Howdy gang! I thought I would share some issues I'm having with a stock '99 XJ Limited 4.0.

It has 336,000 miles on it with an engine rebuild at 300,000 miles. I bought it at a Police impound auction a few years ago as a project, and it had been basically been sitting around until October of last year when my daily driver, a 2014 WK SRT, was stolen in broad daylight, never to be seen again. Over time, I had been trying to go through it to get it mechanically sound enough to put a paint job on it (massive amount of hail damage and peeling clearcoat), and have replaced the torque converter, shift solenoids, rear springs, all four shocks, new brakes, headlights, etc. Since having to use it as my primary vehicle (a motorcycle and a clapped out Miata are no good for Home Depot runs), it has started to run poorly, with a wildly variable idle, intermittent hard starting, hesitation at throttle tip-in and in steady-state driving.

With the help of a smoke generator, I found and repaired SEVERAL vacuum leaks, including the throttle body and (Rusty's) throttle body spacer. I removed the spacer and replaced the throttle body with a BBK unit. While that helped, it still had issues with starting and idling, so it got a new set of spark plugs, ignition wires, and cap and rotor. That also seemed to help, but not much. Replacing the coolant temp sensor, intake air temp sensor, and MAP sensor made no difference, and it eventually threw a code for the front O2 sensor so I replaced both O2 sensors. Eventually, I replaced the idle air control valve, but still no joy. After replacing the throttle position sensor, it seemed to improve and run really well, but has since started to run poorly with hard starting and no idle when cold. It would intermittently run well above idle under load, but seemed a bit down on power, and would idle ok when warm. Resetting the computer every time, it would find a good idle sometimes, but not every time.

I double-checked for vacuum leaks, and found the BBK throttle body leaked at the throttle shaft a little, and fixed that with some new o-rings. Still not running well, it threw a P0304 code for a #4 cylinder misfire about 2 weeks ago, and looking at the fuel injector wiring connectors, some had broken lock tabs, and the wires had some exposed wires at the connectors, so I replaced all of the connectors and injectors. It still didn't run well and threw the same code again, so I replaced the cap, rotor, wires and plugs again since it had been well over a year since I did that, No joy. A few days ago I put a new ignition coil on it with no change.

Searching this forum and a few others, it seemed that the throttle position sensor could still be the issue, so I used an analog voltage meter AND a digital meter to see what the sensor was doing. While it varies the output voltage smoothly, at idle the output voltage is .552 volts DC and 3.29 volts DC at full throttle. Both readings seem low to me, and I wondered what the folks here thought. I also noticed that the (O'Reilly's) TPS did not have the big rubber seal on it like the MOPAR TPSs have, and have read that aftermarket TPSs often don't last very long. The TPS that NAPA sells looks like the MOPAR TPS with the big seal on it, too. I put an o-ring on the O'Reilly TPS housing, and there is no detectable vacuum leak there, but it seems like the output voltage could be causing my problem.

Separately, but perhaps a contributing factor may be the overall electrical system voltage that never gets to 14 volts, but never falls below about 13.2 or so. The battery is less than two years old, and it turns over well every time, and the OBD system test of the alternator checks good. I know it could use a battery cable upgrade, but there have been no other electrical issues at all. I have a battery charger on it now to bring the battery up to full voltage, and will check the TPS output again tomorrow.

I'm ready to throw another TPS on it, but wanted to know what you guys think. Thanks in advance!

Eric

 

How old is your CPS?

Regarding your voltages, post #2 from member tjwalker in this thread (on another forum)

https://www.jeepforum.com/threads/tps-voltage-range.1184574/

 

When the engine is cold, look at the iat temp and your coolant temp with an obd 2 reader..they should be almost identical. Both sensors have control of fuel scheduling/ mixture. Also disconnect your upper o2 sensor and see if it starts better when cold

 

Thank you, but I have seen that post, but I was actually wondering whether the voltages I recorded could be causing the symptoms my vehicle is experiencing, since they are slightly lower than the 3.9 volts at full throttle cited in that post, and that post didn't mention what the normal closed-throttle voltage range should be. The post after that (from the OP) does mention a closed throttle reading of about .5 volts, but the issues he was having were transmission related, not an engine performance issue. Additionally, the OP has not posted what, if anything, solved his problem. I have not tested the CPS, and have never replaced it myself, so I have no idea how old it is, but knowing the limited technical skills of the previous owner, I suspect it has never been replaced.

Thanks you for that. I have previously replaced both of those sensors, just to be safe, since this vehicle has so many miles on it (over 336,000), and the previous owner was not diligent with maintenance of any kind. Given that, I have not recently checked to see if those temp sensors match up, but I will as soon as I can get to it. I think I did check to see if those readings match (a while ago), but honestly cannot recall. Also, I haven't tried disconnecting the forward O2 sensor yet.

Interestingly, I did put the battery on a trickle charger for the last two days, and it finally came up to full charge this morning. I meant to test the CPS for min/max voltage, but thought I would try to start it, just to see what happens with the battery fully charged. I was still suspecting that the overall electrical system voltage wasn't what it should be, given that the gauge has only showed a little over 13 volts for as long as I can remember. Initially, it didn't want to start without giving it some throttle input, but once running, it seemed to be much smoother than it had been previously, and idled ok. It used to run rough and stall while coming up to normal operating temperature, and even after being fully warm would idle erratically and stumble slightly upon throttle tip-in from a stop. So, I decided to drive it around town until it came up to normal operating temperature today to see if ran any better, and besides a slightly high idle speed once it got fully warmed up, it ran just about as well as I could imagine from cold to fully warm; smooth response off-idle, plenty of power, and no surging at a steady speed of 43-45 MPH.

