pressure testing; would be a good way to rule things out, confirming things like the fuel pump/pressure are in spec.

As for LPG; are you referring to https://en.wikipedia.org/wiki/Liquefied_petroleum_gas?
Did you have a conversion added to your car; to run this? That could potentially add a whole other variable to troubleshooting your problem. I don't have a clue; how yours is setup or how your system is adapted to work with your 1.6L. I suppose, a failure or malfunction in that system; could pose a problem as well but maybe a LPG specialist shop, would be needed to check that out? Here is the USA LPG conversions; used to be popular on work trucks for industrial use with ranching, mining and the oil industry.

The throttle body cleaning, is easily done with some appropriate cleaner and then running a procedure with a VW specific scan tool. I usually use a "throttle body cleaner" from CRC and a old tooth brush, to get all the contamination from the assembly. If you don't have a vw scanner; you can try turning the key to "on" position without starting the car, wait a minute or so, until the noises from the throttle body stop. This is considered a "poor man's tba" and while up for debate, many consider this a alternative option to a tba with a vw scan tool.

http://wiki.ross-tech.com/wiki/index.php/Throttle_Body_Alignment_(TBA)

throttle body cleaner (CRC, GumOut, Wurth, Valvoline, Permatex, Liqui Moly, etc)

https://www.google.com/search?q=thr...r&oq=throttle+body+cleaner+&aqs=chrome..69i57.3285j0j7&sourceid=chrome&ie=UTF-8

You might read up in your service manual; how to test your coolant temp sensor and confirm it is ok and then possibly replace it, to see if that is the culprit. The manual, should have the coolant temp specs; when the coolant light goes out, fans kick on and what the "normal" operating temp range is.

How are your fuel trims (normal is usually + or - 10% range) and are there any pending codes?

 

[billymade]

Ok, sounds good; a new coolant temp sensor, is probably not that expensive. Let us know, how the testing goes! Thanks!

PS: I'm not seeing for sure if the scan gauge II reads fuel trims for VW; you might look at VW specific scan tool. There are many "VAG" scan tools out in the marketplace; you might check out OBDEleven for a affordable option that runs on Android based devices or the Ross Tech VCDS for a mature windows based scanning solution.

https://www.scangauge.com/support/x-gauge-commands/volkswagen/ (you might email tech support and see if long and short term fuel trims are supported)

https://www.google.com/search?q=VAG...+scan+tool&oq=VAG+scan+tool&aqs=chrome..69i57.3590j0j9&sourceid=chrome&ie=UTF-8

https://www.google.com/search?q=obd...&oq=obdeleven&aqs=chrome..69i57j69i60j69i59l2.1768j0j7&sourceid=chrome&ie=UTF-8

https://www.google.com/search?q=ros...+tech+vcds+&aqs=chrome..69i57j69i60l3j69i59l2.3000j0j9&sourceid=chrome&ie=UTF-8

 

[billymade]

Hmm, sounds like something unique to the 1.6L; never came in usa spec vw's, let us know how your testing goes, sounds like you may have found the problem! Thanks.

 

[billymade]

Hmm, a "fuel injection pump"; did strangely, sound like a diesel. Hopefully, you can narrow down the possibilities and find the problem soon!

 

[billymade]

Ok, sounds like your coolant temp sensor was bad; good job on the swap. Another thing that can cause a rough idle is a dirty throttle body; clean it and do an alignment.
Otherwise, as noted a vw specific scan tool that can do live data would be helpful in your troubleshooting process.

 

[billymade]

For a vw scan tool; there are many "vag" versions on ebay/amazon (make sure they do live data not just read codes);. Just keep in mind, most offshore asian scan tool offer little to no support and if anything goes wrong, your usually on your own. If you go this route; purchasing through amazon, is a good idea with their good return policies.

Another option is obdeleven that is compatible with android devices (phones/tablets); has a downloadable app and a wireless dongle you plug into your obd II port.

https://www.google.com/search?q=vag+scan+tool&ie=UTF-8&oe=UTF-8&hl=en-us&client=safari

Lastly, the ross tech company sells vcds that runs on a windows pc; it is a well known, mature solution, that has true usa based support and registered user only forums to back up the tool. While it costs more than many other vw compatible scan tool solutions; it will pay for itself in its after sale support and wide use, familiarity in the world wide vw community. VCDS is compatible with the latest vw/audi cars and they offer free software updates; which is unheard of in the automobile repair industry.

https://www.google.com/search?q=ross+tech+vcds&ie=UTF-8&oe=UTF-8&hl=en-us&client=safari

 

[billymade]

The above thread, has a lot of good troubleshooting info; you might start a new thread and discuss your problem more fully and make sure, to put it in the discussion area, related to your engine type. The failed coolant temp sensors; are one of the common, rough idle/cold start issues.

 

[billymade]

Thanks for the feedback; on the final resolution to the idle problem. I replaced the plastic injector cups with billet aluminum versions and rebuilt injectors and seals, i'm hoping a occasional erratic idle, will be solved, as well. In my case, a 1.8T AWP code engine.