1987 Maxima- runs rough at low speeds, won't pass 50/15 emmissions test. High speed ok, black smoke from exhaust when starting after stall at low speeds.

just had problems with two of my cars replaced the oxygen sensor. EGR valve and lastly the Catalytic converter the both cars passed with flying colors

signs of a bad fuel pressure regulator-- the vehicle will run rough, the engine will not start ( usually in the mornings,) many times engine starts but stalls shortly after starting. a bad regulator can result in fouled plugs. replacement is only cure. 1- black soot on plugs-2-black smoke from tailpipe-3- running rough-4- stalling, good-day! repost if need be.

could be a slight air leak on the exhaust like a small rust hole and its drawing in the air ,the proper way is to put a probe up the tailpipe but without knowing what the results were its very difficult to diagnose ,was it too high on O2 as this signifies a hole in the exhaust ,was it too high on CO or CO2 ,You see our test is every year on vehicles over ten years of age and it also includes a mechanical test as well but the emmisions is done with a probe not the OBDII as very few europeon vehicles can be read via the OBDII as they nearly all use encrypted software now from 2005 .

there are many things that cause this, so it is best to have it scanned if light is on. but it may be a bad coolant tempurature sensor the symtoms are --- if it is bad it will run to rich, run rough at idle, and produce black smoke. engine may stall out. a maf sensor may cause this also. just get car scanned to find problem code. if it is white smoke check for oil in anti-freeze, if so it may be a blown head gasket. but if that is bad it will over heat. have a good day!!

Fix your car right away, by clicking the link I attached
JAGUAR XK8 1996 2006 WORKSHOP REPAIR SERVICE MANUAL

CHeck fuel pump. Check oxygen sensor

The codes you provided indicate separate issues. Code 1167 is indicative of either a stuck fuel injector of the O2 sensors. Code 0340 indicates the CMP position sensor. Replacing the O2 sensors will likely clear the 1167 code assuming the fuel injectors are okay. It would be the first course of action. If there's no noticeable change in performance (i.e. stalling problem continues) then the CMP position sensor is likely your overall problem. Best of luck.
Greg

Try this, it is the common reason for this problem.
Here is the common cause of surges, stalls at stops, slow idle speed, erratic idle speed, rough idle and engine hesitation (and other problems), it is in most cases the idle speed control air-bypass valve and or throttle valve and upper intake, these area's get full of gunk and combustion residue over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake

A scan for trouble codes may point you in the right direction quickly, rather than hit & miss, with replacing parts. Some of the autoparts stores will scan for free, and your right where you need to be to get the right part at the same time.

Replace the ECT sensor.
And letme know. If you feel any diference. ok.