I know the battery cables and terminals are not in the best condition, and I can't remember if I had previously replaced the alternator, but have never had an issue with it starting; it's always turned over nicely. I'm thinking the alternator may not be putting out the voltage and amperage that it should, although it passes the OBD Alternator and Battery test with my scan tool. I will remove the alternator and battery, and have them tested as soon as I can. I suppose it's a good idea to replace the battery cables and terminals, too.

Thanks again for the replies, and I will post my progress as soon as there is any.

Eric

 

Yes to those last things you mentioned!
And while you're at it, clean up your grounds in the engine bay if you haven't yet

 

Get out of my head, sir! LOL! Excellent suggestion! My plan is to install all new cables, including the engine cylinder head ground strap, alternator cable, etc., once I get this issue resolved.

As it turns out, the battery tests OK (about a year old, and still under warranty, too!), but the alternator does not. The diodes all schecked good, but the internal voltage regulator is bad, dropping to 12.7 fairly quick under load. Once I have it swapped out, I'll let y'all know if that fixes the problem.

Thanks for the help!

Eric

 

Big 7 cable kit has been ordered.

 

I think you're on the right track mate 👍🏻

 

Well, it turns out that the new alternator works, but I still get only 13.9 volts out of it, which I suppose is ok. Tried to reset the IAC after getting it up to normal operating temperature, but it never really seemed to settle into a smooth idle. I was hoping for a steady 750-850RPM, but after driving it for about 20 minutes, it started to idle rough and low, down to around 500RPM or so, and the voltage was dropping to about 12 (which makes sense with the A/C running full blast, too). I decided to take it down to the VA in Dallas and back, which is a 44 mile round trip, and it started surging a little at speed (70-80MPH) and still wouldn't idle properly. Every time I came to a stop, I had to left foot brake and give it some throttle to stay running. As soon as I pulled out of the VA parking lot, the check engine light came on, but still ran strong enough to motor along at 80-85MPH with power to spare. After getting it back in the garage, my scan tool showed a P0304 code, and the scanner suggested that it was a mechanical problem, and to check for a broken valve spring, etc.

Tomorrow, I'll pull the spark plugs and do a compression test to see if all the cylinders are pumping air like they are supposed to. If that all checks good, I'm out of ideas and patience, so it's off to the Jeep dealer's service department.

I miss my 2014 Grand Cherokee SRT so very much.

 

Not sure what to think of this:

I did finally check the IAT and ECT sensors first thing this morning, and there is a 2* difference between them, with the ECT being higher, but have no idea if that is normal, or not.

I looked at the TPS signal with the throttle closed, and it was reading 16.3%, which seemed high to me, so I adjusted the throttle stop screw until it was not touching the bellcrank (making sure the cables weren't keeping it from closing fully), and got a reading of 9.3%.

I then adjusted the stop screw until the reading started to come back up, with a final reading of 9.5%.

It was still cold (86*), but I disconnected the battery, turned on the ignition and headlights, and let it sit for a minute to reset the computer.

After reconnecting the battery, it had a little trouble starting, needing a little throttle until it fired up, but it quickly settled into an idle of about 1000RPM with the temp gauge at the bottom of the scale.

I put it in gear, at it settled right on down to about 800RPM, and felt fairly smooth.

I drove it around town for about 10 minutes or so, and it ran just about flawlessly, with an idle speed of about 800RPM.
.
I will drive it later for 30 minutes or more at highway speed to see how it does.

Somehow, I expect it to start running bad after it gets heat-soaked, but I won't bet a dime on anything, at this point.

More to follow...

 

Try the 3 Key Trick to see if that makes it start easier.

Turn the key to On 2 times without going to start to allow the fuel pump to load the system. On the 3rd Key On go to start.

 

2 deg split between iat and cts is considered dead on. I was hoping for a 10-15 degree split....not your issue

 

I will try that. I has a new fuel pump in the tank, but can't be sure the pressure regulator has been replaced.

Far out! I thought that was really close, too, but had no knowledge to back it up.

Thank you, gentlemen!

Eric

 

Well, a few days ago I finally got around to replacing all of the power and ground cables with a "Big 7" wire set. I was really hoping that would help, if not solve my problems. No such luck. Perhaps I still have a vacuum leak somewhere.

Today, I decided to try a computer reset, and it seemed to idle much better initially, but after a 20 minute drive above 40mph, it idles erratically and low. After about 30 minutes of mixed driving (highway/side streets), it surges a little under steady load at speed, in addition to continuing to idle rough/erratic/low. With the A/C on in 100*+ weather, it also seems a little down on power, more so than usual.

The only sensors I have not changed are the Crankshaft Position Sensor and the Distributor Sync Sensor. I have read in several forum posts, both here and elsewhere, that the Crankshaft Sensor either works or it doesn't, resulting in a no-start condition when it doesn't. I've seen a few posts and videos about the Camshaft Sync Module possibly causing this issue, but nothing that shows a vehicle actually running poorly, with a new Cam Sync Module fixing it.

Going to try some more troubleshooting as soon as I can, including firing up the smoke machine, and turning the engine over by hand to check the sensor outputs, but my radiation treatments are starting to take some starch out of my sails.

Wish me luck. I feel really close to giving up and taking it to the Jeep Service department to let them drain my wallet. All help is greatly appreciated.

Eric

 

This is short video of the TPS signal at idle. Should it not be rock steady? Is there a normal range for the TPS at idle?