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1987 bucking jeep


NAXJA Forum User
first off it is a 1987 4.0 5spd
it started the other nite when driving home from work it would buck or hickup once then run like it never happend. any thoughts would help

whats new

spark plugs & wires
cap roter
both done weeks before
cps done after with no change

thanks for any input
Test AND adjust your throttle position sensor. A faulty TPS can create bucking symptoms such as you describe. May as well clean your throttle body while you're in the area....

A hiccup from a coil could create this as well. I'd test the coil for primary and secondary resistances.
TSP is always a good bet, they seem to have a lot to do with the timing, but when they fail it tends to be off idle misses, dead spots or the other extreme a 2000 RPM idle. I had a hunting idle (idle that changed all the time) which turned out to be a TPS ground problem at a loose splice.
If it bucks around 2000 RPM, look at the O2 sensor. Seems to be the spot where marginal 02 operation shows up first. Usually under open throttle low vacuum situations (while accelerating).
In the 87 there was a recall for bucking, something about the cam grind and the initial distributor timing. I imagine years of wear on the distributor gears and the timing chain may cause the rotor to move out of the timing envelope, along with the initial cam grind problems. The cam problems seemed to be VIN specific, the fix was to re index the distributor.
Re check the cable on your plugs and distributor cap, make sure there are no gaps and they are seated all the way onto the plugs and into the distributor cap. Odd high voltage shorts to ground can cause it to buck. If the gap on the spark plug end is too large or the cable not seated properly, it can cause occasional shorts to ground or what I call a hard miss.
thanks for the info so far I just got done putting the 2nd new cps in and while I was at it a put a large ground strap from motor to frame ( starts faster)

I will check the tps at work

let you know after work

thanks again

fyi 137000 miles
I did the test on the tps and on my cheap meter it was ok ( not digital)
I think I will try to put a 2nd ground wire on it

it seemed to buck more on the way home tonite it is a 6 mile drive and it would happen 1 or 2 times but tonite it happened 4 or 5 times all between 1200 and 1700 I have yet noticed the problem above 2000 rpm but i do not cruise there
the problem seams to be getting worse
will the cam senser give this same problem? I looked at it and it looks ok
but dont have time to test right now
I did not say this before but it all started after i changed the motor mounts (thought it may have been doing it all the time both mounts were busted) and valve cover gasket (I checked real hard for junk befor putting the cover back on) :helpme:
the o2 sensor test bad I got a infinite reading from wire a to b would this cause the problem?

just reread and saw that 8mud said to test it so off to the part store anything to watch for when buying a o2 sensor
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Also make sure you didn't crack your vacuum line going to the map sensor, when you were working on the jeep. I did that and it made my jeep buck.
If the CPS is bad it won't do anything from what i've heard.
new o2 senser and checked the vac line to the map sensor (put a plug in one end and a mighty vac on the other, it held with no bleed off ) still same problem keep them coming guys im not used to working on a rig with this much stuff on it :wow:

is it posible for the tps to test fine but still be no good?
An '87? Check that your EGR valve isn't acting up. Unplug the Vacuum line to it from the EGR solenoid. If the hiccup goes away replace the solenoid or seal off the vacuum line and run with the EGR disabled (though not legal for smog purposes...)

That was the "hiccup" on my '88 a few years ago. The replacement solenoid was bad from the dealer too. Had to get another one...
I might suggest testing the fuel pressure, with a real gauge. NOT just having fuel at the rail. What you are describing is very similar to what started with my daughter's 90 XJ.

Just another avenue to consider.
A simple way to test if its your fuel pump is to wait till it starts acting up, then turn it off, turn the key to the position just before crank and then off again, 3 times. Turn it over and if the buckings gone that may be your problem.
What can cause bucking? Running lean, likely from low fuel, either the O2 sensor is acting up or the fuel is being starved some other way. Like mentioned, low fuel pressure or some other sort of mechanical restriction. There are a number of sensors that affect fuel flow or injector pulse duration. The O2, MAP and the TPS. The MAP in my experience usually fails full rich.
A timing problem can cause bucking, timing is mostly controlled by the TPS with input from the CPS, speed sensor and MAP.
Ignition problems can also cause bucking, a bad ignition cable can jump to a nearby spark plug cable and fire the wrong spark plug at the wrong time. Or short to ground and fire excess fuel in a cylinder from the last miss fire. Or weak spark can cause the same.
Very white plugs are a sign of a very lean motor. This can also happen from vacuum leaks, enough fuel, but too much air. Often indicated by some plugs being white while others are gray or even dark gray or black. Most of the vacuum hookups on the Renix intake are off center so a vacuum leak can affect some cylinders more than others.
More exotic causes can be bad or sticky valves, coke (soot) in the cylinders that ignites and burns causes odd ignition, and distributor indexing (initial timing). I had an XJ that had been parked for an extended period, the pressure tested good, the flow rate didn't. It finally turned out to be bad fuel, some had turned to jelly in the fuel lines. Moisture or water in the fuel has caused all sorts of ignition problems over the years (not common but it does happen), it seems more common in the early winter months.
The cam position sensor has never really given me any problems at all, though a short would affect the 5 volt sensor supply circuit. I've run for months with mine disconnected and never really noticed any difference at all in the Renix (whole different can of worms in the OBD 1, post 90 models).
Lastly, a good sensor doesn't necessarily mean it is putting out goos info, the connectors and wiring may have excessive resistance.
Sorry about wasting $60 on a new O2 sensor, but at least now you have a good spare to test with next time.
I keep a box full of junk yard sensors, sometimes it's easier just swap them out and eliminate that sensor as a possible problem. But not a good idea if you have to pay too much for the process and start buying and swapping out good sensors. Besides the fact a new sensor isn't necessarily a good sensor, I've had more than a few fail right out of the box.
One third or one quarter gas pedal is a typical dead spot for in a TPS. Though it really doesn't buck, the motor just kind of dies for an instant in my experience. I had a TPS like that in my 87.
I've run out of ideas, there are probably others I'm forgetting. Hope I haven't info overloaded you. :)
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