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Bucking/hesitating

XJeeper87

NAXJA Forum User
Location
FL
While accelerating my xj likes to buck ALOT it only seems to happen on first acceleration when the engine is cold when the engine is hot or u have already accelerated once it works perfect. Before my TPS went out it did this. After my TPS went out it didnt do this. After I replaced the TPS it starts doing it again. So any ideas on whats wrong?. It has Newish plugs/cap/rotor/wires

BTW its an 1987 4.0L XJ Renix
 
After you replaced the TPS, and adjusted it, what numbers did you get?

Auto or manual?
 
Manual 4.88V on A and 0.81V on C

Those numbers are pretty good. If you had 5.0 and .85 that would .17 percent, and yours is .1659 so the difference is negligible.

I would inspect/test the EGR, could be having a problem that shakes itself loose once you get moving (note, skip the transducer info, not for your year):

Warm the engine to normal operating temperature.


Operate engine at idle speed.


Check vacuum at solenoid vacuum source.


Disconnect the hose and attach a vacuum gauge to it.


Vacuum should be at 15 inches.


If vacuum is low, check the line for kinks, twists, or a loose connection at vacuum connector on intake manifold.


If vacuum is ok, remove gauge, reconnect the line and proceed to next step.


Check vacuum at solenoid output port.


Disconnect the line and attach a vacuum gauge to output port. Vacuum reading should be 0 at this side of solenoid.


If vacuum reading is 0, leave the gauge connected and proceed to the next step. However, if vacuum is present check solenoid/ECU operation with the DRB-ii tester.


Disconnect electrical connector at solenoid and note vacuum at output port. Vacuum should now be present at output port.


If vacuum is present, proceed to EGR valve test. However, if vacuum is not present, replace the solenoid. Test the EGR valve as follows:


Leave solenoid electrical connector disconnected. Note the engines idle speed.


The engine should idle roughly or stall. If this occurs the valve is ok. If the idle does not change, proceed to next step.


Disconnect hose from the EGR valve and connect a hand vacuum pump to EGR nipple.


Apply a minimum of 12" of vacuum to the valve and note engine idle. If engine now idles rough inspect the vacuum line between the EGR valve and the solenoid.


If the idle did not change, remove the EGR valve and inspect the valve and the exhaust passage in the manifold for blockage, repair as necessary. If no blockage is present replace the EGR valve.


Vacuum Transducer Testing as follows:


Disconnect the transducer vacuum lines and back pressure line. Remove transducer.


Plug transducer output port. Apply 1-2 PSI air pressure to transducer back pressure port. Air pressure can be supplied with a hand operated air pump or compressed air (regulated to correct pressure). Apply a minimum of 12” of vacuum to the input port.


Replace transducer if it will not hold vacuum.
 
I had the same exact problem you describe, I did have a bad TPS at the time but it NEVER gave me any issues for some reason. Kind of odd because I find bad TPS all the time at work. The BAD TPS only went to 2.5 volts at WOT. With that being said, I put a new engine in my Jeep, all new ignition components, all new engine sensors and what not. I fired up the the motor and it revved fine, only under load it missed. So, common sense would tell me its something I did probably not a component. Come to find out, I was at work and found a TSB from Jeep in ALLDATA. Its for RENIX engines and it worked 105% for me. Basically it has to do with aligning the distributor accurately. First off we know that you CAN NOT change the timing BUT you can have the distributor misaligned and that will effect the sync or timing to what REAL TDC is. Ignor the distributor procudure in the book, the TSB has you do something different.
Step 1 TDC the engine
Step 2 Remove the cap and drill a hole or "window" right after the number one post on the cap, I did mine on the vertical part of the cap towards the top right in line where the rotor passes.
Step 3 Remove the distributor and cut off the "ears" where the hold down bolt passes through
Step 4 Install the cap back on the distributor and install the distributor
Step 5 With the engine at its exact TDC with the crankshaft dampener at the "0" mark for reference, loosen the distibutor and position the "window" you cut .020 of an inch behind the direction of the rotor. Another way of understanding this is the rotors trailing edge should be .020 PAST the inside where the air gap is at the number one post.

I know it may be hard to understand and its even harder for me to explain but if you basically understand the concept of step 5 and to remove the hod down ear and do it TDC'd thats all that matters. Its an odd procedure and it sounds like a joke. I know you'd think there has to be some other way but its true. It worked 105% for my truck and its a published procudure from Jeep for the dealerships. Makes sense after the fact too, the one thing I thouched and did by the book, which wasn't good enough, was that. It doen't take much to make it right. Try that first. I will see if I can find it on google and maybe someone put up the TSB, sorry I can't post the link because I can't get in trouble for posting something off ALLDATA with out permission.
 
Alright, so to make things crystal clear. It only does it ONE time on a cold start? Then before the engine even reaches operating temp it will never do it again till its sat a while? OR will it do it on a cold start and keep doing it UNTIL it warms up? ALSO one more thing, when you say cold start does cold really mean cold OR can it be warm BUT you haven't driven it in about an hour? It could be a time thing is what I am getting to...have you checked fuel pressure? Try hooking up a gauge then key the ignition on to "prime" the fuel pump, don't start it obviously, then just key it off and come back in 30 or 45 minutes. Just check and see what the fuel pressure was when you keyed it on and see what it is when you come back. It shouldn't drop much at all if any. Be sure to do it when the engine is cold that way the fuel pressure wont drop with temp. Let me know if that helps, the more detail you give us on the forum the more detail we can lead you in the right direction.
 
Ok When i go to work i go down about 4 -5 miles on highway wen i first get on highway and accelerate it bucks and hesitates but when i turn on dif highway and accelerate it doesnt do it and it doesnt seem to do it again unless it sits long enough for the engine to become cold again
 
What injectors are you running?
 
You may be suffering from the dreaded "dribble injector" syndrome.

This is when your injectors "dribble" fuel into the intake after shutdown.

Heck, I don't know. However, if I was you, which I'm not, but if I was, I would get those fire starters out of there and upgrade to some Ford (Bosch) 19 lb orange-top injectors. Couldn't hurt.
 
Yeah, check the fuel pressure and see if it leaks down. A leaky injector should show by doing that or something leaky. That 4-5 miles of road could be evil, maybe try taking a different route???
 
haha, it should be the highway from hell because the problem goes away. but in all honesty just check the basics first, make sure the battery and alternator are up to par and have you checked fuel pressure yet to see if it leaks down? you can also ohm out the injectors when they are cold vs hot and see if you come up with anything. a lot of time it wont show potential problems but atleast you'll know if one is open or something obviously wrong. what I have found is just rule out the basics.
 
I havent had the ability to test drive it yet cus my insurance got dropped :( anyways I replaced the negative battery cable and my charging went up from 11-12 to around 14-15
 
you know thats a start, computers don't like to operate at low voltages. Typically 9-10 volts is the bad zone and tends to reset things (PCM memory) 11-12 volts is border line probably ok but still low. 13-14 sounds great to me! honestly its probably not the exact problem BUT its a good start, you gotta get everything up to par. Check your charging system for AC volts too. Leaky diodes cause weird problems with computers, sensors and gauges.
 
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