hesitation at partial throttle

L

littlebill86

NAXJA Forum User
Location
Lancaster, CA
hi guys, im new to this forum as well at to xj's. im a fairly mechanically skilled person, but i cant seem to figure out this problem.

my XJ is a 1988 renix, it has a 4.0L engine. when i went to pick it up from the seller it would not start. i replaced the CPS and it fired right up. when i pulled and would rev smoothly in park. when i pulled out of the driveway all seemed well. as i began to accelerate i wound up through first gear all good, but as the vehicle shifted into second which brought the rpm back down to about 2 thousand it began to misfire and sputter, it will run this way until you smash the pedal to the floor and then it seems to run awesome. I suspected the TPS to be the culprit, replaced it adjusted it and the jeep runs the same way. all vacuum leaks have been dealt with and i sprayed brake cleaner around the manifolds and the throttle body which made no change in the rpm so thats all good....any help would be amazing fella's. thanks in advanced.

here is a picture of the vehicle in question
0528131857.jpg
[/URL][/IMG]
 
Check out Cruiser54 's threads on Renix maintenance, and my old thread The "RenX Files" for a lot of helpful clues to start with. You need to go through, clean and test each and every sensor ground from the sensor to the battery, under the hood. many problems start there.

Get a copy of the Renix fuel injection manual that explains simple volt meter and ohm meter tests you can run.

There are about a half dozen or more things that cause your problem.

Bad connections, grounds and sensors under the hood. Do the meter tests first!!!!! Before you buy any parts!!!

Does not sound like a fuel pressure or flow issue. Check the fuel pressure regulator vacuum nipple for any fuel (if there is fuel there it is a bad FPR).

Find my "Oxygen Sensor Testing" thread here, and test the grounds, the 12 volt heater supply, the sensor heater, and the operating response, as I suspect your problem is the O2 sensor system (not necessarily the sensor, could be wiring, heater relay....).

Test the new TPS for a bad spot with an analog volt meter!!!!

And find a how to thread on testing the EGR valve!!!!!!

Based on you current data, FPR leak, O2 sensor issues, TPS and EGR issues are the prime suspects IMHO.
-------------------------------------
Also, you should test the cranking voltage of the new CPS. Make sure it is at least .50 AC volts. If not, it can cause hard starts someday.
 
Could be a lot of things. First thing I always try to figure out is if it happens cold or warm. After around 140-160 degrees on the temp. gauge it seems you pick up some more sensor inputs for fuel management.

I've heard the arguments, but IMO it is rarely (never) an open loop closed loop system. Though the program does seem to change after the motor heats up and sensor input becomes more important.

I get a new to me XJ and the first thing I do is check out the basics. Compression test, TDC to see if the distributor is set up properly.. Clean all the connectors and grounds. Test and set the TPS. Make sure my ignition system is in good shape, ohm test and inspect the plug cables and pay special attention to the distributor cap and rotor. My distributor cap and rotor seemsto last about half as long as my plugs do on two different Renix.

I had some serious mid range misses and pops on my last XJ (when new to me). Turned out to be a combination of things. My ballast resistor was melted down (in the back where you couldn't see it), my O2 sensor was on it's last legs and I had a TPS ground issue that was intermittent. That ground issue drove me nuts, it was there and then it wasn't, took me forever to chase it down.

The most bang for the buck, getting it to run right, was clean connectors and good grounds.
 
Good advice--find Cruiser's info.

This website is ESSENTIAL: http://www.lunghd.com/

I had a similar problem a few months back--my cap and rotor were bad and crap was interfering with the cam sensor--replaced the cap and rotor AFTER flushing the distributor out with an aerosol can of electrical parts cleaner followed by some compressor air.
 
8Mud said:
Kind of a cross between a low tech computer and a high tech hammer.:gag:
Click to expand...

It is a lot higher tech than the one used to travel to the moon and back. They used a slide rule as a back up and for mid course corrections IIRC.
 
hey guys i have been out doing resistance checks on my ground system. all seems to be ok there, as far as components where should i go from here. the problem really only shows its face at partial throttle after shifting into second gear and does not run good until its wide open
 
Temporarily disconnect the O2 sensor and see if the problem goes away.

Did you use a new TPS sensor?
 
OK guys, i have been going through cruisers refresh list. i put an ohm meter on the tps to ground circuit and found a high resistance reading. i now have that cleaned up and it is reading 1 ohm. i re-adjusted the tps as directed and it runs better on the bottom end all the way to the shift point for second and then crap! it just runs bad, it sounds like it is missing really bad. shut off the throttle and bam back to normal. What am i missing here? are there any really good and nice renix owners near Lancaster ca that i can hook up with? i need a guide at this point i feel lost for the first time owning a car
 
I think one ohm on the ground is still kind of marginal. Strip back a little insulation on the TPS (ECU side) ground wire and make a temporary jumper form the ECU side of the TPS to chassis ground. Try for near zero ohms (I typically get between 0.1 and 0.4 ohms), it can really help the idle also. Clean the spots (pins) you ohm test and the test probe for the meter with a little solvent, just the oil form your fingers can affect your readings.

I never did try running my XJ with the O2 sensor disconnected, worth a try, the default is likely to be full rich and if you are fuel starving at mid throttle, this may lead you in a productive direction. Fuel starvation at mid throttle is possible if the O2 is bad or marginal.

Twice now I've had mid range bucking and missing, once was a bad O2 sensor. The other time it was a shorted O2 wire, it was cooking on the front exhaust manifold. Where it was cooking was near impossible to see without moving the power steering pump. I disconnect the bottom of the cable (ECU temp sender, knock senor and O2) and pulled the harness up the front to see the bad spot. If the power steering pump was ever puled, it is easy to mis-route the front harness when putting things back together again and have it cook on the exhaust manifold, I've seen it on half a dozen different Renix XJ's (most after a trip to the dealership for a new water pump).

