407 very intermittent surging

[moizeau]

Next one. On the way to the ferry and I hit the Cruise Control. Then slowed down behind a lorry. Once the lorry was out of the way I reengaged CC. I had slowed to about 30mph and the car's CC was set to 60. It accelerated like I'd used the kickdown. Then it started surging and slowing +/- 5mph either side of 60. Luckily I hadn't travelled far so limped or rather 'surged and slowed' back to my mothers house. Connected PP, no codes. Spoke to a few friends that suggested it could be a vacuum hose or a valve stuck. I located what I think were vacuum hoses going to a unit, along with an electrical connection, that's perched on top of the alternator. I disconnected the hoses, checked for damage and then reconnected them. Desperate to catch a ferry, I decided to take the car for a confirmation run (that I was doomed). No issues what so ever. Booked a short term RAC cover then drove it 650 miles down England and France without as much as a hiccup. I've not touched the CC since, but I had used it during the 3 days previous, but only engaging it when I was already at the preferred speed. (no massive acceleration) . Not having had a car with CC before I've never really researched how it works. I'm guessing that when I disconnected the hoses, that allowed the 'whatever it is' to go to a home position. I'd be very grateful if someone could fill in the blanks please. Thanks

[xantia_v6]

Which engine?

[moizeau]

2l HDI 136 with the AM6 (oil change on the list, previous owner followed Pugs advice and did nothing)

[xantia_v6]

With an HDI engine, there is no separate physical control system on the engine, the speed control is all done by software in the ECU.
Maybe you should try it again to get a better idea of what causes it to act strangely.

[moizeau]

Cheers for that. So it was cured by fluke then? (snarl) Yes I agree, I need to see if I can make it do it again. I will try the same scenario. Any idea what the doofer on top of the alternator is that I unplugged?

[GiveMeABreak]

I’m not too sure I’m understanding the issue.

The cruise will only function at about 30 MPH, so that is normal. If you have set it at 60, then resume CC and are doing at least 30, then the system will rapidly accelerate to reach the desired speed and as you have an Auto, it may feel like when you use ‘kickdown’. Cruise will attempt to keep the desired speed, but it is normal practice for the speed set to vary a few MPH above or below the set speed..

Normally if there is any problem with the injection / depollution or other engine related systems the cruise control will automatically be deactivated.

[moizeau]

Thanks Marc, the variation in the speed, the surging wasn't normal. It was flat out, over hit the mark then nothing, under hit, then flat out, repeat. not really safe to drive, but it wasn't far. I disabled the CC (on the stick) and it continued surging under normal driving until it was parked, plugged in, no codes, disconnected and reconnected the pipes (whatever they are). Then and now it's fine. Need to call in the Scoobies

[moizeau]

Back again!. After getting home (see 1st post) I then went to Lyon and back which is 400 miles within a couple of days. No problems. Then it only did a short run to for the CT before starting the export procedure. It's not moved for 5 weeks. Just got it insured today so drove it 40 miles. Came out a very slow corner and it seemed to accelerate faster than normal but changing gear at the same revs. Within 10 secs the surging started again. 5 secs of power followed by almost wanting to stall for 5 secs....repeat. Got home and lifted the bonnet and saw 'steam/smoke/vapour' coming from behind the engine cover. By the time I had got the BOLs out to find out that the engine cover is just PULL, the s/s/v had stopped. From above, all looks good, no burning, no oil, no water. Started it, went for a drive. (This time the surging was without the CC engaged.) The only similarity between the two occurrences is the sharp acceleration before the surging. On the drive I tried to make it happen again. Kickdown, italian tune, engage CC and set to 70mph then touch the brake and wait 'til it reaches 35mph then re-engage, change the speed using the stick. It's fine??? The only difference was that when I did the acceleration test using the CC it accelerated smoothly, as in normal driving, whereas the first time it went like a bat out of hell! It feels as if whatever the 'puter is using to accererate is mechanical and gets jammed, then the 'puter realises and uses something else to cut the revs, then realises it's going too slow and accelerates....repeat. Once stood, presumably 'puter resets, everything is good to go. I'm also guessing that whatever is stuck, if that is what is going on, is due to it standing?
Sorry for the long post.
I'll be driving it regularly from now on so we'll see it it re-occurs. In the meantime..any ideas?
Thanks
Pete

[GiveMeABreak]

Pete is this happening under normal driving? Are you accelerating normally and smoothly, or is it surging without cruise?

I’m wondering if it is the throttle potentiometer playing up ... I’ll have a look, I think I have your VIN from last time.

[moizeau]

Thanks for the reply Marc, 1st time accelerating with cruise, today without, both times no codes, but the 'puter was hooked up after everything was back to normal. Is the pot connected by a cable which then converts to electrical, as in the 2.1td?
Thanks again

Pete

[GiveMeABreak]

No on this one the Accelerator pedal potentiometer connects directly to the Engine ECU which controls everything thus:

1261: is the Accelerator Pedal
1320: is the engine ECU

The 4 pin Black plug on the accelerator has the following 4 wires:
1377: Accelerator pedal 1 position info
1378: Accelerator pedal 2 position info
1379: accelerator pedal position earth
1380: + 5 volt accelerator pedal position indicator power supply

Might be worth checking the connections are ok on the connector. There were a few TSBs released for this engine (DW10BTED4) with the Siemens Injection system, but I don't have these I'm afraid - mostly to do with stalling / cold starting / cutting out issues.

[moizeau]

Cheers Marc, I'll stick my head in the footwell later. I guess the diag would throw something up if there was a duff connnection with the engine running, but only when the problem surfaces so I'll plug it in again and go for a drive.
Thanks again

[white exec]

The injection/engine ECU - which incorporates the CC - relies on the gearbox speed (distance) sensor to tell it what speed the vehicle is moving at. Possible that the distance sensor is faulty/bad connection, not an uncommon fault. A clue is sometimes the speedo (often driven from the same sensor) not being stable.

Vehicles with CC and auto boxes can seem to behave oddly if the box changes gear while the CC is attempting to put on speed. Have noticed that some Inj-ECU-operated CCs will call for "full throttle" if CC is resumed with it set to a much higher speed - the car can be in quite a hurry to achieve the higher speed. If an auto box is in the mix too, it can get quite exciting!

If the throttle pot is simply a two-track potentiometer, you should be able to test it for smooth action
with a test meter, either unplugged on resistance, or connected on the output voltages from each track.

Do any road tests on evenness of acceleration with the box manually selected for a single gear, to knock out the effect of gear shifts; think you're doing this anyway.

[GiveMeABreak]

Cheers Marc, I'll stick my head in the footwell later. I guess the diag would throw something up if there was a duff connnection with the engine running, but only when the problem surfaces so I'll plug it in again and go for a drive.
Thanks again

That might be an idea - getting some live data and go for a run to see what is recorded when it does what it is doing. That may give an indication of something - but at present if there are no fault permanent fault codes, it's likely to be intermittent - and these may not be recorded if the conditions and criteria that cause them disappear after a while.

[GiveMeABreak]

I'd expect a P0717, P0720, P0722 or P0727, fault code with any problem with the gearbox or engine speed sensors though - signal or coherence issue. It will be interesting to see what is recorded though