QuestionLet me start by saying this vehicle came to my shop from an engine rebuilder 1000 miles away. It was overheated and this place rebuilt it for whatever reason. It is not over heateing for us but it is FAR from running right. The engine does not start unless you have the idle set way high or you depress the accl pedel. If the rpm's drop below 1100 the engine dies. If you put it in gear it dies (unless you have it reved up above 3000rpm). It has pretty bad random cylender misfires at idle and off idle. We have tried a new MAF sensor which didnt change anything so I swapped back to original which I already have the cap off of to adjust mixture. I put new ngk plugs and bosch wires on. It seems to be running rich but it might just be all the miss fires. No matter what I do to the mixture it always has these misfires. Any help you can offer would be greatly appriciated.
Also the car sat at this engine rebuilders place for more than a year or two before being shipped from ohio to south FL where we are.
AnswerHi Dave,
If that car was brought to me and with that info, this is what I would do.
First drain the gas and put fresh gas in both tanks. Then run a compression test (throttle open) A fresh rebuild should have from 150 PSI to 170 psi on all six cylinders with little difference between cylinders. With that much time since the rebuild I would change the oil and filter and with all plugs out I would spin the starter and watch the oil pressure. After the new oil had primed the oil galleys you should see 35+ PSI oil pressure with just the starter.
If I did not see good compression and oil pressure I would not proceed any further as it is a waste of time.
If they both were good then I would check the ignition system. If you don't have a scope you can just connect all the plugs to the wires and lay them on the head and spin the starter and watch all 6 to see that you are getting a thick spark (about as thick as a pencil lead) and it must be blue in color (not in the sun light). Then I would ohm each wire to see if the resistance was ok (about 4K ohms per foot of wire)(not necessary to test the ohms on the new Bosch wires) then closely examine the cap and rotor for any carbon tracking. Check the firing order (153624).
Then set the ignition timing (about 8 deg BTDC)
Check to see that the throttle plate is closed all the way but the load of the return spring is on the stop screw. (this is important) If you have to reset it use a .002" feeler gauge in between the butterfly and the wall of the throttle body to set the load and opening.
Cold start fuel is supplied by the cold start valve (injector) which is controlled by the thermo-time switch in the coolant manifold (large one toward the front) by grounding the injector. If several attempts are made to start, an electric heater in the time switch will break the circuit or when the engine coolant is warm that will break the circuit also.
The Aux air valve supplies extra air for cold start and cold fast idle and is turned off by an electrical heater inside the Aux air unit.
Fuel pressure is critical so it would be a good idea to check fuel pressure at the fuel rail. You want to see about 32 PSI with the engine running and 42 PSI with the vacuum hose removed from the regulator.
Since the car has been setting a long time and you don't know the history. You should also run a low pressure gauge into one of the exhaust manifold plugs just above the O-2 sensor. You can just find a hose that fits snug into the lug after removing a cap and start it up and rev it up a little. If you leave it too long the hose will melt but it only takes a quick start and rev up to tell if there is a problem. When rev up, if it is much over 1.5 PSI you have a problem. Either a stopped up and melted CAT or a stopped up muffler. GM claims their cars can stand up to 3 PSI but a Jag can not and often when a CAT gets burned the pressure will go over 5 to 7 PSI on a rev up.
I always had to run the tests in that order Compression and oil pressure first to confirm it is a good engine, then ignition second and fuel always last.
let me know.
Howard
