'90 has no acceleration or power??? - JeepForum.com
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post #1 of 8 Old 11-28-2009, 03:20 PM Thread Starter
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'90 has no acceleration or power???

'90 Jeep Cherokee (not a grand) 4 liter 6 cylinder

I was having a lot of problems with it not starting and with it dieing once it started, so I replaced the fuel filter and still had the problems so I replaced the fuel pump and cleaned all the spark plugs. Wow what a difference! The Jeep ran great for about a week or two and then slowly I started having other problems, Jeep dies when coasting down hill (I shrugged this off as no big deal popping it into neutral and starting it right back up) Then the Jeep started to act like it was losing power while going down the road but always recovered and drove fine. This started happening every time I drove. The Jeep also had a very slow crank when starting, it would act like the battery was dead but then would start right up. (not cold outside battery not dead) When driving the Jeep it runs better if i put it into neutral and keep it at a high RPM while braking. Also once I get going down the road it tops out at 45 MPH and stays at the same RPM (unless put into neutral of course) I'm not sure what RPM it's topping out at, just don't remember.

I researched a lot of possible things, the list of what could be wrong with Jeeps is endless it seems. If you can rule out something for me or send me starting in the right direction it would help. I'm 19 and don't know much about working on vehicles but I'm learning & I want to do my own work and need some guidance.
- rotor, cap, coil, crank shaft position sensor, engine control unit, crank angle sensor, wires, plugs, oil, battery, idle control valve, need to clean throttle body, need to clean injectors, relays, check 20 amp fuse for the ECU from the panel under the hood, MAP sensor, Idle air control sensor, Idle air motor sensor, Idle air control valve, throttle position sensor, do a fuel volume check find out what the pump should be pumping in 30 sec or 1 min..., O2 sensors, heated O2 sensor, catalytic converter, cam shaft position sensor, vacuum leak.

but if one of these things were the source of the problem wouldn't it come on suddenly? Not slowly... With the exception of the idle air stuff and the vacuum lines.
Could there be something lodged in my fuel line? Lodged in the pump? (the gas tank is FULL)

I think I've found place where part of my vacuum line is missing, small black tube sucking in air, what now??? How do I figure out where it's supposed to go? How do I find the vacuum line schematics for a 1990 Jeep Cherokee and how do I read them?

Additional note: My Jeep has for a very long time had a high idle problem, idles up really high sometimes when i put it in park. This comes and goes and to me isn't a big deal. Is it possibly related?? I'm thinking Idle air control here.

Thank you for reading this, any advice, help or guidance is really appreciated. Or if anyone could rule out some possibilities from the list I've composed that would be a huge help as well. Again thank you for reading and replying.

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post #2 of 8 Old 11-28-2009, 03:47 PM
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Sounds A LOT like the problems I had with my CPS (crank position sensor) on my first XJ.

Here's how to test it.:

CPS TESTING PROCEDURE 1991 – 2001 4.0L H.O. engines

1. Near the rear of intake manifold, disconnect sensor pigtail harness connector from main wiring harness.
2. Place an ohmmeter across terminals B and C (A-B-C looking into connector left to right with the part with the notch in the middle on the right) Ohmmeter should be set to 1K-to-1OK scale for this test.
3. The meter reading should be open (infinite resistance). Replace sensor if a low resistance is indicated.

CPS TESTING PROCECURE for 1987 – 1990 4.0 L engines

Test # 1

Get a volt/ohm meter and set it to read 0 - 500 ohms. Unplug the CPS and measure across the CPS connector's A & B leads. Your meter should show a CPS resistance of between 125 - 275 ohms. If the CPS is out of that range by much, replace it.

Test # 2

You'll need a helper for this one. Set the volt/ohm meter to read 0 - 5 AC volts or the closest AC Volts scale your meter has to this range. Measure across the CPS leads for voltage generated as your helper cranks the engine. (The engine can't fire up without the CPS connected but watch for moving parts just the same!) The meter should show .5 - .8 VAC when cranking. (That's between 1/2 and 1 volt AC.) If it's below .5vac, replace it.

I'm not gonna post how to test everything that could be wrong, but if you do a quick search you should find what you need for the rest.

If it is indeed the CPS theres tons of guides on how to change it on here, but you can get it swapped at a shop for around the same price you'd pay to do it yourself. Plus, it's a complete pain in the butt to swap out, worth paying the hour of labor a shop will charge you for it imo.

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post #3 of 8 Old 11-28-2009, 05:14 PM
It's the crank sensor!
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You need to adopt a methodical testing strategy to determine the root cause of your problems. You undoubtedly have more than one issue with your XJ.

For the high idle, it is often related to the idle air control. I've included a procedure for cleaning it and the throttle body.

For the engine driveability problems, you must fix that vacuum leak. You also should have the battery LOAD TESTED to ensure you have solid available battery voltage. That may be related to your "slow crank" problem. Many stores will perform this test for no charge. Also ensure that your battery cables and posts are shiny clean.

