An exhausting problem

Q I have had a problem with my Fiat Punto for a couple of years now and replaced or swapped so many parts with new or known, good, used parts to no avail. The car runs and drives fine – the engine boosts well and MPG is fantastic – but as soon as I take my foot off the gas in any gear I get a noise that sounds just like an exhaust blowback. You can trigger the noise by going on and off throttle and it sounds like there is a small hole in the exhaust.

The exhaust is all new, except for the original cat, which is now 21-years-old.

The heat shields and hangers/rubbers are all in good condition and not fouling. The inlet/exhaust manifold gaskets have been replaced, as have all the downpipe and exhaust gaskets. The aftermarket openstyle cone air filter was swapped back to the standard filter and airbox and this made no difference. All engine mountings are brand new. I have no warning lights on the dashboard and a DIY blink test of the ECU has not revealed any fault codes. This suggests to me that the problem results from some kind of mechanical wear.

The car has a rather old Fiat NON-FIRE type 8v 1372cc turbo engine phase 3 (later 130bhp version) and all valve clearances are within factory tolerance, with the exception that exhaust valve 4 is just out of spec at 0.40mm (tolerance for exhaust clearances is 0.50mm +/- 0.05mm, so I’m not sure if this tighter clearance is enough to cause the blowing noise on the overrun? I will of course re-shim it back to spec). On the subject of clearances, is it best to be on the looser side rather than the tighter side? The crankcase breather pipe is clear.

It is on the original IHI water-cooled turbocharger, but the engine hardly uses any oil or water, nor does it smoke or overheat. The engine and turbo have covered 155,000 miles, but have been regularly serviced. The timing is spot on and the cam lobes are all in good condition and all appear to be reaching the top end. Oil pressure when driving is 3 bar and a hot idle shows 1.5 bar.

Examination of the spark plugs during their renewal over the last couple of years has revealed tips that looks fine (light grey/ tan), although the base area has some dry sooty deposits on all four plugs. I don’t remember them having these deposits in earlier years but can’t be 100% sure.

Is it possible the original 1998 catalyst has loose baffles inside and that backpressure is causing them to close up and make the noise? Or could a faulty actuator cause the wastegate to move and create the noise? Or is the turbo shaft sticking, so unless there are enough revs pushing gas through the exhaust side there is a

build-up of gas trying to escape through a non-spinning exhaust wheel? Or is there a problem with either the cylinderhead or dump valve, or a leak in the intake pipework or intercooler?

If the bearings in the turbo were on their way out, I would expect oil to be pulled through the intake and/or smoke, but that isn’t the case. I checked the turbo two years ago for signs of any in/out or axial play, of which there was none, and for cracks in the exhaust housing, but it was crack-free even around the wastegate area. At that time, the shaft seemed to spin freely and there were signs of the blades coming into contact with the housings.

Some people have said it must be a characteristic of my engine but I have owned nine of these cars in the past and none of them showed the same signs. Stuart Clarke

A Regarding your question about tappet clearance, a larger gap is better than a tight gap. The larger gap may emit a noise, but the tighter gap is more likely to cause problems with valves not closing fully.

You have given a very comprehensive description of your problem and I do not suspect that there is an issue with the turbo. It can’t be discounted entirely, but if the turbo was failing I would expect to find oil loss or more serious and constant noises.

You have checked the intercooler pipes and if there was a failure in this area I would expect the boost pressure to drop and a code to appear. It is also unlikely that you have a head gasket problem.

The most likely culprit is, I believe, the catalytic convertor, which may be causing a blockage at certain times. The catalytic convertor does not have baffles, but is a honeycomb unit. This does not necessarily have to be loose for the problem to occur, but may be breaking down and blocking up.