Hey all,

1999 Cherokee Sport

I know theres many threads about no code misfires but I havent found one that matches my exact issue.

My jeep is running crappy. At completely random times, it will begin to misfire and buck hard. It doesnt matter if the engine is warm or cold, if i turn it on and off, low speed or highway, it just comes and goes as it pleases. This has made troubleshooting very difficult. The only pattern I have noticed is that when it starts, it continues to misfire on and off pretty consistently until it goes away like it never happened. Even between rides and warm up/cool downs. When it comes it comes, when it goes it goes. On top of this, the jeep is behaving weird RPM wise. For example, on the highway while maintaining speed, lets say 50mpgh, if I give it some gas, the RPMs will shoot up to where you would expect them to be, maybe by about 1000 or so, and then drop and settle down by 500. So it constantly sounds like the jeep is ready to get up and go but then it pulls back. It doesnt quite feel like its bogging down, and this doesnt occur at all if I floor it. The same goes for when I let OFF the gas after maintaining speed, except it works the opposite. The RPMs will fall, and then all of a sudden raise up slightly again even though I have not given it any gas.

As mentioned, no check engine light for anything. No pending codes either. Engine can buck super hard with many misfires after one another but no CEL. Light it not out.

I have recently changed my TPS, CPS and upstream o2. I unplugged downstream to see if it was the issue and it made no difference. I did buy cheap CPS and TPS but the RPM issue has been present since before I replaced them. (The misfire has not.)

EDIT: Also, my fuel trims are in the negative, roughly -2 to -4 long term. I dont know if this a lot or a little.

Any advice?

 

99 still has a coil and distributor, right?

If so, check the wire leading to the coil. I had exactly your symptoms and it turned out to be a loose connection there. No codes.

 

Yes, coil and dist. I will check out the wire and see if it helps.

 

Update:

I haven't checked out the wire yet but I may have been off on the temperature info. We had our first really cold morning here where I live and it made the misfires far worse, and it went away when the engine got up to temp. On other cases they have still occured with a warmed up engine many times but today was a little different.

 

I've been reading that this could have something to do with my temp sensor? My gauge works fine but I understand that apparently there are two sensors for engine coolant temp. Should I look into replacing one or both?

 

Did you replace the sensors because of having the intermittent misfiring issue. Or, did the misfiring begin after these were replaced? Was the misfiring behavior any differen before? Better or worse after replacing the sensors, or no change? What brand did you put in for these replacement sensors? Mopar is the only brand I would ever recommend for these sensors. The other brands can be bad out of the box even when new, more often than not..

For the fuel trim readings:
-2 and -4 is within normal parameters for each STFT and LTFT value individually on the seperate banks, and for each individual bank when you add their STFT and LTFT together.. When you add the values together on the banks.. For example, a -5 STFT and a +4 LTFT is a total value of -1... When the STFT and LTFT are added together on each bank individually, they should equal less than 10 (either way +/- total fuel trim).

Yours is at -2 and -4 for the LTFTs for each bank? This is not bad, but what it is telling you is that your Jeep is running just slightly on the richer side for whatever reason (normal age and wear & tear s normal).. The PCM is adjusting to this by taking away fuel via the amount of time it allows the fuel injectors pulse length to be.

A misfire is one of three things. It is related to inadequate spark happening for ignition, abnormal air/fuel ratio not conducive to combustion, or a mechanical problem (ie: blown gasket, cracked head, rings, etc..).

Since your trims are showing negative on both banks LTFT, it may possibly be that you could have a leaky fuel injector or two on each bank. (If the trims were negative on just one of the banks, it may mean a leaky fuel injector or two on only that one bank).

I do not know what is going on with your RPMs right now? This problem may or may not be related to the misfiring?

I think if it were me. Seeing how the fuel trims are only a little negative like they are. I would begin by looking to fully verify seeing that there is adequate fueling happening via the fuel rail. I would get a fuel psi test gauge hooked up to the fuel rail to focus my efforts looking there.. Prime it once or twice with the key first. Let it sit for a good amount of time and see if the fuel psi is bleeding off decreasing in value over time. That would indicate possibly having some injector(s) that may be leaking providing a little too much fuel to their corresponding cylinders. You can test the fuel pressure further to try and verify if there is a leakdown towards with and involving the fuel pump vs towards and with involving the fuel injectors, or possibly both?

Testing for fuel psi can be normal when the vehicle is at rest while not under a load. If I was testing for proper fuel psi, I would also leave the gauge hooked up where I could see it, and drive around while varying the load as another way to test for having the proper fueling.

This is where I would start is with fueling to make sure it tests within specifications specific to your Jeep.

