Corsa-B.uk - Vauxhall, Opel and Holden Corsa B Owners Club

Hello

Sorry this is a bit long.

Just read through a previous posting of a mis-fire and I was going to tag onto that, but maybe what I have isn't quite the same and I didn't want to take over somebody else's call for help.

Not having had one of these engined cars before, or not having been in another, I don't know what is normal/abnormal behaviour. The car is my sons. Its a first car and has done 72k miles.

First of all, the car starts fine and at all times. We fitted a new timing chain when we got it as this was noisy. It went straight forwardly.
It also pulls well and will accelerate upto whatever speed you want (ok within reason), and it does not feel limited. It is also doing around 50 miles to the gallon.
However
There is a definite vibration from the engine that feels like a misfire. This effect has possibly been there since we got the car a couple of months ago, but there was always a vibration evident at idle (from the drivers seat anyway, it could not be felt from the passengers point of view) and so I had the front right hand engine mount replaced just before its MOT, which it flew through only a couple of days ago. The new mounting however has highlighted the problem.


A summary of my symptoms and investigation work so far....

Starting fine. All conditions.
Idle speed 900 - 950 when warm. A little faster immediately after startup (say 1100)
Evident mis-firing (or uneven running).
Spark plug ceramics are very white, but all clean (no signs of oil). New BOSCH twin earth electrode plugs fitted.
Car uses no oil or water.
Car pulls OK (without comparison to a similar car it sems OK).
Compression test (tested hot and allowing several cycles to get the peak values) in order cyls 1,2,3 are 9.2, 10, 10.5 (Bar).
No MIL lights showing (and the light does come on before start during the self test period so it does work).
I haven't done the 'paperclip test' yet for engine codes. Certainly my mates or my universal code reader won't work with it.
From the MOT emmissions test, idle CO = 0.07%, fast idle 0.05% (is it running on fresh air?). Fast idle HC = 11 PPM, Lambda = 1.00

Anecdotal impressions and general points ...

There is vibration evidence of a mis-fire or partial at idle and it feels regular. It certainly feels regular mid range, but at higher rpms you wouldn't notice it.
I feel the mis-fire is also evident when you lift off and it overuns back down to idle. If it was an ignition/injector problem I wouldn't notice this would I?
Am I doing the compression test correctly? Should I hold the pressure relief valve on the tester and get an impression of the peaks on individual strokes?
(never done it that way before tho I must say).
I have read about issues with the crank sensor, but it starts fine, every time.
Coil packs can be as problem, but I have no fouled plugs. They all look identical.
Is it a MAF problem?
Is it the Lambda sensor?
Is it the ECU (code reader won't work - tried 2 different ones and neither can establish communication)?
Is the engine toast?

Any help/ideas greatly appreciated.
Cheers... Rob.

Just checked the diagnostic connector. Pin 6 has no terminal in it, so I guess the 'paperclip test' is not available for my car.
I had my paperclip, pad and pen all in hand ready to go as well!

Cheers... Rob.

It would seem from the TIS information (now I've finally found the info I was looking for) that my CO reading is down by a factor of 10, and should be more approaching the 1% value.

Could that be my problem. It would certainly fit with my whiter than white ceramics on my spark plugs.

What could cause this, is it the MAF that is faulty? Would unplugging it prove/change anything or is it the lambda sensor? I am suspicious of the reading of '1.00' for it during the MOT test. Being an analogue electronic engineer, I don't like exact numbers. They make me nervous about it being a correct value.

... Rob.

OK

I got myself one of those £20 plugin code/interface readers from a well known auction site and got the software working on the computer.
Guess what, it all seems hunky dory. The only issue was a fault with the MAF, which I caused as I ran it with the thing unplugged to see if it ran any different. Anyway, I cleared the code and it hasn't come back. All the other ECU related checks came out fine as well. The fact that I communicated with the ECU via this bit of kit also gives me high confidence the ECU is fine.
I also got a 2nd hand coil pack and fitted that, which made no difference.
I also found some info from another owners car, which indicates that my CO readings and Lambda are virtually spot on correct (the car does run on petrol fumes after all).

So I have re-done the compression test (cold) and you can see the results below.



As you can see, the middle cylinder is down on the other two (which are broadly in spec relative to each other), but the question is if it is enough to cause the running I am seeing. Its certainly a lot better than other cars I have had, where a faulty cylinder showed up as less than 50 % the reading of the others.

Any ideas?....

Cheers... Rob.

Going to get some new spark plugs tomorrow.
Although these were new, I did have the experience with my old 6 cylinder 280E Merc where two out of the 6 brand new NGK plugs were faulty. Couldn't believe it two were down!.
Anyway, worth the cost af another set of plugs.
If that doesn't fix it, I'm afraid its down to the scrap guy for it, along with its brand new MOT.

Cheers... Rob.

OK
Time to close this conversation with myself on-line.

I believe I have fixed the problem.... that wasn't there.

I think that the 3 cylinder unit is inherently unbalanced.
To counteract this 'feature', GM have fitted it with extremely pliant engine mountings.

So what I did was buy a spurious engine mounting from that well known online auction, and fitted it, noting only that the new mount was much stiffer than the outgoing one.

Result ... Well you have read the musings up above, all of which point to the 'no fault found' scenario. Basically it all is working rather well.

So..
I have bought a good 2nd hand ORIGINAL (genuine) engine mounting from you know where (cost me the same as my brand new spurious part) and compared the two.

The big obvious difference in construction is that the spurious item has the rubber mounted (flexible) centre fastened on 3 sides (top left and right). The genuine original part has it only mounted in two (left and right) with non bonded rubberised buffer regions above and below the moveable centre only, to limit any extremes of movement. In addition it is very pliant, which suggests a soft rubber, of maybe only 50 Shore hardness (guessing). I would guess that the spurious part was much harder with a more typical type of compound used in the automotive bushing (maybe 100 Shore hardness). I don't know if I cut the top rubber connection if it would then give me the equivalent to the original part. It may well have done, if the compound was soft enough. I may cut it to see what it 'feels' like and make a judgement if it was suitable for re-fitting. That would really just be an excercise for my own edification though.

Any way.
End result on fitting the genuine part is that there is still the slight vibration at idle (what I was originally hoping to erradicate), but absolutely no issues through the rev range. The car pulls (as it always did) very well for a 1.0 litre.

Morals
Don't always expect the cheap spurious part to be the same as the original, even if it looks to be spot on and very well constructed.

The cheap part may well be fine for the 4 cylinder (sweet running) cars. Its just unusual IMHO to find a simple heavy duty part like this that is rather more special than it looks...

... Rob.