I keep getting a yellow check engine but when I hook a scanner up to it it does not pull any codes. The light goes off after a couple days but returns a couple of days later.

I recently replaced the battery, cap and rotor, and cleaned the throttle body valve.

The jeep runs fine while I'm on the gas but whenever I stop after slowing down from speed the idle drops real low and the jeep stumbles but catches itself. Sometimes when I accelerate and stay in first gear the jeep hesitates and the rpms do not want to go up but after about a second the rpms do and it accelerates jerkingly quick.

My plan is to clean the air flow sensor. What else could be causing this.


99 xj 172k miles aw4 I6

 

I would clean the IAC valve and test the TPS. The pintle on the IAC gets dirty and can affect the idle. The TPS is best tested with an analog VOM on the resistance scale should show smooth movement as you move the throttle from closed to wide open. (engine off)
Also, the pre-cat oxygen sensor could need replaced. It can affect idle and overall rpms as well.

 

I'd try another code reader or scan tool. You need to make sure that the problem here isn't the code reader itself.......

Or bring it to an Autozone where they will read the code for no charge.

 

Ya, what tjwalker said.. seems odd to get a light and no codes come up. I'm not certain here, but I think I read that the PCM/ECU can trip the light but not show any codes? That could boil down to a bad connection somewhere.

 

The check engine light keeps going on and off. The only symptoms I ever get is a hesitation then a jerk acceleration. I'm going to look at some stuff when I have time.

 

Cleaned the map sensor and iac today... Lets see if that helps.

 

Stopped at a stop sign today about 30 seconds after starting it after being in a store for 10 minutes. It was only slightly below operating temp.

Idle dropped to 250rpm (first line) and the jeep was shaking like crazy. It stayed at 250 rpms until I pressed the gas and it surged to above 1000 rpms. It then settled just below the line before 1000 rpms.

I'm going to check vac lines tomorrow, I'm going to take the IAC completely out and clean it, and check tps and map if I can get of a multimeter.

Any other suggestion?. It runs fine for days and then does this. No check engine light came on.

 

IAC and TPS are still suspect. Doin both o2 sensors sounds like it might be good. Does it ever suck for air real hard without increasing rpms?

 

Not at all. I really want to hold off on the O2 sensors because of cost.

 

The place where the code reader plugs in could be bad. I had that problem with my old truck. I thought it wasn't showing any codes but I realized my plug in was bad. Fixed it and boom codes. Idk if it'll help, just a suggestion

 

Cherokees have mass air flow sensors??? None of the 10 that I've owned did

 

The Alphabet soup strikes again! She must have meant IAC. Pretty sure the MAP takes care of what a MAF sensor would do. For "airflow", I just check the dog's jowls! <<(My Sharpey is not functioning!)

Never a bad idea to check those manifold bolts. If a couple easy ones are loose, you gotta pull the air cleaner and go over them all at once, starting in the middle and working out. I use short little bursts of starting fluid to look for vacuum leaks. If you hit one the sound of the engine will change.

Also on GP, that little tube from the throttle body to the MAP up on the firewall can get brittle, and any flaw can cause trouble. Being rich or flooding thou is more like it.


1984-1990 w/AMC engine are pre-OBD.

1984-1986 w/GM V6-173 are OBD-I (GM control setup.)

1991-1995 are OBD-I (Chrysler)

1996-2001 are OBD-II (Chrysler)

The "Key Tricks"
Older (OBDI) models - Start with the ignition off. Within five seconds, switch the key on, off, on, off, on. ("On" is NOT "Start" and "Off" is NOT "Lock"). The "check engine" light will flash. Count the flashes. Each code is a two digit code, so a (for example) 23 would be FLASH FLASH (pause) FLASH FLASH FLASH (long pause). It will never flash more than 9 times, so watch for those pauses!

Newer (OBDII) models - Put the key into the ignition, push down the odometer reset button, then turn the key to RUN with your finger on the button - and then release the button. The odometer will go through 1111111 through 999999, then display the car's serial number. It will then show the error codes (it might show them starting with P). If this method does not work, try the OBDI method.

 

Dflintstone; very helpful information as usual!

If I could add this regarding OBDII models The 2 digit code that you will get using this technique is helpful, but it is not as specific as the full PXXXX code, which is only available using a code reader or scan tool, so generally I recommend using the code reader or scan tool. But if you have a CEL and have no access to a reader or scan tool, it makes sense to see if you can access the 2 digit code that can point you in the right direction. Note: the OBDII odometer technique works only for 1996 and early 1997 models, so it might work on one 1997 XJ and not on another. Once Chrysler made the change in '97, you will need either a code reader or a scan tool to retrieve the codes.

 

True, but if it's a decent scanner it'll reflect a communication issue, even though it has power from the OBD port.

I'd recommend another scan tool too. Theres 2 series of codes, Stored & Pending. Most auto-chain code readers only search for active codes, and won't search past "stored" history. Easiest way to tell is to try to scan ABS with the scanner, most generic "Code Readers" will not do ABS.

Also, very good advice on the CPS & TPS though.

 

Is it FLASHING as in " going off and on " with a steady rate when the problem occurs?

If so I thought that was indicative of a somewhat serious misfire condition?

I'm not certain but I think I read that one some threads here before.

You might want to read codes when it is having the problem, or gain access to a code reader/scanner that can record data as the engine is running and take it for a drive while recording the data.