Replacing tuneup hardware if it hasn't been done recently is ALWAYS a good idea. Plugs, wires, cap, rotor can be responsible for lots of driveability issues. Replacement of this hardware is not that expensive and should always be done before moving onto more complex causes.

RENIX vintage XJs such as yours are very susceptible to ground problems. I would recommend verifying and freshening them.

If you continue to have problems after chasing some of the basics mentioned above, then fuel pressure testing with a gauge on the schrader valve on the fuel rail and comparing your readings to factory spec makes some sense.

Testing your way to solutions is your best bet. Don't throw parts at this on a hunch. It becomes expensive quickly and is terribly inefficient.

Good luck; take one problem at a time and post back what you find and further questions.


The Idle Air Control (IAC) is mounted on the back of the throttle body. The valve controls the idle speed of the engine by controlling the amount of air flowing through the air control passage. It consists of a stepper motor that moves a pintle shaped plunger in and out of the air control passage. When the valve plunger is moved in, the air control passage flows more air which raises the idle speed. When the valve plunger is moved out, the air control passage flows less air which lowers the idle speed. Over time and miles, the IAC can get carboned up which can have an adverse affect on idle quality. Cleaning the IAC may restore proper function and is an easy procedure to perform and good preventive maintenance so it is never a bad idea.


*Remove the air filter cover, associated hoses and the rubber boot that goes from the air filter cover to the throttle body. Remove the IAC with a torx driver (2 bolts; one can be kind of hard to get to)

*"Gently" wiggle out the IAC from the throttle body. Gasket on the IAC can be re-used if it is not damaged

*Clean the IAC with a spray can of throttle body cleaner; inexpensive and available at any place that sells auto parts. Throttle body cleaner is recommended rather than carburetor cleaner as it is less harsh, safe for throttle body coatings and is best for this task. Use cleaner, a rag and a toothbrush and or Q-Tips. Be gentle; don’t twist or pull on the pintle that protrudes from the IAC as it is fragile and you could damage it.

*Thoroughly spray clean and flush where the IAC seats in the throttle body with the same spray cleaner

*It is also a good idea to clean the entire throttle body itself, the butterfly valve inside of the throttle body and all associated linkage as long as you have things disassembled

*Reinstall IAC and check idle quality

99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
Without "data", all you have is an opinion!
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post #4 of 8 Old 11-28-2009, 06:31 PM
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Remove, clean and scrape the ground connections at the engine dipstick tube. Reinstall. Remove, clean, scrape and clean all connectors at each end of the braided wire from the driver side firewall to the rear of the head. While you're at it, scrape the paint from underneath the eyelet on the firewall. Many of your symptoms could be caused by poor grounds. The only way to eliminate the grounds as a possiblity is to perform the above procedure. Can't hurt and will probably help.
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post #5 of 8 Old 11-28-2009, 07:09 PM
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Every RENIX owner should have this:

Jeep Cherokee Engines - RENIX (non-HO) Engine Sensor Diagnostics

and this:

Multimeter Tutorial

BUT...sounds like the cat to me. No power or acceleration, falls on it's face.

Where is the vac line in question? MAP line from MAP to throttle body sounds suspect as well.

If you PM me I'll send you the FSM on this fuel injection system, MAP etc.

1990 XJ Laredo - White & Chrome
1988 MJ Pioneer - Metallic Green and Black
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post #6 of 8 Old 11-28-2009, 08:18 PM
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Paradise XJ is providing an excellent tool by giving the RENIX engine sensor diagnostics. Real good place to start AFTER verifying that the grounds are in good shape since most sensors work off of a variable ground to the ECU. Garbage in---------Garbage out when it comes to computers.
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post #7 of 8 Old 11-29-2009, 03:50 PM
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Don't have a vac diagram, I'm sure you can google one, but here are some pics:


Vac line FROM map sensor goes to throttle body. It looks like there are 2 vac holes on the throttle body but the upper one is just a dummy to keep the "plug" in there (I think, either way it's a hole to nowhere. Just one hose from MAP to TB.

Vac line from CCV:

Goes to the fitting on the manifold. the rubber elbow on the right:

The TOP vac line connected to the fitting on the left (brass) goes to the Fuel Pressure Regulator on the end of the fuel rail...

...and also has two other vac lines below. One is a dummy (on my OEM vac harness it's blocked off) and the other goes to the EGR solenoid.

The other brass fitting at the other end of the manifold goes to the vac reservoir (the fat vac line, and through that to the cruise servo, HVAC stuff and vac disco axle) usually behind the front passenger bumper, but I relocated the reservoir when I got rid of the pressure bottle and got a new bumper.

I still have to take some more pictures of the EGR and the little doodad on the fenderwell (like, what IS that anyway).

Hope this helps.

1990 XJ Laredo - White & Chrome
1988 MJ Pioneer - Metallic Green and Black
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post #8 of 8 Old 11-29-2009, 07:56 PM
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I love your "Testa Rossa" valve cover!!!!!!!!!!!!!
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