O/T: 1990 Cherokee engine codes

The patient: 1990 Jeep Cherokee, straight 6, auto, 4x4

The symptoms: runs rough, stalls at idle. This is under normal driving conditions. If you drive more aggressively, the problems decrease/disappear.

Already checked fuel pressure, so pump/filter should be good. Changed TPS and MAP with known-good used parts. Vacuum hoses look good. Cap and rotor good.

Auto parts stores say they can't check vehicles older than 1996 (ie: no OBD I systems, according to them)

There is no 'check engine' light, so I can't cycle the ignition switch to get the codes.

There is an 'emiss maint' (emissions maintenance) light, but it doesn't work. Doesn't come on at all, even when the key is first turned on and it's supposed to blink so you know the system is functional. Bulb is good.

Does this light serve the same purpose as the 'check engine' light for retrieving codes?

Also went to a salvage yard/garage that does have an OBD I scanner. No luck here either. This guy said "1990 is the crossover year from Chevrolet to Chrysler, and the computer won't store any codes".

How is that possible?

Is there any other way to read the codes?

I was thinking it could also be a vacuum leak at the intake, or possibly the oxygen sensor. However, I don't have a spare O2 sensor and don't want to buy a $50 part if that isn't the problem.

Opinions and suggestions are appreciated. Thanks for your time.

Sincerely,

Anthony

To locate a vacuum leak, at idle, spray "small" amount of starter fluid, WD-40 or other spray product that will be drawn through leak source into intake to engine combustion chamber causing speed change because of mixture variance and not cause internal engine damage or exterior explosion. A hand held propane container with rubber hose connection works well for this. Use caution.

your problem could be caused by a host of diff things.emmison/maint light is to warn you of impending problem,it can set code but not always,it may take the place of check engine light.you should be able to read the codes with grounding one of the pins in the diagnostic plugin[on GM vehicles up to 95 anyway]and key on.

I would go to www.alldata.com a online site that covers just about any vehicle on trouble shooting and repair,most if not all shops use it.
about 25 pop a year per vehicle.

I used it to solve a similair problem on a 93 grand am,in my case it was a bad injector coil,but it could be as easy as a bad ground connection.

The Emmision /maint light is only for a reminder to replace oxygen sensor. They come on at certain milage intervals . The "check engine or Service engine " light indicates trouble code has been set . To access stored codes you need a DRBll scan tool or similar , most dealers have them or some better equiped independant shops .
You can have drivability problems and not always have codes set . I have found lots,,,lots of vacuum leaks on these , best way to check is carb spray ,or shop that has a smoke machine . Dirty injectors and throttle body will also cause poor idle problems , Injectors need to be cleaned with a canister type or Motorvac cleaning system . dumping stuff in the tank won't do much good . Throttle position sensor may have a glitch in it at idle or off idle , can check with ohm meter.Unplug connector , hold leads to two of the terminals on the TPS and slowly open throttle , look for any blips or drop outs . Be sure to check fuel presuure with a gauge !!! Needs to be 31 lbs ,,
Might save yourself lot of headaches and $$ by taking it into a Quality shop and pay to have it diagnoised $ 50 - 100 ?
Keep us updated

Check the cps on the back of the engine. could be acting up. (if yours is a 4.0)

Start simple--You can clean the throttle body with a $3 can of carb-cleaning spray. Remove air filter hose, wire or tie throtthe wide open, and spray the inside of the body. DO NOT CONTAMINATE OR SPRAY DIRECTLY AT THROTTLE POSITION SENSOR. Wate a few minutes and clean the inside with an old toothbrush, spray again. You will use about 1/2 the can. Let set for about 30 minutes.


REMEMBER---UNTIE THROTTLE THAT WAS WIRED WIDE-OPEN !! When you crank the engine hold the throttle at about 2,500 rpm until all the black-white smoke clears. I can't remember the number of engines I have used this procedure on but Chrysler products are the worst.

I have a "1990 Jeep Cherokee, straight 6, auto, 4x4" except for the auto trans part, mine is a 5 speed stick. 4.0 litre inline 6 cylinder engine. I've owned it from new. I have about 170,000 miles on mine.

I don't remember having problems to quite the extent that yours sounds like, but I have had it run rough and stall at idle. You may have dirty injectors. Mine clears up with a dose or two of Lucas Fuel Treatment. I've also used Seafoam, but the Lucas seems to work a little faster for me.

I usually let the gas tank get down to about half, add a full tank treatment and run it to a quarter tank, then fill up. The concentrated initial dose seems to do the trick.

It may work for you, it may not, but it wouldn't cost much to give it a try.

Anthony,

We bought two Cherokees with the 4L engine--1st one a '97 several years ago and more recently, a 2001. On both of these, the throttle position sensor went out shortly after we bought the cars. Same symptoms as you described except we were able top read the codes. I believe the last one I bought at Advance Auto Parts for about $30 but don't remember the price. On these engines I needed a torx driver to get the screws out. Had to be careful backing the screws out of the aluminum manifold and used anti-seize when I went back in with the screws...

I had a 1988 Jeep Comanche pickup, 4.0/AT, and I discovered that thru about '91, Chrysler didn't change much from the AMC/Renault "Renix" electronic engine controls. So I borrowed a friend's Snap-On scan tool. As far as extracting "codes," I don't think that's possible with this system, but you can start the engine and read real-time data, and use that to determine what's out of recommended specifications.

So find someone with a scanner and the proper Jeep software for that year, and you should be able to determine the cause and cure for your problem pretty quickly. Better to spend $50-$100 on diagnostics than spend $200-$300 on unnecessary parts...right?

...Not certain what years it applied to, but my '88 Comanche was subject to a bulletin regarding an updated crankshaft position sensor [located on the bellhousing, IIRC]...so it might be wise to see what kind of TSB's apply to the '90 Cherokee as well. I know the new CPS made a big difference in the way my truck ran.

EGR stuck open will give you a very large vacuum leak. Disconnect vacuum line and tap it a little bit, if it clears up, either take it apart and clean or plug line and leave it alone.