P1391 After New Cam, Had to Cut a Distributor Ear
08-26-2013, 06:04 PM
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Year: 1998
Model: Cherokee
Engine: 4.0L
P1391 After New Cam, Had to Cut a Distributor Ear
I had a P1391 code for months after installing a new Melling Camshaft from Autozone (P1391 = Intermittent CPS/CKP Sensor). I had to cut an ear off the base of the distributor to align the Camshaft Position Sensor signaling with what the ECU expected it to be.
This kind of repair wasn’t posted anywhere, so this might help someone. I warn you, however, that my repair was probably unique to my engine and the brand of camshaft I used. (1998 4.0L XJ, Melling camshaft). Before you take this step, read all the other great posts about sensors, distributors, and engine timing.
MAIN PROBLEM – The distributor is supposed to be installed with the engine at TDC on Cylinder #1, and with an alignment pin in the body of the distributor. The distributor twists in and the rotor sits at about 5 o’clock, near the #1 cylinder post. As the distributor seats, the ears on the housing should also align with the bolt hole in the block. Mine did not. I had to remove the alignment pin and turn the distributor body about ˝ cm clockwise to align the ears. At that location, the rotor was still just past 5 o’clock and the engine ran fine, but I had a constant P1391 code. Every time I erase it, it would come back. The body of the distributor affects the location of the Camshaft Position Sensor, which sits in there. The center shaft of the distributor affects the rotor position and also the location of a metal ring that swings through the CPS on each engine rotation.
WHAT GOT ME HERE: I did a partial engine rebuild after a head gasket failure at 230K miles. The rebuild went great. My pistons and block are in great shape. I had a machine shop rebuild and resurface the head. They did new valve guides and springs, etc. I installed new rings, piston rod bearings, timing kit, camshaft, and lifters, among all the gaskets, hoses, plugs, seals and such. The only oddity was the camshaft. The Melling shaft from Autozone was supposed to be the same as OEM, but I think its distributor gears are slightly different. As I installed the distributor, I noticed the ears wouldn’t align with the bolt-down hole. After the rebuild, however, the engine ran great and I did the standard camshaft break-in procedure. I was so happy. Then on about the third test drive, I got the P1391 code. Crap!
ACTIONS: I replaced the camshaft position sensor. No luck. I replaced the crankshaft position sensor, no luck. I tested the wires, all good. I double checked all my connectors and grounds, all good. I took it in for a dealer scan and they blamed the timing chain being loose. I knew that was bogus because I had just replaced the chain and sprockets. Nevertheless, I double checked my timing and the chain. They were excellent.
But I was still nagged by the small misalignment of the distributor. Also, the dealer’s feedback worried me, saying the CPS pulses were “off just a little.” I almost bought an oscilloscope. I tried watching the pulses with a digital voltmeter. I had pulses, but couldn’t correlate the CPS and CKP timing. Apparently, the CKP controls spark, and CPS controls fuel injection. If CPS was triggering a little early, I would have fuel sitting on the intake valve, waiting to be sucked in, but it wouldn’t affect the engine’s runability. That’s what I had, I think. Okay, time to break out the hacksaw for a test. I cut off one ear at the base of the distributor and turned the body counterclockwise about ˝ cm. This aligned the little holes where the plastic installation pin is supposed to be (While the engine is at #1 TDC). Voila!!! No more CP1391. The engine runs the same. All I did was adjust the alignment of the CPS pulse, putting it back where the ECU expects it to be. Wow… aftermarket parts…
If the distributor alignment has changed, didn’t this also affect my rotor position and cylinder firing? If the camshaft worm gears are different enough to force me to cut an ear, why does my engine run okay? I think it’s because the difference is very slight, ˝ cm. The rotor nose is about 1.5 cm wide, so it has plenty of contact with the posts. I haven’t put a timing light on it. But I vaguely remember the centerline of my rotor was barely past the #1 spark plug post with the old camshaft. Now the tail end of the rotor is just ending its contact with the #1 post. It can still fire when it should, so that part is okay. But the slight difference also affected how the metal plate within the distributor spins through the Camshaft Position Sensor, throwing its signal off just enough to trigger the P1391. It took me three months to figure that out.