I also had a dead spot in the TPS on my 87, I tested it at the TPS connector, with the connector unplugged, with an analog ohm meter. The needle on my meter would would die at about the same spot every time I worked the throttle with the engine off. It always happened at around one third throttle.
 
Last edited:
thanks. the jumper to ground is a good idea. that will be on tomorow's list. i will as well pull the front harness up and out and inspect it. thanks guys, this forum is awesome for support, on so many others i would have been shut down by unwilling members
 
The C101 connector can cause resistance in all sorts of circuits besides ground circuits. Clean it before chasing your tail too much more. The ground refreshing needs to be performed also.

Renix Jeep C101 Connector Refreshing
　
The C101 connector on 1987 and 1988 Renix Jeeps was a source of electrical resistance when the vehicles were new. So much so that the factory eliminated this connector in the 1989 and 1990 models. The factory recommended cleaning this connector to insure the proper voltage and ground signals between the ECU and the fuel injection sensors. We can only imagine how this connector has become a larger source of voltage loss and increased resistance over a period of almost 25 years. The C101 connector needs to be cleaned at least once in the lifetime of your vehicle. Chances are it’s never been done before.

Almost every critical signal between the engine sensors, injectors, ECU, and some to the TCU, travel this path through the C101 connector.
The C101 connector is located on the driver’s side firewall above and behind the brake booster. It is held together with a single bolt in it’s center. To get the connectors apart, simply remove the ¼" bolt and pull the halves apart. You will find the connector is packed with a black tar like substance which has hardened over time.
Take a pocket screwdriver or the like and scrape out all the tar crap you can. Follow up by spraying out both connector halves with brake cleaner and then swabbing out the remainder of the tar. Repeat this procedure until the tar is totally removed. This may require 3 or more repetitions. Wipe out the connectors after spraying with a soft cloth.
If you have a small pick or dental tool, tweak the female connectors on the one side so they grab the pins on the opposite side a bit tighter before bolting both halves back together.
　
Revised 03-02-2013
 
Renix Ground Refreshing
The Renix era XJs and MJs were built with an under-engineered grounding system for the engine/transmission electronics. One problem in particular involves the multiple ground connection at the engine dipstick tube stud. A poor ground here can cause a multitude of driveabililty issues, wasted time, failed emission tests, and wasted money replacing components unnecessarily.
The components grounding at the dipstick tube stud are:
Distributor Sync Sensor, TCU main ground, TCU "Shift Point Logic", Ignition control Module, Injectors, ECU main ground which other engine sensors ground through, Oxygen sensor, Knock Sensor, Cruise Control, and Transmission Sync signal. All extremely important stuff.
The factory was aware of the issues with this ground point and addressed it by suggesting the following:
Remove the nut holding the wire terminals to the stud. Verify that the stud is indeed tightened securely into the block. Scrape any and all paint from the stud’s mounting surface where the wires will attach. Must be clean, shiny and free of any oil, grease, or paint.
Inspect the wire terminals. Check to see that none of the terminals are crimped over wire insulation instead of bare wire. Be sure the crimps are tight. It wouldn’t hurt to re-crimp them just as a matter of course. Sand and polish the wire terminals until clean and shiny on both sides. Reinstall all the wires to the stud and tighten the nut down securely.
While you’re in that general area, locate the battery negative cable which is fastened to the engine block just forward of the dipstick stud. Remove the bolt, scrape the block to bare metal, clean and polish the cable terminal, and reattach securely.
Another area where the grounding system on Renix era Jeeps was lacking is the engine to chassis ground. There is a braided cable from the back of the cylinder head that also attaches to the driver’s side of the firewall. This cable is undersized for it’s intended use and subject to corrosion and poor connections at each end.
First off, remove the cable end from the firewall using a 15mm wrench or socket. Scrape the paint off down to bare metal and clean the wire terminal. Reattach securely.
Remove the other end of the cable from the rear of the head using a 3’4" socket. Clean all the oil, paint and crud from the stud. Clean the wire terminal of the cable and reattach securely.
A suggestion regarding the braided cable:
I prefer to add a #4 Gauge cable from the firewall to a bolt on the rear of the intake manifold, either to a heat shield bolt or fuel rail bolt. A cable about 18" long with a 3/8" lug on each end works great and you can get one at any parts store already made up. Napa has them as part number 781116.
A further improvement to the grounding system can be made using a #4 cable, about 10" long with 3/8" terminals at each end. Attach one end of this cable to the negative battery bolt and the other end under the closest 10mm headed bolt on the radiator support just forward of the battery. Napa part number 781115.
For those of us with Comanches, it’s very important to remove the driver’s side taillamp assembly to access the ground for the fuel pump. Remove the screw holding the black ground wire. Scrape the paint from the body and corrosion from the wire terminal. Reattach securely.
If you want to upgrade your grounds and battery cables in general, contact Jon at www.kelleyswip.com. He makes an incredible cable upgrade for a very reasonable price.
　
Revised 03-04-2013
 
You must log in or register to reply here.

Similar threads

N
Whining/grinding; won't move in 2wd
Replies
5
Views
157
Natesgtojudge
N
F
2000 XJ 4wd AUTO - Stumbles when accelerating but ok when TCM removed and shifting manually
Replies
6
Views
189
lawsoncl
L
B
TPS troubles
Replies
2
Views
194
Blaines2399
B
Ecomike
shaking at idle in reverse and forward. 1987 XJ Wagoneer
Replies
9
Views
280
Bent
Bent
R
Idle and TPS issues
Replies
7
Views
217
Tim_MN
Tim_MN
Back
Top