Hey Jeepers - I just resolved an engine hesitation issue upon accelerating - and thought sharing could help a fellow Jeeper...
I have a manual 91 YJ and last year towards the end of summer, the engine would hesitate for a split moment -- enough where you can see the RPM's decrease on your TAC. The hesitation was only upon acceleration, especially when upshifting or on an incline... and never while downshifting. This hesitation could occur one to a few times a day. But the hesitation can cause a signification jerking action if you are in a low gear accelerating to 2,500 - 3,000 RPM. Over a few months, the issue got worse. We were on HWY 50 heading to Lake Tahoe where you hit a few good elevation climbs. The climb seemed to make the issue worse and the hesitation could hit every second for maybe 5 or 6 in a row. I thought it could stall. It almost felt like the engine was loosing compression during those sputters. However when I am on a fairly level road, the hesitation feels like a fuel issue -- fuel pump or dirty plugs. The engine just quits for a moment like the a miss in distributor system. I recently ( a year back) put in new spark plugs and wires, distributor cap and rotor and the engine purrs from cold start to all accelerating points so I didn't think the issue was in the distributor ignition system. So first I inspected the emission vacuum hoses, and then checked fuel - pressure and capacity - all good. As the temps cooled down for fall and then winter, the issue completely went away. I am an electrical engineer, and this provides a strong clue that the issue is an electrical issue. As you know resistance goes up with temperature. I continued to drive the 91 YJ through the winter and into spring. No sense trying to fix an issue that wasn't occurring during the cold. March came quickly and so did the temperatures. I'm in Cali so we actually had some 90 degree days in March. And like clockwork, the engine issue re-appeared which validated the issue is tied to heat and likely an electrical issue. As April hit, the issue got bad where Jeep was almost not drivable. I could drive 10 minutes, stop at a Home Depot, come out and try to drive and get hit with the hesitation to a point where engine felt like it could stall. Since issue is tied to heat, issue most likely tied to some component in engine bay. My top suspects were Crankshaft Position Sensor (CPS) or O2 sensor. The CPS plug is right off the exhaust manifold and sensor bolts to transmission housing near flywheel. This part is extremely sensitive since it is essentially a magnet and coil looking for the notches in the flywheel and no one seems to make a good part anymore. The O2 sensor definitely sees heat as you know. Then we have the Throttle Position Sensor, the MAP and MAT sensors all in the engine bay.
I decided to start with the CPS since engine hesitation was driven more by acceleration. Acceleration... i.e. faster RPM's is a big problem for a failing CPS. A weak magnet and its magnetic field wouldn't detect the flywheel notches as RPM's increased. The way the CPS works is that is generates an electric-magnetic field. The field will detect moving metal - like the notches or teeth on a flywheel . It sends a pulse to the ECU which manages everything. If the CPS sensor goes off, your ECU computer has lost its ability to time your engine and that condition would drive an engine hesitation. And the faster the RPM's the more likely a MISS.
I luckily always have a spare CPS (and every fellow Jeeper should as well) so installed the new one. The new one is OMIX-ADA, p/n 4638128. I noticed that it was lighter in weight as the one I replaced. Parts getting cheaper and cheaper. I will be trying the NTK version next.
Test drove the 91 the next day in the heat of the day. Went on three errands with no issues. I beat on the engine with some hard accelerations. Also drove it again today and no issues. It is a great feeling to finally get this issue resolved and get it on the first component replacement. These engine hesitation issues can result in months of frustration and a lot of costs. So hopefully documenting this case helps someone. Happy Jeeping!
I have a manual 91 YJ and last year towards the end of summer, the engine would hesitate for a split moment -- enough where you can see the RPM's decrease on your TAC. The hesitation was only upon acceleration, especially when upshifting or on an incline... and never while downshifting. This hesitation could occur one to a few times a day. But the hesitation can cause a signification jerking action if you are in a low gear accelerating to 2,500 - 3,000 RPM. Over a few months, the issue got worse. We were on HWY 50 heading to Lake Tahoe where you hit a few good elevation climbs. The climb seemed to make the issue worse and the hesitation could hit every second for maybe 5 or 6 in a row. I thought it could stall. It almost felt like the engine was loosing compression during those sputters. However when I am on a fairly level road, the hesitation feels like a fuel issue -- fuel pump or dirty plugs. The engine just quits for a moment like the a miss in distributor system. I recently ( a year back) put in new spark plugs and wires, distributor cap and rotor and the engine purrs from cold start to all accelerating points so I didn't think the issue was in the distributor ignition system. So first I inspected the emission vacuum hoses, and then checked fuel - pressure and capacity - all good. As the temps cooled down for fall and then winter, the issue completely went away. I am an electrical engineer, and this provides a strong clue that the issue is an electrical issue. As you know resistance goes up with temperature. I continued to drive the 91 YJ through the winter and into spring. No sense trying to fix an issue that wasn't occurring during the cold. March came quickly and so did the temperatures. I'm in Cali so we actually had some 90 degree days in March. And like clockwork, the engine issue re-appeared which validated the issue is tied to heat and likely an electrical issue. As April hit, the issue got bad where Jeep was almost not drivable. I could drive 10 minutes, stop at a Home Depot, come out and try to drive and get hit with the hesitation to a point where engine felt like it could stall. Since issue is tied to heat, issue most likely tied to some component in engine bay. My top suspects were Crankshaft Position Sensor (CPS) or O2 sensor. The CPS plug is right off the exhaust manifold and sensor bolts to transmission housing near flywheel. This part is extremely sensitive since it is essentially a magnet and coil looking for the notches in the flywheel and no one seems to make a good part anymore. The O2 sensor definitely sees heat as you know. Then we have the Throttle Position Sensor, the MAP and MAT sensors all in the engine bay.
I decided to start with the CPS since engine hesitation was driven more by acceleration. Acceleration... i.e. faster RPM's is a big problem for a failing CPS. A weak magnet and its magnetic field wouldn't detect the flywheel notches as RPM's increased. The way the CPS works is that is generates an electric-magnetic field. The field will detect moving metal - like the notches or teeth on a flywheel . It sends a pulse to the ECU which manages everything. If the CPS sensor goes off, your ECU computer has lost its ability to time your engine and that condition would drive an engine hesitation. And the faster the RPM's the more likely a MISS.
I luckily always have a spare CPS (and every fellow Jeeper should as well) so installed the new one. The new one is OMIX-ADA, p/n 4638128. I noticed that it was lighter in weight as the one I replaced. Parts getting cheaper and cheaper. I will be trying the NTK version next.
Test drove the 91 the next day in the heat of the day. Went on three errands with no issues. I beat on the engine with some hard accelerations. Also drove it again today and no issues. It is a great feeling to finally get this issue resolved and get it on the first component replacement. These engine hesitation issues can result in months of frustration and a lot of costs. So hopefully documenting this case helps someone. Happy Jeeping!
