406oc.co.uk

Hi all,

I'm new here... and after much searching (soul, and online), I can't seem to find the answer to a question. I'm hoping that one of you clever folks can help.

I had a local garage change the cam belt on my 2001 406 HDi 90 (150,000 miles) this week. I noticed after the job was finished that the engine was slightly more snappy, but also idled rougher. I was ready to accept that until after about 20 miles of motorway driving (30 miles total driving since the belt change) I lost power at 2800 RPM, requiring recovery.

I've taken it back to the garage at which I had the belt changed. They've plugged it in to the computer which tells them that there's pressure in the intake manifold and no compression in cylinders 2 and 3, meaning a dropped valve in one or both of those cylinders. 1 and 4 are running fine, and the engine will start on these cylinders along, according to them (it wouldn't start for me).

Obviously, my first inclination was that something to due with the belt change put the timing off slightly, causing a piston to smash into a valve or similar. They tell me that this is not the case as the timing belt is still intact, and that having slightly misaligned timing is not possible on this engine due to the pricise lock-out points that are used when changing the belt. Their hypothesis is that the unseated valve (snapped, caughter pins, spring snap etc) occurred independently of the timing belt change and that there is no way that changing the belt could have caused this, as the belt is still in place.

Their explanation seems to make logical sense to me; however, it seems suspect that a system related to timing (i.e. valves) failed so shortly after a change to the timing system (the cam belt), particularly when it was idling roughly after the change.

Has anyone else had a similar experience? Any ideas on how I should approach the garage? Is there any way that I can validate what they've told me?

As an aside, I don't suspect the garage of outright lying to me: they have a reputation of being pretty honest guys. I'm only questioning whether they possibly missed a step with the belt change or there is some fault in their logic.

Thanks in advance.

If you had a "dropped valve" the engine would not turn over due to the valve hitting the piston and stopping the piston from moving (there is a very small gap between a piston and the valves when the piston is at the top of the cylinder), however, there could be a broken rocker or two.You need a correct diagnosis of the problem, taking off the rocker cover would help.

If the belt is still intact, you need to check if the valve timing is still correct, or the garage should, there could be a problem with the tension of the belt, it could have been too slack causing the crank or cam pulley to jump teeth.There are 2 types of belt on these, it's possible the longer belt was fitted by mistake (unlikely, but possible).

I would ask them to show you the timing situation as it is, tell them you want to see the position of the locking pins and if they line up, also if the belt is still correctly tensioned.

I've done more than 50 of these timing belt jobs, they are so straightforward there should not be any problems.

I am not saying that the garage has done anything wrong, merely pointing out possibilities.

HDis aren't really known for dropping valves, they're actually designed to break rockers rather than bend/break valves if the cam belt slips. Also why only 2 cylinders? Unless, as you say, they screwed something up when they were doing it.


Just seen your post Jasper, wouldn't a broken rocker mean the valve is left shut? Why no compression? Unless the rocker was destroyed and is holding the valve open I suppose.

It's a difficult one Steve, I would take the cam cover(rocker cover)off first and have a look.

What the garage appear to be saying is that the problem is a coincidence, again, perfectly possible, thinking about it deeper, loss of compression on 2 cylinders may indicate a cylinder head gasket failure (a break in the gasket between 2 cylinders) causing lack of compression.

It's all down to working backward from the cambelt job....
1)Set up the timing as it should be and check if the timing pins go in the holes, ie, is the timing still set right?
2) Has the belt jumped teeth?
3)Have we got broken rockers causing valves to be open therefore lack of compression.
4)Has the head gasket gone?

When I do these timing belt jobs (on any car), after the timing is set and the tension is correct, my way of checking if all is correct is to put the crank pulley back on and start up the engine, if the tickover is correct then I know all is well. In the past whenever I have had a dodgy tickover (on some very tricky 16 valve timing belt jobs for example), then I know there is something wrong, nearly always a tooth out.

Again, I'm not saying the garage is wrong, just trying to find a logical answer.

Thanks for your replies everyone.

About it being a 'dropped valve' - the guys probably said that just to use layman's terms. Indeed, the engine does turn over (and actually run on two cylinders), so as you say, it can't be that.

