ok, more running probs, bucking and jumping...sometimes.....

Ok, 87 XJ, rebuilt 89 4.0, 4x4, 4door, automatic.

This engine has about 12k on it, the rest of the XJ has 263k, engine bored 60 over, high performance type kit put in.

New parts and work done in the last year or two because i didnt have the money at the time of the rebuild or they just went out:

-TPS and IAC (6 months ago)
-Bosch 19lb. fuel injectors (3 or 4 months ago)
-OEM fuel pump, strainer, flushed tank out, and fuel filter (last week, cause it was losing pressure and pretty much only had enough juice to start motor)
-New power and ground wires off battery to block, chassis, and starter.
-oil change every 3k to 3500 miles
-new air filter about every 6k miles or when dirty.....

Suspects this time are EGR, partially plugged catylitic converter, or something else i have no clue on...last week the CPS tested fine by a reputable mechanic?? Please help!!

Here is the recent problem:

After about an hour of driving the highway or sitting at idle for about 20 minutes (found this out while trying to get air out of cooling system after replacing overflow bottle and thermostat this weekend), it decides to buck and jump, then die.

Try to restart right away, sounds like its not firing all cylinders, very rough, and a puff of blue/white smoke came out of the air box?? Let it cool down, it starts right up and goes with a little hesitation then eventually back to normal.

Also for the last couple weeks the exhaust has sounded like a tired 4 cylinder instead of the throaty inline 6. I just thought it was the weather, but i dont know, it just doesnt sound right....

1. What could this be?
2. If the cat converter is plugged, would putting new exhaust on without the cat hurt anything??? they are too expensive!! (I can get this done, so please dont lecture me about the EPA and exhaust shops......he he, I just want to konw what if any problems i may run into if the cat is left out of the new exhaust)

Thanks,

Cory

Bucking? Are you getting any mild backfire out the throttle body? If so it is most likely timing/spark related. For the most part there are five common catagories that can cause any engine problem. Too much fuel, too little fuel, no spark, timing on the spark, or a restricted exhaust. Running crappy and not being able to rev up fast is a normal symptom of a blocked exhaust.

The most common thing I have seen that is temperature sensitive and will cause the bucking issue is the coil. Bad sensors can cause the computer to jack with things, but the computer is located in the passenger compartment and isn't exposed to the range of temps under the hood. Very few sensors can actually cause the engine to die. Everybody thinks about the CPS or the camshaft sensor, but the MAP sensor can cause it to die or even be a no start.

The best thing to do is have a buddy with a similar year stop over and play sensor swap, or better yet, hit the pull-n-save and grab yourself a set of sensors for trail spares and for troubleshooting.

It did this after 20 minutes idling trying to get the air out of the cooling system, and 70 miles of driving down the highway.

Other than the funny sounding exhaust, the inconsistant bucking and jumping is really bugging me.

If backfiring out the throttle body sounds like a poof and blue/white smoke is what you are asking, then yes it backfired, but ONLY after the bucking, jumping, and dying rodeo session when trying to restart. When the motor cools off it starts ok.

Cory

My last XJ began bucking like that but I had a mechanic fix it (hadn't gotten into the do it yourself mode yet...), so I don't recall what the problem was, but it was exactly the problem you're having, minus the shutting down bit. However, an acquaintance of mine who has a similar year Cherokee (don't remember the exact year) said he was having the same issue. I couldn't recall what happened to mine, so he too took his to a mechanic, and he said it turned out to be the oxygen sensor. Again, however, neither his nor mine were emitting smoke or dying, so that's about all I can say.

I would take a good look at the CPS and the wire. They tend to get a little too cozy with the exhaust manifold and melt. The wire then grounds out against the manifold. The last time mine went, I would get bucking and backfires in the induction system. I would say go ahead and get a new one and swap it out. If it doesn't fix it, at least you have a spare CPS. They will fail, and you could be up the creek without a spare.

I always check the basics first, before I start in with the sensors and stuff. Though a good inspection of the wiring, does usually show up a chaff or rub someplace. The firewall end of the fuel rail and the front of the fuel rail, where the temp., knock and 02 sensor wires go underneath, are often a trouble spot.
Take it on a moderate run up the highway (around 2000 RPM) and pull some plugs, take a look at the color. Light grey is a good thing. Very white is a sign of lean.
Start it up, when it´s dark (really) dark and look for plug and coil wire high voltage leaks.
Look in the distributor cap, inside and out for corrosion and/or carbon traills from shorts.
Pull a plug, open it up to about 0.060 hold it to a good ground and look at your spark. Should be nice and crisp, with very little splatter. Splatter is a sign of weak spark.
Do a by the book pressure test on the fuel rail.
Vacuum test, 14 inches is good, 9 is marginal. Needle stays fairly constant at idle.
Swap your 02 heater relay with another and see if the problem changes.
If when pulling the plugs, you get some with different colors, this can be a tip off of a vacuum leak.
Check and see where your rotor is pointing at TDC (compression) number one cylinder, it should be, mostly after the number one electrode in the distributor.
Most of my bucking problems, have come from a lean fuel condition, sensor related (02) mostly at moderate (2000 or so) RPM´s or vacuum problems.
EGR mostly messed with idle (stalling) hardly noticiable at highway speeds.

Ok, good advice.

Now i have been looking back at things and this one seems to be a temp related issue........Idling for 20 minutes....70 miles down the road(it happened after about a 15 minute jaunt through a town along the highway at the 70 mile mark)......

What do you think if that narrows it down some?

Cory

I had a bucking problem, but it seemed to be a near the same vacuum conditions (same throttle position). And usually at around 2000 RPM, most of it turned out to be the 02 sensor. Though I also had some vacuum issues. May not be just one thing, but a combination of things that add up.
There were a couple of TSB´s, one was a 1/3 throttle problem that was TPS related, another distributor indexing and yet another the wiring in the relay block (fuel pump relay).
A cam psosition sensor test might turn up something, but I haven´t had one go bad yet.
The vacuum line for my MAP came loose, caused the motor to run really poorly at idle and killed my top end.
Might be an easy fix, may take a systematic check of most everything.
I check my compression and vacuum every spring as part of the get ready for summer service. And record the results in the back of my manaul. A vacuum test may help narrow it down, the needle should be fairly steady. A vacuum needle that wags radically, is a sign of valves, if it jumps eratically with little rythm it´s often ignition. If it´s below around 10 or 11 inches, the sensors rarley give good values.
On a cold engine, the engine temp. sensor (lower left of block) and the manifold temp. sensor will have close to the same resistance.
It took me mohths to completely cure my motor issues, there was a bunch of stuff wrong.

8mud,

I have been toying for months with various things, most of the things i have done in the past were becuase of lack of funds when i did the rebuild/engine fiasco or have worn out, leaked, tested bad type of thing....I feel ya man.

Taking it in to the shop tomorrow for an inspection starting with the basics and working up. Probably let them keep it a couple days because every time something happens i cant get within 3 blocks of the shop and EVERYTHING is fine........errrggh.

I will let you know what they find.

Cory