AC Causing a miss
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
Check wiring - wriggle compressor harness around - make sure it isn't close to the plug wires.
Check voltage and make sure the clutch coil isn't sucking the life out of it.
Check injectors. ECM fattens it up to compensate for compressor load. A partially plugged injector could cause a miss.
Remove the belt and crank the compressor shaft (outer hub) by hand. It should turn over. If you have to put a wrench on the bolt, compressor is shot and the ECM can't compensate (completely) for the load.
Check operating pressure with a Manifold Gage Set. Excessive high side - partially plugged orifice - can cause excessive loading.
Check clutch diode.
Try to narrow it down to a single cylinder. Get some 1/4 inch vacuum hose and cut it into 1 inch pieces. Remove plug wires at Distributor and insert the vacuum hose into the cap; re-attach plug wires on top of the vacuum hose piece. Start the engine and observe rpms on Scanner or hand held tach. Touch each piece of hose with a grounded test light to kill the cylinder and note the rpm drop. All should drop the same. If one doesn't, plugged up/weak injector, dead plug, bad wire, head gasket weep/leak, bent valve, at that cylinder. Check compression.
Hook up a scanner and monitor a/c request. Should always show "Yes" with compressor engaged; pulse width should fatten up, MAP should should increase with load.
Check voltage and make sure the clutch coil isn't sucking the life out of it.
Check injectors. ECM fattens it up to compensate for compressor load. A partially plugged injector could cause a miss.
Remove the belt and crank the compressor shaft (outer hub) by hand. It should turn over. If you have to put a wrench on the bolt, compressor is shot and the ECM can't compensate (completely) for the load.
Check operating pressure with a Manifold Gage Set. Excessive high side - partially plugged orifice - can cause excessive loading.
Check clutch diode.
Try to narrow it down to a single cylinder. Get some 1/4 inch vacuum hose and cut it into 1 inch pieces. Remove plug wires at Distributor and insert the vacuum hose into the cap; re-attach plug wires on top of the vacuum hose piece. Start the engine and observe rpms on Scanner or hand held tach. Touch each piece of hose with a grounded test light to kill the cylinder and note the rpm drop. All should drop the same. If one doesn't, plugged up/weak injector, dead plug, bad wire, head gasket weep/leak, bent valve, at that cylinder. Check compression.
Hook up a scanner and monitor a/c request. Should always show "Yes" with compressor engaged; pulse width should fatten up, MAP should should increase with load.