This kind of repair wasn’t posted anywhere, so this might help someone. I warn you, however, that my repair was probably unique to my engine and the brand of camshaft I used. (1998 4.0L XJ, Melling camshaft). Before you take this step, read all the other great posts about sensors, distributors, and engine timing.
MAIN PROBLEM – The distributor is supposed to be installed with the engine at TDC on Cylinder #1, and with an alignment pin in the body of the distributor. The distributor twists in and the rotor sits at about 5 o’clock, near the #1 cylinder post. As the distributor seats, the ears on the housing should also align with the bolt hole in the block. Mine did not. I had to remove the alignment pin and turn the distributor body about ˝ cm clockwise to align the ears. At that location, the rotor was still just past 5 o’clock and the engine ran fine, but I had a constant P1391 code. Every time I erase it, it would come back. The body of the distributor affects the location of the Camshaft Position Sensor, which sits in there. The center shaft of the distributor affects the rotor position and also the location of a metal ring that swings through the CPS on each engine rotation.
WHAT GOT ME HERE: I did a partial engine rebuild after a head gasket failure at 230K miles. The rebuild went great. My pistons and block are in great shape. I had a machine shop rebuild and resurface the head. They did new valve guides and springs, etc. I installed new rings, piston rod bearings, timing kit, camshaft, and lifters, among all the gaskets, hoses, plugs, seals and such. The only oddity was the camshaft. The Melling shaft from Autozone was supposed to be the same as OEM, but I think its distributor gears are slightly different. As I installed the distributor, I noticed the ears wouldn’t align with the bolt-down hole. After the rebuild, however, the engine ran great and I did the standard camshaft break-in procedure. I was so happy. Then on about the third test drive, I got the P1391 code. Crap!
ACTIONS: I replaced the camshaft position sensor. No luck. I replaced the crankshaft position sensor, no luck. I tested the wires, all good. I double checked all my connectors and grounds, all good. I took it in for a dealer scan and they blamed the timing chain being loose. I knew that was bogus because I had just replaced the chain and sprockets. Nevertheless, I double checked my timing and the chain. They were excellent.
But I was still nagged by the small misalignment of the distributor. Also, the dealer’s feedback worried me, saying the CPS pulses were “off just a little.” I almost bought an oscilloscope. I tried watching the pulses with a digital voltmeter. I had pulses, but couldn’t correlate the CPS and CKP timing. Apparently, the CKP controls spark, and CPS controls fuel injection. If CPS was triggering a little early, I would have fuel sitting on the intake valve, waiting to be sucked in, but it wouldn’t affect the engine’s runability. That’s what I had, I think. Okay, time to break out the hacksaw for a test. I cut off one ear at the base of the distributor and turned the body counterclockwise about ˝ cm. This aligned the little holes where the plastic installation pin is supposed to be (While the engine is at #1 TDC). Voila!!! No more CP1391. The engine runs the same. All I did was adjust the alignment of the CPS pulse, putting it back where the ECU expects it to be. Wow… aftermarket parts…
If the distributor alignment has changed, didn’t this also affect my rotor position and cylinder firing? If the camshaft worm gears are different enough to force me to cut an ear, why does my engine run okay? I think it’s because the difference is very slight, ˝ cm. The rotor nose is about 1.5 cm wide, so it has plenty of contact with the posts. I haven’t put a timing light on it. But I vaguely remember the centerline of my rotor was barely past the #1 spark plug post with the old camshaft. Now the tail end of the rotor is just ending its contact with the #1 post. It can still fire when it should, so that part is okay. But the slight difference also affected how the metal plate within the distributor spins through the Camshaft Position Sensor, throwing its signal off just enough to trigger the P1391. It took me three months to figure that out.
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