I've been getting occasional P1494 codes and CEL indicators on my 1999 Cherokee Sport 4.0 (125K miles) and am trying to track down the source of the problem. So far, we've replaced the hoses back at the canister to address the possibility of leaks. We haven't replaced the canister yet or the pump.

However, a new symptom has started and I'm trying to determine if it's related. On a very intermittent basis, the Jeep will surge, then drop off in power repeatedly (usually after a restart that followed a brief drive). The CEL does not come on when that happens but if I shut the engine off, wait a few beats, and then start up again the Jeep runs fine until the next, unpredictable occurrence.

The last time it did it, the surging was causing so much slamming at the rear end as the engine surged that I haven't driven the thing since and have been considering getting rid of it.

Is this likely related to the P1494 error and, if so, what component would most likely cause the surging. The vacuum pump? All suggestions and comments are welcome! I'd like to figure this out and get this Jeep back to a level of reliability.

 

P1494-LEAK DETECTION PUMP SW OR MECHANICAL FAULT


When Monitored: Immediately after a cold start, with battery/ambient temperature between 40 deg. F and 90 deg. F and coolant temperature within 10 deg. F of battery/ambient.


Set Condition: The state of the switch does not change when the solenoid is energized.


POSSIBLE CAUSES


WIRING HARNESS INTERMITTENT
LDP SWITCH SENSE CIRCUIT SHORTED TO GROUND
LDP VACUUM SUPPLY
LEAK DETECTION PUMP
LDP SWITCH SENSE CIRCUIT OPEN
POWERTRAIN CONTROL MODULE


Besides this mumbo jumbo, check your fuel pressure at the fuel rail. Should be 49.2 psi +/- 5 psi.


The PCM does not monitor fuel pressure so it won't set a DTC if off, unless the fuel pressure affects emissions.

TEST ACTION APPLICABILITY

 

Well, it finally acted up again this morning when it was at our store and we could pull it into the garage to see what was going on. It turned out (apparently) to be the vacuum line on the driver side of the engine that's in-line with the spark plugs. The line was off, but close enough to the connection that it must have been sometimes getting the right conditions to run normally.

Hopefully, this will prove to be the solution, but it stopped surging once we reseated that line.

Thanks to CCKen for the helpful reply. I may still need to go down that list if this wasn't the main problem.