Steve, your point about the two cylinders is what's confusing me as well. If the did screw up the belt change (i.e. put the wrong one on or get the tension wrong), wouldn't that affect all four cylinders?

Jasper, I'll have to call the garage to have them answer most of those questions as they have the car. I don't suspect that the head gasket has gone as the engine was otherwise running fine before, and if memory serves, that's not something that catastrophically fails, is it?

As far as 'dodgy tickover' - by that, do you mean rough idling?

Thanks again guys.

If the head gasket blew between 2 cylinders you would have zero compression on 2 cylinders and it would only run on 2 cylinders, you wouldn't get serious water loss if the gasket blew in the centre between the cylinders.The gasket could fail anytime.

Yes, I meant rough idling.My first check after doing a timing belt job is the idling (tickover).

Regarding your problem, as I said, my first check would be the valve timing.

Cool. I'll call the garage on it today--though I can imagine their response:

"the engine was properly pinned in both places whilst doing the change, so putting the timing off is impossible..." etc. etc.

Is this a valid argument, if they use it?

They have a point about pinning, but the tensioning of the belt is where the argument is, even if the timing is correct on fitting the belt, the timing could change if the tension was not correct (too slack), it's possible for the timing gears to move by jumping teeth, which they could do if the tension of the belt is too slack.

As I haven't got the car in front of me, all I can do is give you an educated guess, only the garage will know what happened.

The story with my car started with the garage replacing a cam belt and leaving the tensioner in the locked position, as time went by the belt stretched a little, the previous owner went over a bump, the blet jumped and a piston smashed the head off a valve.

Sounds like a muppet garage, see the owner Jim Henson.

Just to double check, the cam belt was just a routine change? The engine was running perfectly before? If so, the garage needs to put it's own work right, you really shouldn't have taken the car away if you noticed a rough idle as soon as you got in. You say the engine was more snappy and idles rough, probably advanced by a tooth.

Yep, the cam belt was a routine change. The engine was running beautifully before.

I shoudln't have taken it away like that... but these guys are supposed to be good, and I had to pick the car up out-of-hours (i.e. keys left at a pub) so I didn't really have much choice at the time!

I've spoken with a mechanic friend, and he seems to agree with you, poddack, that it's advanced by a tooth. It's extremely unlikely that a cam belt change, followed by a rough idle, snappier acceleration, and then two dropped valves, is a coincidence. If it was one dropped valve, that could be probable, due to fatigue, but the likelyhood of both going at once is remote (unless there is some possible way that a valve going on #2 could possible disturb a valve on #3, though I think that this is unlikely)

I'm going to go see them tomrorow and see what they say.

Something doesn't add up with this problem.

If the timing is out by a tooth the engine would still be running but a little rough and be a little reluctant to start.It would also still be running on all cylinders.
For there to be serious damage such as broken valves and no compression on 2 cylinders the timing would have to be seriously wrong because valves have hit pistons.If valves have hit pistons then there is serious damage to the cylinder head or at least broken cam followers.

I would be very interested to hear what the garage have to say.

I had a major engine problem a few years ago ( rod through block ) when a garage changed my belt on an older c15 engine - when I collected the car after having the belt done I thought it was a bit tappety but power seemed OK'ish - I did query this with the mechanic but he said it sounded OK to him - managed 150 miles before the engine went AWOL - decided to trailer the van back to the dealer and we both fetched the cylinder head off - result was one broken valve and small marks on the top of all pistons from the valves which were hitting them due to incorrect fitting/timing of the cam belt - they replaced the engine FOC - if they are saying new engine get them to fetch the cylinder head off in front of you so that you can see the result - if its affecting 2 cylinders I'm just thinking it may possibly have bent a couple of valve heads rather than breaking them which would/could cause them not to seat correctly and therefore loss of compression - just a thought

That engine was famous for throwing con rods through the block, it was very common.

jasper5 wrote:That engine was famous for throwing con rods through the block, it was very common.

irrelevant in this case as the engine threw a rod due to the valve breaking and hitting the top of the piston at 2500rpm - I'm just trying to explain what would/could happen if a valve had broken on the hdi engine.