I would also pull the plugs to see how they look. They may be able to tell you something more specific based off their appearances?

(edited):
I edited my post to reflect.. because I misread your fuel trim values the first time.

Also, I believe with a scanner hooked up you may be able to look at the value your PCM is seeing/reading from off that sensor in real-time data?

 

I think a '99 only has one. The one in the thermostat housing.
This may not apply to a '99.
But I had a friend with a '96. Was running like crap. He did a complete tune up, with wires, and a new fuel filter. They are not in the tank like a '99.
Anyway it still ran like crap. Drove him nuts till he realized the temp sensor in the thermostat housing was bad and causing it.

 

The coolant temp sensor can be tested with a meter and a manual which has a table of resistances at certain temperatures. And yes, only one coolant temp sensor on a 99; the one in the stat housing.

The maintenance manual recommends a fairly complex way of removing the sensor, placing in a beaker of water, heating, etc and measuring. But I have found that you can get a "general" idea if it's working by testing resistance of the sensor (while installed) both when engine is stone cold and when hot. Or just replace it; they aren't that expensive.

Not saying that is your problem, but from the symptom you just mentioned, it sure could be.

 

If you want to look at and utilize fuel trim values to look for a problem;

You will need to know and look at both of the values, of the STFT and the LTFT in order to do so... One value alone of just the LTFTs is not going to be enough information to make an accurate assessment.

For example, in your case.. If the bank having a LTFT of -4 also has a STFT of say -5 or something? Then, that would be very close to being outside of the normal parameters for on that particular bank. (-4 plus -5) = a total fuel trim value of -9 for on that bank... -10 (or, +10 total trim) is the upper normal limit for the allowable 'total fuel trim' value for on either bank. This would indicate to you that there is greater reason to believe something is going on for this type of value seen on a single bank. Does that make sense to you?

For example, if.... (-2 LTFT plus +2 STFT)= 0 total fuel trim on your bank #1

Then.. for example, if on your other bank #2 you get values of (-4 LTFT plus -5 STFT) = -9 total fuel trim on this bank.

In the above hypothetical example you can see how both the STFT and LTFT values were needed to be known for on each bank, in order to really be able to see how obviously that one bank with a much more negative overall total fuel trim value is showing itself as possibly being the more likely suspected problem, or area of concern!

 

Ussually, a cold engine misfire should be obvious. On an older vehicle the gaskets and all its engine parts with narrow-gap tolerances are sometimes no longer exactly tight enough. If it is slightly worse when cold that could be nothing to worry about.. If it is much much worse when cold and dissapears when hot, as the metal and parts expand with heat, then these tolerances may be becoming too far gone, and may be in need of repairs. Overall, if you have other reasons for a misfire occuring in an older vehicle.. it may just be more pronounced when the engine is cold because of these reasons.

Also, one other thing is that a worse misfire happening when the engine is cold could also be related to possibly having leaky injector(s).

The injector may not be holding fuel pressure after the engine is shut off. When this occurrs with a hot engine the heat of the piston vaporizes the fuel. When the engine gets/is cold again, the fuel sits in the cylinder and causes a misfire to happen on start up until the cylinder clears its self... This will of course be a little difficult to fully clear the cylinder, because it is possibly steady leaking fuel into the cylinder still by the bad injector even worse now with it started and while the engine is running (the fuel pump is on now providing constant state increased psi in the system).

 

Thanks all for such detailed replies, I'm going to dig into this this weekend and answer all the questions when I have the jeep in front of me.

 

OK Gents, I have an update, she threw a code. Without having popped the hood yet I'm going to answer the above questions first and then get to the code.


I replaced the sensors in a vain attempt to troubleshoot the perceived RPM problem, combined with the jeep being randomly sluggish and lack of power. But at that time there were no misfire issues. The misfire problem did not begin immediately after they were replaced. And obviously the rpm issue did not resolve (although the sluggishness does seem to have reduced). I replaced them all maybe 2 or 3 months ago and the misfire problem has only recently popped up. As for replacement sensor brands, I went cheapo. I knew I shouldn't have but money is tight. Heres what I bought:

TPS: https://www.carparts.com/throttle-po...ent/repd314202
CPS: O2: https://www.autozone.com/engine-mana...hText=35034511
What I failed to mention is that I also replaced the crankshaft position sensor, aka pickup coil sensor inside the distributor.
CAM SENSOR: https://www.carparts.com/distributor...ent/repd545401

So yeah. Cheap.

I was not sure that the injectors were the issue, only because I seem to have normal fuel pressure, no long cranking. I had a fuel pump issue a few months ago, jeep was stalling and long crank, and after replacing pump the problem was solved. I'm not ruling this out though.



