Unusual Symptoms After Changing CPS

88 XJ 4.0 AW4.
The other day my CPS finally gave up the ghost. Changed it out and i noticed a couple of problems. After driving it a couple of days. First, it is intermitently very doggy,lacks power. I am having to shift down to be abel to get to speed when going up a small incline. Second, it seems to accelerate especially well once you hit about 2700 RPM. THe lack of power was so bad the other day I thoght that maybe it needed teh computer reset with the new CPS. Pulled the battery cable. WHen I ran it again, it ran fine. Now it is acting up again.
My immediate thought was, check the TPS? Maybe it needs adjustment. What do you think? The TPS only has about 1.5 years on it and 15,000 miles. IT seems like it might be the culprit. THe fact that it went away when I reset teh computer would seem to point to electrical. However, the very noticeable boost in power once I get up to about 2700 rpm is unusual. At any rate, it is butt cold here, I ahve no garage, and I want to see all the options before I decide to work on it.
Thanks
BSD

The most likely reason why your car felt fine for a short time after disconnecting the battery cable was because it was running off a factory fuel calibration -- the onboard computer doesn't "learn" any of the fueling corrections until after the car is driven for a while.

This leads me to believe that there's a sensor somewhere that's feeding the computer bad information; after a time, the incorrect data "skews" the calibration and makes for poor drivability.

Are you throwing any warning lights? I'd start by scanning the codes just to see if anything is lurking in there without tripping the light. On older XJs, you should be able to do this yourself -- if it's anything simliar to my YJ, it's an ON-OFF-ON procedure with the ignition key (without starting the engine). You may want to do a forum search to find the list of trouble codes.

Does the unexplainable power boost at 2700 RPM occur going uphill, downhill, on a level surface...? At varying engine loads the throttle opening varies with a constant RPM; that is, if the boost happens at a certain accelerator position, the actual RPM may vary depending on how loaded the engine is. This can allude to the TPS being at fault, as the TPS signal depends upon the position of the accelerator. You may even want to run a wire from the sesnor to a multimeter and see if the voltage increases uniformly as the pedal is depressed.

What symptom caused you to replace the CPS? Check Engine light, no start, etc.?

This is a RENIX system. They do not throw a code. I replaced the CPS because it was bad. Classic no-start that I was able to get by with for a cople of weeks by tapping on the CPS. THe boost in RPM happens regardless of incline.
Yeah, I was planning on checking the TPS but wanted other feedback so I can be well informed before I tackle it outside in the 10 degree weather and no grage.
BSD

I wonder if the new CPS could be the culprit. I searched but could not find any info on exactly how the CPS works. If it tells the engine where the flywheel is, could it affect the timing? However, if this was the case, surely it would miss. Furthermore, resetting the computer would not have helped. Any thought.
GAB

BSD said:

I wonder if the new CPS could be the culprit. I searched but could not find any info on exactly how the CPS works. If it tells the engine where the flywheel is, could it affect the timing? However, if this was the case, surely it would miss. Furthermore, resetting the computer would not have helped. Any thought.
GAB

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I would try a new CPS, it is the timing for the engine. The signal it produces is time sensitive and can work at some speeds and be off at others.

If the CPS was defective would resetting the computer solve the problem> In addittion, it does not stumble or miss, it just lacks power. If the timing was off, would it not be missing somewhere? It idles fine, starts fine.
Thanks
BSD

If it were the cps, resetting the computer would have no effect, so I don't think it is the cps. More than likely it is either the TPS or possibly the MAP. The key is to figure out if it is speed related, or if it is throttle position related.

old_man said:

If it were the cps, resetting the computer would have no effect, so I don't think it is the cps. More than likely it is either the TPS or possibly the MAP. The key is to figure out if it is speed related, or if it is throttle position related.

Click to expand...

I agree. The CPS produces a signal based on an induced and colapsed magnetic field. It either works, is intermitant, or it doesn't.

Think back when you replaced it. Did you bump or dislodge anything?

O.K. An update
THis time pulling the battery cable did not make a difference. I checked the TPS (mind you it is butt cold out there). Had 4.68 on pin A, 3.90 on pin B and 0.32 wide open throttle. Seemed close but I fine tuned it to 3.83. Checked for movement and it reduced the voltage gradually as I opened up the throttle. Hence, it does not appear to be the TPS. I checked connections and it does not looked like I bumped anything.
Any suggestions? How do I check the MAP sensor?
Thanks
BSD

Is the CPS tight? The CPS can be weak at some rpm ranges.

O.K. One last update. I was checking the connection on the CPS. I can not get it to seat entirely. It is making good connection but the little plastic paw that keeps the two ends of the wiring harness together does not quite engage. Like maybe 1/64 shy.
Also, my XJ had a December 87 manufactur date. The local parts places were not pulling up the right CPS for it; under an 88 listing. I checked and the CPS was identical in look and dimensions to the 87 so I went with that one. DO any of you with parts listing show a different part number?
Thanks
BSD

langer1 said:

Is the CPS tight? The CPS can be weak at some rpm ranges.

Click to expand...

Not sure what you mean. Did I make sure the bolts are in good? Yes.
BSD

BSD said:

O.K. One last update. I was checking the connection on the CPS. I can not get it to seat entirely. It is making good connection but the little plastic paw that keeps the two ends of the wiring harness together does not quite engage. Like maybe 1/64 shy.
Also, my XJ had a December 87 manufactur date. The local parts places were not pulling up the right CPS for it; under an 88 listing. I checked and the CPS was identical in look and dimensions to the 87 so I went with that one. DO any of you with parts listing show a different part number?
Thanks
BSD

Click to expand...

I found the part numbers for the Coolant temp sens, intake air sens, and O2 senc, but not CPS. The three were the same for 87-90, I believe the CPS is too. The part number for my CPS was 83100066 according to a reciept from 1998.

They are the same, do you have it plugged in backwards?

cut the connectors off and solder them together...problem solved.

It is plugged in right. Just to make sure the CPS is not it, explain to me how the CPS works. Is it an all or nothing thing? Or, does it have a variable influence?

The CPS is a hall effect sensor. It converts magnetic field information into electrical pulses.The flywheel is notched, everytime the CPS passes over a notch the magnetic field changes, and it varies the electrical signal sent to the ECU. Moving the CPS closer to the flywheel results in stronger signals. Like mentioned before I think this is an all or nothing kind of thing.

The signal it should produce is a saw tooth pattern 21 small and one taller for TDC. If all peaks are the same the engine will start and run fine but without the taller peak (TDC) the timing will not be correct as the Rpm increases.
I don’t know it that the problem or not because the only way to tell is to look at the wave pattern the CPS is producing with a scope.
But they can work but still produce a bad wave pattern.
If your old one still works you could put it back in and see if the dead spot goes away.
According to your first post this problem started when you replaced the CPS and I just wanted to point out it can be a defective CPS. That particular problem is caused by a defect in the core metal which causes the output to be slow or delayed .
Oh and it's a magnetic problem not an electrical one, so testing with a DMM will not show a problem.

Them CPS can be a PITA. I talked to maybe 20 dealerships today trying to figure out my jeep no start problem in a different thread. Every single one of them said get the OEM one from the dealership. Some are adjustable and use a piece of paper for felt to space the sensor off the flywheel. Im dishing out he 92 bones for a OEM unit today ill let everyone know how it goes.