Evening All,

I would appreciate some expert guidance on this vehicle.

This car has had a misfire for approx 3 months and has gradually got worse over time. The vehicle drives well in 1st and 2nd gear and will rev to 5000rpm with no problem. However, in 3rd gear onwards from about 2500rpm the vehicle misfires (and pops) and has distinct lack of power. The problem is progressively worse with the higher gears selected.

Customer has fitted new plugs and leads plus ignition module which has not improved the situation.

My checks are as follows:

Emissions good. Natural idle: CO 0%, HC 140ppm, CO2 16.1%, o2 0.4% Fast Idle: CO 0%, HC 60ppm, O2 0.2%, CO2 16%, Lambda 1.01

MAF is showing 12.8 KG/H (idle) and 41.6 KG/H (fast idle).

Injection duration appears low for a multi-point system at 0.5ms.

All other sensors appear to be in range.

Now here is the big clue. Manifiold vacuum is low at 12"HG rising to 17"HG at max revs in neutral. The engine sounds ok at idle and high revs I have ruled out air leak based on 02 readings and blocked exhaust on vacuum readings. This thing does not have hydraulic tappets.

So my suspicions at the moment is on the waste-gate. I cannot reach the actuating lever nor apply vacuum to test it. Not having any expertise on turbos, I wonder if this could be the problem.

I would be grateful for any sort of input.

Regards

Pete M

Your manifold pressure readings do not sound right.Possible cam out of time causing low vac.at idle.At max. revs. in neutral you'd expect to see some positive pressure from the turbo,not 17 inches of Hg vacuum.So turbo doesn't seem to be working.The popping and lack of power under high load suggest a fuel delivery problem,so check fuel pressure and volume.

Hi David,

Thanks for your advice. I have discounted (for now) cam timing error because I would have thought that a problem in that area would not "get worse" over the last few months unless there is a tensioner or something about to give up the ghost. I have also discounted fuelling at the moment because I would have thought that a vehicle that can rev and accelerate in 2nd gear to about 5000 rpm without problem would indicate that the fuelling is ok. I am obviously open to thoughts on this.

Now I think about it, the vehicle has a turbo pressure gauge on board and will enquire tomorrow on what readings are being recorded before and after the misfire and will post the results.

Regards

Pete M

Hi Pete,

What is the upstream O2 sensor reading whilst it is playing up?

Regards Mark

Hi Mark,

Thanks for that. The serial comms socket is under the bonnet so it is difficult/impossible to read the upstream O2 readings whilst on the road. However on that subject, I found the scanner was not showing any readings at all and just showed zero no matter what I tried to bring it to life. Because of the excellent gas readings, I assumed the zero reading was one of the "vagaries" of the scanner. I could not plumb into the O2 sensor because of where it is situated, seemingly down the back of the engine somewhere.

On a different subject, this vehicle has covered approx 75K miles and cam belt requires changing at 63K miles. Owner has had vehicle from new and the belt has yet to be changed.

Regards

Pete M

The customer has now reported back regarding the turbo boost gauge. The gauge is divided into increments with the highest reading before it goes into the red as one and a half. Turbo cuts in as per normal with a max boost reading of one and a half. When the mis-fire occurs, the needle then flickers at this point. I am now of the opinion that this vehicle has cam timing error and the overdue belt should be changed as a matter of course and hopefully cure the mis-fire problem. Unless I am advised to the contrary of course...

Regards

Pete M

Hi Pete,

You should advise the cam belt change from the point that it is aver due (and it may break whilst it is in your control)

However i don't think it will fix your misfire.

Go the extra mile and gather the O2 readings when it is misfiring.

Mark carter

Has it been chipped? If yes then i would close the gap down on the plugs even if they are new. As the boost rises stronger and higher in a high gear it blows the spark out. Seen and done many misfires as you describe on turbo cars in the past. HTH

Hi Vince,

Very many thanks for that. Yes the vehicle has been chipped in the past and what you say makes sense to me. I will let you know the outcome.

Regards

Pete M

Evening all,

Could not close gaps on plugs as they were triple electrode. He got his mechanic to put new NGK plugs (single electrode) in and noticed that one of the 4 camshaft cover bolts was loose and could not be secured because of damage to the thread. This was temporary cured by using loc-tite (or similar). Car is now running perfectly. Now, I am not a mechanic but these bolts are very long (about 6") and I am guessing they do more than hold the camshaft cover in place. I seem to recollect that on some vehicles these bolts hold the camshaft carrier in place and if this is the case, maybe could cause the misfire.

Anyway, vehicle booked in for cambelt change and NEW bolt!

Once again thanks for your inputs into this.

Regards

Pete M

Bet it was the plugs