This all makes perfect sense to me. For whatever reason on my cheap car scanner I can only see data from one bank though.




OK onto the code. So today when leaving work it was quite cold, and the misfires were worse than ever. It almost felt like it was ready to stall out. They improved as the engine warmed up but never fully went away. Finally it threw code p1391. Crank or cam sensor.

Error Code P1391 is defined as CMP or CKP Signal Intermittent Condition. It refers to intermittent conditions in the CMP (camshaft position sensor) or the CKP (crankshaft position sensor)

Sigh. I've read over and over about these cheap parts but I did not heed the warning.

So despite the above conversation I think we may have our culprit, one of the two cheapo sensors I threw in it. Unless anyone disagrees, I'm going to start by ordering a better CPS and a whole new distributor.

Do these look quality?
https://www.quadratec.com/products/55102_0013.htm
https://www.quadratec.com/p/crown-au...ine-56027866AC

I do trust both quadratec and crown.

Let me know what you all think.

 

Also, if its truly MOPAR or nothing with this CPS, I'll make it happen.

 

You said your scanner only lets you read just the one bank? I think you and I are using different meanings for the terminology? If you are giving us the values of just one of the banks, that makes the value a little more negative at -6 for the total trim.

Either way, it still makes sense for them being negative on whatever the bank because if there is a problem with either the cam or crank sensors then there may be moments where no sparks are being created to cause the fuel to burn up like it normally would during in combustion. This would be picked up by the oxygen sensors to inform the PCM of the extra fuel escaping into the exhaust. The PCM would decrease the fuel spray to compensate for this via shortening the injectors pulse width.. as negative fuel trim.

The (CPS) Crankshaft Position Sensor is in my opinion the single most important of all the sensors that require having the Mopar brand name. The magnets used by Mopar in this sensor work very well for in the application. The other branded CPS sensors magnets are subpar.

The cam and crank sensors work together with one another. I found this from a 10 year old post and copied it. I think this person explains it pretty well.

(Copied and pasted):
The CPS is a Hall effect transducer that varies its output voltage in response to changes to magnetic field. Inside the sensor there is a little magnet that lies along a piece of wire.
When the magnetic field is disturbed by the passing of the reluctor ring teeth it generates an inductive current in the piece of wire attached to the magnet.
The signal produced by the CPS is an analogic square waveform that varies in frequency in response to the overcoming speed (RPM) of the reluctor ring teeth and grooves. This tells the ECU at what speed the crankshaft is turning.
Another information provided by the CPS is piston top dead center (TDC). On the flywheel there are three big holes that produce a long "zero" signal. The holes are only three because in a six cylinder the pistons move in pairs, therefore you have two pistons at TDC at a time. Now, how does the ECU know which of the two TDC is fireing and which is exhausting?
That's the work of the sync pulse stator (camshaft position sensor) in the distributor. According to the position of the rotor the sync pulse stator recognizes which of the two is fireing and tells the ECU "hey, give fuel to cylinder number X!" and magically the engine turns over and runs all by itself...

(end copy and paste)

 

It is solid advice I believe to scrutinize and replace non mopar sensors for the CPS, TPS, and the Cam... There is report after report of the generic ones here failing immediately and causing all sorts of different difficult to track down problems. You also have codes pointing directly to the sensors for you as well.

Except, codes fool people all of the time while they go around chasing their tails.. Sensors pick up abnormalities. These abnormalities may be the result of a problem located elsewhere many times though too. Nothing beats 'testing'.

The cold start misfire rough running condition being worse like it is when cold has me a little concerned still.. I say this because it is possible that you have something else still going on that these generic sensors are not helping..

You replaced these sensors due to sluggish performance to start with. I wonder why it felt that way to you?

Keep in mind that if there is another problem like leaky fuel injectors (I know I keep using this as an example.. Thats all though).. If the cylinder(s) do not fire off in combustion due to leaky injectors causing excess fuel in the cylinder.. This may be able to throw a code for the cam or crank sensors. They will be effected. If there is misfires due to excess fuel leaking into cylinder(s) the crank sensor will know this because it will be able to sense the absence of increasing crankshaft speed/rpm where it senses this right after combustion moves the piston how it is supposed to do normally.. However, this 'should' result in a code that is specific to a misfire when correlated with the cam sensors intelligence. (something to be aware of). This, and the generic sensors here may not communicate effectively with each other, or the PCM. The OEM sensors do all of this together better.

This and the prior sluggish performance how you mentioned, and the cold engine running much rougher like it is for you makes me wonder just a little.