Car briefly goes jerky after a cold start

[mjt]

Ever since we bought our Mk2.5 in 2014 it's had a tendency to go through a jerky phase a few minutes after starting off from cold. It only lasts for a short time, as little as 30 seconds or less, then it's fine and doesn't miss a beat. I've always put it down to a glitch in the mapping and have lived with it but in the last few weeks it seems to have developed a tendency to stall if the clutch is dipped at the crucial moment.

The engine is a 1.8HE petrol as listed below my sig. It's done just shy of 50K and been serviced every year, having only done 5K - 6K per year. I use an independant as I'm of an age where getting up off the ground is becoming a challenge. One thing I have done, rather than entrust it to the garage, is change the plugs at 34K. I decided to get Ford plugs and was supplied with FoMoCo rather than Motorcraft. The gaps are correctly set at 1.3mm exactly the same as the ones that came out.

Having bought a Tunnelrat ELM327 cable a few months ago I decided it was time to try it out. FORScan reported there were no codes stored other than a spurious IC one. I know about the cluster problem but other than this issue have not had any problems with the car. As it's a 60-plate I think it might be clear of the date range of the affected clusters. Anyway after clearing that one it didn't come back.

I'm just wondering what could possibly cause the jerkiness for such a short period. It seems as though it briefly misfuels as the engine gets to a particular critical temperature. Could it be a HOS? If so it's obviously still working within the acceptable range. I'd be interested to have the thoughts of the experts on here.

 

[mjt]

Wow! 40 views but no responses. I'm just bumping this in the hope that someone might have an insight.

[TomsFocus]

53 minutes ago, mjt said:

Wow! 40 views but no responses. I'm just bumping this in the hope that someone might have an insight.

My guess would be throttle body if it's bad enough to stall.  But I've owned a few cars with this slight & brief judder shortly after cold start, both petrol and diesel, I think that is just the mapping.

[mjt]

22 hours ago, TomsFocus said:

I've owned a few cars with this slight & brief judder shortly after cold start, both petrol and diesel, I think that is just the mapping.

As I said, that was my thought but I get the impression it's been getting worse recently. I'm thinking of trying to get some live data if I can work out how to do it and which PIDs I need to observe. I've only just started using FORScan so on a bit of a learning curve and at my age learning new things becomes more difficult.

[F0CUE]

The plug gap is defo correct so can't be that 

It could be the oil just taking its time to get through the system and get up to temperature or may be a very small air leak and the jerk is the ECU compensating. Check all pipes for softness/ collapsed or holes.

[Dave sport tdci]

My low mileage 56 plate 1.8 tdci always felt jerky even when trying to drive smoothly I thought it was a bad gear box mount or it was because the diesel is a old design and very crude engine ?

However  after having my clocks re soldier it has felt much smoother and far better to drive 

Does your jerkyness get more noticeable in the summer or hot days ?

[Tdci-Peter]

On 7/3/2019 at 9:13 PM, mjt said:

go through a jerky phase a few minutes after starting off from cold. It only lasts for a short time, as little as 30 seconds or less, then it's fine and doesn't miss a beat.

At some stage after a cold start, a petrol engine will try to switch from open loop mode to closed loop mode. In open loop the air/fuel mix is controlled by guesswork, or more technically, by using the sensors measuring air flow to control injector timings by using a map from past use. In closed loop the O2 sensors are used to refine this map continuously to exactly manage the air/fuel ratio for minimum emissions.

Forscan should be able to tell you which mode it is in, and if it is having difficulty in getting into closed loop mode. Calibration errors in the MAP (or MAF sensor if fitted), or in the O2 sensor, or air leaks, or cat problems, or various things could all make the switch difficult. Fuel trims are another guide that Forscan can access. It is quite a complex area, a bit beyond my experience as most of my petrol cars have been carburetor jobs! But there is a lot of info about it on the net.

[mjt]

Thank you Peter, that makes a lot of sense. I'm seriously thinking of trying to get some live data but it may take me some time to get to grips with Forscan. If & when I've got something I'll report back.

I'm guessing the PCM has no way of detecting when the HOS has reached working temperature and simply relies on a time delay. If so it might be switching to the HOS before it's working properly.

If I lived on a road where I could drive straight off at 30mph from cold I suspect I probably wouldn't notice the jerkiness. It's just apparent when running on a small throttle opening and lightly loaded and occurs when I slow for some bends a short distance from my house.

[TomsFocus]

53 minutes ago, mjt said:

Thank you Peter, that makes a lot of sense. I'm seriously thinking of trying to get some live data but it may take me some time to get to grips with Forscan. If & when I've got something I'll report back.

I'm guessing the PCM has no way of detecting when the HOS has reached working temperature and simply relies on a time delay. If so it might be switching to the HOS before it's working properly.

If I lived on a road where I could drive straight off at 30mph from cold I suspect I probably wouldn't notice the jerkiness. It's just apparent when running on a small throttle opening and lightly loaded and occurs when I slow for some bends a short distance from my house.

The PCM uses coolant temp rather than a time delay to determine when to switch to closed loop.  I don't know the actual temp figures for this engine though.

[mjt]

10 minutes ago, TomsFocus said:

The PCM uses coolant temp rather than a time delay to determine when to switch to closed loop.

If that is so this effect must be happening while it's still operating open-loop as I very much doubt the coolant temperature would have moved much in the first two or three minutes.

As far as I can tell ambient temperature doesn't seem to make much, if any, difference.