1993 Jeep Cherokee check engine light on

I have a 1993 Jeep Cherokee Sport and the check engine light has come on for the fist time in the 16 years I have owned it. The Jeep has 238,000 mile on it. Today I brought it to the self service car wash and cleaned the sludge off the engine with Black Magic engine cleaner. I mostly concentrated on the passenger side of the engine (4.0 straight six) since this is where most of the engine sludge was caked on. I washed the rest of the vehicle as well. It started and I drove it home without any problems. Later on that day when I was driving home from the store, the engine began hesitating, it began to buck. When I stopped for a traffic light the engine idled unevenly and felt as though it wanted to stall out. When I went to take off from the light, the engine hesitated and did not want to take the gas. It felt as though it had no power. It did however accelerate but very hesitantly. The check engine light went on. I was able to drive it home, it never stalled out but it always felt as though it was going to. The engine also raced at times when I was stopped, not really high but way above a normal idle. When I got it in my driveway and looked under the hood I checked for the obvious, loose wires, etc, I did notice that the battery cable connections were loose and dirty. I cleaned them and tightened them. I took the Jeep for a drive after that and it ran good but the check engine light is still on and it will not go off. Any ideas on what this could be? Did washing the engine destroy one of the sensors? Could it be a bad O2 sensor? I love this Jeep and plan on keeping it until I die. I have replaced several parts in the past, including the crank shaft position sensor which is only about 2 yrs old so I don't think it could be that. Someone mentioned to me that it could be the cam shaft postiton sensor that is located inside the throttle body air intake. I don't know much about this. Any suggestions on what could be causing this would be very much appreciated.

You probably toasted your TPS (throttle position sensor) - it's on the driver side, but they get cranky when watered. Sounds an awful lot like either the TPS or a vacuum line knocked loose.

The person who told you it could be the cam position sensor in the throttle body intake got it half right, they were describing the throttle position sensor I just suggested but called it the wrong thing (the cam shaft position sensor is under the distributor rotor.)

Another thing you can try to make sure you aren't throwing money at the problem unnecessarily is reading the codes - but finding a place with an OBD-I code reader may be difficult, I haven't had to try yet. Check autozone, CAP, advance auto, etc. Most will do a free scan and tell you what the codes are in the hope of selling you parts to fix the problem.

kastein said:

You probably toasted your TPS (throttle position sensor) - it's on the driver side, but they get cranky when watered. Sounds an awful lot like either the TPS or a vacuum line knocked loose.

The person who told you it could be the cam position sensor in the throttle body intake got it half right, they were describing the throttle position sensor I just suggested but called it the wrong thing (the cam shaft position sensor is under the distributor rotor.)

Another thing you can try to make sure you aren't throwing money at the problem unnecessarily is reading the codes - but finding a place with an OBD-I code reader may be difficult, I haven't had to try yet. Check autozone, CAP, advance auto, etc. Most will do a free scan and tell you what the codes are in the hope of selling you parts to fix the problem.

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Thank you for your reply. I will try to find an auto parts store with an OBD-I code reader. How long does the TPS usually last? Do they usually need replacing or could they be cleaned? Also, how much trouble is it to replace?

TPS lasts till you get it wet, or till it breaks... no real telling for sure. It's basically a variable resistor attached to the throttle valve shaft. After a good bit of use, the resistive element wears out under the spot where the wiper spends most of its time (cruising position.) They always need replacing, they're sealed well enough to keep you from disassembling one without damaging it BUT not well enough to actually keep water out, kinda like a wristwatch that is "water resistant" they can withstand a bit of a sprinkle but are hosed if you really get them wet.

As for replacing them, it's not that hard, I did it in 15 minutes a while ago (because it was raining and I wanted to go back inside.) All you need is a T15 or T20 (forget which) torx bit, preferably on a ratchet rather than a screwdriver because the sensor is mounted on the back of the throttle body and it's a bit limited workspace wise. I find that unplugging a few of the vacuum lines and other sensor wires lets me move things around so I have more space to work.

kastein said:

TPS lasts till you get it wet, or till it breaks... no real telling for sure. It's basically a variable resistor attached to the throttle valve shaft. After a good bit of use, the resistive element wears out under the spot where the wiper spends most of its time (cruising position.) They always need replacing, they're sealed well enough to keep you from disassembling one without damaging it BUT not well enough to actually keep water out, kinda like a wristwatch that is "water resistant" they can withstand a bit of a sprinkle but are hosed if you really get them wet.

As for replacing them, it's not that hard, I did it in 15 minutes a while ago (because it was raining and I wanted to go back inside.) All you need is a T15 or T20 (forget which) torx bit, preferably on a ratchet rather than a screwdriver because the sensor is mounted on the back of the throttle body and it's a bit limited workspace wise. I find that unplugging a few of the vacuum lines and other sensor wires lets me move things around so I have more space to work.

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Thank you for all your help. From what you have explained, I am sure that the TPS is the problem. I will however try to find an OBD-I code reader just to be sure. Do you know about how much $ a new TPS sensor will be?

You do need to zero out the TPS when you install it. The holes in the sensor are slotted for adjustment. Do adjust just get a voltmeter on the signal wire, there will be three wires one is 5v reference, ground, and the signal voltage telling the computer where the throttle is. It will output from 0 to like 4.5 volts, you just need to set it so when the throttle is completely closed you get o volts.

And to know for sure if your old one is bad just put your meter on the signal wire move the throttle open and closed slowly and see if it is cleanly going from 0-5V.

On a 93 the holes aren't going to be slotted, you bolt the sucker down and call it a day. TPS adjustments apply to RENIX only AFAIK.

Sensor will run you 30-50 bucks at any auto parts store.

Late comer to the party but he may have moisture in the distributor.

You can get the OBDI code by turning the ignition on, off, on, off, and then on. The CEL will flash the code in two digits per code. 12 is recent battery disconnect, and 55 is normal end of cycle. Others are significant. I'm lazy tonight, but look for a post or profile of Forum member 5-90, and follow the links in his sig for a nice summary of the codes.

It can take a while for a code to clear even if the problem goes away, so be patient. If it's running well now, I would not worry too much. As Joe says, it might have been wet ignition, and just have taken some time to get the moisture out.

OBD I method and codes courtesy of 5-90: http://www.geocities.com/JeepI6Power/obd1.html

Geocities link is running on borrowed time, www.kelleyswip.com/obd1.html is the new link

kastein said:

Geocities link is running on borrowed time, www.kelleyswip.com/obd1.html is the new link

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Thanks for the link, I needed to bookmark it.

kastein said:

On a 93 the holes aren't going to be slotted, you bolt the sucker down and call it a day. TPS adjustments apply to RENIX only AFAIK.

Sensor will run you 30-50 bucks at any auto parts store.

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Thank you for all your help kastien. It was exactly what you predicted, the TPS was NFG. I replaced it with a new one. A $36 fix. I bought the sensor from Advanced. They were very helpful people, I highly recommend them. Installing it was very easy, just take out the 2 screws take off the old one, put on the new one and tighten the srcrews. It was a T-20 torque head bit. Thank you to everyone who replied to my post. I am so glad I found this website!

No problem! Glad you got it fixed. Advanced Auto Parts and NAPA are probably my favorite parts chains at this point.