1988 XJ 4.0 flutters/misses under load @low rpm...2 trips to

[Mudshark]

This problem began on the first test drive I took after installing a new longblock in my 1988 Cherokee Chief. At between 1200-1800 rpms under a load (gradually accelerating from a stop or driving uphill), the engine would continuously miss. The tach would drop sharply then immediately recover, like there was a momentary interruption in spark. If I gave it lots of gas and accelerated quickly, it ran great. The old engine had 182k on it and had lost all oil pressure. It sat in a barn for almost two years until late this May. I replaced the distributor cap, rotor, pick-up, plugs, wires, throttle positioning sensor, O2 sensor, coil, module, fuel pump and filter, and the computer until I finally succumbed to defeat and took it to a dealer. They told me the computer (that I had bought from a parts store) and EGR valve were bad. I had them replace the parts, and other than a constant overheating battle we still wage, she ran like a champ....for about 5000 miles. (I have recently installed a 3 row GDI radiator, but on the steep roads I often drive it still runs hot, and if a hi-flow thermostat housing doesn't work, I will convert to the open cooling system).



A couple of weeks ago the same problem began occuring; an occasional miss at low rpms. It only happened once or twice each time it occured, then it would straighten out. Last week, it suddenly started dying while idling at traffic lights, and if it was warm, it was nearly impossible to start. I took it to a different dealer (I had moved to Utah), and they told me the vac line going to the EGR valve was split. Also, the top of the plastic reservoir had a crack that spewed coolant onto the coil and distributor when it was hot. They fixed these problems as well as replacing my fuel pump and sending unit (long story short- my tank leaked because I screwed up my sending unit installing an aftermarket pump). The dealer told me my overheating problems should be fixed because my cooling system was now under pressure. I picked it up from the dealer on Friday. The jeep ran perfect for 500 miles, but while driving on a washboard road in the Uinta Mountains this weekend, it started missing again at low rpms; I had to keep it above 2000 rpms to keep it running regularly. When I got off the washboard and onto pavement, it still missed for about a minute before it straightened out. While driving on a long, steep section of highway (5-7% grade), it seriously started missing in the 1600-2200 rpm range. I had to slow to 55 mph and take it out of overdrive keeping it above 2200 rpm. On the way down that pass and then going over the next pass (which was less steep) it only missed a few times, and otherwise ran great. I have never replaced the CPS. I am beginning to think I might have a short somewhere. If this is the case, where and how should I look? Both Jeep dealers did a diagnostic check. It still ran hot this weekend (210-220 on the highway). I am going out of my mind! [addsig]

	Crank position sensor A crank sensor is a component used in an internal combustion engine to monitor the position or rotational speed of the crankshaft. This information is used by engine management systems to control ignition system timing and other engine parameters. Before electronic crank sensors were available, the distributor would have to be manually adjusted to a timing mark on the engine.

[MudderChuck]

Sounds like almost exactly the same problem I had with my Chev K-1500. I finally nailed it down to a very slight coolant leak through the head gasket to a couple of cylinders. The hot gas getting into the cooling system partially melted a couple of plastic peices (heater core and radiator). When the coolant pressure got up a little, coolant would get into the cylinders and cause a missfire at moderate RPM´s, It would occasionally idle a little rough. And would sometimes want to guit when seriously decelerating. Took me forever to figure out what was going on, didin´t leak enough to affect compression readings. But did foam up the coolant some, and I would occasionaly notice a few bubbles in the radiator at idle.

I finally hollowed out an old spark plug, silver soldered a metal valve stem to the end (after removing the core) put a breaker bar on the harmonic balancer bolt. And put about thirty pounds of air to each cylunder at TDC for that cylinder (when both valve rockers are loose or as determined by a compression gauge). I was looking for a leaky valve, had the radiator cap off and some coolant blew out of the top pf the radiator. Figured I´d found the problem. [img]modules/phpBB_14/images/smiles/icon_smile.gif[/img]

Worth looking at. Even if it doesn´t find the proglem, you can check your valves for leakage, with the throttle and choke proped open, you can hear the valves leaking with about 10 to 15 pounds of pressure. Don´t confuse the sound, with the air passing the rings a little and going into the oil gallery (this is normal).

Be careful to much air will turn over the motor and the breaker bar is hard to hold or if it turns in reverse can back out your harmonic balancer bolt.[addsig]