High Revs on Cold Start

I have an intermittent engine start problem when the engine is cold. Car is a Mk4 Golf TDI (ATD Engine).

What happens is the engine starts but instantly revs high. On starting, the revs will immediately go to anything between 1,200 and 2,000 RPM and then drop slowly. The revs will drop to the normal 900RPM idle – generally within a few seconds, the longest I have seen it take is 15 seconds or so. Sometimes the revs will increase by 100-200 for a few seconds and drop again (similar to gently pressing the accelerator). Generally after 30 seconds to a minute of running, the revs maintain a (normal) steady 900 RPM idle.

Other things I know are as follows:

1. Car has been fine all summer but the temperatures here in the UK are beginning to drop and when this happened, this issue arose (albeit intermittent). This may however be coincidence.

2. Only happens when the engine is cold. When starting when hot or warm it's fine.

3. The real clue it is going to rev high on starting is that the glow plug light does not extinguish when turning on the ignition (glow plug pre-heat). The light normally goes out after approximately 3-5 seconds. If the glow plug light goes out, the car starts as it should. If the light stays on I see this problem, but when the engine starts the glow plug light goes out.

4. There is no smoke or soot on starting.

5. The car's performance and MPG are as expected.

I'm hoping this has been seen before or is a known issue.

Any advice or pointers to what's at fault would be great.

Thanks

That's odd.
The glow plug light will stay on more than 5 seconds when it gets cold.
I've read here that they stay on anyway, even after the light goes off.
Interesting to know how it starts when it gets to -10c.
A lot of cars do this normally, but I don't think these diesels do.
Is it a PD, VE or other?
There are a few different temperature sensors on our cars, perhaps one of those is out.

Ambient temperature is around 5 to 10 degrees Celcius at the moment, so not really cold. I've tried leaving it for 10 seconds to go out, which is normally more than enough....

Didn't see an issue when the temperature was above 10 degrees Celcius, but this may be coincidence.

It's a PD.

I wonder whether this has anything to do with a messed up temperature sensor? These are located under the rain tray (for you I think it's on the passenger side).

From what I can see, the ambient air temperature sensor (reading displayed on the LED matrix panel of the dash board) is working ok as it's readings are as expected.

The coolant temperature gauge on the dash is also reading as expected. Once warmed up, the coolant it as a steady 90 degrees Celcius.

UhOh - which temperature sensor are you referring to?

The cts does the dash and ecu but the ecu side if it in your case sounds like it's not working hence the high revs when starting cold. Mine did the exact same thing.

I'm 99% sure it's your sensor that's shot

I was thinking that it was the ambient temp sensor that might be capable of causing this behavior. Not sure if a bad CTS would do it, though perhaps it could: I've only experienced it failing on the gauge side.

Can you scan for codes/errors? I think that everyone should either have VCDS or, at least, a ScanGauge or UltraGauge: all will read/show codes.

i get the odd "high idle" on my BEW...but only to 1100RPM tops for a second or two, then back to 900 when its cold here. its been below freezing the last few days, i'd guess an avg of -4*c for the week. key on, glow plugs for 5-7 seconds, hit the key when the light goes out, 3 cranks and shes going.

i have a cam position sensor error that comes and goes, only shows on VCDS (courtesy mjhandy ), my ScanGaugeII shows nothing.

Ambient temp sensor has zero affect on the revs, it's all down to the cts. My gauge also read perfect but it's because it's two sensors on one housing. You can have a perfect signal to the gauge and a wonky signal to the ecu or vice versa.

My revs did exactly the same, gauge read fine.

It's one of those fairly definite kinda faults like the no glow plug light and no start being the 109 etc.

So yeh, 99% certain it's gonna be the cts. Put it this way if my car did this I'd go straight out and buy a new sensor.

If it happened all the time I'd say maybe clutch switch or something but only doing it when the engines cold tells me the ecu is most likely getting a duff signal.

Seatman said:

if my car did this I'd go straight outside buy a new sensor

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Me too. The glow plug light behavior is a pretty damning clue.

Coolant temp sensors are pretty cheap, but if you can track down someone with VCDS you can read both sides of your particular CTS to check for sure.

Thanks everyone for your responses

The car was in the garage for its MOT in October and I asked for it to be scanned for faults - no faults logged.

Ok, so it looks like the culprit could well be the Coolant Temperature Sensor, which sounds entirely reasonable. Worth changing to rule it out, if nothing else.

Where is the CTS located - on the engine block?

CharlieTDI said:

The car was in the garage for its MOT in October and I asked for it to be scanned for faults - no faults logged.

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A scan would detect an open CTS or a shorted CTS, but not a CTS that's lying about the actual temperature.

No way for the car to know it's not actually that cold outside.

I'd first look to disconnect the CTS. I'd expect that if there's an CTS issue that disconnecting it should cause a different behavior.

Anyone here think that the N108 could cause these kinds of problems? I'd think there'd be a code set, but...

My 99 MkIV is doing the high idle thing also. Already replaced the CTS (the one in the upper hose), but problem continues. Sometimes the idle has a definite 1-second thump to it, like one cylinder has an injector leak.

Finally, the coolant temp sensor has been changed for a new OEM sensor. The old sensor had the socket made of black plastic. From what I can tell from looking in the engine bay, the new one is green.

Verdict?

So far so good. It's only been a few days, but glow plug light now goes out as it should and the engine starts up normally. Also, no more creeping revs when cold and idling - rock solid idle at ~900RPM.

Thought I'd let you guys know as it's always good to find out the fix.

Happy days!

Awesome for you to post a follow-up!

CharlieTDI said:

Finally, the coolant temp sensor has been changed for a new OEM sensor. The old sensor had the socket made of black plastic. From what I can tell from looking in the engine bay, the new one is green.

Verdict?

So far so good. It's only been a few days, but glow plug light now goes out as it should and the engine starts up normally. Also, no more creeping revs when cold and idling - rock solid idle at ~900RPM.

Thought I'd let you guys know as it's always good to find out the fix.

Happy days!

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Glad you figured it out and posted the solution.
It may help others with similar problems.