Posted by Colin Hedger on February 1, 2011, 8:21 pm
Year of Manufacture *: 1997 Engine Size and Code *: 4.4 V8 ( Optional ) Scanner Used: Launch
Have this 4.4 V8 with an occasional , slight misfire.No codes logged . Emissions fine at 1500 RPM plus. The car has seperate cats and exhaust system for each bank of cylinders,(no link pipes) leading to a seperate back box with twin exit pipes on each side (4 pipes total). At idle,with the sample probe up one exit pipe on one side , the O2 level is higher than i expected ,so i experimented with blocking the other exit pipe (on the SAME side) with a rag.I thought that might bring the O2 level down, but instead that GREATLY increases the oxygen reading.Am i missing something? can someone explain? This occurs on whichever bank/exhaust side you test. Gas readings below:
N/S (idle, probe in 1 exit, other open)
CO 0.02 %vol CO2 15.3 %vol HC 0000 O2 1.15 %vol Lambda 1.052
N/S (idle, probe in 1 exit, other blocked)
CO 0.02 %vol CO2 10.1 %vol HC 0023 O2 07.4 %vol Lambda 1.516
O/S (idle, probe in 1 exit, other open)
CO 0.00 %vol CO2 15.3 %vol HC 0000 O2 1.05 %vol Lambda 1.048
O/S (idle, probe in 1 exit, other blocked)
CO 0.00 %vol CO2 09.3 %vol HC 0000 O2 08.5 %vol Lambda 1.644
Either side @ 2000RPM exits open
CO 0.00 %vol CO2 16.1 %vol HC 0000 O2 0.10 %vol Lambda 1.004
Hello Colin, could it be because the exhausts are completely seperated that when you block one exit off on the same back box it is creating to much back pressure which would in turn affect inlet manifold vaccum and make it run uneven (engine not breathing properly )
You say it has intermittent missfire, coils and plugs ok ??? Can you watch your pre cat lamda voltages through live data and see how they are switching,it will give you an insight to how the mixture is at idle.
Looking at the gas readings at idle without you blocking off the pipes the hc are 0 so doesnt seem like a coil or plug missfire, but cats can mask this to a degree.
Exhaust system sound (no minor leaks)??
Check inlet manifold vaccum with a guage , good engine 21/22 inches of mercury or if you have a smoke machine give it a go .
Could be a dirty hot wire on air mass meter , can make the engine run a little lean at idle.
on February 1, 2011, 10:36 pm, in reply to "