Car failed MOT , same as last year only do not know what dealer did to pass it . Customer said new cat was fitted last year , perhaps this masked the problem and allowed a pass. In between MOTs car has only done about 4000 miles.

On this year fail the main dealer suggested a blown head-gasket. The customer took it to another garage who tested for gasket failure and found none. But after all their tests including inspection of the cat by dropping the exhaust, declared a fueling problem and went no further.

The car also has a very low idle on cold start 500-600 rpm almost cutting out and needs revving to keep it stalling for the first few minutes. Also looks like someone has cut wiring to idle control valve signal wires back to ECU only to re-connect it with butt crimps (very poor work but continuity fine) , perhaps they were trying to test the Idle control or trying to fudge the idle.

On my exam only one fault code present PO443 Evap Canister Purge Valve Circuit malfunction. Code cleared and did not come back. Suspect dealer+ previous garage working on trying to solve MOT emissions disconnected the sensor and caused fault, as testing the valve showed no malfunction.

The MOT results from previous year were almost identical to this years test .CO 8%/ HC 291ppm/lambda 0.772. The second garage to have a look at it put in a new Lamda sensor and CO went down to 5% on a re-test.

I managed to test the ECT, MAP, Evap Canister Purge, Idle control motor and two of the 4 Injectors were tested all good. Injectors 1 & 4 tested for voltage and current ramp 72 Volt peak/ 0.8 Amp peak/ 3.0 mS on time/engine hot. The O2 sensor stuck on high 0.9V indicating a fixed Rich mixture. With WOT the O2 would start to dip on opening the throttle then recover back to Rich.

I still needed to somehow test the two middle injectors do fuel tests and smoke tests for leaks etc.

But I ran out of time and as I was finishing up I found by accident that the car would hunt up and down on revs if I held the throttle at 1800 rpm . Not a misfire as such but clearly engine revs going up and down consistently. I then lifted all spark leads and found number 3 had developed a hole in the side (sparking quite well through insulation) and number one lead was putting out quite a weak spark.

Customer wanted to replace leads/sparks/coil pack and do an emission only test to see if this solves the problem. So I stopped short of completing my diagnosis. He will take it back to MOT station to re-test once new parts were fitted. I hope for his sake and mine that this solves the mystery.

My question about all this is: If the throttle/idle control unit is as I suspect at fault for the low revs when cold, is it also likely to affect the emissions as well? The car idles well once warmed up and gives no sign of a misfire as such either at idle or on WOT.

All ideas, thoughts, experiences welcome. My money is on one of the two untested injectors.

George